5-1

This chapter covers the main systems found on smallairplanes. These include the engine, propeller, andinduction systems, as well as the ignition, fuel,lubrication, cooling, electrical, landing gear, autopilot,and environmental control systems. A comprehensiveintroduction to gas turbine engines is included at theend of this chapter.

POWERPLANTThe airplane engine and propeller, often referred to as apowerplant, work in combination to produce thrust.The powerplant propels the airplane and drives thevarious systems that support the operation ofan airplane.

RECIPROCATING ENGINESMost small airplanes are designed with reciprocatingengines. The name is derived from the back-and-forth,or reciprocating, movement of the pistons. It is thismotion that produces the mechanical energy needed toaccomplish work. Two common means of classifyingreciprocating engines are:

1. by cylinder arrangement with respect to thecrankshaft—radial, in-line, v-type or opposed, or

2. by the method of cooling—liquid or air-cooled.

Radial engines were widely used during World War II,and many are still in service today. With these engines,a row or rows of cylinders are arranged in a circularpattern around the crankcase. The main advantage of aradial engine is the favorable power-to-weight ratio.

In-line engines have a comparatively small frontal area,but their power-to-weight ratios are relatively low. Inaddition, the rearmost cylinders of an air-cooled,in-line engine receive very little cooling air, so theseengines are normally limited to four or six cylinders.V-type engines provide more horsepower than in-lineengines and still retain a small frontal area. Furtherimprovements in engine design led to the developmentof the horizontally-opposed engine.

Opposed-type engines are the most popular reciprocat-ing engines used on small airplanes. These enginesalways have an even number of cylinders, since acylinder on one side of the crankcase “opposes” acylinder on the other side. The majority of theseengines are air cooled and usually are mounted in ahorizontal position when installed on fixed-wingairplanes. Opposed-type engines have high power-to-weight ratios because they have a comparatively small,lightweight crankcase. In addition, the compactcylinder arrangement reduces the engine’s frontal areaand allows a streamlined installation that minimizesaerodynamic drag.

Powerplant—A complete engine and propeller combinationwith accessories.

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The main parts of a reciprocating engine include thecylinders, crankcase, and accessory housing. Theintake/exhaust valves, spark plugs, and pistons arelocated in the cylinders. The crankshaft and connectingrods are located in the crankcase. [Figure 5-1] Themagnetos are normally located on the engine accessoryhousing.

The basic principle for reciprocating engines involvesthe conversion of chemical energy, in the form of fuel,into mechanical energy. This occurs within thecylinders of the engine through a process known as thefour-stroke operating cycle. These strokes are calledintake, compression, power, and exhaust. [Figure 5-2]

1. The intake stroke begins as the piston starts itsdownward travel. When this happens, the intakevalve opens and the fuel/air mixture is drawn intothe cylinder.

2. The compression stroke begins when the intakevalve closes and the piston starts moving back tothe top of the cylinder. This phase of the cycle isused to obtain a much greater power output fromthe fuel/air mixture once it is ignited.

3. The power stroke begins when the fuel/airmixture is ignited. This causes a tremendouspressure increase in the cylinder, and forces thepiston downward away from the cylinder head,creating the power that turns the crankshaft.

4. The exhaust stroke is used to purge the cylinderof burned gases. It begins when the exhaust valveopens and the piston starts to move toward thecylinder head once again.

Even when the engine is operated at a fairly low speed,the four-stroke cycle takes place several hundred timeseach minute. In a four-cylinder engine, each cylinderoperates on a different stroke. Continuous rotation of acrankshaft is maintained by the precise timing of thepower strokes in each cylinder. Continuous operationof the engine depends on the simultaneous function ofauxiliary systems, including the induction, ignition,fuel, oil, cooling, and exhaust systems.

PROPELLERThe propeller is a rotating airfoil, subject to induceddrag, stalls, and other aerodynamic principles thatapply to any airfoil. It provides the necessary thrust topull, or in some cases push, the airplane through the air.The engine power is used to rotate the propeller, which

Intake Valve Exhaust Valve

Cylinder

Piston

Crankshaft

Connecting Rod

Crankcase

Figure 5-1. Main components of a reciprocating engine.

Intake Compression

Power Exhaust

IntakeValve

ExhaustValve

SparkPlug

Piston

ConnectingRod

Crankshaft

1 2

3 4

Figure 5-2. The arrows in this illustration indicate thedirection of motion of the crankshaft and piston during thefour-stroke cycle.

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in turn generates thrust very similar to the manner inwhich a wing produces lift. The amount of thrustproduced depends on the shape of the airfoil, the angleof attack of the propeller blade, and the r.p.m. of theengine. The propeller itself is twisted so the blade anglechanges from hub to tip. The greatest angle ofincidence, or the highest pitch, is at the hub while thesmallest pitch is at the tip. [Figure 5-3]

The reason for the twist is to produce uniform lift fromthe hub to the tip. As the blade rotates, there is adifference in the actual speed of the various portions ofthe blade. The tip of the blade travels faster than thatpart near the hub, because the tip travels a greaterdistance than the hub in the same length of time.Changing the angle of incidence (pitch) from the hubto the tip to correspond with the speed producesuniform lift throughout the length of the blade. If thepropeller blade was designed with the same angle ofincidence throughout its entire length, it would beinefficient, because as airspeed increases in flight, theportion near the hub would have a negative angle ofattack while the blade tip would be stalled. [Figure 5-4]

Small airplanes are equipped with either one of twotypes of propellers. One is the fixed-pitch, and the otheris the controllable-pitch.

FIXED-PITCH PROPELLERThe pitch of this propeller is set by the manufacturer,and cannot be changed. With this type of propeller, thebest efficiency is achieved only at a given combinationof airspeed and r.p.m.

There are two types of fixed-pitch propellers—theclimb propeller and the cruise propeller. Whether theairplane has a climb or cruise propeller installeddepends upon its intended use:

• The climb propeller has a lower pitch, thereforeless drag. Less drag results in higher r.p.m. andmore horsepower capability, which increasesperformance during takeoffs and climbs, butdecreases performance during cruising flight.

• The cruise propeller has a higher pitch, thereforemore drag. More drag results in lower r.p.m. andless horsepower capability, which decreasesperformance during takeoffs and climbs, butincreases efficiency during cruising flight.

The propeller is usually mounted on a shaft, which maybe an extension of the engine crankshaft. In this case,the r.p.m. of the propeller would be the same as thecrankshaft r.p.m. On some engines, the propeller ismounted on a shaft geared to the engine crankshaft. Inthis type, the r.p.m. of the propeller is different thanthat of the engine. In a fixed-pitch propeller, thetachometer is the indicator of engine power.[Figure 5-5]

A tachometer is calibrated in hundreds of r.p.m., andgives a direct indication of the engine and propellerr.p.m. The instrument is color-coded, with a green arcdenoting the maximum continuous operating r.p.m.Some tachometers have additional markings to reflect

Angle of Incidence—For a propeller, it is the angle formed by the chordline and the reference plane containing the propeller hub. For a wing, itis the angle formed by the chord line of the wing and the longitudinalaxis of the airplane.

Figure 5-3. Changes in propeller blade angle from hub to tip.

Mod

erat

e

Travel Distance – Moderate Speed

Sho

rt

Travel Distance

Slow Speed

Gre

ater Travel Distance – Very High Speed

2500 r.p.m.

2500 r.p.m.

2500 r.p.m

Figure 5-4. Relationship of travel distance and speed ofvarious portions of propeller blade.

Figure 5-5. Engine r.p.m. is indicated on the tachometer.

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engine and/or propeller limitations. Therefore, themanufacturer’s recommendations should be used as areference to clarify any misunderstanding oftachometer markings.

The revolutions per minute are regulated by thethrottle, which controls the fuel/air flow to the engine.At a given altitude, the higher the tachometer reading,the higher the power output of the engine.

When operating altitude increases, the tachometer maynot show correct power output of the engine. Forexample, 2,300 r.p.m. at 5,000 feet produce lesshorsepower than 2,300 r.p.m. at sea level. The reasonfor this is that power output depends on air density. Airdensity decreases as altitude increases. Therefore, adecrease in air density (higher density altitude)decreases the power output of the engine. As altitudechanges, the position of the throttle must be changed tomaintain the same r.p.m. As altitude is increased, thethrottle must be opened further to indicate the samer.p.m. as at a lower altitude.

ADJUSTABLE-PITCH PROPELLERAlthough some older adjustable-pitch propellers couldonly be adjusted on the ground, most modernadjustable-pitch propellers are designed so that you canchange the propeller pitch in flight. The firstadjustable-pitch propeller systems provided only twopitch settingsa low-pitch setting and a high-pitchsetting. Today, however, nearly all adjustable-pitchpropeller systems are capable of a range ofpitch settings.

A constant-speed propeller is the most common type ofadjustable-pitch propeller. The main advantage of aconstant-speed propeller is that it converts a highpercentage of brake horsepower (BHP) into thrusthorsepower (THP) over a wide range of r.p.m. andairspeed combinations. A constant-speed propeller ismore efficient than other propellers because it allowsselection of the most efficient engine r.p.m. for thegiven conditions.

An airplane with a constant-speed propeller has twocontrols—the throttle and the propeller control. Thethrottle controls power output, and the propellercontrol regulates engine r.p.m. and, in turn, propellerr.p.m., which is registered on the tachometer.

Once a specific r.p.m. is selected, a governorautomatically adjusts the propeller blade angle asnecessary to maintain the selected r.p.m. For example,after setting the desired r.p.m. during cruising flight, anincrease in airspeed or decrease in propeller load willcause the propeller blade angle to increase as necessaryto maintain the selected r.p.m. A reduction in airspeedor increase in propeller load will cause the propellerblade angle to decrease.

The range of possible blade angles for a constant-speedpropeller is the propeller’s constant-speed range and isdefined by the high and low pitch stops. As long as thepropeller blade angle is within the constant-speed rangeand not against either pitch stop, a constant enginer.p.m. will be maintained. However, once the propellerblades contact a pitch stop, the engine r.p.m. willincrease or decrease as appropriate, with changes inairspeed and propeller load. For example, once aspecific r.p.m. has been selected, if aircraft speeddecreases enough to rotate the propeller blades untilthey contact the low pitch stop, any further decrease inairspeed will cause engine r.p.m. to decrease the sameway as if a fixed-pitch propeller were installed. Thesame holds true when an airplane equipped with aconstant-speed propeller accelerates to a fasterairspeed. As the aircraft accelerates, the propeller bladeangle increases to maintain the selected r.p.m. until thehigh pitch stop is reached. Once this occurs, the bladeangle cannot increase any further and enginer.p.m. increases.

On airplanes that are equipped with a constant-speedpropeller, power output is controlled by the throttle andindicated by a manifold pressure gauge. The gaugemeasures the absolute pressure of the fuel/air mixtureinside the intake manifold and is more correctly ameasure of manifold absolute pressure (MAP). At aconstant r.p.m. and altitude, the amount of powerproduced is directly related to the fuel/air flow beingdelivered to the combustion chamber. As you increasethe throttle setting, more fuel and air is flowing to theengine; therefore, MAP increases. When the engine isnot running, the manifold pressure gauge indicatesambient air pressure (i.e., 29.92 in. Hg). When theengine is started, the manifold pressure indication willdecrease to a value less than ambient pressure (i.e., idleat 12 in. Hg). Correspondingly, engine failure or powerloss is indicated on the manifold gauge as an increasein manifold pressure to a value corresponding to theambient air pressure at the altitude where the failureoccurred. [Figure 5-6]

The manifold pressure gauge is color-coded to indicatethe engine’s operating range. The face of the manifoldpressure gauge contains a green arc to show the normaloperating range, and a red radial line to indicate theupper limit of manifold pressure.

For any given r.p.m., there is a manifold pressure thatshould not be exceeded. If manifold pressure isexcessive for a given r.p.m., the pressure within thecylinders could be exceeded, thus placing undue stresson the cylinders. If repeated too frequently, this stresscould weaken the cylinder components, and eventuallycause engine failure.

Manifold Absolute Pressure (MAP)—The absolute pressure of thefuel/air mixture within the intake manifold, usually indicated in inchesof mercury.

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You can avoid conditions that could overstress thecylinders by being constantly aware of the r.p.m.,especially when increasing the manifold pressure.Conform to the manufacturer’s recommendations forpower settings of a particular engine so as to maintainthe proper relationship between manifold pressureand r.p.m.

When both manifold pressure and r.p.m. need to bechanged, avoid engine overstress by making poweradjustments in the proper order:

• When power settings are being decreased, reducemanifold pressure before reducing r.p.m. If r.p.m. isreduced before manifold pressure, manifoldpressure will automatically increase and possiblyexceed the manufacturer’s tolerances.

• When power settings are being increased,reverse the order—increase r.p.m. first, thenmanifold pressure.

• To prevent damage to radial engines, operating timeat maximum r.p.m. and manifold pressure must beheld to a minimum, and operation at maximumr.p.m. and low manifold pressure must be avoided.

Under normal operating conditions, the most severewear, fatigue, and damage to high performancereciprocating engines occurs at high r.p.m. and lowmanifold pressure.

INDUCTION SYSTEMSThe induction system brings in air from the outside,mixes it with fuel, and delivers the fuel/air mixture tothe cylinder where combustion occurs. Outside airenters the induction system through an intake port onthe front of the engine cowling. This port normally con-tains an air filter that inhibits the entry of dust and otherforeign objects. Since the filter may occasionally

become clogged, an alternate source of air must beavailable. Usually, the alternate air comes from insidethe engine cowling, where it bypasses a cloggedair filter. Some alternate air sources functionautomatically, while others operate manually.

Two types of induction systems are commonly used insmall airplane engines:

1. the carburetor system, which mixes the fuel andair in the carburetor before this mixture enters theintake manifold, and

2. the fuel injection system, which mixes the fueland air just before entry into each cylinder.

CARBURETOR SYSTEMSCarburetors are classified as either float-type orpressure-type. Pressure carburetors are usually notfound on small airplanes. The basic difference betweena pressure carburetor and a float-type is the pressurecarburetor delivers fuel under pressure by a fuel pump.

In the operation of the float-type carburetor system, theoutside air first flows through an air filter, usuallylocated at an air intake in the front part of the enginecowling. This filtered air flows into the carburetor andthrough a venturi, a narrow throat in the carburetor.When the air flows through the venturi, a low-pressurearea is created, which forces the fuel to flow through amain fuel jet located at the throat. The fuel then flowsinto the airstream, where it is mixed with the flowingair. See figure 5-7 on page 5-6.

The fuel/air mixture is then drawn through the intakemanifold and into the combustion chambers, where it isignited. The “float-type carburetor” acquires its namefrom a float, which rests on fuel within the floatchamber. A needle attached to the float opens andcloses an opening at the bottom of the carburetor bowl.This meters the correct amount of fuel into thecarburetor, depending upon the position of the float,which is controlled by the level of fuel in the floatchamber. When the level of the fuel forces the float torise, the needle valve closes the fuel opening and shutsoff the fuel flow to the carburetor. The needle valveopens again when the engine requires additional fuel.The flow of the fuel/air mixture to the combustionchambers is regulated by the throttle valve, which iscontrolled by the throttle in the cockpit.

MIXTURE CONTROLCarburetors are normally calibrated at sea-levelpressure, where the correct fuel-to-air mixture ratio isestablished with the mixture control set in the FULLRICH position. However, as altitude increases, thedensity of air entering the carburetor decreases, whilethe density of the fuel remains the same. This creates a

Figure 5-6. Engine power output is indicated on the manifoldpressure gauge.

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progressively richer mixture, which can result inengine roughness and an appreciable loss of power. Theroughness normally is due to spark plug fouling fromexcessive carbon buildup on the plugs. Carbon buildupoccurs because the excessively rich mixture lowers thetemperature inside the cylinder, inhibiting completecombustion of the fuel. This condition may occurduring the pretakeoff runup at high-elevation airportsand during climbs or cruise flight at high altitudes. Tomaintain the correct fuel/air mixture, you must lean themixture using the mixture control. Leaning the mixturedecreases fuel flow, which compensates for thedecreased air density at high altitude.

During a descent from high altitude, the opposite istrue. The mixture must be enriched, or it may becometoo lean. An overly lean mixture causes detonation,which may result in rough engine operation,overheating, and a loss of power. The best way tomaintain the proper mixture is to monitor the enginetemperature and enrichen the mixture as needed.Proper mixture control and better fuel economy forfuel-injected engines can be achieved by use of anexhaust gas temperature gauge. Since the process ofadjusting the mixture can vary from one airplane toanother, it is important to refer to the Airplane FlightManual (AFM) or the Pilot’s Operating Handbook(POH) to determine the specific procedures for agiven airplane.

CARBURETOR ICINGOne disadvantage of the float-type carburetor is itsicing tendency. Carburetor ice occurs due to the effectof fuel vaporization and the decrease in air pressure inthe venturi, which causes a sharp temperature drop inthe carburetor. If water vapor in the air condenses whenthe carburetor temperature is at or below freezing, icemay form on internal surfaces of the carburetor,including the throttle valve. [Figure 5-8]

FUEL/AIR MIXTUREThe blend of fuel and air is routedto the combustion chambers to beburned.

THROTTLE VALVEThe flow of the fuel/air mixture iscontrolled by the throttle valve. Thethrottle valve is adjusted from thecockpit by the throttle.

DISCHARGE NOZZLEFuel is forced through thedischarge nozzle into the venturiby greater atmosphericpressure in the float chamber.

VENTURIThe shape of the venturi createsan area of low pressure.

AIR INLETAir enters the carburetorthrough the air inlet.

AIR BLEEDThe air bleed allows air to be mixed with fuel beingdrawn out of the discharge nozzle to decreasefuel density and promote fuel vaporization.

FUEL INLETFuel is received intothe carburetor throughthe fuel inlet.

FLOAT CHAMBERFuel level is maintainedby a float-type device.

FUEL

MIXTURE NEEDLEThe mixture needle controls fuel tothe discharge nozzle. Mixture needleposition can be adjusted using themixture control.

Figure 5-7. Float-type carburetor.

To Engine

Incoming Air

Ice

Ice

Venturi

Fuel/AirMixture

Ice

Figure 5-8. The formation of carburetor ice may reduce orblock fuel/air flow to the engine.

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The reduced air pressure, as well as the vaporization offuel, contributes to the temperature decrease in thecarburetor. Ice generally forms in the vicinity of thethrottle valve and in the venturi throat. This restrictsthe flow of the fuel/air mixture and reduces power. Ifenough ice builds up, the engine may cease to operate.

Carburetor ice is most likely to occur whentemperatures are below 70°F (21°C) and the relativehumidity is above 80 percent. However, due to thesudden cooling that takes place in the carburetor, icingcan occur even with temperatures as high as 100°F(38°C) and humidity as low as 50 percent. Thistemperature drop can be as much as 60 to 70°F.Therefore, at an outside air temperature of 100°F, atemperature drop of 70°F results in an air temperaturein the carburetor of 30°F. [Figure 5-9]

The first indication of carburetor icing in an airplanewith a fixed-pitch propeller is a decrease in enginer.p.m., which may be followed by engine roughness. Inan airplane with a constant-speed propeller, carburetoricing usually is indicated by a decrease in manifoldpressure, but no reduction in r.p.m. Propeller pitch isautomatically adjusted to compensate for loss ofpower. Thus, a constant r.p.m. is maintained. Althoughcarburetor ice can occur during any phase of flight, it isparticularly dangerous when using reduced powerduring a descent. Under certain conditions, carburetorice could build unnoticed until you try to add power. Tocombat the effects of carburetor ice, engines withfloat-type carburetors employ a carburetor heat system.

CARBURETOR HEATCarburetor heat is an anti-icing system that preheatsthe air before it reaches the carburetor. Carburetor heatis intended to keep the fuel/air mixture above thefreezing temperature to prevent the formationof carburetor ice. Carburetor heat can be used to meltice that has already formed in the carburetorprovided that the accumulation is not too great. The

emphasis, however, is on using carburetor heat as apreventative measure.

The carburetor heat should be checked during theengine runup. When using carburetor heat, follow themanufacturer’s recommendations.

When conditions are conducive to carburetor icingduring flight, periodic checks should be made to detectits presence. If detected, full carburetor heat should beapplied immediately, and it should be left in the ONposition until you are certain that all the ice has beenremoved. If ice is present, applying partial heat orleaving heat on for an insufficient time might aggravatethe situation. In extreme cases of carburetor icing, evenafter the ice has been removed, full carburetor heatshould be used to prevent further ice formation. Acarburetor temperature gauge, if installed, is veryuseful in determining when to use carburetor heat.

Whenever the throttle is closed during flight, theengine cools rapidly and vaporization of the fuel is lesscomplete than if the engine is warm. Also, in thiscondition, the engine is more susceptible to carburetoricing. Therefore, if you suspect carburetor icingconditions and anticipate closed-throttle operation,adjust the carburetor heat to the full ON position beforeclosing the throttle, and leave it on during theclosed-throttle operation. The heat will aid invaporizing the fuel, and help prevent the formation ofcarburetor ice. Periodically, open the throttle smoothlyfor a few seconds to keep the engine warm, otherwisethe carburetor heater may not provide enough heat toprevent icing.

The use of carburetor heat causes a decrease in enginepower, sometimes up to 15 percent, because the heatedair is less dense than the outside air that had beenentering the engine. This enriches the mixture. Whenice is present in an airplane with a fixed-pitch propellerand carburetor heat is being used, there is a decrease inr.p.m., followed by a gradual increase in r.p.m. as theice melts. The engine also should run more smoothlyafter the ice has been removed. If ice is not present, ther.p.m. will decrease, then remain constant. Whencarburetor heat is used on an airplane with aconstant-speed propeller, and ice is present, a decreasein the manifold pressure will be noticed, followed by agradual increase. If carburetor icing is not present, thegradual increase in manifold pressure will not beapparent until the carburetor heat is turned off.

It is imperative that a pilot recognizes carburetor icewhen it forms during flight. In addition, a loss ofpower, altitude, and/or airspeed will occur. Thesesymptoms may sometimes be accompanied byvibration or engine roughness. Once a power loss isnoticed, immediate action should be taken to eliminate

Rel

ativ

e H

umid

ity

20°F(-7°C)

32°F(0°C)

70°F(21°C)

100°F(38°C)

100%

50%

80%

60%

70%

90%High Carburetor

Icing Potential

Carburetor Icing Possible

Outside Air Temperature

Figure 5-9. Although carburetor ice is most likely to formwhen the temperature and humidity are in ranges indicatedby this chart, carburetor ice is possible under conditionsnot depicted.

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ice already formed in the carburetor, and to preventfurther ice formation. This is accomplished byapplying full carburetor heat, which will cause afurther reduction in power, and possibly engineroughness as melted ice goes through the engine. Thesesymptoms may last from 30 seconds to severalminutes, depending on the severity of the icing. Duringthis period, the pilot must resist the temptation todecrease the carburetor heat usage. Carburetor heatmust remain in the full-hot position until normalpower returns.

Since the use of carburetor heat tends to reduce theoutput of the engine and also to increase the operatingtemperature, carburetor heat should not be used whenfull power is required (as during takeoff) or duringnormal engine operation, except to check for thepresence or to remove carburetor ice.

CARBURETOR AIR TEMPERATURE GAUGESome airplanes are equipped with a carburetor airtemperature gauge, which is useful in detectingpotential icing conditions. Usually, the face of thegauge is calibrated in degrees Celsius (°C), with ayellow arc indicating the carburetor air temperatureswhere icing may occur. This yellow arc typicallyranges between -15°C and +5°C (5°F and 41°F). If theair temperature and moisture content of the air are suchthat carburetor icing is improbable, the engine can beoperated with the indicator in the yellow range with noadverse effects. However, if the atmospheric conditionsare conducive to carburetor icing, the indicator must bekept outside the yellow arc by application ofcarburetor heat.

Certain carburetor air temperature gauges have a redradial, which indicates the maximum permissiblecarburetor inlet air temperature recommended by theengine manufacturer; also, a green arc may be includedto indicate the normal operating range.

OUTSIDE AIR TEMPERATURE GAUGEMost airplanes also are equipped with an outside airtemperature (OAT) gauge calibrated in both degreesCelsius and Fahrenheit. It provides the outside orambient air temperature for calculating trueairspeed, and also is useful in detecting potentialicing conditions.

FUEL INJECTION SYSTEMSIn a fuel injection system, the fuel is injected eitherdirectly into the cylinders, or just ahead of the intakevalve. A fuel injection system is considered to be lesssusceptible to icing than the carburetor system. Impacticing on the air intake, however, is a possibility ineither system. Impact icing occurs when ice forms onthe exterior of the airplane, and blocks openings suchas the air intake for the injection system.

The air intake for the fuel injection system is similar tothat used in the carburetor system, with an alternate airsource located within the engine cowling. This sourceis used if the external air source is obstructed. Thealternate air source is usually operated automatically,with a backup manual system that can be used if theautomatic feature malfunctions.

A fuel injection system usually incorporates these basiccomponents—an engine-driven fuel pump, a fuel/aircontrol unit, fuel manifold (fuel distributor), dischargenozzles, an auxiliary fuel pump, and fuel pressure/flowindicators. [Figure 5-10]

The auxiliary fuel pump provides fuel under pressureto the fuel/air control unit for engine starting and/oremergency use. After starting, the engine-driven fuelpump provides fuel under pressure from the fuel tankto the fuel/air control unit. This control unit, whichessentially replaces the carburetor, meters fuel basedon the mixture control setting, and sends it to the fuelmanifold valve at a rate controlled by the throttle. Afterreaching the fuel manifold valve, the fuel is distributedto the individual fuel discharge nozzles. The dischargenozzles, which are located in each cylinder head, injectthe fuel/air mixture directly into each cylinderintake port.

Some of the advantages of fuel injection are:

• Reduction in evaporative icing.

• Better fuel flow.

• Faster throttle response.

• Precise control of mixture.

• Better fuel distribution.

• Easier cold weather starts.

FuelManifoldValve

FuelDischargeNozzle

Fuel/AirControlUnit

Engine-DrivenFuel Pump

Fuel Tank

AuxiliaryFuel Pump

Figure 5-10. Fuel injection system.

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Disadvantages usually include:

• Difficulty in starting a hot engine.

• Vapor locks during ground operations onhot days.

• Problems associated with restarting an enginethat quits because of fuel starvation.

SUPERCHARGERS ANDTURBOSUPERCHARGERSTo increase an engine’s horsepower, manufacturershave developed supercharger and turbosuperchargersystems that compress the intake air to increase itsdensity. Airplanes with these systems have a manifoldpressure gauge, which displays manifold absolutepressure (MAP) within the engine’s intake manifold.

On a standard day at sea level with the engine shutdown, the manifold pressure gauge will indicate theambient absolute air pressure of 29.92 in. Hg. Becauseatmospheric pressure decreases approximately 1 in. Hgper 1,000 feet of altitude increase, the manifoldpressure gauge will indicate approximately 24.92 in.Hg at an airport that is 5,000 feet above sea level withstandard day conditions.

As a normally aspirated aircraft climbs, it eventuallyreaches an altitude where the MAP is insufficient for anormal climb. That altitude limit is the aircraft’sservice ceiling, and it is directly affected by theengine’s ability to produce power. If the induction airentering the engine is pressurized, or boosted, by eithera supercharger or a turbosupercharger, the aircraft’sservice ceiling can be increased. With these systems,you can fly at higher altitudes with the advantage ofhigher true airspeeds and the increased ability tocircumnavigate adverse weather.

SUPERCHARGERSA supercharger is an engine-driven air pump orcompressor that increases manifold pressure and forcesthe fuel/air mixture into the cylinders. The higher themanifold pressure, the more dense the fuel/air mixture,and the more power an engine can produce. With anormally aspirated engine, it is not possible to havemanifold pressure higher than the existing atmosphericpressure. A supercharger is capable of boostingmanifold pressure above 30 in. Hg.

The components in a supercharged induction systemare similar to those in a normally aspirated system, withthe addition of a supercharger between the fuelmetering device and intake manifold. A supercharger isdriven by the engine through a gear train at one speed,two speeds, or variable speeds. In addition,superchargers can have one or more stages. Each stageprovides an increase in pressure. Therefore,superchargers may be classified as single stage, twostage, or multistage, depending on the number of timescompression occurs.

An early version of a single-stage, single-speedsupercharger may be referred to as a sea-levelsupercharger. An engine equipped with this type ofsupercharger is called a sea-level engine. With thistype of supercharger, a single gear-driven impeller isused to increase the power produced by an engine at allaltitudes. The drawback, however, is that with this typeof supercharger, engine power output still decreaseswith an increase in altitude, in the same way that it doeswith a normally aspirated engine.

Single-stage, single-speed superchargers are found onmany high-powered radial engines, and use an airintake that faces forward so the induction system cantake full advantage of the ram air. Intake air passesthrough ducts to a carburetor, where fuel is metered inproportion to the airflow. The fuel/air charge is thenducted to the supercharger, or blower impeller, whichaccelerates the fuel/air mixture outward. Onceaccelerated, the fuel/air mixture passes through adiffuser, where air velocity is traded for pressureenergy. After compression, the resulting high pressurefuel/air mixture is directed to the cylinders.

Some of the large radial engines developed duringWorld War II have a single-stage, two-speedsupercharger. With this type of supercharger, a singleimpeller may be operated at two speeds. The lowimpeller speed is often referred to as the low blowersetting, while the high impeller speed is called the highblower setting. On engines equipped with a two-speedsupercharger, a lever or switch in the cockpit activatesan oil-operated clutch that switches from one speed tothe other.

Under normal operations, takeoff is made with thesupercharger in the low blower position. In this mode,the engine performs as a ground-boosted engine, andthe power output decreases as the aircraft gainsaltitude. However, once the aircraft reaches a specifiedaltitude, a power reduction is made, and thesupercharger control is switched to the high blowerposition. The throttle is then reset to the desired

Service Ceiling—The maximum density altitude where the best rate-of-climb airspeed will produce a 100 feet-per-minute climb at maximumweight while in a clean configuration with maximum continuous power.

Supercharger—An engine-driven air compressor used to provideadditional pressure to the induction air so the engine can produce addi-tional power.

Sea-Level Engine—A reciprocating aircraft engine having a ratedtakeoff power that is producible only at sea level.

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manifold pressure. An engine equipped with this typeof supercharger is called an altitude engine.[Figure 5-11]

TURBOSUPERCHARGERSThe most efficient method of increasing horsepower ina reciprocating engine is by use of a turbosupercharger,or turbocharger, as it is usually called. A drawback ofgear-driven superchargers is that they use a largeamount of the engine’s power output for the amount ofpower increase they produce. This problem is avoidedwith a turbocharger, because turbochargers arepowered by an engine’s exhaust gases. This means aturbocharger recovers energy from hot exhaust gasesthat would otherwise be lost.

Another advantage of turbochargers is that they can becontrolled to maintain an engine’s rated sea-levelhorsepower from sea level up to the engine’s criticalaltitude. Critical altitude is the maximum altitude atwhich a turbocharged engine can produce its ratedhorsepower. Above the critical altitude, power outputbegins to decrease like it does for a normally aspiratedengine.

Turbochargers increase the pressure of the engine’sinduction air, which allows the engine to develop sealevel or greater horsepower at higher altitudes. Aturbocharger is comprised of two main elements—a

turbine and a compressor. The compressor sectionhouses an impeller that turns at a high rate of speed. Asinduction air is drawn across the impeller blades, theimpeller accelerates the air, allowing a large volume ofair to be drawn into the compressor housing. Theimpeller’s action subsequently produces high-pressure,high-density air, which is delivered to the engine. Toturn the impeller, the engine’s exhaust gases are used todrive a turbine wheel that is mounted on the oppositeend of the impeller’s drive shaft. By directing differentamounts of exhaust gases to flow over the turbine,more energy can be extracted, causing the impeller todeliver more compressed air to the engine. The wastegate is used to vary the mass of exhaust gas flowinginto the turbine. A waste gate is essentially anadjustable butterfly valve that is installed in the exhaustsystem. When closed, most of the exhaust gases fromthe engine are forced to flow through the turbine. Whenopen, the exhaust gases are allowed to bypass theturbine by flowing directly out through the engine’sexhaust pipe. [Figure 5-12]

Since the temperature of a gas rises when it iscompressed, turbocharging causes the temperature ofthe induction air to increase. To reduce thistemperature and lower the risk of detonation, manyturbocharged engines use an intercooler. Anintercooler is a small heat exchanger that uses outsideair to cool the hot compressed air before it enters thefuel metering device.

SYSTEM OPERATIONOn most modern turbocharged engines, the position ofthe waste gate is governed by a pressure-sensingcontrol mechanism coupled to an actuator. Engine oildirected into or away from this actuator moves thewaste gate position. On these systems, the actuator isautomatically positioned to produce the desired MAPsimply by changing the position of the throttle control.

Other turbocharging system designs use a separatemanual control to position the waste gate. With manualcontrol, you must closely monitor the manifoldpressure gauge to determine when the desired MAP hasbeen achieved. Manual systems are often found onaircraft that have been modified with aftermarketturbocharging systems. These systems require specialoperating considerations. For example, if the wastegate is left closed after descending from a high altitude,it is possible to produce a manifold pressure thatexceeds the engine’s limitations. This condition is

Brake

Horsepower

S.L. Density Altitude

Normally Aspirated Engine

High BlowerLow Blower

Two SpeedSuperchargedEngine

Figure 5-11. Power output of normally aspirated engine com-pared to a single-stage, two-speed supercharged engine.

Altitude Engine—A reciprocating aircraft engine having a rated takeoffpower that is producible from sea level to an established higher altitude.

Turbocharger—An air compressor driven by exhaust gases, whichincreases the pressure of the air going into the engine through thecarburetor or fuel injection system.

Waste Gate—A controllable valve in the exhaust system of areciprocating engine equipped with a turbocharger. The valve iscontrolled to vary the amount of exhaust gases forced through theturbocharger turbine.

Intercooler—A device used to remove heat from air or liquid.

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referred to as an overboost, and it may produce severedetonation because of the leaning effect resulting fromincreased air density during descent.

Although an automatic waste gate system is less likelyto experience an overboost condition, it can still occur.If you try to apply takeoff power while the engine oiltemperature is below its normal operating range, thecold oil may not flow out of the waste gate actuatorquickly enough to prevent an overboost. To helpprevent overboosting, you should advance the throttlecautiously to prevent exceeding the maximummanifold pressure limits.

There are system limitations that you should be awareof when flying an aircraft with a turbocharger. Forinstance, a turbocharger turbine and impeller canoperate at rotational speeds in excess of 80,000 r.p.m.while at extremely high temperatures. To achieve highrotational speed, the bearings within the system mustbe constantly supplied with engine oil to reduce thefrictional forces and high temperature. To obtainadequate lubrication, the oil temperature should be inthe normal operating range before high throttle settingsare applied. In addition, you should allow theturbocharger to cool and the turbine to slow downbefore shutting the engine down. Otherwise, the oilremaining in the bearing housing will boil, causinghard carbon deposits to form on the bearings and shaft.These deposits rapidly deteriorate the turbocharger’sefficiency and service life. For further limitations, referto the AFM/POH.

HIGH ALTITUDE PERFORMANCEAs an aircraft equipped with a turbocharging systemclimbs, the waste gate is gradually closed to maintainthe maximum allowable manifold pressure. At somepoint, however, the waste gate will be fully closed, andwith further increases in altitude, the manifold pressurewill begin to decrease. This is the critical altitude,which is established by the airplane or enginemanufacturer. When evaluating the performance of theturbocharging system, if the manifold pressure beginsdecreasing before the specified critical altitude, theengine and turbocharging system should be inspectedby a qualified aviation maintenance technician toverify the system’s proper operation.

IGNITION SYSTEMThe ignition system provides the spark that ignites thefuel/air mixture in the cylinders and is made up ofmagnetos, spark plugs, high-tension leads, and theignition switch. [Figure 5-13]

Exhaust GasDischarge Waste Gate

This controls the amount of exhaustthrough the turbine. Waste gate positionis actuated by engine oil pressure.

TurbochargerThe turbocharger incorporates aturbine, which is driven by exhaustgases, and a compressor thatpressurizes the incoming air.

Throttle BodyThis regulates airflow to the engine.

Intake ManifoldPressurized air from theturbocharger is suppliedto the cylinders.

Exhaust ManifoldExhaust gas is ducted throughthe exhaust manifold and isused to turn the turbine whichdrives the compressor.

Air IntakeIntake air is ducted to the turbochargerwhere it is compressed.

Figure 5-12.Turbocharger components.

Overboost—A condition in which a reciprocating engine has exceededthe maximum manifold pressure allowed by the manufacturer.

OFFR L BOTH

START

UpperMagneto Wires

LowerMagneto Wires

UpperSparkPlugs

Lower SparkPlugs

Left Magneto Right Magneto

LowerSparkPlugs

UpperSparkPlugs

2

3

1

4

Figure 5-13. Ignition system components.

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A magneto uses a permanent magnet to generate anelectrical current completely independent of theaircraft’s electrical system. The magneto generatessufficiently high voltage to jump a spark across thespark plug gap in each cylinder. The system begins tofire when you engage the starter and the crankshaftbegins to turn. It continues to operate whenever thecrankshaft is rotating.

Most standard certificated airplanes incorporate a dualignition system with two individual magnetos, separatesets of wires, and spark plugs to increase reliabilityof the ignition system. Each magneto operatesindependently to fire one of the two spark plugs in eachcylinder. The firing of two spark plugs improvescombustion of the fuel/air mixture and results in aslightly higher power output. If one of the magnetosfails, the other is unaffected. The engine will continueto operate normally, although you can expect a slightdecrease in engine power. The same is true if one of thetwo spark plugs in a cylinder fails.

The operation of the magneto is controlled in thecockpit by the ignition switch. The switch hasfive positions:

1. OFF

2. R—Right

3. L—Left

4. BOTH

5. START

With RIGHT or LEFT selected, only the associatedmagneto is activated. The system operates on bothmagnetos with BOTH selected.

You can identify a malfunctioning ignition systemduring the pretakeoff check by observing the decreasein r.p.m. that occurs when you first move the ignitionswitch from BOTH to RIGHT, and then from BOTH toLEFT. A small decrease in engine r.p.m. is normalduring this check. The permissible decrease is listed inthe AFM or POH. If the engine stops running when youswitch to one magneto or if the r.p.m. drop exceeds theallowable limit, do not fly the airplane until theproblem is corrected. The cause could be fouled plugs,broken or shorted wires between the magneto and theplugs, or improperly timed firing of the plugs. It shouldbe noted that “no drop” in r.p.m. is not normal, and inthat instance, the airplane should not be flown.

Following engine shutdown, turn the ignition switch tothe OFF position. Even with the battery and masterswitches OFF, the engine can fire and turn over if youleave the ignition switch ON and the propeller ismoved because the magneto requires no outside sourceof electrical power. The potential for serious injury inthis situation is obvious.

Loose or broken wires in the ignition system also cancause problems. For example, if the ignition switch isOFF, the magneto may continue to fire if the ignitionswitch ground wire is disconnected. If this occurs, theonly way to stop the engine is to move themixture lever to the idle cutoff position, then have thesystem checked by a qualified aviation maintenancetechnician.

COMBUSTIONDuring normal combustion, the fuel/air mixture burnsin a very controlled and predictable manner. Althoughthe process occurs in a fraction of a second, the mixtureactually begins to burn at the point where it is ignitedby the spark plugs, then burns away from the plugsuntil it is all consumed. This type of combustion causesa smooth buildup of temperature and pressure andensures that the expanding gases deliver the maximumforce to the piston at exactly the right time in the powerstroke. [Figure 5-14]

Detonation is an uncontrolled, explosive ignition ofthe fuel/air mixture within the cylinder’s combustionchamber. It causes excessive temperatures andpressures which, if not corrected, can quickly lead tofailure of the piston, cylinder, or valves. In less severecases, detonation causes engine overheating,roughness, or loss of power.

Detonation is characterized by high cylinder headtemperatures, and is most likely to occur whenoperating at high power settings. Some commonoperational causes of detonation include:

Magneto—A self-contained, engine-driven unit that supplies electricalcurrent to the spark plugs; completely independent of the airplane’selectrical system. Normally, there are two magnetos per engine.

Normal Combustion Explosion

Figure 5-14. Normal combustion and explosive combustion.

Detonation—An uncontrolled, explosive ignition of the fuel/air mixturewithin the cylinder’s combustion chamber.

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• Using a lower fuel grade than that specified bythe aircraft manufacturer.

• Operating with extremely high manifoldpressures in conjunction with low r.p.m.

• Operating the engine at high power settings withan excessively lean mixture.

• Detonation also can be caused by extendedground operations, or steep climbs where cylinder cooling is reduced.

Detonation may be avoided by following these basicguidelines during the various phases of ground andflight operations:

• Make sure the proper grade of fuel is being used.

• While on the ground, keep the cowl flaps (ifavailable) in the full-open position to provide themaximum airflow through the cowling.

• During takeoff and initial climb, the onset ofdetonation can be reduced by using an enrichedfuel mixture, as well as using a shallower climbangle to increase cylinder cooling.

• Avoid extended, high power, steep climbs.

• Develop a habit of monitoring the engineinstruments to verify proper operation accordingto procedures established by the manufacturer.

Preignition occurs when the fuel/air mixture ignitesprior to the engine’s normal ignition event. Prematureburning is usually caused by a residual hot spot in thecombustion chamber, often created by a small carbondeposit on a spark plug, a cracked spark plug insulator,or other damage in the cylinder that causes a part toheat sufficiently to ignite the fuel/air charge.Preignition causes the engine to lose power, andproduces high operating temperature. As withdetonation, preignition may also cause severeengine damage, because the expanding gases exertexcessive pressure on the piston while still on itscompression stroke.

Detonation and preignition often occur simultaneouslyand one may cause the other. Since either conditioncauses high engine temperature accompanied by adecrease in engine performance, it is often difficult todistinguish between the two. Using the recommendedgrade of fuel and operating the engine within its propertemperature, pressure, and r.p.m. ranges reduce thechance of detonation or preignition.

FUEL SYSTEMSThe fuel system is designed to provide an uninterruptedflow of clean fuel from the fuel tanks to the engine. Thefuel must be available to the engine under allconditions of engine power, altitude, attitude, andduring all approved flight maneuvers. Two commonclassifications apply to fuel systems in smallairplanesgravity-feed and fuel-pump systems.

The gravity-feed system utilizes the force of gravity totransfer the fuel from the tanks to the engineforexample, on high-wing airplanes where the fuel tanksare installed in the wings. This places the fuel tanksabove the carburetor, and the fuel is gravity fed throughthe system and into the carburetor. If the design of theairplane is such that gravity cannot be used to transferfuel, fuel pumps are installedfor example, onlow-wing airplanes where the fuel tanks in the wingsare located below the carburetor. [Figure 5-15]

Preignition—The uncontrolled combustion of the fuel/air mixture inadvance of the normal ignition.

Selector Valve

Strainer

Carburetor

Primer

Left Tank Right Tank

Left Tank Right Tank

Selector Valve

FUEL-PUMP SYSTEM

GRAVITY-FEED SYSTEM

Vent

Strainer

Engine-Driven Pump

Electric Pump

Primer

Carburetor

Figure 5-15. Gravity-feed and fuel-pump systems.

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FUEL PUMPSAirplanes with fuel pump systems have two fuelpumps. The main pump system is engine driven, and anelectrically driven auxiliary pump is provided for usein engine starting and in the event the engine pumpfails. The auxiliary pump, also known as a boost pump,provides added reliability to the fuel system. Theelectrically driven auxiliary pump is controlled by aswitch in the cockpit.

FUEL PRIMERBoth gravity fed and pump systems may incorporate afuel primer into the system. The primer is used to drawfuel from the tanks to vaporize it directly into thecylinders prior to starting the engine. This isparticularly helpful during cold weather, when enginesare hard to start because there is not enough heatavailable to vaporize the fuel in the carburetor. It isimportant to lock the primer in place when it is not inuse. If the knob is free to move, it may vibrate outduring flight and can cause an excessively richmixture. To avoid overpriming, read the priminginstructions for your airplane.

FUEL TANKSThe fuel tanks, normally located inside the wings of anairplane, have a filler opening on top of the wingthrough which they can be filled. A filler cap coversthis opening. The tanks are vented to the outside tomaintain atmospheric pressure inside the tank. Theymay be vented through the filler cap or through a tubeextending through the surface of the wing. Fuel tanksalso include an overflow drain that may stand alone orbe collocated with the fuel tank vent. This allows fuelto expand with increases in temperature withoutdamage to the tank itself. If the tanks have been filledon a hot day, it is not unusual to see fuel coming fromthe overflow drain.

FUEL GAUGESThe fuel quantity gauges indicate the amount of fuelmeasured by a sensing unit in each fuel tank and isdisplayed in gallons or pounds. Aircraft certificationrules only require accuracy in fuel gauges when theyread “empty.” Any reading other than “empty” shouldbe verified. Do not depend solely on the accuracy ofthe fuel quantity gauges. Always visually check thefuel level in each tank during the preflight inspection,and then compare it with the corresponding fuelquantity indication.

If a fuel pump is installed in the fuel system, a fuelpressure gauge is also included. This gauge indicatesthe pressure in the fuel lines. The normal operatingpressure can be found in the AFM/POH, or on thegauge by color coding.

FUEL SELECTORSThe fuel selector valve allows selection of fuel fromvarious tanks. A common type of selector valvecontains four positions: LEFT, RIGHT, BOTH, andOFF. Selecting the LEFT or RIGHT position allowsfuel to feed only from that tank, while selecting theBOTH position feeds fuel from both tanks. The LEFTor RIGHT position may be used to balance the amountof fuel remaining in each wing tank. [Figure 5-16]

Fuel placards will show any limitations on fuel tankusage, such as “level flight only” and/or “both” forlandings and takeoffs.

Regardless of the type of fuel selector in use, fuelconsumption should be monitored closely to ensurethat a tank does not run completely out of fuel. Runninga fuel tank dry will not only cause the engine to stop,but running for prolonged periods on one tank causesan unbalanced fuel load between tanks. Running a tankcompletely dry may allow air to enter the fuel system,which may cause vapor lock. When this situationdevelops, it may be difficult to restart the engine. Onfuel-injected engines, the fuel may become so hot itvaporizes in the fuel line, not allowing fuel to reachthe cylinders.

FUEL STRAINERS, SUMPS, AND DRAINSAfter the fuel selector valve, the fuel passes through astrainer before it enters the carburetor. This strainerremoves moisture and other sediments that might be inthe system. Since these contaminants are heavier thanaviation fuel, they settle in a sump at the bottom of thestrainer assembly. A sump is defined as a low point in afuel system and/or fuel tank. The fuel system maycontain sump, fuel strainer, and fuel tank drains, someof which may be collocated.

The fuel strainer should be drained before each flight.Fuel samples should be drained and checked visually

Figure 5-16. Fuel selector valve.

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for water and contaminants. Water in the sump ishazardous because in cold weather the water can freezeand block fuel lines. In warm weather, it can flow intothe carburetor and stop the engine. If water is present inthe sump, it is likely there is more water in the fueltanks, and you should continue to drain them until thereis no evidence of water. In any event, never take offuntil you are certain that all water and contaminantshave been removed from the engine fuel system.

Because of the variation in fuel systems, you shouldbecome thoroughly familiar with the systems that applyto your airplane. Consult the AFM or POH for specificoperating procedures.

FUEL GRADESAviation gasoline, or AVGAS, is identified by anoctane or performance number (grade), whichdesignates the antiknock value or knock resistance ofthe fuel mixture in the engine cylinder. The higher thegrade of gasoline, the more pressure the fuel canwithstand without detonating. Lower grades of fuel areused in lower-compression engines because these fuelsignite at a lower temperature. Higher grades are used inhigher-compression engines, because they must igniteat higher temperatures, but not prematurely. If theproper grade of fuel is not available, use the next highergrade as a substitute. Never use a lower grade. This cancause the cylinder head temperature and engine oiltemperature to exceed their normal operating range,which may result in detonation.

Several grades of aviation fuel are available. Care mustbe exercised to ensure that the correct aviation grade isbeing used for the specific type of engine. The properfuel grade is stated in the AFM or POH, on placards inthe cockpit, and next to the filler caps. Due to its leadcontent, auto gas should NEVER be used in aircraftengines unless the aircraft has been modified with aSupplemental Type Certificate (STC) issued by theFederal Aviation Administration.

The current method to identify aviation gasoline foraircraft with reciprocating engines is by the octane andperformance number, along with the abbreviationAVGAS. These aircraft use AVGAS 80, 100, and100LL. Although AVGAS 100LL performs the same asgrade 100, the “LL” indicates it has a low lead content.Fuel for aircraft with turbine engines is classified asJET A, JET A-1, and JET B. Jet fuel is basicallykerosene and has a distinctive kerosene smell.

Since use of the correct fuel is critical, dyes areadded to help identify the type and grade of fuel.[Figure 5-17]

In addition to the color of the fuel itself, thecolor-coding system extends to decals and variousairport fuel handling equipment. For example, allaviation gasolines are identified by name, using whiteletters on a red background. In contrast, turbine fuelsare identified by white letters on a black background.

FUEL CONTAMINATIONOf the accidents attributed to powerplant failure fromfuel contamination, most have been traced to:

• Inadequate preflight inspection by the pilot.

• Servicing aircraft with improperly filtered fuelfrom small tanks or drums.

• Storing aircraft with partially filled fuel tanks.

• Lack of proper maintenance.

Fuel should be drained from the fuel strainer quickdrain and from each fuel tank sump into a transparentcontainer, and then checked for dirt and water. Whenthe fuel strainer is being drained, water in the tank maynot appear until all the fuel has been drained from thelines leading to the tank. This indicates that waterremains in the tank, and is not forcing the fuel out ofthe fuel lines leading to the fuel strainer. Therefore,drain enough fuel from the fuel strainer to be certainthat fuel is being drained from the tank. The amountwill depend on the length of fuel line from the tank tothe drain. If water or other contaminants are found inthe first sample, drain further samples until notrace appears.

80AVGAS

100AVGAS

RED

GREEN

AVGAS 80

AVGAS 100

100LLAVGAS

BLUEAVGAS 100LL

JETA

COLORLESSOR STRAW

JET A

FUEL TYPEAND GRADE

COLOR OFFUEL

EQUIPMENTCOLOR

Figure 5-17. Aviation fuel color-coding system.

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Water may also remain in the fuel tanks after thedrainage from the fuel strainer had ceased to show anytrace of water. This residual water can be removed onlyby draining the fuel tank sump drains.

Water is the principal fuel contaminant. Suspendedwater droplets in the fuel can be identified by a cloudyappearance of the fuel or by the clear separation ofwater from the colored fuel, which occurs after thewater has settled to the bottom of the tank. As a safetymeasure, the fuel sumps should be drained before everyflight during the preflight inspection.

Fuel tanks should be filled after each flight, or at leastafter the last flight of the day to prevent moisturecondensation within the tank. Another way to preventfuel contamination is to avoid refueling from cans anddrums. Refueling from cans or drums may result in fuelcontamination.

The use of a funnel and chamois skin when refuelingfrom cans or drums is hazardous under any conditions,and should be discouraged. In remote areas or inemergency situations, there may be no alternative torefueling from sources with inadequate anticontamina-tion systems, and a chamois and funnel may be the onlypossible means of filtering fuel. However, the use of achamois will not always ensure decontaminated fuel.Worn-out chamois will not filter water; neither will anew, clean chamois that is already water-wet or damp.Most imitation chamois skins will not filter water.

REFUELING PROCEDURESStatic electricity is formed by the friction of air passingover the surfaces of an airplane in flight and by the flowof fuel through the hose and nozzle during refueling.Nylon, dacron, or wool clothing is especially prone toaccumulate and discharge static electricity from theperson to the funnel or nozzle. To guard against thepossibility of static electricity igniting fuel fumes, aground wire should be attached to the aircraft beforethe fuel cap is removed from the tank. The refuelingnozzle then should be grounded to the aircraft beforerefueling is begun, and should remain groundedthroughout the refueling process. When a fuel truck isused, it should be grounded prior to the fuel nozzlecontacting the aircraft.

If fueling from drums or cans is necessary, properbonding and grounding connections are important.Drums should be placed near grounding posts, and thefollowing sequence of connections observed:

1. Drum to ground.

2. Ground to aircraft.

3. Drum to aircraft.

4. Nozzle to aircraft before the fuel cap is removed.

When disconnecting, reverse the order.

The passage of fuel through a chamois increases thecharge of static electricity and the danger of sparks. Theaircraft must be properly grounded and the nozzle,chamois filter, and funnel bonded to the aircraft. If acan is used, it should be connected to either thegrounding post or the funnel. Under no circumstancesshould a plastic bucket or similar nonconductivecontainer be used in this operation.

STARTING SYSTEMMost small aircraft use a direct-cranking electric startersystem. This system consists of a source of electricity,wiring, switches, and solenoids to operate the starterand a starter motor. Most aircraft have starters thatautomatically engage and disengage when operated,but some older aircraft have starters that aremechanically engaged by a lever actuated by the pilot.The starter engages the aircraft flywheel, rotating theengine at a speed that allows the engine to start andmaintain operation.

Electrical power for starting is usually supplied by anon-board battery, but can also be supplied by externalpower through an external power receptacle. When thebattery switch is turned on, electricity is supplied to themain power bus through the battery solenoid. Both thestarter and the starter switch draw current from the mainbus, but the starter will not operate until the startingsolenoid is energized by the starter switch being turnedto the “start” position. When the starter switch is releasedfrom the “start” position, the solenoid removes powerfrom the starter motor. The starter motor is protectedfrom being driven by the engine through a clutch in thestarter drive that allows the engine to run faster than thestarter motor. [Figure 5-18]

When starting an engine, the rules of safety andcourtesy should be strictly observed. One of the mostimportant is to make sure there is no one near thepropeller. In addition, the wheels should be chockedand the brakes set, to avoid hazards caused byunintentional movement. To avoid damage to thepropeller and property, the airplane should be in an areawhere the propeller will not stir up gravel or dust.

OIL SYSTEMSThe engine oil system performs several importantfunctions, including:

• Lubrication of the engine’s moving parts.

• Cooling of the engine by reducing friction.

• Removing heat from the cylinders.

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• Providing a seal between the cylinder wallsand pistons.

• Carrying away contaminants.

Reciprocating engines use either a wet-sump ordry-sump oil system. In a dry-sump system, the oil iscontained in a separate tank, and circulated through theengine by pumps. In a wet-sump system, the oil islocated in a sump, which is an integral part of theengine. [Figure 5-19]

The main component of a wet-sump system is the oilpump, which draws oil from the sump and routes it tothe engine. After the oil passes through the engine, itreturns to the sump. In some engines, additionallubrication is supplied by the rotating crankshaft,which splashes oil onto portions of the engine.

An oil pump also supplies oil pressure in a dry-sumpsystem, but the source of the oil is a separate oil tank,located external to the engine. After oil is routedthrough the engine, it is pumped from the variouslocations in the engine back to the oil tank by scavengepumps. Dry sump systems allow for a greater volumeof oil to be supplied to the engine, which makes themmore suitable for very large reciprocating engines.

The oil pressure gauge provides a direct indication ofthe oil system operation. It measures the pressure in

BatteryContactor(Solenoid)

StarterContactor

ExternalPowerRelay

++

To groundthroughmasterswitch

OFF

LR

B

S

STARTER

IGNITIONSWITCH

ExternalPowerPlug

MAIN

BUS

+

-

Figure 5-18.Typical starting circuit.

Pressure Oil From Oil Pump

OIL TEMP

Sump Oil and Return OilFrom Relief Valve

Low PressureOil Screen

Oil Coolerand Filter

Oil Sump

Engine and Accessory Bearings

Oil Filler Capand Dipstick

Oil Pressure Relief Valve

Oil PressureGauge

Oil Temperature Gauge

High Pressure Oil Screen

OIL PRESS

TOP VIEW

Oil Pump

OIL TEMP

100 245

OIL PRESS

0 60 115

Figure 5-19. Wet-sump oil system.

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pounds per square inch (p.s.i.) of the oil supplied to theengine. Green indicates the normal operating range,while red indicates the minimum and maximumpressures. There should be an indication of oil pressureduring engine start. Refer to the AFM/POH formanufacturer limitations.

The oil temperature gauge measures the temperature ofoil. A green area shows the normal operating range andthe red line indicates the maximum allowabletemperature. Unlike oil pressure, changes in oiltemperature occur more slowly. This is particularlynoticeable after starting a cold engine, when it may takeseveral minutes or longer for the gauge to show anyincrease in oil temperature.

Check oil temperature periodically during flightespecially when operating in high or low ambient airtemperature. High temperature indications mayindicate a plugged oil line, a low oil quantity, a blockedoil cooler, or a defective temperature gauge. Lowtemperature indications may indicate improper oilviscosity during cold weather operations.

The oil filler cap and dipstick (for measuring the oilquantity) are usually accessible through a panel in theengine cowling. If the quantity does not meet themanufacturer’s recommended operating levels, oilshould be added. The AFM, POH, or placards near theaccess panel provide information about the correct oiltype and weight, as well as the minimum and maximumoil quantity. [Figure 5-20]

ENGINE COOLING SYSTEMSThe burning fuel within the cylinders produces intenseheat, most of which is expelled through the exhaustsystem. Much of the remaining heat, however, must beremoved, or at least dissipated, to prevent the enginefrom overheating. Otherwise, the extremely highengine temperatures can lead to loss of power,excessive oil consumption, detonation, and seriousengine damage.

While the oil system is vital to internal cooling of theengine, an additional method of cooling is necessaryfor the engine’s external surface. Most small airplanesare air cooled, although some are liquid cooled.

Air cooling is accomplished by air flowing into theengine compartment through openings in front of theengine cowling. Baffles route this air over fins attachedto the engine cylinders, and other parts of the engine,where the air absorbs the engine heat. Expulsion of thehot air takes place through one or more openings in thelower, aft portion of the engine cowling. [Figure 5-21]

The outside air enters the engine compartment throughan inlet behind the propeller hub. Baffles direct it to thehottest parts of the engine, primarily the cylinders,which have fins that increase the area exposed tothe airflow.

The air cooling system is less effective during groundoperations, takeoffs, go-arounds, and other periods ofhigh-power, low-airspeed operation. Conversely,high-speed descents provide excess air and canshock-cool the engine, subjecting it to abrupttemperature fluctuations.

Figure 5-20. Always check the engine oil level during the pre-flight inspection.

Fixed CowlOpening

Baffle

Cylinders

AirInlet

Baffle

Figure 5-21. Outside air aids in cooling the engine.

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Operating the engine at higher than its designedtemperature can cause loss of power, excessive oilconsumption, and detonation. It will also lead toserious permanent damage, such as scoring thecylinder walls, damaging the pistons and rings, andburning and warping the valves. Monitoring thecockpit engine temperature instruments will aid inavoiding high operating temperature.

Under normal operating conditions in airplanes notequipped with cowl flaps, the engine temperature canbe controlled by changing the airspeed or the poweroutput of the engine. High engine temperatures can bedecreased by increasing the airspeed and/or reducingthe power.

The oil temperature gauge gives an indirect anddelayed indication of rising engine temperature, butcan be used for determining engine temperature if thisis the only means available.

Many airplanes are equipped with a cylinder-headtemperature gauge. This instrument indicates a directand immediate cylinder temperature change. Thisinstrument is calibrated in degrees Celsius orFahrenheit, and is usually color-coded with a green arcto indicate the normal operating range. A red line onthe instrument indicates maximum allowable cylinderhead temperature.

To avoid excessive cylinder head temperatures,increase airspeed, enrich the mixture, and/or reducepower. Any of these procedures help in reducing theengine temperature. On airplanes equipped withcowl flaps, use the cowl flap positions to control thetemperature. Cowl flaps are hinged covers that fit overthe opening through which the hot air is expelled. If theengine temperature is low, the cowl flaps can be closed,thereby restricting the flow of expelled hot air andincreasing engine temperature. If the enginetemperature is high, the cowl flaps can be opened topermit a greater flow of air through the system, therebydecreasing the engine temperature.

EXHAUST SYSTEMSEngine exhaust systems vent the burned combustiongases overboard, provide heat for the cabin, and defrostthe windscreen. An exhaust system has exhaust pipingattached to the cylinders, as well as a muffler and amuffler shroud. The exhaust gases are pushed out ofthe cylinder through the exhaust valve and then throughthe exhaust pipe system to the atmosphere.

For cabin heat, outside air is drawn into the air inletand is ducted through a shroud around the muffler. Themuffler is heated by the exiting exhaust gases and, inturn, heats the air around the muffler. This heated air isthen ducted to the cabin for heat and defrostapplications. The heat and defrost are controlled in thecockpit, and can be adjusted to the desired level.

Exhaust gases contain large amounts of carbonmonoxide, which is odorless and colorless. Carbonmonoxide is deadly, and its presence is virtuallyimpossible to detect. The exhaust system must be ingood condition and free of cracks.

Some exhaust systems have an exhaust gastemperature probe. This probe transmits the exhaustgas temperature (EGT) to an instrument in the cockpit.The EGT gauge measures the temperature of the gasesat the exhaust manifold. This temperature varies withthe ratio of fuel to air entering the cylinders and can beused as a basis for regulating the fuel/air mixture. TheEGT gauge is highly accurate in indicating the correctmixture setting. When using the EGT to aid in leaningthe fuel/air mixture, fuel consumption can be reduced.For specific procedures, refer to the manufacturer’srecommendations for leaning the mixture.

ELECTRICAL SYSTEMAirplanes are equipped with either a 14- or 28-voltdirect-current electrical system. A basic airplaneelectrical system consists of the following components:

• Alternator/generator

• Battery

• Master/battery switch

• Alternator/generator switch

• Bus bar, fuses, and circuit breakers

• Voltage regulator

• Ammeter/loadmeter

• Associated electrical wiring

Engine-driven alternators or generators supply electriccurrent to the electrical system. They also maintain asufficient electrical charge in the battery. Electricalenergy stored in a battery provides a source ofelectrical power for starting the engine and a limitedsupply of electrical power for use in the event thealternator or generator fails.

Cowl Flaps—Shutter-like devices arranged around certain air-cooledengine cowlings, which may be opened or closed to regulate the flow ofair around the engine.

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Most direct current generators will not produce asufficient amount of electrical current at low enginer.p.m. to operate the entire electrical system. Therefore,during operations at low engine r.p.m., the electricalneeds must be drawn from the battery, which canquickly be depleted.

Alternators have several advantages over generators.Alternators produce sufficient current to operate theentire electrical system, even at slower engine speeds,by producing alternating current, which is converted todirect current. The electrical output of an alternator ismore constant throughout a wide range of enginespeeds.

Some airplanes have receptacles to which an externalground power unit (GPU) may be connected to provideelectrical energy for starting. These are very useful,especially during cold weather starting. Follow themanufacturer’s recommendations for engine startingusing a GPU.

The electrical system is turned on or off with a masterswitch. Turning the master switch to the ON positionprovides electrical energy to all the electricalequipment circuits with the exception of the ignitionsystem. Equipment that commonly uses the electricalsystem for its source of energy includes:

• Position lights

• Anticollision lights

• Landing lights

• Taxi lights

• Interior cabin lights

• Instrument lights

• Radio equipment

• Turn indicator

• Fuel gauges

• Electric fuel pump

• Stall warning system

• Pitot heat

• Starting motor

Many airplanes are equipped with a battery switch thatcontrols the electrical power to the airplane in amanner similar to the master switch. In addition, analternator switch is installed which permits the pilot toexclude the alternator from the electrical system in theevent of alternator failure. [Figure 5-22]

With the alternator half of the switch in the OFFposition, the entire electrical load is placed on thebattery. Therefore, all nonessential electrical equipmentshould be turned off to conserve battery power.

A bus bar is used as a terminal in the airplane electricalsystem to connect the main electrical system to theequipment using electricity as a source of power. Thissimplifies the wiring system and provides a commonpoint from which voltage can be distributed throughoutthe system. [Figure 5-23]

Fuses or circuit breakers are used in the electrical systemto protect the circuits and equipment from electricaloverload. Spare fuses of the proper amperage limitshould be carried in the airplane to replace defective orblown fuses. Circuit breakers have the same function asa fuse but can be manually reset, rather than replaced, ifan overload condition occurs in the electrical system.Placards at the fuse or circuit breaker panel identify thecircuit by name and show the amperage limit.

An ammeter is used to monitor the performance of theairplane electrical system. The ammeter shows if thealternator/generator is producing an adequate supply ofelectrical power. It also indicates whether or not thebattery is receiving an electrical charge.

Ammeters are designed with the zero point in thecenter of the face and a negative or positive indicationon either side. [Figure 5-24] When the pointer of theammeter on the left is on the plus side, it shows thecharging rate of the battery. A minus indication meansmore current is being drawn from the battery than isbeing replaced. A full-scale minus deflection indicatesa malfunction of the alternator/generator. A full-scalepositive deflection indicates a malfunction of theregulator. In either case, consult the AFM or POH forappropriate action to be taken.

Figure 5-22. On this master switch, the left half is for thealternator and the right half is for the battery.

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Not all airplanes are equipped with an ammeter. Somehave a warning light that, when lighted, indicates adischarge in the system as a generator/alternatormalfunction. Refer to the AFM or POH for appropriateaction to be taken.

Another electrical monitoring indicator is a loadmeter.This type of gauge, illustrated on the right in figure5-24, has a scale beginning with zero and shows the loadbeing placed on the alternator/generator. The loadmeterreflects the total percentage of the load placed on the

PRIMARY

BUS

AVIONICS

BUS

To Fuel Quantity Indicators

To Flashing Beacon

To Pitot Heat

To Radio Cooling Fan

To Strobe Lights

To Landing and Taxi Lights

To Ignition Switch

To Wing Flap System

To Red Doorpost Maplight

To Low-Voltage Warning Light

To Instrument, Radio, Compassand Post Lights

To Oil Temperature Gauge

To Turn Coordinator

To Low-Vacuum Warning Light

Switch/Circuit Breaker toStandby Vacuum Pump

To White Doorpost Light

To Audio Muting Relay

To Control Wheel Maplight

To Navigation Lights

To Dome Light

To Radio

To Radio

To Radio or Transponderand Encoding Altimeter

To Radio

FUELIND.

BCNPITOT

PULLOFF STROBE

RADIOFAN

LDGLTS

FLAP

INSTLTS

STBY VAC

RADIO 1

RADIO 2

RADIO 3

NAVDOME

RADIO 4

Low-VoltageWarning Light

Low Volt Out

Power In

Sense (+)

Field

Sense (-)

GroundG

F

B

Pull Off

ALT

AlternatorControl

Unit

ALT

BAT

+

-

ToWingFlap

CircuitBreaker

RL

Battery

BatteryContactor

Magnetos

Ground ServicePlug Receptacle

StarterContactor

Starter

Flight HourRecorder

Oil PressureSwitch

Clock orDigital Clock

Ammeter

AlternatorField Circuit

Breaker

Master Switch

Alternator

CODE

Circuit Breaker (Auto-Reset) Fuse Diode Resistor

Circuit Breaker (Pull-Off,Push-to-Reset)

To Inst LTS Circuit Breaker

Capacitor (Noise Filter)

Circuit Breaker (Push to Reset)

Figure 5-23. Electrical system schematic.

Figure 5-24. Ammeter and loadmeter.

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generating capacity of the electrical system by the elec-trical accessories and battery. When all electrical com-ponents are turned off, it reflects only the amount ofcharging current demanded by the battery.

A voltage regulator controls the rate of charge to thebattery by stabilizing the generator or alternatorelectrical output. The generator/alternator voltageoutput should be higher than the battery voltage. Forexample, a 12-volt battery would be fed by agenerator/alternator system of approximately 14 volts.The difference in voltage keeps the battery charged.

HYDRAULIC SYSTEMSThere are multiple applications for hydraulic use inairplanes, depending on the complexity of the airplane.For example, hydraulics are often used on smallairplanes to operate wheel brakes, retractable landinggear, and some constant-speed propellers. On largeairplanes, hydraulics are used for flight controlsurfaces, wing flaps, spoilers, and other systems.

A basic hydraulic system consists of a reservoir, pump(either hand, electric, or engine driven), a filter to keepthe fluid clean, selector valve to control the direction offlow, relief valve to relieve excess pressure, and anactuator.

The hydraulic fluid is pumped through the system to anactuator or servo. Servos can be either single-acting ordouble-acting servos based on the needs of the system.This means that the fluid can be applied to one or bothsides of the servo, depending on the servo type, andtherefore provides power in one direction with asingle-acting servo. A servo is a cylinder with a pistoninside that turns fluid power into work and creates thepower needed to move an aircraft system or flightcontrol. The selector valve allows the fluid direction tobe controlled. This is necessary for operations like theextension and retraction of landing gear where the fluidmust work in two different directions. The relief valveprovides an outlet for the system in the event ofexcessive fluid pressure in the system. Each systemincorporates different components to meet theindividual needs of different aircraft.

A mineral-based fluid is the most widely used type forsmall airplanes. This type of hydraulic fluid, which is akerosene-like petroleum product, has good lubricatingproperties, as well as additives to inhibit foaming andprevent the formation of corrosion. It is quite stablechemically, has very little viscosity change withtemperature, and is dyed for identification. Sinceseveral types of hydraulic fluids are commonly used,

make sure your airplane is serviced with the typespecified by the manufacturer. Refer to the AFM, POH,or the Maintenance Manual. [Figure 5-25]

LANDING GEARThe landing gear forms the principal support of theairplane on the surface. The most common type oflanding gear consists of wheels, but airplanes can alsobe equipped with floats for water operations, or skis forlanding on snow. [Figure 5-26]

The landing gear on small airplanes consists of threewheelstwo main wheels, one located on each side ofthe fuselage, and a third wheel, positioned either at thefront or rear of the airplane. Landing gear employing arear-mounted wheel is called a conventional landinggear. Airplanes with conventional landing gear areoften referred to as tailwheel airplanes. When the thirdwheel is located on the nose, it is called a nosewheel,and the design is referred to as a tricycle gear. Asteerable nosewheel or tailwheel permits the airplaneto be controlled throughout all operations while onthe ground.

TRICYCLE LANDING GEAR AIRPLANESA tricycle gear airplane has three main advantages:

Servo—A motor or other form of actuator which receives a small signalfrom the control device and exerts a large force to accomplish thedesired work.

Reservoir Double-Acting

Cylinder

Motion

Selector Valve

C.V. C.V.Pump

Hydraulic Fluid Supply

Hydraulic Pressure Return Fluid

System ReliefValve

Figure 5-25. Basic hydraulic system.

Figure 5-26. The landing gear supports the airplane duringthe takeoff run, landing, taxiing, and when parked.

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1. It allows more forceful application of the brakesduring landings at high speeds without resultingin the airplane nosing over.

2. It permits better forward visibility for the pilotduring takeoff, landing, and taxiing.

3. It tends to prevent ground looping (swerving) byproviding more directional stability duringground operation since the airplane’s center ofgravity (CG) is forward of the main wheels. Theforward CG, therefore, tends to keep the airplanemoving forward in a straight line rather thanground looping.

Nosewheels are either steerable or castering. Steerablenosewheels are linked to the rudders by cables or rods,while castering nosewheels are free to swivel. In bothcases, you steer the airplane using the rudder pedals.However, airplanes with a castering nosewheel mayrequire you to combine the use of the rudder pedalswith independent use of the brakes.

TAILWHEEL LANDING GEAR AIRPLANESOn tailwheel airplanes, two main wheels, which areattached to the airframe ahead of its center of gravity,support most of the weight of the structure, while atailwheel at the very back of the fuselage provides athird point of support. This arrangement allowsadequate ground clearance for a larger propeller and ismore desirable for operations on unimproved fields.[Figure 5-27]

The main drawback with the tailwheel landing gear isthat the center of gravity is behind the main gear. Thismakes directional control more difficult while on theground. If you allow the airplane to swerve whilerolling on the ground at a speed below that at which therudder has sufficient control, the center of gravity willattempt to get ahead of the main gear. This may causethe airplane to ground loop.

Another disadvantage for tailwheel airplanes is the lackof good forward visibility when the tailwheel is on ornear the surface. Because of the associated hazards,specific training is required in tailwheel airplanes.

FIXED AND RETRACTABLE LANDING GEARLanding gear can also be classified as either fixed orretractable. A fixed gear always remains extended andhas the advantage of simplicity combined with lowmaintenance. A retractable gear is designed tostreamline the airplane by allowing the landing gear tobe stowed inside the structure during cruising flight.[Figure 5-28]

BRAKESAirplane brakes are located on the main wheels and areapplied by either a hand control or by foot pedals (toeor heel). Foot pedals operate independently and allowfor differential braking. During ground operations,differential braking can supplement nosewheel/tail-wheel steering.

AUTOPILOTAutopilots are designed to control the aircraft and helpreduce the pilot’s workload. The limitations of theautopilot depend on the complexity of the system. The

Figure 5-27.Tailwheel landing gear.

Figure 5-28. Fixed and retractable gear airplanes.

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5-24

common features available on an autopilot are altitudeand heading hold. More advanced systems may includea vertical speed and/or indicated airspeed holdmode. Most autopilot systems are coupled tonavigational aids.

An autopilot system consists of servos that actuate theflight controls. The number and location of theseservos depends on the complexity of the system. Forexample, a single-axis autopilot controls the aircraftabout the longitudinal axis and a servo actuates theailerons. A three-axis autopilot controls the aircraftabout the longitudinal, lateral, and vertical axes; andthree different servos actuate the ailerons, the elevator,and the rudder.

The autopilot system also incorporates a disconnectsafety feature to automatically or manually disengagethe system. Autopilots can also be manuallyoverridden. Because autopilot systems differ widely intheir operation, refer to the autopilot operatinginstructions in the AFM or POH.

PRESSURIZED AIRPLANESWhen an airplane is flown at a high altitude, itconsumes less fuel for a given airspeed than it does forthe same speed at a lower altitude. In other words, theairplane is more efficient at a high altitude. In addition,bad weather and turbulence may be avoided by flyingin the relatively smooth air above the storms. Becauseof the advantages of flying at high altitudes,many modern general aviation-type airplanes arebeing designed to operate in that environment. It isimportant that pilots transitioning to such sophisticatedequipment be familiar with at least the basicoperating principles.

A cabin pressurization system accomplishes severalfunctions in providing adequate passenger comfort andsafety. It maintains a cabin pressure altitude ofapproximately 8,000 feet at the maximum designedcruising altitude of the airplane, and prevents rapidchanges of cabin altitude that may be uncomfortable or

cause injury to passengers and crew. In addition, thepressurization system permits a reasonably fastexchange of air from the inside to the outside of thecabin. This is necessary to eliminate odors and toremove stale air. [Figure 5-29]

Pressurization of the airplane cabin is an acceptedmethod of protecting occupants against the effects ofhypoxia. Within a pressurized cabin, occupants can betransported comfortably and safely for long periods oftime, particularly if the cabin altitude is maintained at8,000 feet or below, where the use of oxygenequipment is not required. The flight crew in this typeof airplane must be aware of the danger of accidentalloss of cabin pressure and must be prepared to deal withsuch an emergency whenever it occurs.

In the typical pressurization system, the cabin, flightcompartment, and baggage compartments areincorporated into a sealed unit that is capable ofcontaining air under a pressure higher than outsideatmospheric pressure. On aircraft powered by turbineengines, bleed air from the engine compressor section isused to pressurize the cabin. Superchargers may be usedon older model turbine powered airplanes to pump airinto the sealed fuselage. Piston-powered airplanes mayuse air supplied from each engine turbocharger througha sonic venturi (flow limiter). Air is released from thefuselage by a device called an outflow valve. Theoutflow valve, by regulating the air exit, provides aconstant inflow of air to the pressurized area.[Figure 5-30]

To understand the operating principles ofpressurization and air-conditioning systems, it isnecessary to become familiar with some of the relatedterms and definitions, such as:

• Aircraft altitude—the actual height above sealevel at which the airplane is flying.

• Ambient temperature—the temperature in thearea immediately surrounding the airplane.

Altitude (ft) Pressure (p.s.i.) Altitude (ft) Pressure (p.s.i.)

Sea Level

2,000

4,000

6,000

8,000

10,000

12,000

14,000

14.7

13.7

12.7

11.8

10.9

10.1

9.4

8.6

16,000

18,000

20,000

22,000

24,000

26,000

28,000

30,000

8.0

7.3

6.8

6.2

5.7

5.2

4.8

4.4

STANDARD ATMOSPHERIC PRESSURE

At an altitude of 28,000 feet, standard atmospheric pressure is 4.8 p.s.i. By adding this pressure to the cabin pressure differential of 6.1 p.s.i.d., a total air pressure of 10.9 p.s.i. is obtained.

The altitude where the standard air pressure is equal to 10.9 p.s.i can be found at 8,000 feet.

Figure 5-29. Standard atmospheric pressure chart.

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• Ambient pressure—the pressure in the areaimmediately surrounding the airplane.

• Cabin altitude—used to express cabin pressurein terms of equivalent altitude above sea level.

• Differential pressure—the difference in pressurebetween the pressure acting on one side of a walland the pressure acting on the other side of thewall. In aircraft air-conditioning and pressurizingsystems, it is the difference between cabinpressure and atmospheric pressure.

The cabin pressure control system provides cabinpressure regulation, pressure relief, vacuum relief, andthe means for selecting the desired cabin altitude in theisobaric and differential range. In addition, dumping ofthe cabin pressure is a function of the pressure controlsystem. A cabin pressure regulator, an outflow valve,and a safety valve are used to accomplish thesefunctions.

The cabin pressure regulator controls cabin pressure toa selected value in the isobaric range and limits cabinpressure to a preset differential value in the differentialrange. When the airplane reaches the altitude at whichthe difference between the pressure inside and outsidethe cabin is equal to the highest differential pressure for

which the fuselage structure is designed, a furtherincrease in airplane altitude will result in acorresponding increase in cabin altitude. Differentialcontrol is used to prevent the maximum differentialpressure, for which the fuselage was designed, frombeing exceeded. This differential pressure isdetermined by the structural strength of the cabin andoften by the relationship of the cabin size to theprobable areas of rupture, such as window areasand doors.

The cabin air pressure safety valve is a combinationpressure relief, vacuum relief, and dump valve. Thepressure relief valve prevents cabin pressure fromexceeding a predetermined differential pressure aboveambient pressure. The vacuum relief prevents ambientpressure from exceeding cabin pressure by allowingexternal air to enter the cabin when ambient pressureexceeds cabin pressure. The cockpit control switchactuates the dump valve. When this switch ispositioned to ram, a solenoid valve opens, causing thevalve to dump cabin air to atmosphere.

The degree of pressurization and the operating altitudeof the aircraft are limited by several critical designfactors. Primarily the fuselage is designed to withstanda particular maximum cabin differential pressure.

Several instruments are used in conjunction with thepressurization controller. The cabin differentialpressure gauge indicates the difference between insideand outside pressure. This gauge should be monitoredto assure that the cabin does not exceed the maximumallowable differential pressure. A cabin altimeter is alsoprovided as a check on the performance of the system.In some cases, these two instruments are combined intoone. A third instrument indicates the cabin rate of climbor descent. A cabin rate-of-climb instrument and acabin altimeter are illustrated in Figure 5-31 onpage 5-26.

Decompression is defined as the inability of theairplane’s pressurization system to maintain its designedpressure differential. This can be caused by amalfunction in the pressurization system or structuraldamage to the airplane. Physiologically, decompressionsfall into two categories; they are:

• Explosive Decompression—Explosive decom-pression is defined as a change in cabin pressurefaster than the lungs can decompress; therefore, itis possible that lung damage may occur. Normally,the time required to release air from the lungswithout restrictions, such as masks, is 0.2 seconds.Most authorities consider any decompression thatoccurs in less than 0.5 seconds as explosive andpotentially dangerous.

Air Scoops

Heat ShroudCabin

Heat Valve

Heat Exchanger

TurbochargerCompressor Section

Flow Control Venturi

Forward Air Outlet

Floor Level Outlets

To CabinAltitudeController

Outflow ValveSafety/Dump Valve

BLUE – Ambient Air

RED – Compressor Discharge Air

ORANGE –Pressurization Air

CODE

BROWN – Pre-heatedAmbient Air

GREEN – ConditionedPressurization Air

Pressurized Cabin

Figure 5-30. High performance airplane pressurizationsystem.

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• Rapid Decompression—Rapid decompression isdefined as a change in cabin pressure where thelungs can decompress faster than the cabin; there-fore, there is no likelihood of lung damage.

During an explosive decompression, there may benoise, and for a split second, one may feel dazed. Thecabin air will fill with fog, dust, or flying debris. Fogoccurs due to the rapid drop in temperature and thechange of relative humidity. Normally, the ears clearautomatically. Air will rush from the mouth and nosedue to the escape of air from the lungs, and may benoticed by some individuals.

The primary danger of decompression is hypoxia.Unless proper utilization of oxygen equipment isaccomplished quickly, unconsciousness may occur in avery short time. The period of useful consciousness isconsiderably shortened when a person is subjected to arapid decompression. This is due to the rapid reductionof pressure on the body—oxygen in the lungs isexhaled rapidly. This in effect reduces the partialpressure of oxygen in the blood and therefore reducesthe pilot’s effective performance time by one-third toone-fourth its normal time. For this reason, the oxygenmask should be worn when flying at very high altitudes(35,000 feet or higher). It is recommended that thecrewmembers select the 100 percent oxygen setting onthe oxygen regulator at high altitude if the airplane isequipped with a demand or pressure demand oxygensystem.

Another hazard is being tossed or blown out of theairplane if near an opening. For this reason, individualsnear openings should wear safety harnesses or seatbeltsat all times when the airplane is pressurized and theyare seated.

Another potential hazard during high altitudedecompressions is the possibility of evolved gasdecompression sicknesses. Exposure to wind blasts andextremely cold temperatures are other hazards onemight have to face.

Rapid descent from altitude is necessary if theseproblems are to be minimized. Automatic visual andaural warning systems are included in the equipment ofall pressurized airplanes.

OXYGEN SYSTEMSMost high altitude airplanes come equipped with sometype of fixed oxygen installation. If the airplane doesnot have a fixed installation, portable oxygenequipment must be readily accessible during flight. Theportable equipment usually consists of a container,regulator, mask outlet, and pressure gauge. Aircraftoxygen is usually stored in high pressure systemcontainers of 1,800 – 2,200 pounds per square inch(p.s.i.). When the ambient temperature surrounding anoxygen cylinder decreases, pressure within thatcylinder will decrease because pressure varies directlywith temperature if the volume of a gas remainsconstant. If a drop in indicated pressure on asupplemental oxygen cylinder is noted, there is noreason to suspect depletion of the oxygen supply, whichhas simply been compacted due to storage of thecontainers in an unheated area of the aircraft. Highpressure oxygen containers should be marked with thep.s.i. tolerance (i.e., 1,800 p.s.i.) before filling thecontainer to that pressure. The containers should besupplied with aviation oxygen only, which is 100percent pure oxygen. Industrial oxygen is not intendedfor breathing and may contain impurities, and medicaloxygen contains water vapor that can freeze in theregulator when exposed to cold temperatures. To assuresafety, oxygen system periodic inspection andservicing should be done.

An oxygen system consists of a mask and a regulatorthat supplies a flow of oxygen dependent upon cabinaltitude. Regulators approved for use up to 40,000 feetare designed to provide zero percent cylinder oxygenand 100 percent cabin air at cabin altitudes of 8,000feet or less, with the ratio changing to 100 percentoxygen and zero percent cabin air at approximately34,000 feet cabin altitude. Regulators approved up to45,000 feet are designed to provide 40 percent cylinderoxygen and 60 percent cabin air at lower altitudes, with

Cabin Pressure Altitude Indicator (Thousands of Feet)

Cabin Differential Pressure Indicator (Pounds Per Square Inch Differential)

Maximum Cabin Differential Pressure Limit

Cabin Rate-Of-ClimbIndicator

Cabin/DifferentialPressure Indicator

Figure 5-31. Cabin pressurization instruments.

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the ratio changing to 100 percent at the higher altitude.Pilots should avoid flying above 10,000 feet withoutoxygen during the day and above 8,000 feet at night.[Figure 5-32]

Pilots should be aware of the danger of fire when usingoxygen. Materials that are nearly fireproof in ordinaryair may be susceptible to burning in oxygen. Oils andgreases may catch fire if exposed to oxygen, andcannot be used for sealing the valves and fittings ofoxygen equipment. Smoking during any kind ofoxygen equipment use is prohibited. Before each flight,the pilot should thoroughly inspect and test all oxygenequipment. The inspection should include a thoroughexamination of the aircraft oxygen equipment,including available supply, an operational check of thesystem, and assurance that the supplemental oxygen isreadily accessible. The inspection should beaccomplished with clean hands and should include avisual inspection of the mask and tubing for tears,cracks, or deterioration; the regulator for valve andlever condition and positions; oxygen quantity; and thelocation and functioning of oxygen pressure gauges,flow indicators and connections. The mask should bedonned and the system should be tested. After anyoxygen use, verify that all components and valves areshut off.

MASKSThere are numerous types of oxygen masks in use thatvary in design detail. It would be impractical to discussall of the types in this handbook. It is important that themasks used be compatible with the particular oxygensystem involved. Crew masks are fitted to the user’sface with a minimum of leakage. Crew masks usuallycontain a microphone. Most masks are the oronasal-type, which covers only the mouth and nose.

Passenger masks may be simple, cup-shaped rubbermoldings sufficiently flexible to obviate individualfitting. They may have a simple elastic head strap or

the passenger may hold them to the face.

All oxygen masks should be kept clean. This reducesthe danger of infection and prolongs the life of themask. To clean the mask, wash it with a mild soap andwater solution and rinse it with clear water. If amicrophone is installed, use a clean swab, instead ofrunning water, to wipe off the soapy solution. The maskshould also be disinfected. A gauze pad that has beensoaked in a water solution of Merthiolate can be usedto swab out the mask. This solution should containone-fifth teaspoon of Merthiolate per quart of water.Wipe the mask with a clean cloth and air dry.

DILUTER DEMAND OXYGEN SYSTEMSDiluter demand oxygen systems supply oxygen onlywhen the user inhales through the mask. An automixlever allows the regulators to automatically mix cabinair and oxygen or supply 100 percent oxygen,depending on the altitude. The demand mask providesa tight seal over the face to prevent dilution withoutside air and can be used safely up to 40,000 feet. Apilot who has a beard or mustache should be sure it istrimmed in a manner that will not interfere with thesealing of the oxygen mask. The fit of the mask aroundthe beard or mustache should be checked on the groundfor proper sealing.

PRESSURE DEMAND OXYGEN SYSTEMSPressure demand oxygen systems are similar to diluterdemand oxygen equipment, except that oxygen issupplied to the mask under pressure at cabin altitudesabove 34,000 feet. Pressure demand regulators alsocreate airtight and oxygen-tight seals, but they alsoprovide a positive pressure application of oxygen to themask face piece that allows the user’s lungs to bepressurized with oxygen. This feature makes pressuredemand regulators safe at altitudes above 40,000 feet.Some systems may have a pressure demand mask withthe regulator attached directly to the mask, rather thanmounted on the instrument panel or other area withinthe flight deck. The mask-mounted regulator eliminatesthe problem of a long hose that must be purged of airbefore 100 percent oxygen begins flowing intothe mask.

CONTINUOUS FLOW OXYGEN SYSTEMContinuous flow oxygen systems are usually providedfor passengers. The passenger mask typically has areservoir bag, which collects oxygen from thecontinuous flow oxygen system during the time whenthe mask user is exhaling. The oxygen collected in thereservoir bag allows a higher aspiratory flow rateduring the inhalation cycle, which reduces the amountof air dilution. Ambient air is added to the supplied

Figure 5-32. Oxygen system regulator.

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oxygen during inhalation after the reservoir bagoxygen supply is depleted. The exhaled air is releasedto the cabin. [Figure 5-33]

SERVICING OF OXYGEN SYSTEMSCertain precautions should be observed wheneveraircraft oxygen systems are to be serviced. Beforeservicing any aircraft with oxygen, consult the specificaircraft service manual to determine the type ofequipment required and procedures to be used. Oxygensystem servicing should be accomplished only whenthe aircraft is located outside of the hangars. Personalcleanliness and good housekeeping are imperativewhen working with oxygen. Oxygen under pressureand petroleum products create spontaneous resultswhen they are brought in contact with each other.Service people should be certain to wash dirt, oil, andgrease (including lip salves and hair oil) from theirhands before working around oxygen equipment. It isalso essential that clothing and tools are free of oil,grease, and dirt. Aircraft with permanently installedoxygen tanks usually require two persons toaccomplish servicing of the system. One should bestationed at the service equipment control valves, andthe other stationed where he or she can observe theaircraft system pressure gauges. Oxygen systemservicing is not recommended during aircraft fuelingoperations or while other work is performed that couldprovide a source of ignition. Oxygen system servicingwhile passengers are on board the aircraft isnot recommended.

ICE CONTROL SYSTEMSIce control systems installed on aircraft consist ofanti-ice and de-ice equipment. Anti-icing equipment isdesigned to prevent the formation of ice, while de-icingequipment is designed to remove ice once it hasformed. Ice control systems protect the leading edge ofwing and tail surfaces, pitot and static port openings,fuel tank vents, stall warning devices, windshields, andpropeller blades. Ice detection lighting may also beinstalled on some airplanes to determine the extent ofstructural icing during night flights. Since manyairplanes are not certified for flight in icing conditions,refer to the AFM or POH for details.

AIRFOIL ICE CONTROLInflatable de-icing boots consist of a rubber sheetbonded to the leading edge of the airfoil. When icebuilds up on the leading edge, an engine-drivenpneumatic pump inflates the rubber boots. Someturboprop aircraft divert engine bleed air to the wing toinflate the rubber boots. Upon inflation, the ice iscracked and should fall off the leading edge of thewing. De-icing boots are controlled from the cockpitby a switch and can be operated in a single cycle orallowed to cycle at automatic, timed intervals. It isimportant that de-icing boots are used in accordancewith the manufacturer’s recommendations. If they areallowed to cycle too often, ice can form over thecontour of the boot and render the boots ineffective.[Figure 5-34]

Many de-icing boot systems use the instrument systemsuction gauge and a pneumatic pressure gauge toindicate proper boot operation. These gauges haverange markings that indicate the operating limits forboot operation. Some systems may also incorporate anannunciator light to indicate proper boot operation.

Proper maintenance and care of de-icing boots isimportant for continued operation of this system. Theyneed to be carefully inspected prior to a flight.

Another type of leading edge protection is the thermalanti-ice system installed on airplanes with turbineengines. This system is designed to prevent the buildupof ice by directing hot air from the compressor sectionof the engine to the leading edge surfaces. The systemis activated prior to entering icing conditions. The hotair heats the leading edge sufficiently to prevent theformation of ice.

Oronasal Mask

Rebreather Bag

Figure 5-33. Continuous flow mask and rebreather bag.

Figure 5-34. De-icing boots on the leading edge of the wing.

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An alternate type of leading edge protection that is notas common as thermal anti-ice and de-icing boots isknown as a weeping wing. The weeping-wing designuses small holes located in the leading edge of thewing. A chemical mixture is pumped to the leadingedge and weeps out through the holes to prevent theformation and buildup of ice.

WINDSCREEN ICE CONTROLThere are two main types of windscreen anti-icesystems. The first system directs a flow of alcohol tothe windscreen. By using it early enough, the alcoholwill prevent ice from building up on the windshield.The rate of alcohol flow can be controlled by a dial inthe cockpit according to procedures recommended bythe airplane manufacturer.

Another effective method of anti-icing equipment isthe electric heating method. Small wires or otherconductive material is imbedded in the windscreen.The heater can be turned on by a switch in the cockpit,at which time electrical current is passed across theshield through the wires to provide sufficient heat toprevent the formation of ice on the windscreen. Theelectrical current can cause compass deviation errors;in some cases, as much as 40°. The heated windscreenshould only be used during flight. Do not leave it onduring ground operations, as it can overheat and causedamage to the windscreen.

PROPELLER ICE CONTROLPropellers are protected from icing by use of alcohol orelectrically heated elements. Some propellers areequipped with a discharge nozzle that is pointed towardthe root of the blade. Alcohol is discharged from thenozzles, and centrifugal force makes the alcohol flowdown the leading edge of the blade. This prevents icefrom forming on the leading edge of the propeller.Propellers can also be fitted with propeller anti-iceboots. The propeller boot is divided into two sections—the inboard and the outboard sections. The boots aregrooved to help direct the flow of alcohol, and they arealso imbedded with electrical wires that carry currentfor heating the propeller. The prop anti-ice system canbe monitored for proper operation by monitoring theprop anti-ice ammeter. During the preflight inspection,check the propeller boots for proper operation. If a bootfails to heat one blade, an unequal blade loading canresult, and may cause severe propeller vibration.[Figure 5-35]

OTHER ICE CONTROL SYSTEMSPitot and static ports, fuel vents, stall-warning sensors,and other optional equipment may be heated by

electrical elements. Operational checks of theelectrically heated systems are to be checked inaccordance with the AFM or POH.

Operation of aircraft anti-icing and de-icing systemsshould be checked prior to encountering icingconditions. Encounters with structural ice requireimmediate remedial action. Anti-icing and de-icingequipment is not intended to sustain long-term flight inicing conditions.

TURBINE ENGINESThe turbine engine produces thrust by increasing thevelocity of the air flowing through the engine. It consistsof an air inlet, compressor, combustion chambers, tur-bine section, and exhaust. [Figure 5-36] The turbine

PROP ANTI-ICE BOOTThe boot is divided into two sections: inboard and outboard. When the anti-ice is operating, the inboard section heats on each blade, and then cycles to the outboard section. If a boot fails to heat properly on one blade, unequal ice loading may result, causing severe vibration.

PROP ANTI-ICE AMMETERWhen the system is operating, the prop ammeter will show in the normal operating range. As each boot section cycles, the ammeter will fluctuate.

Inboard Section

Outboard Section

Figure 5-35. Prop ammeter and anti-ice boots.

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engine has the following advantages over areciprocating engine: less vibration, increased aircraftperformance, reliability, and ease of operation.

TYPES OF TURBINE ENGINESTurbine engines are classified according to the type ofcompressors they use. The compressor types fall intothree categories—centrifugal flow, axial flow, andcentrifugal-axial flow. Compression of inlet air isachieved in a centrifugal flow engine by acceleratingair outward perpendicular to the longitudinal axis ofthe machine. The axial-flow engine compresses air bya series of rotating and stationary airfoils moving theair parallel to the longitudinal axis. The centrifugal-axial flow design uses both kinds of compressors toachieve the desired compression.

The path the air takes through the engine and howpower is produced determines the type of engine. Thereare four types of aircraft turbine engines—turbojet,turboprop, turbofan, and turboshaft.

TURBOJETThe turbojet engine contains four sections: compressor,combustion chamber, turbine section, and exhaust. Thecompressor section passes inlet air at a high rate ofspeed to the combustion chamber. The combustionchamber contains the fuel inlet and igniter forcombustion. The expanding air drives a turbine, whichis connected by a shaft to the compressor, sustainingengine operation. The accelerated exhaust gases fromthe engine provide thrust. This is a basic application ofcompressing air, igniting the fuel-air mixture,producing power to self-sustain the engine operation,and exhaust for propulsion.

Turbojet engines are limited on range and endurance.

They are also slow to respond to throttle applications atslow compressor speeds.

TURBOPROPA turboprop engine is a turbine engine that drives apropeller through a reduction gear. The exhaust gasesdrive a power turbine connected by a shaft that drivesthe reduction gear assembly. Reduction gearing isnecessary in turboprop engines because optimumpropeller performance is achieved at much slowerspeeds than the engine’s operating r.p.m. Turbopropengines are a compromise between turbojet enginesand reciprocating powerplants. Turboprop engines aremost efficient at speeds between 250 and 400 m.p.h.and altitudes between 18,000 and 30,000 feet. Theyalso perform well at the slow airspeeds required fortakeoff and landing, and are fuel efficient. The mini-mum specific fuel consumption of the turboprop engineis normally available in the altitude range of 25,000feet to the tropopause.

TURBOFANTurbofans were developed to combine some of the bestfeatures of the turbojet and the turboprop. Turbofanengines are designed to create additional thrust bydiverting a secondary airflow around the combustionchamber. The turbofan bypass air generates increasedthrust, cools the engine, and aids in exhaust noisesuppression. This provides turbojet-type cruise speedand lower fuel consumption.

The inlet air that passes through a turbofan engine isusually divided into two separate streams of air. Onestream passes through the engine core, while a secondstream bypasses the engine core. It is this bypassstream of air that is responsible for the term “bypassengine.” A turbofan’s bypass ratio refers to the ratio ofthe mass airflow that passes through the fan divided bythe mass airflow that passes through the engine core.

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Figure 5-36. Basic components of a turbine engine.

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TURBOSHAFTThe fourth common type of jet engine is the turboshaft.It delivers power to a shaft that drives something otherthan a propeller. The biggest difference between aturbojet and turboshaft engine is that on a turboshaftengine, most of the energy produced by the expandinggases is used to drive a turbine rather than producethrust. Many helicopters use a turboshaft gas turbineengine. In addition, turboshaft engines are widely usedas auxiliary power units on large aircraft.

PERFORMANCE COMPARISONIt is possible to compare the performance of areciprocating powerplant and different types of turbineengines. However, for the comparison to be accurate,thrust horsepower (usable horsepower) for thereciprocating powerplant must be used rather thanbrake horsepower, and net thrust must be used for theturbine-powered engines. In addition, aircraft designconfiguration, and size must be approximatelythe same.

BHP— Brake horsepower is the horsepower actually delivered to the output shaft. Brake horsepower is the actual usable horsepower.

Net Thrust— The thrust produced by a turbojet orturbofan engine.

THP— Thrust horsepower is the horsepowerequivalent of the thrust produced by a turbojet or turbofan engine.

ESHP— Equivalent shaft horsepower, with respect to turboprop engines, is the sum of the shaft horsepower (SHP) delivered to the propeller and the thrust horsepower (THP) produced bythe exhaust gases.

Figure 5-37 shows how four types of engines comparein net thrust as airspeed is increased. This figure is forexplanatory purposes only and is not for specificmodels of engines. The four types of engines are:

• Reciprocating powerplant.

• Turbine, propeller combination (turboprop).

• Turbine engine incorporating a fan (turbofan).

• Turbojet (pure jet).

The comparison is made by plotting the performancecurve for each engine, which shows how maximumaircraft speed varies with the type of engine used. Sincethe graph is only a means of comparison, numericalvalues for net thrust, aircraft speed, and drag arenot included.

Comparison of the four powerplants on the basis of netthrust makes certain performance capabilities evident.In the speed range shown to the left of Line A, thereciprocating powerplant outperforms the other threetypes. The turboprop outperforms the turbofan in therange to the left of Line C. The turbofan engineoutperforms the turbojet in the range to the left of LineF. The turbofan engine outperforms the reciprocatingpowerplant to the right of Line B and the turboprop tothe right of Line C. The turbojet outperforms thereciprocating powerplant to the right of Line D, theturboprop to the right of Line E, and the turbofan to theright of Line F.

The points where the aircraft drag curve intersects thenet thrust curves are the maximum aircraft speeds. Thevertical lines from each of the points to the baseline ofthe graph indicate that the turbojet aircraft can attain ahigher maximum speed than aircraft equipped with theother types of engines. Aircraft equipped with theturbofan engine will attain a higher maximumspeed than aircraft equipped with a turboprop orreciprocating powerplant.

TURBINE ENGINE INSTRUMENTSEngine instruments that indicate oil pressure, oil temper-ature, engine speed, exhaust gas temperature, and fuelflow are common to both turbine and reciprocatingengines. However, there are some instruments that areunique to turbine engines. These instruments provideindications of engine pressure ratio, turbine dischargepressure, and torque. In addition, most gas turbineengines have multiple temperature-sensing instruments,called thermocouples, that provide pilots withtemperature readings in and around the turbine section.

Aircraft Drag

Airspeed

Net

Thr

ust

ACB

D

E F

TurbojetTurbofan

Turboprop

Reciprocating

Figure 5-37. Engine net thrust versus aircraft speed and drag.

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ENGINE PRESSURE RATIOAn engine pressure ratio (EPR) gauge is used toindicate the power output of a turbojet/turbofan engine.EPR is the ratio of turbine discharge to compressorinlet pressure. Pressure measurements are recorded byprobes installed in the engine inlet and at the exhaust.Once collected, the data is sent to a differentialpressure transducer, which is indicated on a cockpitEPR gauge.

EPR system design automatically compensates for theeffects of airspeed and altitude. However, changes inambient temperature do require a correction to beapplied to EPR indications to provide accurate enginepower settings.

EXHAUST GAS TEMPERATUREA limiting factor in a gas turbine engine is thetemperature of the turbine section. The temperature ofa turbine section must be monitored closely to preventoverheating the turbine blades and other exhaustsection components. One common way of monitoringthe temperature of a turbine section is with an exhaustgas temperature (EGT) gauge. EGT is an engineoperating limit used to monitor overall engineoperating conditions.

Variations of EGT systems bear different names basedon the location of the temperature sensors. Common tur-bine temperature sensing gauges include the turbine inlettemperature (TIT) gauge, turbine outlet temperature(TOT) gauge, interstage turbine temperature (ITT)gauge, and turbine gas temperature (TGT) gauge.

TORQUEMETERTurboprop/turboshaft engine power output is measuredby the torquemeter. Torque is a twisting force appliedto a shaft. The torquemeter measures power applied tothe shaft. Turboprop and turboshaft engines aredesigned to produce torque for driving a propeller.Torquemeters are calibrated in percentage units,foot-pounds, or pounds per square inch.

N1 INDICATORN1 represents the rotational speed of the low pressurecompressor and is presented on the indicator as apercentage of design r.p.m. After start the speed of thelow pressure compressor is governed by the N1 turbinewheel. The N1 turbine wheel is connected to the lowpressure compressor through a concentric shaft.

N2 INDICATORN2 represents the rotational speed of the high pressurecompressor and is presented on the indicator as apercentage of design r.p.m. The high pressurecompressor is governed by the N2 turbine wheel. The

N2 turbine wheel is connected to the high pressurecompressor through a concentric shaft. [Figure 5-38]

TURBINE ENGINE OPERATIONALCONSIDERATIONSBecause of the great variety of turbine engines, it isimpractical to cover specific operational procedures.However, there are certain operational considerationsthat are common to all turbine engines. They are enginetemperature limits, foreign object damage, hot start,compressor stall, and flameout.

ENGINE TEMPERATURE LIMITATIONSThe highest temperature in any turbine engine occursat the turbine inlet. Turbine inlet temperature istherefore usually the limiting factor in turbineengine operation.

THRUST VARIATIONSTurbine engine thrust varies directly with air density.As air density decreases, so does thrust. While bothturbine and reciprocating powered engines are affectedto some degree by high relative humidity, turbineengines will experience a negligible loss of thrust,while reciprocating engines a significant loss of brakehorsepower.

FOREIGN OBJECT DAMAGEDue to the design and function of a turbine engine’s airinlet, the possibility of ingestion of debris alwaysexists. This causes significant damage, particularly tothe compressor and turbine sections. When this occurs,it is called foreign object damage (FOD). Typical FODconsists of small nicks and dents caused by ingestionof small objects from the ramp, taxiway, or runway.However, FOD damage caused by bird strikes or iceingestion can also occur, and may result in totaldestruction of an engine.

Prevention of FOD is a high priority. Some engineinlets have a tendency to form a vortex between theground and the inlet during ground operations. Avortex dissipater may be installed on these engines.Concentric Shaft—One shaft inside another shaft having a common core.

Low PressureCompressor (N1)

High PressureCompressorDrive Shaft

High PressureCompressor (N2)

Low PressureCompressorDrive Shaft

Figure 5-38. Dual-spool axial-flow compressor.

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Other devices, such as screens and/or deflectors, mayalso be utilized. Preflight procedures include a visualinspection for any sign of FOD.

TURBINE ENGINE HOT/HUNG STARTA hot start is when the EGT exceeds the safe limit. Hotstarts are caused by too much fuel entering thecombustion chamber, or insufficient turbine r.p.m. Anytime an engine has a hot start, refer to the AFM, POH,or an appropriate maintenance manual for inspectionrequirements.

If the engine fails to accelerate to the proper speed afterignition or does not accelerate to idle r.p.m., a hungstart has occurred. A hung start, may also be called afalse start. A hung start may be caused by aninsufficient starting power source or fuel controlmalfunction.

COMPRESSOR STALLSCompressor blades are small airfoils and are subject tothe same aerodynamic principles that apply to anyairfoil. A compressor blade has an angle of attack. Theangle of attack is a result of inlet air velocity and thecompressor’s rotational velocity. These two forcescombine to form a vector, which defines the airfoil’sactual angle of attack to the approaching inlet air.

A compressor stall can be described as an imbalancebetween the two vector quantities, inlet velocity andcompressor rotational speed. Compressor stalls occurwhen the compressor blades’ angle of attack exceedsthe critical angle of attack. At this point, smoothairflow is interrupted and turbulence is created withpressure fluctuations. Compressor stalls cause airflowing in the compressor to slow down and stagnate,sometimes reversing direction. [Figure 5-39]

Compressor stalls can be transient andintermittent or steady state and severe. Indications of atransient/intermittent stall are usually an intermittent“bang” as backfire and flow reversal take place. If thestall develops and becomes steady, strong vibration anda loud roar may develop from the continuous flowreversal. Quite often the cockpit gauges will not show amild or transient stall, but will indicate a developedstall. Typical instrument indications includefluctuations in r.p.m., and an increase in exhaust gastemperature. Most transient stalls are not harmful to theengine and often correct themselves after one or twopulsations. The possibility of engine damage, whichmay be severe, from a steady state stall is immediate.Recovery must be accomplished quickly by reducingpower, decreasing the airplane’s angle of attack andincreasing airspeed.

Although all gas turbine engines are subject tocompressor stalls, most models have systems thatinhibit these stalls. One such system uses variable inletguide vane (VIGV) and variable stator vanes, whichdirect the incoming air into the rotor blades at anappropriate angle. The main way to prevent airpressure stalls is to operate the airplane within theparameters established by the manufacturer. If acompressor stall does develop, follow the proceduresrecommended in the AFM or POH.

FLAMEOUTA flameout is a condition in the operation of a gasturbine engine in which the fire in the engineunintentionally goes out. If the rich limit of the fuel/airratio is exceeded in the combustion chamber, the flamewill blow out. This condition is often referred to as arich flameout. It generally results from very fast engineacceleration, where an overly rich mixture causes thefuel temperature to drop below the combustiontemperature. It also may be caused by insufficientairflow to support combustion.

Another, more common flameout occurrence is due tolow fuel pressure and low engine speeds, whichtypically are associated with high-altitude flight. Thissituation also may occur with the engine throttled backduring a descent, which can set up the lean-conditionflameout. A weak mixture can easily cause the flame todie out, even with a normal airflow through the engine.

Any interruption of the fuel supply also can result in aflameout. This may be due to prolonged unusualattitudes, a malfunctioning fuel control system,turbulence, icing or running out of fuel.

Symptoms of a flameout normally are the same asthose following an engine failure. If the flameout is dueto a transitory condition, such as an imbalance betweenfuel flow and engine speed, an airstart may be

Normal InletAirflow

Distorted InletAirflow

Figure 5-39. Comparison of normal and distorted airflow intothe compressor section.

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attempted once the condition is corrected. In any case,pilots must follow the applicable emergencyprocedures outlined in the AFM or POH. Generally,

these procedures contain recommendations concerningaltitude and airspeed where the airstart is most likely tobe successful.

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