AVIATION HIGH SCHOOL Revision H

Basic / Advanced Jets Dec. 2007

a.n.cInformation Sheet On : The Fuel & Ignition Systems of a Gas Turbine Engine.

NAME :______________________________________________. DATE :_____________________

Lesson :________ AIM : How do Fuel & Ignition meet in the Combustion Chamber.

PART ATopic : The Engine’s Ignition system

Since ignition always precedes fuel in the start sequence of a gas turbine engine, we will begin ourinvestigation with the engine’s ignition system.

WARNING: Always follow all maintenance manual procedures, especially with regard to safety, when working on any ignition system.

I. Purpose: the primary purpose of the turbine engine’s ignition system is to provide a high intensity spark for ignition of the fuel and air mixture during engine start.Note: There are several different ignition system used on gas turbine engines. To attempt to describe all of these systems is beyond the scope of this lesson. For the description and use of any given ignition system, the student should refer to the aircraft or engine’s maintenance manual.

II. Gas Turbine Engine Ignition System.A. Used primarily during start.B. Once the engine is started ignition is normally turned off.

Once ignited, combustion of the air / fuel mixture inside the combustion chamber becomes self-sustained andan ignition source is no longer required. Therefore, most turbine engine ignition systems are normally operatedonly for brief periods @ start-up.

C. The ignition system is also used as a precaution against engine flame-out during critical operations or under not ideal operating conditions.

1. For critical situation like take-off, landing and bad weather or turbulence, ignition is put oncontinuously as a precaution against engine flame-out. When used in this way, the prescribed limitations on

ignition use time must be observed to prevent over-heating the ignition system components, and thereby enhancingthe life span of the ignition system. For example: The duty cycle on the Pratt &Whitney’s JT9D engine’signition system is 10 minutes on then 20 minutes off for cooling. On some engines like the V2500, used on theAirbus 319, a low tension ignition system is used for in-flight continuos ignition. This system requires no coolingoff period and maybe used continuously throughout the flight.

D. Two ignition systems, minimum, are required per engine for safety (by the FAA) .1. Pilot or mechanic can select either one or both to fire.

E. The ignition system provides approximately one spark discharge minimum per second.

III. Type of Ignition SystemA. The Capacitor Discharge type most common type used on modern gas turbine engines because of

its high voltage and high heat intensity spark.1. A high heat intensity spark is needed for high altitude, cold temperature starts because fuel

volatility ( its ability to form a readily burnable vapor) decreases with high altitudes and cold temperatures sothe ignition system has to produce a high voltage to “spark” across a wide igniter gap to ground.

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B. There are two types of Capacitor Discharge Ignition Systems. They are:1. The Low Tension System which supplies 28 volts DC to each exciter unit. & 2. The High Tension System which supplies 115 VAC to each exciter unit.

C. Ignition systems may be of the intermittent or extended duty cycles.

Definition: A Joule is the unit of work or energy expended by an electrical current of one amp passing through a resistance of one ohm. One Joule equals 1 watt per second. This is an enormous amount of energy considering the spark jumps the igniter’s gap in approximately 40 millionths of a second (.000040 sec.) Ignition systems stored energy can be from 2000 to 25000 volts & 200 amps.

D. Ignition systems may be of the 2 , 4 or 20 joule varieties.

IV. Ignition System Components (in each system) ( 2 systems per engine) A. Low tension lead-- delivers low voltage / low amperage input from A/C electrical system to the ignitionsystem..

B. Exciter Box- uses capacitors & transformers to create high voltage / high amperage pulses ( sparks). 1. High voltage pulses are created inside the exciter box at the triggering transformer.

C. High tension lead- delivers high intensity voltage / current from the exciter box to igniter plug D. Igniter plug ( similar to a spark plug) provides a discharge gap for the spark to ground. 1. Normally, 2 per engine. ( 1 in each system).

LOW TENSION INPUT lead from A/C’s electrical system

( 115 VAC or 28 VDC)

V. Operation of the Ignition System. A. Low voltage flows into exciter.B. Exciter box stores an ever increasing build-up of current.C. When current is high enough it flows through a transformer that creates an out put voltage of

18,000 to 26,000 volts.D. Voltage is high enough to “JUMP” igniter plug gap.E. Normal spark rate is one to two sparks per second.

VI. Igniter Plugs.A. Provides a discharge path to ground for the current stored in the exciter’s storage capacitor.B. Igniters use high temperature material and air cooling to provide satisfactory igniter life under

extreme temperatures.1. Outer shell of the igniter is made of nickel-chromium because it is corrosion resistant and

doesn’t expand that much with heat.2. The center electrode is made of tungsten due to its heat and erosion resistance.

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Name :___________________________________________. Date :___________________

Ignition System. ( continued)

D. There are two types of igniter plugs. They are the low tension igniter & the high tension igniter.1. The low tension igniter provides a low resistance path for the energy stored in the exciter

box. It operates at around 2000 volts.2. The high tension igniter plug has a large air gap and requires about 25,000 volts to provide the ionization that allows the spark to bridge the gap to ground. (the engine’s case is the ground)

Self-ionizing or Shunted Gap Low Tension Igniter

High Tension Igniter (Recessed or constrained gap )

High Tension Igniter

Notice the sem-conductor material between the center electrode & ground electrode. ( on low tension igniter)

Low Tension Igniter

E. Most engines today have one high output / high tension igniter of perhaps 20 joules and a low output or low tension igniter of perhaps 4 joules. Both operate during engine start but only the low tension igniter operates during continuous ignition.

1. Under abnormal adverse start conditions such as very cold weather or high altitude relights, an igniter may require as much as 12 joules so a high tension system is used.

2. When an igniter is used as a precaution against engine flame-out on take-off, landing or in bad weather, continuous low tension ignition of only 3 to 6 joules is required and may be used for hours.

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F. Low tension ignition system igniter is the self-ionizing or shunted-gap igniter.1. The firing end of this igniter contains a semi-conductor material that bridges the gap between

the center electrode and ground. This allows charge build up in the capacitor of the exciter box.G. The high tension igniter plugs are the Surface Gaped Igniter and the Recessed or Constrained gap igniter.

1. Surfaced gap igniter protrudes into the combustion chamber and is subject to high heat. Many are secondary air cooled as a result.

2. The Recessed or Constrained-gap igniter plug operates at cooler temperatures because it does not

have to project into the combustion chamber. Spark has to jump out from the center electrode toground.

H. Spark igniters for the low and high tension systems are not interchangeable.I. Glow plug igniters are similar in appearance to old type automobile cigarette lighter.

1. Glow plugs are used on smaller turbine & turbo-prop engines.2. They are not considered igniters in the strictest sense ( they do not spark) but they use a

higher resistive coil of wire that glows yellow hot when 24 to 28 volts is supplied to the coil. The coil generates high heat that is capable of igniting the air / fuel mixture.

J. Do not use an igniter plug if it has been dropped as damage to insulating material may resultK. Always use a new gasket when installing a new plug or re-installing an old plug.

VII. Auto-Ignition / Auto RelightA. Used on turbo-prop & turbo-shaft engines.B. Provides ignition for re-light should engine flame-out.C. Senses burner or compressor discharge pressures. If either falls below a certain value, as

would happen in a flame-out, the auto ignition would automatically provide a relight sparkD. System is de-energized for normal shut-down of the engine.

VIII. Gas Turbine vs. Reciprocating Piston Engine Ignition SystemsA. Turbine engines require a high-energy, high -intensity spark for ignition at high altitude & cold

temperature re-starts. B. Timing of when spark occurs very important in reciprocating engine. In turbines spark not timed

to occur at a particular time.C. Turbine ignition necessary , normally, for short time during engine start. In reciprocating engines

spark event must occur in each cycle for as long as the engine is operating.D. In turbine engines, the ignition system is normally turned off after start.

In a piston engin turning ignition off is how the engine is normally shut down.E. Igniter plugs are less susceptible to fouling than reciprocating engine spark plugs because its high

heat intensity spark cleans the igniter

IX. Safety Precautions When Working on the Turbine Engine Ignition System.A. Follow all safety precautions of the Aircraft or Engine’s Maintenance Manual when working on

the ignition system.B. Ignition switch off. Tag switch with Do Not Operate. C. Pull and tag circuit breakersD. If removing high tension leads from exciter box or igniter plug DO the following:

1. Disconnect input lead then wait the time prescribed in the MM. ( 1 to 5 minutes). This allows any residual energy to bleed off from exciter capacitors.

2. Then disconnect the igniter lead and ground the center electrode to the engine case to discharge any capacitor voltage.

E. Exercise care in handling exciter boxes and igniter plugs as they may contain or be coated with radioactive materials like iridium or selisium. Page 4 of 11 Notebook page # : ______

Name :__________________________________________________. Date :______________

Part B Topic : The Engine’s Fuel System

I. The Fuel System A. Supplies the engine with the correct amount of fuel in all operating conditions B. Must provide the means to increase / decrease power at will to obtain the thrust required for any operating condition. This is accomplished by varying the flow of fuel into the combustion chambers. C. Fuel system must deliver the fuel to combustion chambers not only in the right quantity but also in the right condition. Fuel must be atomized ( made into a fine spray) so that it will ignite and burn efficiently. Fuel Injection nozzles will atomize the fuel.

Introductory Note: The following written schematic is based on the fuel system of the Pratt & Whitney JT9D engine as it is used on the Boeing 747 aircraft. Please note that the location and nomenclature of engine fuel system components may vary between different engines.

II. Gas Turbine Fuels.A. Are a blend of kerosene type fuels.

1. Kerosene has more heat energy per gallon than gasoline. The Reason for this is because kerosene weighs more per gallon than gasoline.

B. All turbine fuels, unlike the gasoline grades of piston engine fuels which are dyed different colors to aid in the recognition of different type grades, are colorless or have a light straw color

II. From the Fuel Tank to the Combustion Chamber(s). The Boeing-747's main fuel tanks are in itswings

We will now follow the fuel as it flows from the aircraft’s main fuel tanks,( through various fuel systemcomponents), to the combustion chamber

A. Boost Pump: provides a positive flow (fuel under pressure) from the aircraft’s fuel tank(s) to the engine driven fuel pump ( that is located on the accessory drive gearbox ).

1. The boost pump is located in the fuel tank. Each main tank has two boost pumps.2. The boost pumps are electrically driven.3. The centrifugal type boost pump is the most commonly used.

B. Fuel flows from the aircraft tanks through stainless steel lines to the aircraft’s engines.C. Fuel Shut-Off Valve or Fuel spar valve. 1. Shuts off fuel to the engine in case of fire

2. Must be on the aircraft side of the firewall. 3. Located at the forward wing spar.

D. Engine Driven Fuel Pump : 1. Located on the main accessory drive gearbox 2. Delivers more fuel to the fuel control that is required by the engine at all times and operating conditions.

a. Fuel control’s internal bypass valve will return any un-used, un-metered fuel back tothe inlet side of the fuel pump.

3. The fuel pumps internal pressure relief valve will direct excess fuel / fuel pressure back to the inlet side of the fuel pump.

E. Fuel Heater : 1. Located on the fuel pump. 2. Uses compressor bleed air to heat fuel up and to de-ice the fuel filter.

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F. Fuel Filter / Strainer : 1. You must have a filter between the fuel tank outlet or engine’s fuel inlet and the fuel metering / control device. (As per the FAR’s). 2. The Filter ( on the JT9D) is in the fuel pump housing. 3. Removes contaminants and dirt from the fuel. 4. Filter Bypass Valve : If the fuel filter becomes clogged, the bypass valve will open and allow unfiltered fuel to Bypass the clogged filter on its way to the fuel control. Dirty fuel is better than no fuel 5. Micron type filter or micro-filter usually of cellulose fiber is the one most commonly used because it has greater filtering action than a screen or mesh filter. G. Fuel Control / Fuel Metering Unit. Tries to maintain an air to fuel mixture ratio of 15 : 1 ( 15 parts air to one part fuel by weight). 1. Its purpose is to a. Meter / Control fuel to the engine combustion chambers.

1. A lean or rich air to fuel mixture along with normal airflow can cause a lean die-out or rich flame-out of the engine. b. Changes engine thrust by controlling engine RPM . More fuel = higher RPM c. Prevents over-heating at turbine blades. d. Prevent the flame from going out in the combustion chamber.

e. Control acceleration / deceleration rates of the engine to prevent lean or rich flame outs.1. If the engine is accelerated or decelerated too fast there can be a mis-match

between air flow and fuel flow causing a rich or lean flame-out of the engine.2. Fuel control’s / Fuel metering unit’s bypass valve will return un-used / un-metered fuel back to the

inlet side of the fuel pump.3. The most common types of Fuel controls / Fuel Metering Units in use today are the

a. The Hydro-Mechanical Fuel control, b. The electronically supervised Hydro-Mechanical Fuel Control.

Supervision is provided by the Electronic Engine Control Computer or EEC and the c. Electronically controlled hydro-mechanical fuel metering unit. Here, the fuel metering unit

is controlled by the full authority Electronic Engine Control or EEC. ( EEC is part of the Full Authority Digital Engine Control System.) This is the most commonly used fuel control / fuel metering unit on today’s gas turbine engines.

4. Simple Operation of the Fuel Control or Fuel Metering System. Strictly speaking , the pilot of a Boeing 747 does not directly control his engines. The fuelmetering system does. Through the power levers on the flight deck , the pilot tells either the fuel control unitor the FADEC’s EEC how much thrust he wants. Before either system responds , the fuel control or FADEC,via sensors, will look at conditions inside and outside of the engine. Based on what it sees the fuel control orthe FADEC’s EEC will meter the proper increase or decrease in fuel flow to the combustion chamber(s) toprovide the required thrust.. REMEMBER , the fuel control or FADEC varies thrust by controlling engine RPM.

H. Types of Engine Controls (fuel metering units.) 1. Hydro-Mechanical Fuel Control: Here the fuel control uses fuel pressures and internal mechanical linkage to control fuel metering to the burner. Mechanical inputs from the pilot tells the fuel control when to operate & what thrust is desired.

a. The hydro-mechanical fuel control meters fuel in accordance with the throttle / power lever position to provide the precise amount of fuel necessary for the desired thrust.b. Meters fuel by weight ( pounds or kilos) rather than volume ( gallons or liters) because

volume varies with temperature.Page 6 of 11 Notebook page # :______

Name :___________________________________________. Date :___________________Fuel System ( continued)

c. To perform its function, the fuel control unit monitors or has mechanical inputs from : throttle position, engine R.P.M , compressor inlet air temperature and pressure and burner or compressor discharge pressure.

2. Electronically supervised - Hydro-Mechanical Fuel Control Unit.a. This fuel metering system consists of a hydro-mechanical fuel control and a supervisory

Electronic Engine Control or E.E.C b. With this type of fuel metering system, the fuel control unit controls most engine operations

like starting, idle, acceleration, deceleration and shut down, but its operation is supervised by the EEC computer.

c. With this system, initial thrust output is controlled by the fuel control unit , but the EEC will adjust the fuel control to obtain the most effective engine operation.

d. The EEC’s most important function is to limit engine RPM and EGT temperature to prevent over-speed or over-temp occurrences. It does this by limiting the amount of fuel that the fuel control can send to the combustion chamber(s).

e. Any malfunction in the supervisory EEC that adversely affects engine operation will cause an immediate reversion back to engine control by the hydro-mechanical fuel control unit.

3. Full Authority Digital Engine Control or FADEC At the heart of the FADEC is a two channel computer known as the EEC or Electronic Engine Control. One of the basic purposes of the FADEC and the EEC is to reduce flight crew workloads particularly duringcritical operations such as takeoff, when the complete attention of the flight crew to the task of flying is mostimportant. To reduce crew workloads, the FADEC’s EEC accomplishes the following:

a. The FADEC’s EEC automatically accelerates the engine to the desired thrust ( EPR) then maintains thrust as the aircraft changes altitude or ambient conditions change.

b. The FADEC’s EEC performs virtually all the functions necessary to operate the engine during all phases of flight.

c. The FADEC eliminates the need for the hydro-mechanical fuel control. In this system , the hydro- mechanical fuel control is replaced by a digitally controlled fuel metering unit. The fuel metering unit meters fuel by responding to digital control signals sent to it from the FADEC’s EEC.

d. The FADEC’s EEC receives inputs from engine RPM sensors, burner pressure. throttle lever position, bleed-air status, aircraft altitude, total inlet temperature & pressure, total outlet temperature & pressure, variable stator vane position, fuel flow rate, fuel & oil temperature, the thrust management computers & the digital air computers.

e. The FADEC sends digital command signals to the fuel metering unit, the bleed valves, the variable stator vane actuators, the turbine case cooling valve actuator, air/oil heat exchanger anti-icing valves, anti-surge bleed valves and the fuel / oil cooler bypass valve and the active clearance control or ACC.

I. The ACC or active clearance control is a FADEC / EEC controlled system that aidsengine efficiency by controlling the clearance between the tips of the compressor and turbine blades and theengine case. The FADEC commands cool fan discharge air to be sent to tubes surrounding the compressorand turbine section cases. This air shrinks the case around the compressor and turbine and thereby ensuresthat the compressor and turbine blade to case clearances are kept to a minimum. More of the compressor andturbine gas flow is now forced to flow over the blades. Engine efficiency is thus increased. Note: The ECC indirectly controls fuel by sending digital signals to the fuel metering unit which actually meters the

fuel. Page 7 of 11 Notebook page # :______

From the fuel metering/ fuel metering devices, the fuel then flows to the:

I. Fuel Pressure Relief Valve. Excess fuel, not metered by the fuel control into the combustion chambers, goes back to inlet side of the fuel pump via the pressure relief valve.

From the pressure relief valve fuel then flows to the:

J. Fuel flow transmitter: generates an electrical signal dependent on how much fuel flows through it. Tells the pilots how much fuel , in pounds per hour, are flowing into the combustion chamber(s). From the transmitter, fuel then flows to the:

K. Fuel Oil Heat Exchanger : here, fuel is used to cool the engine oil. Fuel is heated by the oil. Oil increases the temperature of the fuel while the fuel decreases the temperature of the oil From here fuel flows through the Pressurization & Dump valves to the fuel nozzles. ( see “O” & “P” for more information P & D valves :

L. Fuel Nozzles : inject fuel into the combustion chambers in a precisely patterned very , very fine atomized spray.

1. Located / inserted at the front of the combustion chamber ; usually mounted in the diffuser case.2. Sprays atomized fuel into the combustion chamber.

a. The nozzle atomizes the fuel ( makes a very fine spray) by a high pressure differential at the nozzle outlet. ( like using a spray bottle on the spray setting rather than stream)Note: the ideal condition of fuel as it is sprayed into the combustion chamber would be a vapor. ( like in a piston engine’s cylinder). Because the conditions inside the gas turbine engine ( like colder fuel temps), are different than inside cylinder, the best we can hope for in the turbine engine fuel is that it is finely atomized.

M. Types of Fuel Nozzles1. Simplex Nozzle-- old type nozzle. Discharges fuel through a single orifice. Provides one spray pattern

2. Duplex Nozzle---has two separate spray patterns from it know as the primary flow and secondary flow.

a. Primary Flow spray pattern is on all the time. ( From engine start up to shut down) b. Secondary Flow spray supplements primary flow at engine speeds above engine idle c. Duplex fuel nozzles require flow dividers ( also known as Pressurization valves. See following )

d. The principal advantages of the duplex fuel nozzle are better fuel atomization than the old type simple nozzle and a more uniform flow pattern.

3. Both simplex and duplex designs work under very high differential pressures across the nozzle.4. Air Blast Fuel Nozzles– ( a newer Simplex design). Air blast nozzles produce finer fuel droplets.

a. Uses a high velocity airflow to blast the fuel, the air blast nozzles more completely atomize the fuel than can be accomplished with fuel pressure along (like the duplex nozzles).

b. Air blast nozzles have the advantage of using lower fuel pressures.

N. Flow Divider / Pressurization Valve ( 2 different names for same thing)1. Divides fuel flow between primary and secondary fuel flows. (Creates primary & secondary fuel flows)

2. Primary fuel flow is on all the time the engine is operating. Secondary fuel flow supplements the primary flow at engine speeds above idle.

3. At engine speeds of idle and below, the flow divider / pressurization valve is closed allowing only primary fuel to flow into combustion chamber.

4. At engine speeds above idle, increased fuel pressure will open flow divider / pressurization valve allowing secondary fuel to flow from the nozzle to supplement the primary fuel flow.

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Fuel System (continued)

O. Location of the Flow Divider / Pressurization Valve. On the B747 ‘s JT9D engine, on which this written schematic is based , the pressurization valve is located inside the P & D valve housing. ( See “P” below)

However, the pressurization valve or flow divider may be found:1. Inside the duplex fuel nozzle

a. Duplex nozzles with internal flow dividers / pressurization valves will have only one fuel inlet line. Primary and secondary flows will be created inside the nozzle

2. Inside the Pressurization and Dump Valve ( The P & D valve ) b. Duplex nozzles with external flow dividers/ pressurization valves will have two fuel inlet

lines; One for primary flow and one for secondary flow. Primary and secondary fuel flowsare created before ( up stream ) of the fuel nozzles in the Pressurization and dump valve

3. If the flow divider / pressurization valve is inside the nozzle itself, the duplex nozzle will be supplied by only one fuel inlet line. If the flow divider / pressurization valve is inside the pressurization and dump valve ( upstream of the fuel nozzle) the duplex fuel nozzle will be supplied by two fuel inlet lines- one for primary flow and the other for secondary fuel flow.

P. Pressurization & Dump Valve ( P & D Valve ) located immediately before or upstream of the fuel nozzles.

1. Contains two separate valves: a dump vale and a pressurization valve2. Pressurization valve controls primary and secondary fuel flows to the fuel nozzles. (see “P” below)3.. Dump valve : a. Provides a positive shut off of fuel to fuel nozzles at engine shut down

b. Opens to drain fuel from fuel manifolds at engine shut down to prevent fuel from boiling off in the lines and leaving coke (carbon) 7 lacquer deposits behind.

4. P & D Valve Positions. a. The dump valve is open with the engine shut down to drain fuel; it is closed with the engine operating..

b. The pressurization valve is closed when the engine is shut down or at idle. The pressurization valve opens, by fuel pressure, at engine speeds above idle to allows secondary fuel to flow.After the Pressurization & Dump valve, the fuel flow splits into two separate lines called manifolds. Onemanifold (the smaller) is for primary fuel flow while the larger manifold is for secondary fuel flow. Thesemanifolds supply fuel to the fuel nozzles

III. Mixing Fuel and AirA. Accomplished by swirl vanes which surround each fuel nozzle.B. Swirl vanes swirl primary air that mixes with the fuel. Provides for good mixingC. Swirling of fuel & air also slows the progression of the combustion process toward the rear of the

combustion chamber and the turbine section.

IV. Fuel Control MaintenanceA. Hydro-Mechanical Fuel Control

1. Line maintenance on the fuel control is usually limited to its replacement or adjustment.a. Adjustments are limited to idle speed adjustment , maximum speed / maximum EPR

(thrust) adjustment and a specific gravity adjustment for the fuel used.. 1. Jet A kerosene fuel is the most common turbine engine fuel. If an alternate to this fuel

is used the fuel control’s specific gravity setting should be checked or adjusted. 2. Adjusting the Fuel Control’s specific gravity will help it determine the amount of heat

energy present in the fuel since specific gravity is a measure of the fuel’s density.Page 9 of 11 Notebook page # :_____

b. The process of adjusting the fuel control is know as trimming.In early the days of the turbo-jet engine, thrust was adjusted by varying the area of the exhaust nozzle. Thiswas done by trimming the end of the tail pipe to increase the nozzle area (to decrease thrust) or by installingsmall metal tabs called “mice” to decrease the nozzle area ( to increase thrust).. These trimming proceduresare no longer followed.

V. Trimming a Hydro-mechanical Fuel Control

Introduction: Trimming is adjustments made to the fuel control to assure proper RPM & Power outputs from the engine. The primary purpose & the proper trimming of the fuel control unit is done to ensure the availability of maximum rated thrust output of the engine whenever needed. A secondary purpose of trimming a fuel control is to set ground idle & flight idle RPM.

A. Trim checks are completed whenever the engine’s thrust output is suspect or after maintenancetasks as prescribed by the manufacturer’s maintenance manual.1. Trimming an engine is required after the following maintenance tasks:

a. Engine change b. Fuel Control change and c. Throttle linkage adjustment 2. The Fuel Control may also need to be trimmed when a deterioration of engine efficiency in

producing thrust occurs as a result of old age.B. The ideal conditions for trimming a turbine engine’s fuel control are no wind, low humidity and

low moisture with a standard day temperature ( 59°F.) and pressure (29.92" of Hg.)1. If wind is present, trim into the wind. However,2. Do not trim engine if wind speed is above 25 MPH.

Excessive high wind speed into the engine causes a false high compressor inlet pressure to a degree where itwill affect cockpit EPR ( Engine Pressure Ratio) readings. Increased wind into the inlet will increase the totalpressure at the face of the compressor which will cause a rise in the turbine discharge pressure. This willcause what appears to be higher EPR readings . As a result the engine will be under trimmed and later, incalm air, will not be able to produce its take-off power nor its idle RPM..

C. To trim a fuel control accomplish the following:1. Review the specific trimming procedures in the aircraft’s or engine’s maintenance manual.2. Measure the barometric pressure and temperature at the engine’s inlet. This is required to

correct the engine’s performance readings to standard day sea level conditions.a. To obtain the temperature reading, it is a common & preferred practice to hang a

thermometer in the shade of the nose wheel well and wait for the temperature to stabilizeb. Temperature & barometric pressure may also be obtained from the airport’s air traffic

control tower or weather station.D. Determine / calculate the required thrust output and RPM readings for the local field conditions (of

temperature and barometric pressure) using the trim tables located in the engine’s Jet Run-Up Handbook or in the aircraft’s Maintenance Manual ( ATA chapter 71) . Note: Depending on engine type, either EPR or NI RPM can be used for thrust determinations.

E. Aircraft should be facing into the wind.( Wind not to exceed 25MPH)F. Engine Bleed Air Off. G. Prepare the fuel control for the trim as per the maintenance manual

1. In order to save wear on the engine and also to save fuel, engines are generally trimmed at less than take-off power or at part power.

2. Install part power stop on throttle lever quadrant on the flight deck..a. This stop provides an obstruction in the path of the throttle lever to prevent them from

being advanced to take-off power.3. Or deploy part power trim stop on the fuel control itself.

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Name :_____________________________________________. Date :_________

Fuel System (continued)

H. Run the engine at idle with all bleeds off.1. If necessary, adjust the fuel control’s idle adjust screw until idle EPR and idle RPM are

within required limits.I. Run the engine with throttle lever(s) at the at part power stop and check for the required engine

EPR and RPM. (All bleeds off). 1. Adjust the fuel control’s maximum power trim screw if necessary to obtain proper readings

Note: In order to stabilize cams, springs and linkages within the fuel control unit, all final adjustments must be made in the increase direction.

J. After trimming of the engine is complete, remove the part power stop at the fuel control then perform a take-off power check.Note: If the part power stop was deployed on the fuel control to do the trim check,, it must be retracted prior to performing a full power check or before the aircraft is placed back in service.

1. Check for correct throttle spring back. Correct spring back is indicated when the fuel control reaches its internal maximum power stop before the cockpit lever reaches its most advanced most power stop. The throttle lever(s) will spring back when advanced to the most power setting stop and then released.

VI. FADEC Maintenance.A. Engine trimming is not required.

1. The FADEC or full authority EEC automatically adjusts for changes in performance.B. The FADEC includes extensive and continuous self-test routines.

1. Built in test equipment (BITE) can detect and isolate faults within the EEC or to its inputs or outputs.

2. The FADEC is able to isolate problem and indicate to maintenance personnel whether the fault is within itself or in a sensor or actuator

VI. Engine Run-Up Danger Zones

WARNING : Running gas turbine engines are an inherent danger to personnel & equipment.A. Exercise extreme caution & be constantly alert when approaching or leaving the proximity of a

running turbine engine.B. The minimum danger zone behind a running engine at idle is 100 feet. On large engines running

at idle, that danger zone can extend back to over 500 feet..C. At take-off power, the danger zone behind a running engine can be in excess of 1000 feet.

D. At take-off power, temperatures of over 100 ° F. exist at distances of over 200 feet behind the engine.

E. At take-off power the danger zone around the engine’s inlet extends out to a radius of 25 feet. At idle power that inlet danger area radius extends out to 15 feet.

Important Note : Always wear appropriate ear protection when in the vicinity of a running gas turbine engine, even if it is running at idle power. The most effective type of ear protection is the muff type which covers both the ear canal opening and the mastoid bone structure behind the ear.

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