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INTRODUCTION TO AIRCRAFT

FUEL SYSTEM

Copyright 2009 Tata Technologies. All rights reserved. All other trademarks are trademarks of their respective owners.

Introduction

All powered aircraft require fuel on board to operate the engine(s). The

fuel must be available to the engine under all conditions of engine

power, altitude, attitude, and during all approved flight maneuvers.

An aircraft fuel system allows the crew to pump, manage, and deliver

fuel to the propulsion system of an aircraft. Fuel systems differ greatly

from aircraft to aircraft due to the relative size and complexity of the

aircraft in which they are installed. They are designed to provide an

uninterrupted flow of clean fuel from the fuel tanks to the engine.

The problem of safely storing and delivering fuel to an aircraft's hungry

power plant has been an issue for airplane designers since the first

flights at Kitty Hawk.


Glossary

Abbr. Full form

AAR Air to Air Refuelling

CT Centre Tank

FVV Float Vent Valves

MSLW Maximum Structural Landing Weight

MTOW Maximum Take-Off Weight

NACA National Advisory Committee for Aeronautics

NRV Non Return Valve

ST Wing Surge Tank

WT Wing Tank

WW World War


The Fuel System Design Rules

There are some golden rules which needs to be followed for

designing a safe and efficient fuel system:

The design of the Gauging System should be independent of the Transfer Control System.

The means used to call attention to a fault with the Transfer Control System shall be independent of the Transfer Control

System.

The means used to call attention to a fault with the Gauging System should be independent of the Gauging System.

The means used to call attention to a fault with the Transfer Control System should be independent of the means used to call

attention to a fault with the Gauging System.


The Fuel System Parts

Fuel Tanks (Wing Tanks, Centre Tank, Trim Tank etc)

Venting System

Refuel, Defuel and Ground Transfer System

Engine Feed System

Fuel Transfer System

Fuel Scavenge System

Water Scavenge System

Jettison (as a Customer option)


Fuel Tanks

The fuel tanks are designed to maximize fuel availability and to minimize heat loss from the fuel.

The fuel stored in the wing tanks provides wing bending moment relief, thus optimizing aircraft structural weight.

Generally, Fuel is stored in

two types of tanks: CT

and WT, which are

located as shown.

Aircrafts also have Surge

tanks to help in venting.

Sometimes, airplanes also

have trim tanks which

help to adjust the centre

of gravity.


Venting System

The vent system maintains the tank pressures within the structural limits

during normal ground operation, refuel overflow, and any flight

maneuver. It also avoids fuel spillage during refuel and throughout the

operational attitude/acceleration envelope.

To allow proper fuel flow, each fuel tank must be vented from the top

part of the expansion space. Vent outlets must be located and

constructed in a manner that minimizes the possibility of being

obstructed by ice or other foreign matter. There must be no point in

any vent line where moisture can accumulate either on the ground or

during level flight.

Fuel tank vents may not terminate at a point where the discharge of fuel

from the vent outlet constitutes a fire hazard or from which fumes may

enter personnel compartments


Valves Used In Venting

Float vent valves (FVV) are float operated valves which allow air to flow

between the tanks and the vent pipes and prevent fuel entering the

vent pipes. A rising fuel level raises the float and closes the orifice

thus preventing movement of fuel into the vent pipe. These valves

require no external power. These valves are located in the tanks to

ensure that they are in the air bubble (ullage) over a range of aircraft

attitudes. Multiple Float Vent Valves are used, where necessary, to

cover the appropriate aircraft attitude range.

Vent line fuel drain valves are breather valves which allow fuel to pass

from the vent pipe into the tank but prevent flow of air or fuel in the

other direction (part of vent line assembly). The sphincter valve is a

self sealing rubber boot that is operated by fuel pressure. The

breather valve requires no external power source.


Wing Surge Tank (ST)

If a refuel overflow occurs, the FVV closes and fuel passes through the

main vent pipe into the ST. If the overflow continues, the fuel fills the

ST and overflows out of the aircraft via the NACA duct / flame arrestor

assembly.

At low fuel quantities air is vented from the tank via both the bifurcated

inlet in the WT & the FVV, into the ST and out of the aircraft through

the NACA duct / flame arrestor assembly which if blocked, the

overpressure protector relieves pressure preventing the tank from

becoming over-pressurised.

An ST Non Return Valve (NRV) is installed in each wing, and allows fuel

to drain inboard from the ST into the WT main cell but prevents flow

outboard. This valve ensures that any fuel that has spilt into the ST

via the vent system is recovered back into the WTs thus minimizing

unusable fuel and preventing cumulative build-up to the spill level. It

is a flap valve that requires no external power source.


Refuel

Refuel is the process of filling the aircraft with fuel. There are two types

of refueling processes as given below:

Over-the-wing refueling

Accomplished by opening the fuel

tank cap on the upper surface of the

wing or fuselage, if equipped with

fuselage tanks. The fueling nozzle is

carefully inserted into the fill opening

and fuel is pumped into the tank.

When finished, the cap is secured and

subsequent tanks are opened and

refilled until the aircraft has the

desired fuel load onboard.

Pressure refueling

It occurs at the bottom, front, or rear of the

fuel tank. A pressure refueling nozzle locks

onto the fueling port at the aircraft fueling

station. Fuel is pumped into the aircraft

through this secured and sealed connection.

Gauges are monitored to ascertain when the

tanks are properly loaded. An automatic

shutoff system may be part of the aircraft

system. It closes the fueling valve when the

tanks are full.


The Fuel System facilitates both manual and automatic refuels.

Automatic refuel distributes the uplifted fuel to a predetermined post

refuel distribution within the fuel tanks. This ensures a predictable

aircraft's centre of gravity.

Manual refuel allows fuel to be uplifted to any tank at the ground crew's

discretion, the amount of fuel added to each tank being covered by

aircraft procedures. Manual refuel provides an alternative to auto

refuel thereby mitigating any failures associated with auto refuel, and

can be useful when trouble shooting system problems.

Refuel ...


Defuel allows fuel to be removed from any or all of the fuel tanks. This

can be achieved using the aircraft fuel pumps and/or via suction

provided at the refuel coupling. Suction defuel being used when

electrical power is not available for the aircraft pumps or in

combination with the aircraft pumps to reduce defuel time. Suction

defuel is also known as Pressure defuel.

Ground transfer enables fuel to be moved from any tank to any other

tank.

Manual and automatic refuels, defuel and ground transfers between

tanks can be controlled from the external refuel panel. A customer

option to control the refuel automatically from the cockpit is also

available.

Defuel & Ground Transfer System


Fuel Valves

There are many types fuel valves used in an aircraft fuel system. They

are used to shut off fuel flow or to route the fuel to a desired location.

Light aircraft fuel systems may include only one valve, the selector valve.

It incorporates the shutoff and selection features into a single valve.

Large aircraft fuel systems have numerous valves. Most simply open and

close and are know by different names related to their location and

function in the fuel system (e.g., shutoff valve, transfer valve,

crossfeed valve). Fuel valves can be manually operated, solenoid

operated, or can be operated by electric motor. Ex: the refuel

coupling isolation valves are solenoid valves to ensure no fuel will be

spilled due to loss of power.


Engine Feed

The ability of the fuel system to provide fuel at a rate of flow and

pressure sufficient for proper engine operation is vital in aircraft.

Two common classifications apply to fuel systems in aircraft as shown:

Fuel-Pump Feed System

Aircraft with fuel-pump systems have two fuel

pumps in each wing. The main pump system is

engine driven with an electrically driven auxiliary

pump provided for use in engine starting and in

the event the engine pump fails. The auxiliary

pump, also known as a boost pump, provides

added reliability to the fuel system

Gravity-Feed System

The gravity-feed system utilizes the

force of gravity to transfer the fuel

from the tanks to the engine.

If the design of the aircraft is such

that gravity cannot be used to

transfer fuel, fuel pumps are

installed.


Fuel Transfers & Crossfeed

Crossfeed allows for fuel from one wing tank to be burned by the engine

installed on the opposite wing. In some cases, the fuel is routed directly

from the tank to the engine while in others, it is transferred from one wing

tank to the opposite wing tank before feeding it to the engine. The

crossfeed provision allows the pilot to use all of the fuel on board and to

maintain lateral balance limitations in the event of single engine

operations.

Fuel transfer from the wing tanks to the centre tank is required to mitigate

certain failure conditions and maximize fuel availability by providing an

alternative route from wing tank to engine. This transfer is achieved by

opening the tank inlet valves of the refuel gallery on one side of the

aircraft, which depending on fuel head differences will allow a gravity

transfer of fuel from the wing tank to the centre tank via the refuel gallery.

Control is independently provided to initiate gravity transfer from both wing

tanks.


Scavenge System

Fuel scavenge is provided to help satisfy the economic requirement to

minimize unusable fuel and operating weight empty. When the CT

becomes empty, the CT engine feed pumps are automatically

switched off and fuel is scavenged from the CT low points that occur

in cruise. Jet pumps are used to scavenge this fuel from the CT into

the WTs. The motive flow for these jet pumps are provided by the WT

main and standby engine feed pumps.

Similarly, water scavenge is provided to help satisfy the economic

requirement to minimize water drain operations. Jet pumps are used

to scavenge water from each tank to prevent significant quantities of

water collecting in the fuel tanks. The discharge of each jet pump is

directed toward the remote inlet of their respective engine feed

pump(s).


Fuel System Indicators

Aircraft fuel systems utilize various indicators. All systems are required to

have some sort of fuel quantity indicator. Fuel flow, pressure, and

temperature are monitored on many aircraft. Valve position indicators

and various warning lights and annunciations are also used.

Fuel Quantity Indicating Systems

All aircraft fuel systems must have

some form of fuel quantity indicator.

These devices vary widely

depending on the complexity of the

fuel system and the aircraft on

which they are installed.

Fuel Flowmeters

A fuel flowmeter indicates an engines fuel use in real time. This can be useful to the pilot for ascertaining

engine performance and for flight planning

calculations. The types of fuel flow meter used on an

aircraft depends primarily on the power-plant being

used and the associated fuel system.

Fuel Temperature Gauges

Monitoring fuel temperature can inform the pilot when

fuel temperature approaches that which could cause

ice to form in the fuel system.

Fuel Pressure Gauges

Monitoring fuel pressure can give

the pilot early warning of a fuel

system related malfunction.


Jettison

The Fuel System facilitates both

manual and automatic refuels

As shown in the figure, fuel being

jettisoned free of the airframe on

a transport category aircraft. Fuel

jettison systems are also referred

to as Fuel Dump Systems.

Aircraft have two major types of weight limits: the MTOW and

the MSLW which is almost always being the lower of the two. This

allows an aircraft on a normal, routine flight to take off at the higher

weight, consume fuel en route, and arrive at a lower weight.

A situation could occur in which a landing is desired before sufficient

fuel has burned off to lighten the aircraft. Fuel jettison systems are

required on the aircraft so that fuel can be jettisoned in flight to avoid

structural damage cause by landing the aircraft when it is too heavy.


Military Aircraft Fuel Systems

Military aircraft fuel systems face the additional problems of avoiding

catastrophic results from hits to the fuel system by enemy fire.

Most propeller-driven aircraft of the World War era used piston engines that

burned an aviation-grade gasoline for fuel. Diesel engines were not unknown,

but were rarely used for any military aircraft outside of airships due to inferior

performance. Most fuel systems therefore used carburetors to introduce fuel

into the engine. Towards the end of WWI, the self-sealing fuel tanks began to

come into use to minimize damage from hits to the fuel tank.

Jet fighters came to military service at the very end of WWII. Jets are more fuel-

hungry than most propeller aircraft and require different fueling mechanisms

in the engine itself. Virtually all jet engines use injection fuel systems, and

many designs from the 1950s onward incorporate the ability to dump raw fuel

into the engine aft of the engine's compression chamber to improve thrust.

This procedure is known colloquially as afterburning. Fuel storage continues

to be based on self-sealing tanks, with improvements in materials as

technology evolves.


In addition to internal fuel storage, many military aircrafts have the ability

to use external fuel tanks to extend their range. These are known as

drop tanks, since many of them are jettisoned from the aircraft once

expended or if an emergency occurs.

Drop Tanks

Drop tanks are commonly used on bombers or attack aircraft. Escort

fighters may use such devices for long-range missions to preserve

their fuel stores for air combat maneuvering near the target. Use of

drop tanks in active military service dates back to at least the Spanish

Civil War in the 1930s, though experimental models may have been

tested before this time.


Air to Air Refuelling

A significant advance in military aircraft fuel systems was the

introduction of air-to-air refueling capability.

Using air-to-air refueling allows an airplane to take off with a significantly

higher weapons load or extend its range or loiter time over the

battlefield.

Experiments with air-to-air refueling began in the 1920s with individual

biplane fighters transferring fuel to one another. While many systems

developed since the 1950s use a large tanker aircraft to fuel other

aircraft, a recent trend towards the use of "buddy store" refueling

systems mounted on standard fighters has brought the concept of

aerial refueling back to its origins.


There are two types of AAR techniques:

I. Flying Boom

II. Probe & Drogue

Air to Air Refuelling ...

Flying Boom requires navigating a giant telescoping tube into a

receptacle located near the front of the receiver plane. After the boom

latches in, the tanker starts pumping fuel.

Probe & Drogue method uses a hose that drops into a basket and fuels.


www.wikipedia.org

http://military.answers.com

A350 Fuel SDD

Bibliography

NOTE: After going through this presentation, atleast one of the ATA

28 - System Design Document (SDD) should be referred for a

complete understanding of the aircraft Fuel System.

Documents

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