Introduction to Airframe Systems II-Student's Copy

8/2/2019 Introduction to Airframe Systems II-Student's Copy

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Introduction to Airframe Systems II (AEC 205)

1. Aim

The aim of this course is to broaden the knowledge horizon of selected

students to the ab initio of airframe systems, their role and system

components integration.

2. What is a System?

Before delving into the course proper, it is pertinent that the student

understands the meaning of a System as it relates to the introductory

aspects of the basic airframe systems considering the known fact that

the term System is used by various fields of endeavor.

The Oxford English dictionary defines a System as a complex whole,

set of connected things or parts, organized body of either material or

immaterial things.

In view of the above, we could adduce that a System may be referred

to various things depending on the context for which it is used.

However, we shall narrow our definition of the subject matter to aviation

related topics taken into further consideration that even in aviation; its

definition may also vary. A System may be described in the context of

this lecture to be a collection of connected parts/items on board an

aircraft, present to perform a specific function or sets of functions.

It should be noted here that the aforementioned definition is limited to

what the aerospace personnel recognizes or perhaps understands as a

system, in which case the airframe systems.


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3. Airframe Systems Breakdown

Having considered the meaning of a System as it relates to this course

of study, it is essential to identify the various airframe systems that we

are concerned with. These systems can be broken down into three (3)useful categories:

a. Power Generation, Regulation and Distribution Systems.

I. Pneumatic (Air) Systems.

II. Hydraulic Systems.

III. Electrical Systems.

b. Power User Systems.

I. Aircraft Environmental Control Systems.

II. Flight Control Actuation Systems.

III. Aircraft Ice Protection Systems.

IV. Aircraft Emergency Systems.

c. Aircraft Fuel Systems.

However, not all the systems listed above would be treated during the

period of the course except for the aircraft fuel system, pneumatic/air

system, flight control and actuation system.

4. Course Module and Specific Learning Outcomes

Introduction to airframe systems for 200 level ND students shall cover

the following topics with its subsequent learning outcomes as follows:


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4.1 Aircraft Fuel Systems.

This module is aimed at acquainting the student with the basic types and

function of a simple aircraft fuel system not excluding the major sub-

systems and components that make up the system. Furthermore, thismodule seeks to enhance the students knowledge on the variety of

aviation fuels that are commercially available as well as their

characteristics.

4.1.1 Learning Outcomes

At the end of this module, the student would be expected to explicitly:a. Define in unequivocal terms the function of a simple aircraft

fuel system.

b. Identify the major subsystems of an aircraft fuel system and

be able to briefly explain the functions of at least 3 subsystems.

c. Identify the major components of a fuel system as well as

their functions.

d. Identify the categories of aviation fuels and their peculiar

characteristics.

e. Differentiate between a Gravity-Feed and Pressure-Feed

Fuel System. The student should be able to sketch a simple

gravity-feed fuel system.

f. Identify the difference between integral fuel tanks and

bladder fuel cells or tanks.

4.2 Pneumatic (Air) System

This course module seeks to provide the student with a preliminary

description of the aircraft pneumatic system and further offer an

appreciation of why they take their present form. By conducting a

preliminary run through of the pneumatic system, the student should be


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able to comprehend the basic functions of a pneumatic (air) system and

the vital role it plays in the smooth operation of the aircraft.


At the end of this module, the student would be expected to:

a. Identify and sketch a simple pneumatic system.

b. Identify major components of an air system.

c. Give a brief explanation of the functions of the major

components identified in item (b) above.d. Explain in simple terms the role the pneumatic system plays

in providing cabin pressurization, air conditioning and cooling for

an aircraft.

4.3 Flight Control Actuation System

This course module shall provide an introduction to the role andfunctions of an aircraft flight control actuation system. At the end of the

program/module, the student would be able to appreciate the various

forms of flight control techniques employed from the early days of the

biplanes flown by the pioneers to present day methods due to

technological advancement in the aviation industry.


At the end of the module, the student would be expected to have learnt

and appreciated the following:

a. The basic principles of flight control.

b. Differentiate between primary and secondary flight control

surfaces.


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c. Identify and briefly explain the two (2) major flight control

linkage systems used by conventional aircraft.

d. Identify the major components essential for flight control

function.

e. Provide alternate means of controlling flights.

5. Aircraft Fuel Systems & Aviation Fuels

5.1 Introduction

The basic function of an aircraft fuel system is to provide a reliablesupply of fuel to the engines. Another definition states that the primary

function of the fuel system is to provide a uniform flow of clean fuel

under constant pressure to the carburettor or fuel metering device. This

supply of fuel must be sufficient to meet the rigorous demands of the

power plant at varying altitudes and attitudes of flight. It is imperative to

state here that this function is flight safety critical requiring somewhat

complex sub-systems and equipment necessary for the smooth

operation of the entire system.

This lecture note describes the various types of fuel system and sub-

systems in conjunction with the major components within each of the

sub-system. Aviation fuels are also briefly explained as well as

challenges dealing with fuel contamination and its effect on the system

as a whole.

5.2 Types of Fuel Systems

5.2.1 Gravity Feed Fuel System

In this type of system, the fuel system employs the force of gravity to

project fuel to flow to the engine fuel control mechanism or power plant.

In order to ensure this method functions properly, the base of the fuel

tank must be high enough to guarantee sufficient fuel pressure head at

the inlet of the fuel control component/metering device on the power

plant or engine. This may be easily achieved in high-wing aircraft by


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locating the fuel tank in the wings. Fig 0-1 below gives a brief

description of gravity feed system. In this case, the fuel flows by gravity

from the tanks of both wings through the fuel feed lines to the fuel

selector valve. The fuel then flows through the fuel strainer or filter down

to the fuel control component or carburetor/metering device. Also, fuel

from the primer is tapped from the main fuel filter. The diagram further

depicts a vent line attached to the fuel tank. In subsequent paragraphs,

we shall deal with each individual component for clarification purposes.

Figure 0-1: Gravity Feed and Fuel Pump System (courtesy of www.free-online-private-pilot-ground school.com).


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5.2.2 Fuel-Pump/Pressure Feed System

This type of fuel system employs the use of pumps to move fuel from the

tanks to the engine fuel control component due to the location of the

tanks being too low for adequate fuel head pressure to be generated.This type of fuel system is best achieved for low-wing aircraft where the

tanks are located at almost the same level as the engine fuel control

component. Also, this may be as a result of the distance of the tanks

from the power plant location. Fig0-1 above gives a brief illustration of a

pressure-feed fuel system. In this mode of operation, the fuel flows from

the separate fuel pipes to the selector valve (a switch over device used

to ensure sufficient pressure of fuel flow i.e. switches from active

pressure relief device to stand-by). After passing through the selectorvalve, the fuel flows to the electrical fuel pump. The engine driven pump

is located parallel to the electrical fuel pump to enable the fuel move by

either means without the need for a by-pass valve. Though not

indicated in the diagram, is a fuel boost pump which is responsible for

supplying fuel to the engine for initial starting while the engine-driven

pump provides the necessary fuel pressure for smooth operation.

Aircraft especially large aircraft with medium to high powered engines

require a fuel-pump or pressure feed system irrespective of the location

of the fuel tanks due to the large fuel capacity delivery at constant

pressure to the engine.

There are five (5) major sub-systems of which all aircraft fuel system

should have. They include:

a. The Engine Feed

b. Fuel Transfer

c. Refuel/Defuel

d. Vent

e. Fuel Quantity Measurement

In some advanced fuel systems, the under listed sub-systems are

incorporated as follows:

f. System Pressurization


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g. Auxiliary Fuel Tanks

h. Fuel Jettison

i. In-flight Refueling

In order for the student to adequately understand the basic functions of

each sub-system, it is essential that knowledge of the major components

that play a vital role in each of these fuel sub-systems is grasped.

These components are:

a. Fuel Storage Compartment/Tanks

b. Pumps

c. Valves

d. Level Sensors and Gauging Probes

6 Fuel System Components

6.1 Fuel Tanks

Fuel tanks may vary in type and design based on the available

technology at the time, the size and shape of the tank area specific to its

intended operation. Consequently, fuel tanks may be divided into three

(3) basic types; integral, bladder and rigid removable tanks.

6.1.1 Integral Tanks

In simple terms, an integral tank is one which forms a part of the aircraftstructure. This provides the advantage whereby the structural members

of the wing such as the ribs, spars, stringers/stiffeners and skin provides

tank boundaries (sealing materials are applied to areas where this

members are joined) hence reducing the overall mass of the fuel

system. However, the downside of this arrangement is that the shapes

of the tanks are not ideal, being of large platform and of shallow depth

making it vulnerable to fuel location changes within tanks with changes

in aircraft attitude.


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Currently on most transport aircraft (military or civil), most of the fuel is

carried in integral wing tanks. However, for military strike aircraft, it may

be impracticable to store all the fuel (if any) in the wings. This further

compounds the complexity of the fuel tank location in the fuselage of a

strike aircraft. Tanks are usually numerous and irregular in shape in

order to efficiently utilize the volume within the fuselage due to

competition for space with many other structures and system

components. This has an advantage of improving survivability of the

fuel system due to higher number of tanks and the protective effect of

surrounding components.

Integral tanks are usually manufactured using the same material as the

surrounding aircraft structure, sealed with a fuel proof sealingcompound. Access panels in the skin must be provided for tank

conditioning and components inspection. Additional tanks may be

located in the fin and tail plane. Tanks in the tail surfaces may be used

to control the aircraft centre of gravity just as the case of wing tanks.

Fig0-2 below gives a brief description of an integral central fuel tank with

structural partitions.

Figure 0-2: Schematic of an Integral Central Fuel Tank indicatingstructural partitions (courtesy ofwww.tc.engr.wisc.edu).
http://www.tc.engr.wisc.edu/http://www.tc.engr.wisc.edu/http://www.tc.engr.wisc.edu/


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6.1.2 Rigid Removable Tanks

A rigid removable fuel tank is one which is installed in a cubicle or

compartment within the aircraft structure which had been designed at

the initial stages to accommodate the weight of the tank. The tank mustbe fuel-tight, however the partition for which it is being held need not be

fuel-tight. This type of tank is mainly fabricated or constructed using

aluminum components welded together. The tank must be smaller than

the compartment; hence optimized utilization of available space within

the aircraft structure is not achieved. They are held firmly in the

compartment by several padded straps or screws. Also, an access

panel is used to cover the fuel tank. The tank incorporates a fuel feed

line; drain point, vent tube and a fuel quantity indicator. They can beremoved for possible replacement, repair or routine inspections. The

section/compartment in which the removable tank is installed is

structurally sound and does not require the tank for structural integrity.

This type of tank can be found on most general aviation aircraft such as

the Cessna172, Beech-craft etc. Fig 0-3 below shows an example of a

rigid removable tank.

Figure 0-3: Rigid Removable Fuel Tank (courtesy of AircraftMaintenance Engineering-Mechanical; available at www.aviamech.blogspot.com).


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6.1.3 Bladder Fuel Cells

A bladder fuel cell or tank is basically a reinforced flexible as well as a

collapsible rubberized bag which is located in a non-fuel-tight partition

designed to structurally carry the weight of the fuel. The bladder is rolledup and installed into the compartment through a fuel-filler neck or access

panel and unrolled. The bladder is secured by means of metal buttons or

snaps which attach the tank to the top, bottom and sides of the partition

or compartment set aside for the bladder. The bladder fuel cell

incorporates components such as vents, drain, quantity indicator etc. A

possible plus to this arrangement or concept of fuel storage is the

possibility of storing as much fuel as possible in the aircraft structure.

These types of tanks are found on many medium-to high-performancelight aircraft including turbo-prop and turbine powered aircraft.

Figure 0-4: Rubber Bladder-type Fuel Cell (courtesy of aero parts and

supply incorporated).


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6.2 Pumps

Aircraft fuel pumps are categorized into four main types, namely:

a. Transfer pumps.

b. Booster (back-up) pumps.

c. Jet pumps.

d. Engine driven pumps.

It is worthy to note that most fuel pumps are typically powered by either

DC or AC electric motors not excluding the fact that they can also be

powered hydraulically. Fuel pumps are generally designed to be bothcooled and lubricated using fuel itself. Safety measures have been

incorporated in most fuel pumps to enable the device run/operate dry for

an indefinite period. We shall now briefly describe in preliminary detail

the functions of the types of fuel pumps listed above for the sake of

clarity.

6.2.1 Transfer PumpsFuel transfer pumps are primarily responsible for transmitting fuel

between an aircrafts multiple fuel tanks in order to ensure that the fuel

feed requirement of the engine is met and sustained. The transfer

pumps perform a secondary task whereby fuel is transferred from one

tank location to another so as to control the aircrafts centre of gravity. A

transfer pump does not function continually as they are only operated

whenever the need arises. A typical aircraft system would incorporate

more than one transfer pump in case of failure of either pumps

(redundancy). Fig0-6 depicts a typical booster pump as shown below:


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Figure 0-5: Aircraft Fuel Transfer Pump (courtesy of weldonpumps.com).

6.2.2 Booster Pumps

Booster fuel pumps are basically used to enhance sufficient flow of fuel

at a pre-defined high pressure to the engine mounted pump (or engine).

In some cases, these types of pumps are referred to as engine feed

pumps. A necessary reason for this is that at a sufficiently high

pressure, aeration is prevented from occurring i.e. air is prevented from

being present in the fuel stream that could cause an engine to flame -

out consequently leading to loss of power. Furthermore, cavitations or

bubbles (especially at high altitudes for military aircraft) due to low fuel

pressure vapor and high temperatures are prevented by boosting the

delivery pressure. Cavitation is a process whereby a combination of

relatively high temperatures coupled with an increase in engine demand

at high altitudes results in a situation where the fuel begins to vaporize

(combination of low fuel vapor pressure and high temperature). Booster

pumps are usually electrically driven with delivery pressures ranging

from 70kN/m2 to 300kN/m2 and are designed to operate continuously

during flight. Fig0-7 below is a pictorial example of a typical booster

pump:


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Figure 0-6: Aircraft Fuel Booster Pump (courtesy of aircraft systems 2ndedition).

6.2.3 Jet Pumps

The jet or ejector pump functions using the venturi/ejector principle effect

of a constricted (converging/diverging) nozzle. Fuel is scavenged from

remote areas within the fuel tank and supplied under a pre-determined

pressure to an operating engine fuel control unit. Under normal

conditions, the engine-driven fuel pumps supply the engine fuel-controldevice with more volume than is needed in order to ensure the engine is

not starved of fuel. Consequently, the excess fuel from this pump is

directed back to the motive flow intake/inlet of the ejector pump. This

returned fuel in most cases is at high pressure of the order of

approximately 2068.9kN/m2 but at a lower volume. Once the motive

fluid exits the nozzle of the ejector in the venturi area, the pressure in

that constricted area drops (206.9kPa) with increment in the velocity of

the fluid ( the pressure energy of a fluid in motion is converted to velocityenergy with a drop in pressure around the constriction). The fluid in

motion continues along the venturi and sucks fuel from the tank with it

routing it to the engine-driven fuel pump at a volume sufficient enough

for the engine-driven pump.

Ejector pumps are reliable since they do not require any moving parts;

nevertheless they require a high pressure flow from another pump with a

low efficiency of about 25% (released fluid flow is limited in pressure).


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Figure 0-7: Schematic description of an ejector motive pump (courtesyofwww.physicsforum.com).

Fig0-8 above gives a schematic diagram of how an ejector motive pumpoperates using the venturi principle while Fig0-9 below is sample picture

of an ejector pump.

Figure 0-8: A pictorial view of a Fuel Ejector Pump(www.gasgoo.com/auto-products/fuel-system)

Engine Driven Fuel Pump

The purpose of the engine driven fuel pump is to deliver a continuoussupply of fuel at the proper pressure at all times during engine operation.The pump widely used at the present time is the positive displacement,rotary vane-type pump.
http://www.physicsforum.com/http://www.physicsforum.com/http://www.gasgoo.com/auto-products/fuel-systemhttp://www.gasgoo.com/auto-products/fuel-systemhttp://www.gasgoo.com/auto-products/fuel-systemhttp://www.gasgoo.com/auto-products/fuel-systemhttp://www.physicsforum.com/


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6.2.4 Fuel Transfer Valves

The major role a valve plays in an aircraft fuel system is to control the

direction and quantity of fuel flow within the system. It is important to

note that fuel systems valves with the exception of check valves areelectrically powered and controlled. Some valves may be designed to

be in an open or closed position (or variably controlled) which may be

achieved using solenoids or a small electric motor. Valves in a fuel

system vary depending on their function and can be categorized under

the following classes:

a. Check Valves/Non-Return Valves (NRVs). A check valve

is a device designed to prevent flow reversal i.e. allow fuel flow in

only one direction. They are basically two-port valves allowing

fluid to enter one port and leave the other port. These are

generally the uncomplicated type of valve in the fuel system being

self-actuating as others are a little more complex requiring external

power and control signaling. While others may be in an open or

closed position, others may require variable provisional control so

as to provide rate of flow control (using a metering device). Fig0-10

is schematic view describing how a check valve operates while Fig

0-11 is a simple pictorial view of a fuel pump check valve.


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Figure 0-9: Skeletal view of a simple disc NRV describing how thevalve operates in open and close positions (courtesy ofengineering products catalogue).

Figure 0-10: Example of a NRV.

It is important to note that there are various types of NRVs with

different shapes and sizes which is dependent on its function.

b. Cross-Feed Valves. Cross feed valves are utilized toconvey and control the flow of fuel from one side of the aircraft to

the other. Peradventure an engine on the starboard section (right

tank) of an aircraft wing is faulty and needs to be shut down, fuel

may be transferred from the right section/tank to the port (left tank)

for utilization. Fig0-11 below depicts the positioning of a cross-

feed valve in a simple fuel system schematic:


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Figure 0-11: Simplified Depiction of an Aircraft Fuel Systemindicating the location of the cross-feed valve.

The fuel manifolds (a chamber or pipe with several openings for

receiving or distributing a fluid or gas) are arranged so that any

fuel tank pump can supply either engine. A cross-feed valve

isolates the left fuel manifold from the right. This valve is normally

closed providing fuel feed from tank to engine. The valve may be

opened any time it becomes necessary to feed an engine from an

opposite fuel tank. Only one open cross-feed valve is required for

successful cross-feed operation.


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Figure 0-12: Transfer Valve (courtesy of aircraft systems 2ndedition).

c. Fuel Vent Valves. The primary function of a fuel ventvalve is to expel air from the fuel tanks during refueling process or

vent excess fuel from the tanks in-flight. In situations where the

fuel system is designed to be non-pressurized, during fuel

utilization/burn or defueling, air is permitted to enter the tanks to

replace the volume of fuel burned.

Figure 0-13: A typical fuel vent valve (courtesy of aircraft systems2nd edition).

d. Refuel/Defuel Valves. This type of valve is operated during

the refueling process in that the valves allow the fuel to flow from

the refueling point/gallery into the fuel tanks. Once the requiredamount of fuel is reached in the selected tank, the valves are

controlled to shut. The valves perform a similar role during

defueling except that the fuel flows in the reverse direction (permits

flow reversal from a fueling mode to a defueling mode).


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Figure 0-14: Fuel/defuel Valve (courtesy ofwww.gnyequipment.com).

e. Shut-off Valves (SOV). Shut-off valves are used to disallowfuel from flowing from one tank to another or to the engine as thecase may be. Shut-off valves perform the obvious function ofshutting off fuel flow when required. This might involve stemmingthe flow of fuel to an engine, or it may involve the prevention of fueltransfer from one tank to another.f. Fuel Dump Valves. Fuel dump valves allow excess fuel to

be jettisoned overboard especially during a state of emergency.

Due to the critical nature of these valves, it is necessary that they

remain in closed position in normal flight operation except when

the urgent need to activate the valve is warranted. This is to avoid

an inadvertent release of fuel into the atmosphere. Fig0-15 below

shows a stripped schematic of a fuel dump valve. The numberingsindicating the component piece are as follows:

(1). Coupling.

(2). Packing, same as (6).

(3). Elbow.

(4). Lock-nut.

(5). Retainer.
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Figure 0-15: A typical fuel dump valve (courtesy of aviationmaintenance and misc manuals).

6.2.5 Level Sensors and Gauging Probes

The importance of being able to monitor/detect when an aircraft fuel

tank(s) is either full or empty as well as being able to measure the level

of fuel in various tanks cannot be overemphasized. This may be

successfully achieved by incorporating different types of level sensors

and gauges. Critical tank levels i.e. empty or full can be monitored by

level sensors working independently of level gauges that may also be

used, thus providing some level of redundancy to the system.

6.2.5.1 Level Sensors

Just as the name implies, level sensors are utilized to accurately indicatethe actual level of fuel in tanks especially at critical tank level conditions

such as full or empty. The most common types of sensors are: float

operated; optical; zener diode; capacitance level sensors; ultrasonic

sensors and thermistors. We shall briefly describe the float level and

zener sensors for further clarification.

a. Float Level Sensors. Float level sensors work under the

theory of buoyancy in that as the level of the fuel in the tank variesin position, so also does the position of the float change. Also, it


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may be possible to install the float in conjunction with limit switches

to indicate when the tank is at its critical level. Furthermore, the

float may be physically or mechanically linked with the refueling

valve to activate the close position in the event of the tank being

filled to its desired level. As much as the float sensor is a simple

device, it may be susceptible to jam since it incorporates moving

parts.

Figure 0-16: Typical clayton aircraft fuel float and magnetic floatlevel sensor (courtesy of Gafsusa Aviation & aircraft parts andwww.orbitz.com).

b. Zener Diode Level Sensor. Zener diode (a diode is a semi-

conductor device with two terminals that typically permits the flow of

current in only one direction and also controls the flow of electricity) level

sensors are solid state electronic devices capable of measuring fuel tank

levels to values as accurate up to a couple of millimeters. They work on

the principle of the zener diode reference voltage sensitivity to

temperature. The sensor comprises two zener diodes assembled in a

small cylindrical housing, one operating at a relatively high level current

to induce a self heating effect while the other at a lower reference

current producing a negligible heating effect. Once these diodes are

immersed in the fluid, the fuel tends to cool the heating effect created by

the diode with the higher current. Remote signal conditioning electronics

monitor the two-diode assembly as it is immersed or uncovered from

fuel, to derive a switching signal based on the current change in the

heated diode with respect to the reference diode. Some sensors may be

positioned to determine when the tank is full while others to detect when
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the tank is empty. The response time when sensing from air to liquid is

approximately less than 2 seconds (refueling) while from liquid to air is

less than 7 seconds (low level warning). Its major advantage is the high

level of accuracy and reliability concerns due to absence of moving

parts. Fig0-17 depicts a fuel quantity probe with multiple zener diode

level sensors as shown below:

Figure 0-17: Probe with multiple zener diode level sensors(courtesy of GE aviation formerly Smiths Aerospace).

6.2.5.2 Fuel Gauging Probes

The major role of an aircraft fuel gauging probe is to monitor fuel

quantity within a given tank on board an aircraft. Fuel quantity

measurement may be achieved by incorporating various probes

operating under the principle of fuel capacitance measurement at

selected locations within the tank. Considering the fact that air and fuel

have different dielectric values or constants (a dimensionless constant

that indicates how easily a material can be polarized by imposition of an

electric field on an insulating material), the amount of fuel left in the tank

can be deduced by inferring the capacitance level of the probes. A

dielectric material is a substance that is a poor conductor of electricity

(electrical insulator) but an efficient supporter of electro-static fields

when polarized by an electric field. When a dielectric is placed in an

electric field, current does not flow through it as would a conductor. The

fundamental principle of capacitance gauging is the difference in the


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dielectric properties/constants of air and fuel. This phenomenon is

employed by configuring a capacitor as two concentric tubes arranged

vertically or near vertically in a fuel tank. As the fuel level changes, the

amount of the probe immersed in fuel changes and subsequently the

ratio of air to fuel and therefore the capacitance. These probes are

located in pre-determined positions such that in the event of attitude

changes due to effect of roll and pitch conditions, its effect is minimized.

The probes must be positioned to cope with the changes in the fuel level

induced by pitch and roll attitude. Fuel quantity indication systems

(FQIS) are utilized to provide adequate tank level warning to both air and

ground crew. Figs 0-18 and 0-19 show an example of a gauge probe

and level sensor respectively.

Figure 0-18: Types of Fuel Probe Units (courtesy of aircraft systems 2ndedition).


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Figure 0-19: Solid State Level Sensors (courtesy of aircraft systems 2ndedition).

6.2.5.3 Capacitance Gauging

The industry has more or less universally acknowledged this method of

gauging as the way to gauge fuel quantity more precisely. Although

capacitance gauging dates back to a 1924 French Patent, it has

progressively improved and advanced as new technology and materials

have become accessible over the successive 80 years. While the

sensors are relatively unsophisticated, the long successof capacitance

gauging systems is directly related to their compatibility and prolonged

existence in therelative aggressive environment of the fuel tank.

6.2.5.4 Capacitance Principles

Capacitance is the physical property of an item to accumulate charge

and is developed by applying a potential difference (voltage) across a

non-conducting medium (dielectric). A capacitive component (capacitor)

is formed by placing a non-conducting medium between two conducting

plates. The charge is configured as lines of electrical field across the


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dielectric. Fig0-20 is a schematic diagram illustrating the capacitance

probe concept as shown below:

Figure 0-20: Capacitance Probe Concept (courtesy of aircraft fuelsystems 1st edition)

6.3 Engine Feed Systems

The major function of the engine feed system is to supply fuel at a pre-defined pressure to the aircraft engine and APU if fitted. The system is

commonly designed such that fuel is fed from the tanks to the engine by

means of booster pumps. Normally, each tank is equipped with at least

two booster pumps that are identical. In cases where the aircraft

incorporates centre tanks (large aircraft), fuel is usually fed from the

centre tanks before the fuel in the wing tanks are utilized. Transfer

valves are installed on each wing and are activated in the likely event

where fuel is needed say from the outer tank to the inner tank.

Redundancy is mostly incorporated in this sub-system for safety reasons

in that in the event of failure of a single pump, the other pump is capable

of providing the maximum requirements of an engine. Also, it is possible

to feed an engine with fuel from either side of the aircraft by activating

the cross-feed valve. The Airbus A320 has a centre tank that feeds fuel

to the engine directly except during take-off and fuel recirculation when

the boost pumps are switched off automatically. The wing tanks operate

permanently at a lower pressure as compared to the centre tank.


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Consequently, when the centre pump stops, fuel is fed from the wing

tanks. The A321 being a simplified version of the A320 varies slightly in

that fuel is transferred to the wing tanks instead of flowing directly to the

engines. When the transfer valves are opened, fuel tapped from the

wing pumps is transmitted into the centre tank through jet pumps. This

further creates a depressurization which sucks in fuel from the centre

tank into the wing tanks. A transfer valve automatically closes when the

affected wing tank is overfilled or when the centre tank is empty.

Fig0-20 below are detailed schematics of an engine feed system

showing the main feed tank, pumps, lines and control valves in their

appropriate locations. The inboard section of the tank is where the feed

system equipment is located and this section is bounded by a semi-

sealed rib with flapper check valves that allow fuel to migrate inboard

only. This has the effect of trapping fuel inboard which is desirable. The

fuel boost pumps are located together on the lower skin of the collector

cell. Two boost pumps are typically installed to allow dispatch of the

aircraft with only one boost pump operative. Thereis also a suction feed

check valve in the collector cell to allow the engine to suck fuel from the

tank in the unlikely event of loss of both feed pumps. In this situation the

suction capabilityof the engine fuel system will be limited to altitudes of

about 20,000 ft or lower. The actualvalue of this operational limit will be

established during flight testing of the aircraft as part ofthe certification

process.A scavenge ejector pump is shown in the figure which is used

to charge the collector cell.


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Figure 0-21: Engine feed system detailed schematics (courtesy ofaircraft fuel systems 1st edition).

6.4 Fuel Transfer System

The function of the fuel transfer system is to move fuel from the main

wing and fuselage tanks to the collector tank/box. In some aircraft, twotransfer pumps are provided in each wing tank and another two in the


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fuselage or centre tanks. These pumps are normally activated by the

level of fuel in the tanks they supply i.e. once the fuel reaches a certain

level as measured by the fuel gauging system or level sensors, the

transfer pumps begin to run until a pre-defined fuel level within the tank

is attained. Transfer pumps are electrically operated at 115VAC 3-

phase electrical power driving an induction motor. As discussed in

earlier sessions, the duty cycle of transfer pumps is not continuous as

compared to booster pumps since it activates only at fuel tank level

demand requirements. Figure0-21 shows a schematic of a typical

override transfer system in a traditional three tank aircraft. Here centre

tank fuel is consumed first by employing centre tank transfer pumps that

produce significantly higher feed line pressures than the main feed boost

pumps are capable of. So while the feed tank boost pumps operate

continuously their outlet check valves are maintained closed by the

override pump pressure so that all of the feed flow to the engine comes

from the centre tank. Once the centre tank fuel has been depleted, the

centre tank boost pumps are switched off allowing feed flow to be

provided from the main feed tank boost pumps that automatically take

over the engine feed task.

Figure 0-22: OverrideTransfer System Schematic (courtesy of aircraft

fuel systems 1st edition).


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6.5 Refuel/Defuel System

In the case of the aircraft refueling system, fuel is fed into the various

tanks by means of a refueling receptacle connected to refueling tanker

being a pressurized external source supplying the fuel. From thereceptacle, the fuel enters the refueling gallery/manifold which

distributes the incoming fuel to the designated tanks. At the point where

the tanks are filled up, the refueling shut-off valve is activated to prevent

entry of anymore fuel (over filling and over pressurization is prevented).

In a very simple system, the refueling shut-off valve may be a float

operated mechanical valve whereas in some complex fuel system, a fuel

management system is used to control the refuel valve by electrical

means such solenoid operated or motorized valves. Some other aircraft

allow over-wing/gravity filling by supplying fuel at a high point in the

system such as over the wings. However, this method is unpopular with

medium or large aircraft as it is time consuming except in situations

where there are no external fuel pressure fed bowsers such as in remote

airstrips or desert regions. The fuel is poured via access panels over

wing. The defueling process is basically the reverse procedure except

that it rarely occurs except during maintenance inspections.

6.6 Vent System

During the refueling process or fuel transfer, large amounts of air can be

displaced by the incoming fuel very quickly especially when conducting

pressure refueling. Pressure refueling involves relatively high positive

pressure within the order of 50psi to speed up the refueling process.

Pressures of such magnitude of force being exerted over a large area iscapable of damaging the fuel tank and the excess air is required to be

expelled overboard by means of a pressure relief vent valve. In some

cases, the vent system may allow air into the tanks as the fuel is being

burned. This is however not the case for pressurized fuel tanks.

Pressurization of the fuel tanks by means of ram air is in some aircraft

implemented to assist fuel transfer around the system while gravity feed

and fuel transfer pumps are sufficient for some non-pressurized fuel

system. It is worthy to note that in some high performance fighter


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aircraft, ram air is not totally sufficient to pressurize the fuel system;

hence pressure reduced bleed air is used to pressurize the system to an

acceptable level by means of a pressure reducing valve (PRV).

In many large transport aircraft, excess air is vented by means ofpressure relief vent valves located at the top of the tanks. Also, air and

fuel may be expelled via pipes into the surge tank. Float valves are

situated at the pipe inlets in the main tanks to prevent large quantities of

fuel being vented. Surge tanks located mostly at the wing tips allows

fuel venting to occur without spillage. At the low end of functional

complexity are float operated vent valves. These valves are relatively

simple devices used to allow air to enter the vent lines and to close

when exposed to fuel to prevent fuel from entering the vent system and,ultimately spillage overboard. Most float vent valves are direct acting

(not pilot operated or pressure assisted) devices. They rely on the float

buoyancy to close the valve and the float weight to cause the valve to re-

open. A float vent valve in one of the simplest forms is illustrated in

Figure0-22 below:

Figure 0-23: Direct acting float vent valve (courtesy of aircraft fuelsystems 1st edition).


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6.7 Fuel Quantity Measurement Systems

Fuel quantity measurement often requires a complex arrangements

comprising series of probes and warning sensors. In most cases, low

level sensing incorporates some level of redundancy by placing severalmeasurement probes and level sensors in the tanks. Probe and sensor

signals must be processed using on-board computing for ease of

interpretation by the cockpit crew. Fuel quantity measurement systems

using capacitance probes may be implemented in two ways namely:

a. AC system.

b. DC system.

6.7.1 AC System

In an AC System, information is conveyed using capacitance probes by

means of an AC voltage signal modulated by the measured tank level

(or capacitance) and fuel quantity. Although, the AC signaling technique

is simpler hence more reliable and less expensive than a DC system, it

is susceptible to Electro-Magnetic Interference (EMI- a form ofdisturbance that affects an electrical circuit due to the production of

electric currents across a conductor moving through a magnetic field

causing a production of voltage across the conductor also known as

Electro-Magnetic Induction) requiring relatively heavy and expensive

shielding cables (coaxial) and connectors to transmit signal making it

difficult to maintain and slightly complex to install. This in the long run

makes the DC system lesser in weight at top level aircraft weight

requirements.

6.7.2 DC System

In a DC system, the probes are fed by a constant voltage and frequency

signal from a probe drive unit. A rectified signal showing tank fuel level

is fed to the processor as an analogue DC waveform. Considering the

need for added and more complex components in the fuel tanks as

compared to the AC system, the tendency to move towards this


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measuring solution is higher due to its weight saving potentials and

superior EMI performance. Most large transport aircraft utilize the DC

fuel measurement system.

6.8 Fuel Jettison

The fuel jettison system would be required to expel fuel overboard in

large quantities in a relatively short period of time (in the order of ten

minutes). Most aircraft are designed to have a larger maximum take-off

weight as against the maximum allowable landing weight. In situations

where a scheduled flight is suddenly cut short after take-off, fuel may be

required to be jettisoned in order to reduce the aircrafts all -up weightthus enabling a safe landing (meeting the certified landing weight).

Furthermore, in the event of a serious malfunction such as engine

failure, it may be necessary to eject fuel subsequently reducing weight in

order to remain airborne. The jettison system comprises a combination

of fuel lines, valves and pumps working in synergy as well as using large

pipes to reject the fuel overboard.

In most transport aircraft, fuel is jettisoned on the lower part/wing

undersides towards the tip trailing edges while in fighter aircraft, this is

typically situated on the fuselage and close to the engine feed points in

the system. The fuel jettison system and its operation must be clear of

fire hazards and the fuel must be discharged clear of any part of the

aircraft structure. Furthermore, during the jettison operation, fuel fumes

must not be perceived in the airplane as well as the aircraft controllability

must not be jeopardized. The system must be built to guard against

inadvertent operation by incorporating master jettison valves located well

downstream in the system close to the fuel jettison point. The fuel

jettison controls in the cockpit must be protected from spurious

operation.

Depending on the aircraft make, the force required for fuel jettison is by

gravity or by centrifugal pumps in the fuel tanks. Fig0-23 below shows a

schematic of the jettison system (including the defueling system).


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Figure 0-24: Jettison and Defuel System Schematic (courtesy of aircraft

fuel systems 1st edition & airframe Systems 2nd edition).


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6.9 Use of Fuel as a Heat Sink

In most high performance aircraft, the fuel performs a vital role of acting

as a heat sink for heat generated within the aircraft during flight. In case

of the Concorde aircraft, the kinetic heat is produced by air friction duringprolonged flight at very high speeds up to Mach 2 in the cruise phase.

In the case of fighter aircraft prolonged operation at high speeds is not

likely because of the punitive fuel consumption. The aircraft will generate

a lot of heat, particularly from the hydraulic and environmental control

system, which needs to be sunk in the fuel. Fuel cooling systems

utilize the aircraft fuel as the heat sink. The mode of operation is such

that fuel flowing from tanks to the engines is routed via one side of a

heat exchanger. The aircraft heat load to be cooled such as the engineoil flows through the other side of the heat exchanger.

However, such a system is limited in some ways in that the fuel flow rate

varies with engine throttle settings. Consequently, when the engine is

on low throttle setting, the fuel flow rate is low hence leading to an

ineffective heat exchange process. This may also lead to a rise in

temperature of the fuel if subjected as a heat sink. Furthermore,

towards the end of the flight, fuel is nearly depleted and the heatcapacity of the fuel as a heat sink is low again leading to a significant

rise in temperature. This could be further mitigated by employing ram air

as coolant for the fuel.

6.10 Fuel System Contamination

Contaminants in the likes of sand and rust may enter the fuel system.Hence, the placement of filters is needed to prevent the unwanted

entities from reaching the engines. These filters may be inspected and

cleaned or replaced as the case maybe during scheduled maintenance

inspections.

At high altitude operations, water in the form of ice may be present in the

fuel where the fuel will fall below zero degrees Celsius. If some

dissolved water is present in the fuel, this poses no immediate threat

provided it stays dissolved. Nonetheless, water may enter the fuel tanks


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in the form of vapor through the vents and some may condense during

flight on the fuel surface and on other cold surfaces. These may

consequently freeze forming ice particles that can lead to filters being

clogged or blocked. A clogged filter may result in pressure differential

which may activate a by-pass valve to allow the contaminated fuel flow

into the engines. Also, a pressure differential is created which activates

a small pump on detection that subsequently injects methyl alcohol into

the filter thus melting the ice and clearing the blockage.

Water tends to accumulate inside the fuel tanks over time and can be

drained during maintenance after settling in the tanks. Furthermore, the

build-up of water may mean a suitable breeding ground for micro-

bacterial growth which may result in the corrosion of system componentsas well as the aircraft skin panels. It may even adversely toil with the

fuel gauging system. Bacteria may be removed and prevented from

returning by using chemicals such as Biobar JF. These days, aircraft

fuels are being treated with additives to prevent bacteria from forming.

6.11 Aviation Fuels

Aviation fuels that are commercially available in their wide variety share

certain common features. For instance, they must have sufficient

stability to be stored and transferred safely. Also, they must not react to

corrode or perish fuel system components. In broader terms, aviation

fuels may be considered into two categories namely; fuels for piston

engine aircraft and those for jet powered aircraft.

6.11.1 Fuels for Piston Engined Aircraft

Aviation Gasoline (AVGAS) is basically the term used for fuels that are

used for piston operated engines. The most important property when

considering AVGAS is the anti-knock rating. Just like all internal

combustion engines, aircraft piston engines are designed to operate

effectively and efficiently at a defined knock rating. AVGAS utilized must

meet the engines defined value; although fuels having higher knock

rating can be exploited without complications. On the contrary, fuels


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with lower knock rating than the engine was designed for must not be

used for any reason.

A second important feature of AVGAS is the fuels volatility to permit

successful cold starting. The fuel must have a high volatility quotient tobe able to vaporize for combustion at the lowest operating temperature.

Currently, three variants of AVGAS are widely available namely;

AVGAS80, AVGAS100 and AVGAS100LL. Amongst the three listed,

AVGAS100LL (LL denotes a lower lead content) is the latest and best

selling variant. It was created in a bid to come up with a universally

acceptable variant.

6.11.2 Fuels for Turbine Engined Aircraft

AVTUR (Aviation Turbine Fuel) is the name given to the various

categories of fuel used by turbine engines. As opposed to piston

engines, jet engines are able to run reliably and efficiently well on

basically all AVTUR variants and even run using AVGAS although with

limited performance. The most important consideration when

highlighting the properties of AVTUR is that it must be capable of beingsupplied to the engines over a wide range of operating conditions that

the modern gas turbine engine is currently subjected to. AVTUR fuels

are all kerosene based.

There are basically three main types of AVTUR fuel used by both

military and civil operations. They are; Jet A used at North American

airports, Jet A-1 used world-wide including North America (also an

AVTUR fuel known as Jet No.3 supplied at airports in mainland China

although its properties are closely related to Jet A-1) and lastly Jet TS-1

normally supplied at airports in Russia and other parts of the eastern

block. Jet TS-1 has been seen to possess superior low temperature

performance as compared to Jet A-1; however it has a low flash point

making it inferior to Jet A-1 from a safety point of view.

Jet A-1 has become the fuel of choice for military aircraft with additives

to prevent ice formation and effects of corrosion. This fuel is known as

Jet A-1 (FSII) or AVTUR/FSII (Fuel System Icing Inhibitors), and by the


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USAs military as JP-8(+) with improved thermal stability additive.

Corrosion inhibitors are also added to guard against corrosion of ferrous

metal components in the fuel system and also improve the lubricating

properties of the fuel.

Introduction to Airframe Systems II-Student's Copy

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