Date post: | 05-Apr-2018 |
Category: |
Documents |
Upload: | olajide-yusuf-olamide |
View: | 228 times |
Download: | 0 times |
of 38
8/2/2019 Introduction to Airframe Systems II-Student's Copy
1/38
Restricted
1
Restricted
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.
8/2/2019 Introduction to Airframe Systems II-Student's Copy
2/38
Restricted
2
Restricted
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:
8/2/2019 Introduction to Airframe Systems II-Student's Copy
3/38
Restricted
3
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
4/38
Restricted
4
Restricted
able to comprehend the basic functions of a pneumatic (air) system and
the vital role it plays in the smooth operation of the aircraft.
4.2.1 Learning Outcomes
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.
4.3.1 Learning Outcomes
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.
8/2/2019 Introduction to Airframe Systems II-Student's Copy
5/38
Restricted
5
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
6/38
Restricted
6
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
7/38
Restricted
7
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
8/38
Restricted
8
Restricted
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.
8/2/2019 Introduction to Airframe Systems II-Student's Copy
9/38
Restricted
9
Restricted
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/8/2/2019 Introduction to Airframe Systems II-Student's Copy
10/38
Restricted
10
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
11/38
Restricted
11
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
12/38
Restricted
12
Restricted
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:
8/2/2019 Introduction to Airframe Systems II-Student's Copy
13/38
Restricted
13
Restricted
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:
8/2/2019 Introduction to Airframe Systems II-Student's Copy
14/38
Restricted
14
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
15/38
Restricted
15
Restricted
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/8/2/2019 Introduction to Airframe Systems II-Student's Copy
16/38
Restricted
16
Restricted
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.
8/2/2019 Introduction to Airframe Systems II-Student's Copy
17/38
Restricted
17
Restricted
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:
8/2/2019 Introduction to Airframe Systems II-Student's Copy
18/38
Restricted
18
Restricted
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.
8/2/2019 Introduction to Airframe Systems II-Student's Copy
19/38
Restricted
19
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
20/38
Restricted
20
Restricted
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.
http://www.gnyequipment.com/http://www.gnyequipment.com/http://www.gnyequipment.com/8/2/2019 Introduction to Airframe Systems II-Student's Copy
21/38
Restricted
21
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
22/38
Restricted
22
Restricted
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
http://www.orbitz.com/http://www.orbitz.com/http://www.orbitz.com/8/2/2019 Introduction to Airframe Systems II-Student's Copy
23/38
Restricted
23
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
24/38
Restricted
24
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
25/38
Restricted
25
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
26/38
Restricted
26
Restricted
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.
8/2/2019 Introduction to Airframe Systems II-Student's Copy
27/38
Restricted
27
Restricted
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.
8/2/2019 Introduction to Airframe Systems II-Student's Copy
28/38
Restricted
28
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
29/38
Restricted
29
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
30/38
Restricted
30
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
31/38
Restricted
31
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
32/38
Restricted
32
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
33/38
Restricted
33
Restricted
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).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
34/38
Restricted
34
Restricted
Figure 0-24: Jettison and Defuel System Schematic (courtesy of aircraft
fuel systems 1st edition & airframe Systems 2nd edition).
8/2/2019 Introduction to Airframe Systems II-Student's Copy
35/38
Restricted
35
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
36/38
Restricted
36
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
37/38
Restricted
37
Restricted
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
8/2/2019 Introduction to Airframe Systems II-Student's Copy
38/38
Restricted
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.