Aircraft ECU and associated electrical systems 7720

Question 1

requirments of a fuel system

Answer 1

hold sufficient fuel required for the flying task

supply the correct amount of fuel to all engines

supply fuel at the correct pressure

to be able to refuel from pressure or gravity refuellers

emergency jettison

isolate fuel from engines in an emergency

monitor fuel usage or fuel contents

transfer fuel to maintain the (C of G)

indicate fuel flow failure or low level

to be able to defuel by suction or gravity to any level required

Question 2

purpose of the fuel managemnt panel

Answer 2

to allow the pilot to control fuel system

Question 3

fuel tank construction

Answer 3

crashworthy construction, made from a polyurethene elastomeric material, which has self sealing properties

Question 4

float switch assembly

Answer 4

fitted in each fuel tank and consists of a tube fitted with four float switches

float switch contains a reed switch operated by magnets

HLFS operates at 100% (tank full) and is used to close the refuel/defuel valve during refuel and provide a tank full indication

80% float switch is a transfer threshold float switch to allow automatic transfer of fuel in the fuel transfer automatic mode

17% float switch operates to indicate when the fuel level in the tank is at a LOW alert level

0% closes the refuel/defuel valve during defuel and switches off the transfer pumps in the transfer tank

Question 5

purpose of the fuel distribution manifold

Answer 5

located on the transmission decking

controls fuel flow xfeed between 1,2,3 tanks

Question 6

purpose of the jet/scavenge pumps

Answer 6

the jet pumps draws fuel from the bottom of the tank to ensure that all fuel is used up,

mechanical pump which feeds pressurised fuel from the booster pump manifold, into the primary inlet,

this comes out the jet at high speed creating a venturi effect

low pressure in the secondary inlet sucks fuel from the bottom of the tanks

Question 7

booster pump operation CT diagram

Answer 7

booster pumps have their own pressure switch and controlled by its own indicator push button

when working normally the pump will be blank

a failure will cause the indicator to show OFF in green and low pressure will show LOW in yellow

with AC power on and booster pumps OFF the indicator will indicate OFF in green

when the booster pump push switch is pressed ON, the booster pump relay will energise, and its contacts will change over and three phase electrical power will be fed to the booster pump,

initially the pump pressure will be low and the pressure switch will cause a yellow LOW caption on the switch indicator to be illuminated

as the booster pump runs up, the pressure will increase and the contacts in the pressure switch will change over and a low caption will be removed

Question 8

how is the pilot made aware of a booster pump failure

Answer 8

an indication will appear in the IDU as a YELLOW LOW caption

booster pump is selected to OFF by pressing the indicator switch

the pilot will need to consider reducing altitude and be aware of possible performance limitations

checked regularly to ensure contents do not fall below 45kgs reducing the possibility of cavitation

Question 9

merlin auto fuel operation

Answer 9

under normal conditions the No1, 2 and 3 fuel tanks motorised ball transfer valves are closed

until the contents of the fuel tanks has fallen to 80%

then float switch in the fuel tank will open the motorised ball transfer valve and allow fuel to enter the tank from the transfer tank

when the transfer tank is empty the LOW LEVEL FLOAT SWITCH (LLFS) operates and swithces the transfer pumps off

transfer pumps in the transfer tank supply fuel to the main fuel gallery pipe which runs along the left side of the cabin

fuel transfer continues until the No 1, 2 and 3 fuel tanks high level float switch (HLFS) operate to close their respective motorised ball transfer valve

Question 10

high level float switch failure

Answer 10

if a HLFS fails to close its mortorised ball valve, the the auto system will cause an overflow of fuel

overspill switches fitted above the normal fuel tank full switches will operate and cause a SPILL indication on the IDU to show that overflow is taking place

the transfer pumps can then be manually selected OFF by switching the AUTO/MANUAL switch to the manual position

Question 11

xfeed switch to pos 1, 2, 3

Answer 11

if all the xfeed switches are selected to pos 1. No 1 tank will supply all three engines and the indications for engines 2 and 3 will indicate TNK1

same for all engines

Question 12

fuel xfeed operation during engine failure

Answer 12

if no one engine had failed. No 1 tank could be left with fuel when the other tanks were low,

the selection of No 2 and No 3 crossfeed switches to pos 1 would allow No1 tank to then feed the servicable No2 and No3 engines the crossfeed indicators for engines 2 and 3 would then indicate TNK1

Question 13

what happens to the merlin fuel indications at 600kg, 200kg and 146kg

Answer 13

if contents of individual tanks fall below 200kg or the total content falls below 600kg the digital readout will change from green to yellow with a yellow box around it

when a reed switch in the LLFS falls below the 20min level, (146kg) a FUEL LOW caution caption appears in the WCA window

Question 14

describe pressure refuel merlin

Answer 14

pressure refuelled from a bowser which takes 9 mins

connecting the pressure refuel hose to the aircraft refuel point, causes a microswitch to operate allowing a DC supply to the REFUEL/OFF/DEFUEL switch on the control panel

when the REFUEL/OFF/DEFUEL SWITCH is selected to refuel, the refuel/defuel valves in No 1,2 and 3 fuel tanks are energised open allowing fuel from the main gallery into No1, 2, and 3 fuel tanks

as the fuel tanks 1,2 and 3 become full, their HLFS operate and remove electrical power from the refuel/defuel valves causing the valves to de-energise and close to shut off the supply of the fuel to No1 2, and 3 fuel tanks

with No1,2 and 3 tanks full and refuel/defuel valves close, power is then fed to transfer tanks refuel/defuel valves which opens to allow fuel from the main fuel gallery to refuel the transfer tank

as transfer tank becomes full HLFS de-energises refuel/de-fuel valves, the refuel control panel will illuminate to indicate tans are full

Question 15

describe HIFR

Answer 15

located on the cabin bulkhead aft if the cabin door, provides the controls necessary to pressure refuel the aircraft in flight

the HIFR hose is a lightweight hose and cant operate the microswitchon the AC refuel point,

but the tanks full/refuel on indicator push switch bi-passes the normal refuel switch and when pressed, the ON light is displayed and the refuel system is allowed to operate

pressing the full/refuel switch to off will stop the refuel process

Question 16

Describe defuel merlin

Answer 16

connect the pressure defuel hose to the aircraft refuel point, causes a microswitch to operate and a DC supply is fed to the REFUEL/OFF/DEFUEL

when the REFUEL/OFF/DEFUEL switch is selected to DEFUEL, the refuel/defuel valve in the transfer tank is energised open and allows fuel to be drawn from the transfer tank

when the transfer tank is empty, the LLFS operates causing the transfer tank refuel/defuel valaves to be energised open and allow the fuel to be dran out via the main fuel gallery,

as the No1, 2 and 3 fuel tanks empty, theirLLFS make and refuel/defuel valves are de-energised closed to prevent the ingress of air which would stop the defuel process

when each fuel tank has been defueled to its lowest point using the defuel bowser, the residual fuel is then drained using the manual fuel drained valves located at the base of the bowser

Question 17

describe fuel jettison

Answer 17

select master dump switch to open

a supply is fed to the master jettison switch, limit switch of the jettison valve, through coils of the motor and motor armature, to the limit switch of the jettision valve and master switch to earth

when the jettison valve is open the limit switch changes over, removing the supply to the motor and making the circuit to illuminate the yellow OPEN caption on the indicator and prepare the circuit for shut selection

when the jettison pump push switches are pressed to ON, the jettison pump relays are energised and contacts change over to feed 115v 3Ø to jettison pumps,

initially the pressure switch will cause a yellow LOW caption to be illuminated on the push switch indicator

when the jettison pumps are running the pressure switch contact will change over and a green ON caption will be illuminated on the push switch indicator

when the MASTTER DUMP switch is selected to shut, a supply is fed through the master jettison switch and the limit switch of the jettison valve, through the shut field coil of the motor and the motor armature, through the limit switch of the jettison valve and the master jettison switch to earth

when the jettison valve is shut, the limit switch changes over remeoving the supply of the motor and extinguishing the captions on the indicator and preparying the circuit for an open selection

Question 18

describe fuel gauging system

Answer 18

allows the pilot to know exactly how much fuel is left on board

Question 19

why is fuel measure in kg

Answer 19

so all of the weight of the aircraft can be calculated prior to take off

this ensures that take off limits are not exceeded

Question 20

tank probe construction

Answer 20

two primary probes and a secondary probe installed in each fuel tank,

made of two concentric tubes, one inside the other,

an oscillator unit is attached to the outer tube

installed at angles so they dont penatrate the fuel tanks during a crash,

Question 21

tank probe operation

Answer 21

vairiable capacitors with the fuel acting as a dielectric (higher the fuel level the greater the capacitence)

the changing level of capacitance relative to the level of fuel causes a change in the frequency output of the local oscillator this is fed to the fuel indication electronic unit (FIEU)

FIEU processes the fuel quantity signals and feeds the information to the aircraft maintenance management computers (AMMC) and cockpit display system (CDS)

Question 22

explain compensator

Answer 22

a secondary probe with a compensator unit on the bottom is fitted in the tanks 1,2 and 3

the compensator is an extra capacitor with two tubes and its own oscillator.

its purpose is to provide a reference and compensate for changes of specific gravity (density) of the fuel

the probe outputs are fed to the fuel indication electronic unit (FIEU) the fuel contents signals are processed for display on the cockpit display system (CDS)

Question 23

how is fuel level displayed

Answer 23

fuel contents afe in KG in a histogram format with corrsponding digital readout

Question 24

fuel signal failure wildcat

Answer 24

in the event of a signal failure of the SMD of the fuel tanks

a red F will Flash within the relevent display 5 times at 1 hertz and then remain steady

Question 25

whats the LOW pressure drop caption and level

Answer 25

displays a GREEN ON caption when they are selected on and a YELLOW LOW caption when below 0.14 bar

buttons are black when pumps are off

Question 26

wildcat quantity guaging system

Answer 26

fuel contents unit (FCU) converts the signal produced ny the probes into a DC voltage proportional to the fuel quantity

located in the nose bay and interfaces with the fuel probes to convert the fuel probe signals to a dc voltage

Question 27

wildcat fuel operation

Answer 27

the wildcat fuel system is configured to keep the CofG within limits during refuel/defuel and flight

fuel is used from the main tanks first and as the fuel drains to 2/3 level the Cof G moves towards the fwd limit

fwd tank drains down to the mid position CofG moving rwd

main tanks now drain to the 1/3 level with CofG moving fwd

the remaining fuel in the fwd tank then empties with the CofG returning rwd

the main fuel tank now drains until empty with Cof G moving to the center

the collector tanks are then the last to be used

fuel LOW indication are shown at 100 kg for both or 50kg for one, these are yellow box around indications and histograms

low level float switch is activated at 47kg of fuel and FUEL LOW 1 or 2 will display on IDU

Question 28

fuel safety precaution

Answer 28

switch of all electrical supplies

ensure all tansk are drained and ensure area is well ventilated

only safety torches are to be used inside tanks

fit blanks to open pipes/conduits

deal with spillages immediately using ready available spill kits

no HP air blasts in the pressence of fuel vapor

fire extinguisher are to be available

appropriate clothing is to be worn

barrier cream is to be used

the TWO-MAN rule is observed

vehicles must not approach with 15 metres of an aircraft undergoing repairs to the fuel system

Question 29

functions of the EECU

Answer 29

controls the engine start sequence and acceleration of the engine up to ground idle

controls the rate of acceleration of the engine and rotor speed when the engine condition swithc is seleceted from GI to flight

controls the rotor speed - default datum speed is 102%

keeps the engine torque matched by sharing torque information with the ECU’s

it limits the output of the engine so that limits of the transmission are not exceeded

it causes the engine to shut down if there is an overspeed of Ng or Np

it feeds engine speed, temperature and torque information to the EIS

Question 30

master? hot standby?

Answer 30

the EECU share their engine power information so that he output power of all engines is equal

eac EECU has two identical control channel A and B, both channel are equally capable of engine control

the operating channel is selected by the pilot, the selected channel becomes the MASTER channel and the other channel become the HOT STANDBY channel

if a failure occures in the master channel control is automatically transfered to the hot standby channel and an illumination on the selector pannel will show the transfer has happened

Question 31

inputs and outputs of the EECU

Answer 31

controls the acceleration start sequence and acceleration of the engines up to ground idle

controls the rate of acceleration of the engine and rotor speed when the engine and rotor speed when the engine conditon switch is selected from GI to FLIGHT

controls the rotor speed - default datum speed is 102%

keeps the engines torque matched by sharing torque in formation with the other ECU’s

it limits the output power of the engine so that limits of the transmission are not exceeded

it causes the engine to shut down if there is an overspeed of Ng or Np

it feed engine speed, temperature and torque information to the EIS

Question 32

pilots inputs to the EECU

Answer 32

engine condition switch VENT/OFF/GI/FLIGHT

control channel selection A/B

collective pitch (collective pitch resolver)

overspeed test switch (simulates engine overspeed)

contingency switch to overcome power limits

accessory drive selection (No1 engine)

neutral selection (No3 engine)

Nf trim

Question 33

engine fadec sensor inputs

Answer 33

Gas turbine speed (Ng)

power turbine speed (Np)

turbine inlet temeperature (TIT)

engine Torque

position of the Throttle

Question 34

how are EECU failures be indicated to pilot

Answer 34

a failure of an EECU will be indicated on the WCA, EECU FAIL attention getters flash with voice warning

other failures will be indicated as a caution captions ENG 1, 2 and 3 ECU SBY, ENG1, 2 and 3 ECU CTR

Question 35

wildcat NP needed for lift off

Answer 35

from ground idle to 104%(Np) for takeoff

Question 36

advantages of FADEC

Answer 36

better aircraft handling , less vibration, less component wear/damage and less pilot fatigue

Question 37

turbojet operation

Answer 37

compressor draws air into the engine and compresses it.

fuel is fed to the burners in the combustion chamber and lit by igniter plugs during the start cycle

some of the air from the compressor is used to support combustion

the rest of the air is heated and it expands to produce thrust which forces its way rearwards to form the thrust stream to drive a fixed wing A/C

Question 38

turbo prop operation

Answer 38

turboprop has a free turbine which is used to drive an aircraft propeller

turboprop engines deliver more power and great efficiency at lower flying speeds than a traditional jet propulsion engine

they offer greater economic advantage on smaller aircraft and over shorter distances

Question 39

turbo shaft operation

Answer 39

a turbo shaft engine has free turbine which is used to produce shaft power to drive machinery instead of producing thrust,

turboshaft engines are most used in applications that require a small, powerfull, light weight engine

Question 40

4 requirements, starting system

Answer 40

A motor - to impart force to overcome inertia and friction of the rotating assembly of the engine amd its ancillary equipment

fuel system - to introduce the fuel/air mixture into the engine appropriatly metered for combustion

ignition system - able to provide a means of igniting the initial charge of fuel/air mixture

a control system - to programme the start sequence and to prevent design parameters from being exceeded

Question 41

2 types of starter motor

Answer 41

electrical - wildcat

air turbine (low pressure) - merlin

Question 42

ch c pg6 fig 7 diagram

basic starter circuit merlin

Answer 42

a basic E- starting circuit for the engine consists of a starting motor and an igniter unit operated by start contactors and an igniter relay,

with the start master switch made an electrical supply will be available at the start button

start button pressed, start contactor energises, allowing DC power to the starter motor which turns the engine compressor, igniter relay energises supplying DC power to the HEIU which produces power required for the igniter plug

fuel is supplied to the engine and should start accelerate to ground idle speed

once the start cycle has started the pilot can release the button and start cycle will be held on by the engine speed sesning unit, this continues to feed power to the start motor and igniter unitl the engine starts or start cycle times out

a successful engine start will be detected by the engine speed sensing unit which removes the supply to the hold circuit and terminates the start cycles

Question 43

HEIU operation

Answer 43

when the HEIU is switched on, a dc current flows through the primary coil.

the primary coil acts as an electromagnet opening the contact.

the open circuited primary looses its magnetic effect and the contacts close under the influence of the spring.

this action is repeated at high speed and the pulses of current in the primary coil induce high voltage pulses into the secondary coil

the pulses in the secondary are half wave rectified by the diode bank and cause the reservoir capacitor to charge,

when the charge reached the break down value of the discharge gap, the capacitor will discharge to earth through the choke and igniter plug.

this action repeats itself. producing high energy sparks at the igniter plug at a rate of 100 - 120 per minute

Question 44

all components of the HEIU explained

Answer 44

parallel/series rectifier bank rectifies the charging current

the discarge resistors dissipate any charge remaining on the reservoir capacitor when the unit is switched off

the safety resistors provide a safe path for the energy if HT lead is disconnected

the choke extends the spark duration

the discharge gap ensures the reservoir capacitor has charged to the correct level before energy arc the gap

primary capacitor stops sparking at the contacts and ensures high voltage pulses are induced into the secondary

Question 45

HEIU safety prec

Answer 45

never touch an igniter unit output lead or the igniter plug whilst the HEIU is operating

wait at least one minute after switching off before touching any part of the system (to allow charge to dissipate)

always disconnect the LT lead at the HEIU (DC supply) before working on the system

always disconnect the LT lead before the HT lead when disconnecting the HEIU

always reconnect the HT lead before the LT lead when reconnecting the HEIU

Question 46

ESU operation

Answer 46

batter master switch ON

engine master start switch ON

fuel switch selected to ON

press the start/stop button

ON caption will illuminate and start signal supplied to the ESU

ESU checks that all conditions are correct and energises the APU start contactor

power is supplied to the starter motor and the APU START caption is illuminated

starter motor accelerates the APU rotor to a speed of 5%, at this point the start fuel valve and igniter operates

at 14% APU rotor speed the main fuel valve opens and the rotor is further accelerated by the starter, with assistance from the engine combustion

at 70% rotor speed to the start fuel valve and ignition system are shut down and the APU START caption is extinguished

at 90% +5 seconds APU rotor speed, the main fuel valve is opened fully

at 90% +10 seconds APU rotor speed the ESU initiates a ‘ready for service’ signal indicating that the APU is operating at the correct conditons to supply bleed air and operate the APU AC generator

the ‘APU READY’ caption on the control panel is illuminated and the APU ‘ready’ advisory caption is displayed on the WCA window

Question 47

merlin No 1 starter cycle

Answer 47

the ON solenoid of the FUEL SHUT OFF VALVE to energise

the ignition exciter to energise

the press/start valve in the engine fuel system to energise

the solenoid of the starting pressure shut of valve (SPRSOV) is energised OPEN allowing air pressure through to the engine

the start relay is energised causing the shut off valve in the environmental control system to close. (ensures all air is available for starting)

operation of the SPRSOV causes a green ‘START’ caption on the PPSD on IDU

Question 48

when motor ECU without fuel

Answer 48

replacement of a component in the starter systems

a compressor wash needs to be carried out

the fuel system or an oil system needs to be primed or bled

the engine needs to be vented to clear engine casing of compressor wash fluid post compressor wash, and unused fuel post flight

Question 49

when FADEC shuts down engines

Answer 49

when the engine start sequence has been initiated, the ECU will control the engine fuel flow and acceleration bleed-air vlave

the ECU will auto shut engine if following conditions apply

EMSS is selected OFF

the engine does not light up within 30 seconds

the Ng is less than 50% within 60 seconds

the PTIT exceeds 850ºC

Question 50

how long should pilot vent engines

Answer 50

15 seconds is the recommended cranking period for venting the engines

Question 51

cautions when starting wildcat engines

Answer 51

wait 3 minutes between attempts to start and/or vent runs to allow the starter to cool (2 minutes if first start of the day)

after three starter/vent operations, a period of 30 minutes must elapse before another attempt to start is made

no more than three successive abortive attempts to start should be made using the same internal battey within a 24 hour period, to prevent overheating

vent runs should be terminated after 15 seconds

Question 52

SMD how does it display info

Answer 52

the systems managment display (SMD) shows fuel contents in the top left window

the ECU in the control of the engine is indicated in green and the serviceable standby in white

engine indications Ng, PTIT, Np, Nr and Tq are displayed in the centre window,

the WCA window on the right

the bottom windows display the temperatures and pressures of the engines and gearbox.

hydraulic pressures of the three systems on the bottom right window

Question 53

EXP NG sensor

Answer 53

Gas generator speed (Ng) the speed of the gas generator is sensed by an NG sensor which is mounted on the accessory gearbox.

the sensor is a magnetic variable reluctance pickup speed sensor, which senses speed of the 76 tooth gear which drives the oil pump.

the sensor has two output windings that feed separate signals to the two ECUs for speed governing and cockpit indications

Question 54

EXP power turbo speed sensor

Answer 54

the speed of the turbine is sensed by an Np sensor which is mounted on the engine rear support.

the sensor is a duel coil magnetic variable reluctance pickup speed sensor,

it senses the speed of the power tubine shaft from the passage of the lugs on the shaft as it rotates

the sensor has two output windings that feed separate signals to the two ECUs for speed governing and cockpit indications

Question 55

EXP PTIT

Answer 55

the PTIT harness is made up of five dual thermocouple sensors which are electrically connected in parallel and arranged concentrically at the rear of the combustion chamber.

the thermocouples provide two outputs feeding separate signals to the ECUs for engine control and cockpit indications

Question 56

Thermocouple principle

Answer 56

when two dissimilar metals are joined at their ends to form a closed loop and heat is applied to one end, an EMF is produced.

the magniture of the EMF will depend upon the difference in temperature between the two ends of the loop and types of dissimilar metals used

the end of the loop to which heat is applied is called the HOT JUNCTION and the other end COLD JUNCTION

AC thermocouples use Alumel and chromel

alumel is a mixture of aluminium and nickel and chromel is a mixture of chromium and nickel

Question 57

ECU torque

Answer 57

engine torque is a measure of the output power being developed by the engine.

the engine torque sensor is a dual coil variable reluctance magnetic pickup probe, which measure the engine torque on the power turbine output shaft, from which torque signals are fed to each ECU for engine control and cockpit display

the engine torque sensor is excited by two pairs of blades,

one pair of blades is fitted to the power turbibe shaft and the other is fitted to the reference shaft

the power turbine shaft twists linearly as it delivers power to the transmission.

the sensor probe detects the difference in angle between the blades to produce the torque signals (only the power turbine shaft twists relative to the output power, the reference shaft does not twist

Question 58

Oil PY/temp/low px

Answer 58

engine oil pressure and gearbox oil pressure are sensed by oil pressure transmitters on the engines and the gearbox which are displayed in the cockpit

oil temeprature of th engines and gearbox are sensed by oil temperature probes on the engines and gearboxes and displayed in the cockpit

oil pressure switches sense low pressure in the engines gearboxes and feed the caption on the WCA window

Question 59

wildcat relight parameters

Answer 59

EMSS is selected to FLY or GI

the ECU BIT detects no failure that would result in an unsafe start

the starter master switch is set to ON

one channel remains functional

Ng is greater than 50%

Question 60

indications on wildcat start

Answer 60

a green START and ignition (IGN) legend is displayed

a small red circle is also displayed at 850ºC on the PTIT scale of the starting engine to display the maximum temperature