Chapter 2 - Full Authority Digital Engine Control (FADEC)

Question 1

What does a FADEC monitor and control in order to achieve the required thrust?

Answer 1

• Engine pressure ratio
• N₁ low pressure compressor or fan speed

Question 2

How are FADECs interconnected?

Answer 2

By a Cross-Channel Data Link (CCDL)

Question 3

What properties does the ADC feed to the FADEC?

Answer 3

• Altitude
• Mach number
• Airspeed
• TAT

Question 4

How can the FADEC provide maximum efficiency in the compressor?

Answer 4

By changing the air flow geometry by adjusting variable inlet guide vanes and variable geometry vanes within the first few stages of the compressor

Question 5

What provides the feedback in a FADEC closed loop system?

Answer 5

Engine speed or EPR

Question 6

How else can a FADEC control compressor and/or turbine efficiency?

Answer 6

Automatic adjustment of blade clearance

Question 7

What is usually across the contacts of discrete signals?

Answer 7

10 kΩ resistor

Question 8

When will the Permanent Magnet Alternator (PMA) take over supply to the FADEC from the essential bus?

Answer 8

At 50% N₂ and above

Question 9

Describe the PMA?

Answer 9

• Driven by the accessory drive gearbox
• Four separate windings
• Two 3 phase and two single phase
• 3 phase power to channel A and channel B
• Single phase windings feed the redundant exciters in the ignition system
• One phase of the 3 phase supply is used to derive a secondary N₂ signal

Question 10

What is the purpose of the current limited excitation voltage fed to the discrete signals?

Answer 10

Allow the FADEC to detect open and short circuit failure conditions in the aircraft engine wiring

Question 11

What is engine transient rate limiting?

Answer 11

Acceleration and deceleration limits which restrict the rate of change of commanded fuel flow

Prevents surge on acceleration or blowout on deceleration

Question 12

What is controlled in CVG scheduling?

Answer 12

The IGVs and the first 5 stages of CVG

Question 13

What is the maximum allowable temperature of the ITT?

Answer 13

1690°F or 921°C

Question 14

At what values will engine protection shut down occur?

Answer 14

N₁ exceeds 105% (approx 9135 RPM) or N₂ exceeds 105.5% (approx 16750 RPM) for overspeed

N₂ is less than 54% (approx 8500 RPM)

Question 15

What are the cockpit discrete inputs?

Answer 15

• Thrust ratings
• Thrust mode selector panel
• Engine start/stop
• Ignition on/off
• Anti-ice on/off
• FADEC reset
• Alternate FADEC select

Question 16

What are the aircraft discrete inputs?

Answer 16

• Air/ground (WOW)
• Landing gear down
• Thrust reverser deployed
• Thrust reverser stowed

Question 17

What are the engine sensor inputs?

Answer 17

• N₁ primary fan speed
• N₁ secondary fan speed
• N₂ HP rotor speed
• P_2.5 compressor inlet pressure
• T_2.5 compressor inlet temperature
• ITT
• P₀ ambient static air pressure
• Compressor variable geometry position feedback

Question 18

At the stated speeds, what is required for the engine protection and shutdown?

Answer 18

Both FADECs to agree

Question 19

For ground starts, which exciter is used?

Answer 19

Selected alternatively for consecutive ground starts

Question 20

What is the primary supply for the ignition system?

Answer 20

Single phase winding in the PMA which provides 20-40 VAC at all N₂ speeds above 10%

Question 21

How often do the ignition capacitors discharge?

Answer 21

Four to seven times per second

Question 22

Why would the ignition switch be selected to ON?

Answer 22

As a safety precaution when flame out is possible

Question 23

What position is the exciter relay in to provide a spark?

Answer 23

De-energised

Question 24

What are the four thrust rating modes?

Answer 24

Maximum take-off - T/O-1 - 100% N₁
Maximum continuous - CON - 86% N₁
Maximum climb - CLB - 76% N₁
Maximum cruise - CRZ - 70% N₁

Question 25

Which thrust rating modes are inhibited on the ground?

Answer 25

CON, CLB and CRZ

Question 26

What height of pressure altitude is applicable to CON, CLB and CRZ?

Answer 26

CON - 300 ft, landing gear not locked down CLB & CRZ - 500 ft, landing gear not locked down, both engines operating 1700 ft for all above the take-off altitude, both engines operating for CLB & CRZ

Question 27

Why does the FADEC use the REF TO TEMP setting?

Answer 27

In place of the TAT supplied by the ADC to compute the corrected fan speed during take-off

Question 28

What is the purpose of TAKE-OFF DATA STORE?

Answer 28

Used to calculate N₁ target during take-off

Question 29

Under what conditions will the RESET/ALTN switch not change to the alternate FADEC?

Answer 29

If the selected FADEC is not capable of controlling the engine

Question 30

What happens if there is indication of an inadvertent thrust reverser deployment?

Answer 30

The engine thrust is reduced to IDLE

Question 31

What provides the position feedback signal to the FADEC from the main metering valve?

Answer 31

Linear variable inductance transducer (LVIT)

Question 32

How is the main metering valve (servo valve) biased?

Answer 32

Closed

Question 33

What does the pressure raising valve (PRV) ensure?

Answer 33

By spring loading, a minimum gear pump pressure at low engine speeds

Question 34

When is the Latching Shut Off Valve (LSOV) START solenoid energised, thus allowing the opening of the PRV?

Answer 34

At 23% N₂

Question 35

What determines the angle of the inlet guide vanes and the first five variable geometry compressor vanes?

Answer 35

Fuel pressure fed to the actuator from the CVG control valve, operated by a torque motor under FADEC control

Question 36

Within the FADEC computer, what will signal a lane change is necessary if a fault is detected?

Answer 36

Passing demand and engine feedback through a comparator unit

Question 37

What does 'engine steady state' limiting mean?

Answer 37

Fuel flow is limited to prevent overspeed of the LP and HP rotors There is also an HP underspeed logic to prevent fuel flow being reduced to a value which would result in an N₂ underspeed

Question 38

When would both exciters in the ignition systems operate?

Answer 38

For in-flight restarts

Question 39

Why is a tri-lane potentiometer used in the throttle lever circuit?

Answer 39

In case of failure If one potentiometer fails, the other two will still send valid information to the 'majority wins' circuit in the computer

Question 40

In the Lane Controller system, what receives the input signal from the throttle lever?

Answer 40

Full Authority Fuel Controller (FAFC)