Chapter 2 - Full Authority Digital Engine Control (FADEC) Flashcards
What does a FADEC monitor and control in order to achieve the required thrust?
- Engine pressure ratio
- N1 low pressure compressor or fan speed
How are FADECs interconnected?
By a Cross-Channel Data Link (CCDL)
What properties does the ADC feed to the FADEC?
- Altitude
- Mach number
- Airspeed
- TAT
How can the FADEC provide maximum efficiency in the compressor?
By changing the air flow geometry by adjusting variable inlet guide vanes and variable geometry vanes within the first few stages of the compressor
What provides the feedback in a FADEC closed loop system?
Engine speed or EPR
How else can a FADEC control compressor and/or turbine efficiency?
Automatic adjustment of blade clearance
What is usually across the contacts of discrete signals?
10 kΩ resistor
When will the Permanent Magnet Alternator (PMA) take over supply to the FADEC from the essential bus?
At 50% N2 and above
Describe the PMA?
- 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
What is the purpose of the current limited excitation voltage fed to the discrete signals?
Allow the FADEC to detect open and short circuit failure conditions in the aircraft engine wiring
What is engine transient rate limiting?
Acceleration and deceleration limits which restrict the rate of change of commanded fuel flow
Prevents surge on acceleration or blowout on deceleration
What is controlled in CVG scheduling?
The IGVs and the first 5 stages of CVG
What is the maximum allowable temperature of the ITT?
1690°F or 921°C
At what values will engine protection shut down occur?
N1 exceeds 105% (approx 9135 RPM) or N2 exceeds 105.5% (approx 16750 RPM) for overspeed
N2 is less than 54% (approx 8500 RPM)
What are the cockpit discrete inputs?
- Thrust ratings
- Thrust mode selector panel
- Engine start/stop
- Ignition on/off
- Anti-ice on/off
- FADEC reset
- Alternate FADEC select
What are the aircraft discrete inputs?
- Air/ground (WOW)
- Landing gear down
- Thrust reverser deployed
- Thrust reverser stowed
What are the engine sensor inputs?
- N1 primary fan speed
- N1 secondary fan speed
- N2 HP rotor speed
- P2.5 compressor inlet pressure
- T2.5 compressor inlet temperature
- ITT
- P0 ambient static air pressure
- Compressor variable geometry position feedback
At the stated speeds, what is required for the engine protection and shutdown?
Both FADECs to agree
For ground starts, which exciter is used?
Selected alternatively for consecutive ground starts
What is the primary supply for the ignition system?
Single phase winding in the PMA which provides 20-40 VAC at all N2 speeds above 10%
How often do the ignition capacitors discharge?
Four to seven times per second
Why would the ignition switch be selected to ON?
As a safety precaution when flame out is possible
What position is the exciter relay in to provide a spark?
De-energised
What are the four thrust rating modes?
Maximum take-off - T/O-1 - 100% N1
Maximum continuous - CON - 86% N1
Maximum climb - CLB - 76% N1
Maximum cruise - CRZ - 70% N1
Which thrust rating modes are inhibited on the ground?
CON, CLB and CRZ
What height of pressure altitude is applicable to CON, CLB and CRZ?
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
Why does the FADEC use the REF TO TEMP setting?
In place of the TAT supplied by the ADC to compute the corrected fan speed during take-off
What is the purpose of TAKE-OFF DATA STORE?
Used to calculate N1 target during take-off
Under what conditions will the RESET/ALTN switch not change to the alternate FADEC?
If the selected FADEC is not capable of controlling the engine
What happens if there is indication of an inadvertent thrust reverser deployment?
The engine thrust is reduced to IDLE
What provides the position feedback signal to the FADEC from the main metering valve?
Linear variable inductance transducer (LVIT)
How is the main metering valve (servo valve) biased?
Closed
What does the pressure raising valve (PRV) ensure?
By spring loading, a minimum gear pump pressure at low engine speeds
When is the Latching Shut Off Valve (LSOV) START solenoid energised, thus allowing the opening of the PRV?
At 23% N2
What determines the angle of the inlet guide vanes and the first five variable geometry compressor vanes?
Fuel pressure fed to the actuator from the CVG control valve, operated by a torque motor under FADEC control
Within the FADEC computer, what will signal a lane change is necessary if a fault is detected?
Passing demand and engine feedback through a comparator unit
What does ‘engine steady state’ limiting mean?
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
When would both exciters in the ignition systems operate?
For in-flight restarts
Why is a tri-lane potentiometer used in the throttle lever circuit?
In case of failure
If one potentiometer fails, the other two will still send valid information to the ‘majority wins’ circuit in the computer
In the Lane Controller system, what receives the input signal from the throttle lever?
Full Authority Fuel Controller (FAFC)
