FADEC Flashcards
What are the primary components of the FADEC system?
The Digital Electronic Control Unit (DECU) and Hydromechanical Metering Assembly (HMA).
What’s the difference between a soft fault and a hard fault?
The hard fault throws the caution light
What do the INC/DEC switches do?
They fine tune the engine N2 speed. Used to set the rotor RPM, match torque indications or both. Only work in reversionary mode.
What’s back up power do?
B/U PWR (Back-up power) Switch controls the airframe power (DC bus 1 or 2) to the FADEC system primary channel.
A power turbine overspeed is detected at what?
114.8% N2
The maximum trip point is reduced by 1% per every 10%/sec of N2 acceleration.
What is the temperature limiting in primary mode?
Temperature limiting (After light off PTIT > 760°C).
a. Reduces fuel flow to 100 PPH minimum.
b. Over temperature during start (Abort start).
c. Fuel shutoff if PTIT > 816°C.
d. Associated ENG 1/2 FAIL indication.
e. ECL to STOP resets the start mode.
What is the only function of the reversionary mode that impacts the primary mode ?
The shutoff feature. Reversionary mode will affect complete shutoff of fuel when ECL is moved to STOP.
What are the 3 electrical connections to the HMU?
- Alternator connector - supplies electrical power to the primary channel and gas producer N1 B speed signal.
- Primary stepper motor connector - primary mode control functions and gas producer N1 A speed signal.
- Reversionary stepper motor connector - reversionary mode control functions.
What are the 3 pneumatic ports of the HMU?
- Compressor Discharge Pressure (P3) sensing. (P3 servomechanism and DECU).
- Modulated air pressure control port (bleed band control).
- Ambient relief port (bleed band control).
What are the 3 fuel ports of the HMU?
- Fuel inlet.
- Fuel Out Metered.
- Fuel Out Start.
What are the similarities and differences of the HMU primary and reversionary modes?
Fuel Pump, Metering Valve, and Pressurizing/Shutoff Valve all work in PRI and REV
When there is a signal disruption to the Decu or an issue with the Primary Stepper Motor, a switchover will take place instantaneously and the Reversionary Stepper Motor will take over.
In the primary mode, the wiper is positioned by the primary stepper motor, which is controlled by command from the DECU. In reversionary, the “mechanical multiplier” is what positions the wiper and its based off of Wf/p3 servomechanism.
In Reversionary mode, the wiper (wiper covers and changes size of metering valve office which effects downstream pressure heading to engine) is controlled mechanically by linkage, with inputs from the Wf (fuel pressure signal) and P3 servomechanism (muscle that opens bleed band).
Primary uses Solenoid Valve for Bleed Band Operation and Reversionary uses a piston actuated (servo) air bleed valve to open/close bleed band.
The p3 line to the HMU splits, one line is used for a signal to the bleed band actuator and the other is used as the muscle to open the bleed band (via a DECU actuated solenoid). In reversionary, the signal is no longer P3 but instead it uses fuel pressure (wf) . It still uses the second P3 line as muscle; however, instead of using a solenoid to open it uses a servo (P3 servo) and DECU isn’t needed.
What is the PLA feedback potentiometer?
The reversionary stepper motor SHAFT directly drives the “Power Lever Angle” feedback potentiometer.
Provides the DECU with the reversionary stepper motor position.
What is the Primary/Reversionary Change-Over Solenoid?
With a hard fault in the DECU primary channel. Or if the reversionary mode is selected by the pilot, the solenoid is de-energized and the reversionary mode is enabled.
Fuel pressure is ported to the Wf/P3 servo and to the hydromechanical speed sensor for reversionary mode operation.
Where are the 2 N1 speed sensors in the HMA for PRI/REV modes?
Within the HMU a Hydromechanical magnetic speed sensor provides the N1 speed signal for the primary channel in the DECU. (N1a)
The built-in three phase alternator provides a redundant N1 speed signal to the DECU primary and reversionary channel. (N1b)
The N1 sensor on back of oil pump is just used for gauge and not for DECU.
What is the purpose of the engine fuel pump?
The FPU increases fuel pressure. Providing fuel pressure for engine operation and the operation of components with-in the HMU.
Purpose: Receive fuel from the low-pressure components of the engine fuel system. Provide fuel under high pressure for engine operation and provide fuel under pressure to be used as the hydraulic medium for the operation and cooling of the components with-in the HMU.
The HMU Controls fuel pressure/flow for engine operation.
