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The FADEC system is a Built In Test Equipment (BITE) system. This means it is able to detect its own internal faults and also external faults.
The system is fully redundant and built around the two-channel ECU. All control inputs are dual, and the valves and actuators are fitted with dual sensors to provide the ECU with feedback signals. Some indicating parameters are shared, and all monitoring parameters are single.
CCDL:
To enhance system reliability, all inputs to one channel are made available to the other, through a Cross Channel Data Link (CCDL). This allows both channels to remain operational even if important inputs to one of them fail.
Active / Stand-by:
The two channels, A and B, are identical and permanently operational, but they operate independently from each other. Both channels always receive inputs and process them, but only the channel in control, called the Active channel, delivers output commands. The other is called the Stand-by channel.
Channel selection and fault strategy:
Active and Stand-by channel selection is performed at ECU power-up and during operation.
The BITE system detects and isolates failures, or combinations of failures, in order to determine the health status of the channels and to transmit maintenance data to the aircraft.
Active and Stand-by selection is based upon the health of the channels and each channel determines its own health status. The healthiest is selected as the Active channel.
When both channels have an equal health status, Active / Stand-by channel selection alternates with every engine start, as soon as N2 is greater than 11000 RPM.
Failsafe control:
If a channel is faulty and the Active channel is unable to ensure an engine control function, this function is moved to a position which protects the engine, and is known as the failsafe position.
EFG
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CTC-201-004-02
EFFECTIVITY
INTRO Page 9
ALL CFM56-5B ENGINES FOR A319-A320-A321
ENGINE SYSTEMS Jan 03
CFMI PROPRIETARY INFORMATION
EFG FADEC SYSTEM INTRODUCTION CFM56-5B TRAINING MANUAL
Closed loop control operation.
In order to properly control the various engine systems, the ECU uses an operation known as closed loop control.
The ECU calculates a position for a system component : - The Command.
The ECU then compares the Command with the actual position of the component (feedback) and calculates a position difference : - The Demand.
The ECU, through the HMU, sends a signal to a component (valve, actuator) which causes it to move.
With the movement of the system valve or actuator, the ECU is provided with a feedback of the component’s position.
The process is repeated until there is no longer a position difference.
The result completes the loop and enables the ECU to precisely control a system component on the engine.
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