Gas Turbines - Fuel Systems — Page 385, Lesson 484

We’re in the middle of the FADEC discussion now, so let’s pick up with a failure scenario and then walk through the full system architecture. First, the failure case. If a channel in command sends a signal to a compressor bleed valve and that valve refuses to function properly, the channel in command will attempt to move it to the optimum safe position. And it will give a bleed valve failure warning on the ECAM or EICAS display. ECAM is the Electronic Centralised Aircraft Monitor, EICAS is the Engine Indication and Crew Alerting System — either way, that’s the cockpit display that alerts the crew. So the key point: the EEC doesn’t just give up when an actuator misbehaves. It actively drives the valve to a safe position and tells the crew about it. Now, what does a typical FADEC actually comprise? Let me go through the list, because this is the heart of the system. First, a pilot thrust lever input, giving thrust lever angle — TLA — to the EEC. That’s your command from the cockpit. Next, Cathode Ray Tube displays — CRT displays — giving indications of EPR, N1, N2, N3, EGT, FF, VIB, and oil pressure. Let me unpack those. EPR is engine pressure ratio. N1, N2, N3 are the rotational speeds of the compressor spools — low, intermediate, and high pressure. EGT is exhaust gas temperature. FF is fuel flow. VIB is vibration. And oil pressure is exactly what it sounds like. All of those parameters are shown to the crew on the CRT displays. Then we have a dual channel electronic controller — that’s the EEC itself. Dual channel means there are two independent control lanes for redundancy. Next, a dedicated engine driven alternator providing the principle power supply. So the FADEC doesn’t rely on the aircraft’s main electrical bus — it has its own alternator driven by the engine, so it keeps working even if other power fails. Then, actuators to operate VIGV, VSV, ACS, and bleed valve. VIGV is variable inlet guide vanes. VSV is variable stator vanes. ACS is the active clearance control system. And bleed valve is the compressor bleed valve we just talked about. So the EEC commands all of these through actuators. Next, position feedback to the EEC from engine sub-systems. So the EEC doesn’t just send commands — it gets feedback on where each actuator actually is. Then, the Fuel Metering Unit — FMU — which has integrated into it the HP fuel pump. HP is high pressure. So the fuel metering and the high pressure fuel pump are combined in one unit. Next, an input to thrust reverser control, if fitted. So the FADEC also interfaces with the reverser system when the engine has one. Then, a facility for engine health monitoring — EHM — and data collection. That’s the system that records engine condition over time for maintenance. And finally, sensors to feedback engine parameters. So the whole loop is: sensors feed parameters to the EEC, the EEC computes, commands actuators, gets position feedback, and displays results to the crew. Now, one important contrast I want you to hold onto. Earlier we saw the supervisory EEC — that’s the one where, in the event of EEC failure, there is provision for manual reversion. The pilot can take over manually. FADEC is different — it’s full authority. There’s no manual reversion; the EEC has complete control of the engine at all times. That’s the fundamental difference between the two architectures. So to tie it together: FADEC is a full-authority digital engine control. It takes your thrust lever angle, reads all the engine parameters, and through its dual-channel EEC, commands the variable geometry and fuel system, monitors itself with feedback, and keeps the crew informed on the displays. And it has its own power supply and health monitoring to keep it self-sufficient. That’s the complete FADEC structure.