Gas Turbines - Fuel Systems — Page 389, Lesson 485

Let’s start with the big picture. The FADEC — that’s the Full Authority Digital Engine Control — is the brain of the modern gas turbine. And the EEC, the Electronic Engine Control, is the computer inside that brain. So when I say “inputs to the FADEC EEC,” I mean the signals that this computer reads to decide how much fuel the engine needs. The two main inputs are the pilot’s thrust lever angle — we call it TLA — and a preset EPR or N1 coming from the Flight Management System, the FMS. Let me unpack that. The thrust lever angle is simply the position of the throttle in the cockpit. EPR is Engine Pressure Ratio, and N1 is the low-pressure compressor speed — either one can be the target the engine is trying to hold. The FMS is the flight management computer that plans the flight, and it can preset that target for you. Now here’s the key change with FADEC. In older engines you had a traditional hydro-mechanical fuel-pump system — a purely mechanical and hydraulic setup. With FADEC, that becomes redundant. Instead, all the parameters that would have affected pump delivery are now fed to a fuel metering valve — the FMV. That valve sits inside the Fuel Metering Unit, the FMU, as a bypass valve. Its job is to vary the delivery rate to the burners — the combustion chambers — according to the inputs the EEC receives. And any excess fuel that isn’t needed is fed back to the low-pressure side of the system. So the FMV is the final gatekeeper that decides exactly how much fuel goes to the fire. Let me list the full set of inputs to the FADEC EEC, because you’ll need these cold. We have the thrust lever angle, TLA. Altitude, from ground level upwards. Mach number — that’s your speed relative to the speed of sound. Ambient temperature — the outside air temperature. Air inlet temperature — the temperature of the air actually entering the engine. Air demand, which comes from compressor bleeds — that’s air being bled off the compressor for things like cabin pressurisation or anti-icing. And finally EGT — Exhaust Gas Temperature, the temperature of the gases leaving the turbine. Every one of these feeds into the EEC so it can compute the correct fuel flow. Now, what does FADEC actually do with all that? It performs three families of functions. Let’s go through them. First: engine control and overspeed protection. This includes fuel control regulation for all stages of flight — climb, cruise, descent, all of it. It includes power management control — managing the thrust you’ve asked for. And it includes control of that Fuel Metering Valve, the FMV, within the Fuel Metering Unit, the FMU. So the FMV is the actuator, and FADEC is the controller. Second: engine stall and surge protection. This is about keeping the airflow through the compressor stable. It controls the Variable Bleed Valve — VBV — which dumps air to prevent surge. It controls the Variable Stator Vanes — VSV — which are adjustable vanes that guide air into the compressor stages. Then, if fitted, it controls the Rotor Active Clearance Control and start bleed system — RACSB. And if fitted, the High Pressure Turbine Active Clearance Control — HPTACC — and the Low Pressure Turbine Active Clearance Control — LPTACC. Those clearance controls manage the gap between the turbine blade tips and the casing, because that gap changes with temperature and affects efficiency and surge margin. Third: engine and aircraft integration. This is where the engine talks to the rest of the aeroplane. FADEC handles automatic and manual starting and restarting of the engine. It handles thrust reverser operation — the buckets that reverse the exhaust for braking on landing. It handles auto-thrust — the automatic throttle control. It collects engine indication data and engine maintenance data — the readings you see on the flight deck and the data used for troubleshooting. And it collects condition monitoring data — the long-term trend information that tells maintenance how the engine is ageing. So the whole story is: the EEC reads a set of inputs — TLA, altitude, Mach, temperatures, bleed demand, EGT — and from those it commands the FMV inside the FMU to meter the exact fuel flow to the burners, while also managing surge protection and integrating with the aircraft systems. Excess fuel returns to the low-pressure side. That’s the FADEC architecture in a nutshell.