Autothrottle — Page 401, Lesson 497

I want to walk you through the autothrottle system as it's configured on this Airbus-style aircraft, because this is where the engine control story really comes together. We're going to look at how the system is powered, how thrust is commanded, and then dive into the fuel control architecture that actually delivers that thrust. Let's start with power, because nothing works without it. Each channel of the autothrottle system is powered by the aircraft's A/C supply — that's the alternating current electrical bus — before and during the initial engine start. But here's the clever part: once the engine is spooling up, above 12% engine rpm, each channel switches over to its own individual internal magnetic alternator. So you have a self-sustaining power source once the engine is turning fast enough. That 12% rpm is your threshold — below it, you're on aircraft A/C; above it, you're on the internal alternator. Now, thrust control. Each engine has its own dedicated FADEC — that's the Full Authority Digital Engine Control. The FADEC is the brain that manages the engine. Thrust selection happens in one of two ways. In manual mode, you move the thrust levers yourself, and that lever position tells the FADEC what thrust you want. In automatic mode, the Flight Management and Guidance System — the FMGS — commands the thrust instead. Either way, the thrust rating limit is provided by the FADEC, and it computes that limit based on the thrust lever position. So even in automatic mode, the FADEC is still looking at where the levers are to know what rating limit to apply. That's an important point: the lever position is always the reference for the rating limit, whether you're flying manually or on autopilot. Let's move to fuel control, because this is where the actual engine performance is managed. On top of the high-pressure fuel pumps and the shut-off system, there are two main systems controlling engine performance. The first is the Hydromechanical Unit, or HMU. The HMU is modulated by the FADEC — the FADEC sends it commands. The HMU does three jobs: it controls the flow of fuel to the combustion chamber, it controls fuel hydraulic signals that go to actuators, and it provides over-speed protection. So it's both a fuel metering device and a safety device. The second system is the Fuel Metering Valve, the FMV. The FMV takes the FADEC's orders and transforms them — through a torque motor and servo valve — into actual fuel flow to the engine nozzle. So the FADEC thinks in electrical signals, and the FMV converts those into hydraulic fuel flow. Now, there's a feedback loop here: the FMV has a resolver, and that resolver provides an electrical feedback signal proportional to the FMV position. That's how the FADEC knows exactly where the valve is. And there's a bypass valve that regulates a constant pressure drop across the FMV. Why does that matter? Because it ensures that the metered fuel flow is proportional to the FMV position. If the pressure drop stayed constant, then the flow is a clean function of valve position — that's the whole point of that bypass valve. Now, what is the FADEC actually trying to achieve? It computes the fuel flow necessary to hold a target N1. N1 is the fan speed — the low-pressure spool speed. So the FADEC's primary goal is to hold N1 at the commanded value. To obtain that N1, the N2 — that's the core or high-pressure spool speed — is allowed to vary, but it must not exceed N2 min or N2 max. So N2 is free to float within those limits to help achieve the N1 target. The FADEC also varies the N2/N1 relationship — that's the ratio between the two spool speeds — for several reasons: to maintain RPM under varying load conditions, to maintain bleed air production, and to avoid engine stalls or flameout. So it's constantly juggling that ratio to keep the engine healthy. And here's a fascinating detail specific to the Airbus FADEC: it even modulates the cooling airflow around the engine. By doing that, it can expand or contract the engine casing, which controls the compressor and turbine clearances at nominal settings. So the FADEC is managing thermal expansion to keep blade tip clearances tight and efficient. That's a level of control you don't see on every system. Finally, the thrust levers themselves. And this is a key contrast: the thrust levers are only moved manually. Unlike many other aircraft — and the excerpt cuts off right there, but the point is clear — on this aircraft, the levers are not physically driven by the autothrottle. The autothrottle commands thrust through the FMGS and FADEC, but the levers stay where the pilot put them. That's a fundamental design difference you need to remember. Let me show you the layout of this system so you can see how the FADEC, HMU, and FMV all connect. So to tie it together: the FADEC is the central computer, the HMU and FMV are its two main fuel control actuators, the resolver gives it feedback, the bypass valve keeps the metering linear, and the whole thing is powered by A/C until 12% rpm, then by the internal alternator. And the thrust levers are your manual input — they set the rating limit, but they don't move on their own.