Autothrottle — Page 401, Lesson 497

Let’s start with the autothrottle system on this Airbus, and I’ll take you through it piece by piece. First, the power supply. Each autothrottle channel is powered by the aircraft’s AC supply — that’s the alternating current electrical system — before and during the initial engine start. Then, once the engine is running, above 12% engine rpm, each channel switches to its own individual internal magnetic alternator. So you have two distinct power sources: the aircraft AC bus for the early phase, and a dedicated magnetic alternator driven by the engine once rpm is up. That 12% rpm is the threshold you need to remember. Now, thrust control. Each engine has its own dedicated FADEC — that’s the Full Authority Digital Engine Control. The FADEC is the brain that actually manages the engine. Thrust selection happens in one of two ways. In manual mode, you move the thrust levers, and that position tells the FADEC what thrust you want. In automatic mode, the Flight Management and Guidance System — the FMGS — does the selecting instead. Either way, the FADEC applies a thrust rating limit based on the thrust lever position. So the lever position defines the maximum rating the FADEC will allow, whether you’re in manual or automatic thrust. Let’s move to fuel control. Beyond the high-pressure fuel pumps and the shut-off system, there are two main systems that control engine performance. The first is the Hydromechanical Unit — the HMU. The FADEC modulates it. The HMU does three jobs: it controls fuel flow 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 against overspeed. The second system is the Fuel Metering Valve — the FMV. The FMV takes the FADEC’s commands and, through a torque motor and servo valve, converts them into actual fuel flow to the engine nozzle. So the FADEC sends an electrical order, the torque motor and servo valve translate that into a mechanical valve position, and that sets the fuel going to the nozzle. The FMV has a resolver — that’s a position sensor — which gives an electrical feedback signal proportional to the FMV’s position. So the FADEC always knows exactly where the valve is. There’s also a bypass valve. Its job is to regulate 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 flow is a direct function of valve opening — a clean, predictable relationship. That’s the whole point of the bypass valve. Now, how does the FADEC actually decide how much fuel to deliver? It computes the fuel flow necessary to hold a target N1. N1 is the low-pressure compressor speed — the fan speed, essentially. To achieve that target N1, the FADEC allows N2 — the high-pressure compressor speed — to vary, but it must stay within N2 min and N2 max. So N1 is the controlled target, and N2 is free to move within its limits. The FADEC also varies the N2/N1 ratio — the relationship between the two spool speeds — to do several things: maintain RPM under varying load conditions, maintain bleed air production, and avoid engine stalls or flameout. So it’s constantly adjusting that ratio to keep the engine stable and delivering what’s needed. And here’s a clever Airbus-specific detail. The FADEC even modulates the cooling airflow around the engine. By doing that, it can expand or contract the engine casing. Why? To control the compressor and turbine clearances at nominal settings. So it’s actively managing the physical clearances inside the engine by controlling thermal expansion — that’s a fine level of control. Finally, the thrust levers themselves. On this Airbus, the thrust levers are only moved manually. That’s a contrast to many other aircraft where the autothrottle physically moves the levers. Here, the levers stay where you put them — the FADEC does the thrust management electronically, not by moving the levers for you. So to tie it together: the FADEC is the central controller, powered by AC then a magnetic alternator above 12% rpm. It takes thrust selection from the levers in manual or the FMGS in automatic, applies a rating limit, and then manages fuel through the HMU and FMV to hold target N1 while keeping N2 within limits and adjusting the N2/N1 ratio for stability. And the levers themselves stay manually positioned throughout.