Autothrottle — Page 401, Lesson 495

We're starting a new topic now: the autothrottle, and I want to walk you through the first part of it, which covers flexible take-off, turbulence, and the FADEC system. Let's begin with flexible take-off. Imagine you're at a light weight, on a long runway, and you don't need full engine power to get airborne. Using full power would just wear the engines out faster. So instead, we use reduced power. That's the whole idea behind the Flexible Take-off mode — it reduces engine wear and increases engine life. In Airbus aircraft, there's a specific detent position on the thrust levers labelled FLEX TO, which is where you'd set the levers for this mode. Now, the most basic way to achieve that reduced thrust is simply to manually set a lower rpm. But with the autothrottle, we have a more elegant method. We select a temperature on the control panel that is higher than the actual ambient airfield temperature. Why does that work? Because the thrust computation system uses that higher temperature to calculate a lower limiting EPR or N1. EPR is engine pressure ratio, and N1 is the low-pressure compressor speed — both are measures of engine thrust. So by fooling the system into thinking it's hotter outside, it computes a lower thrust limit, and you get reduced power for take-off. Next, let's talk about turbulence. In light to moderate turbulence, the autothrottle can stay engaged — unless performance is poor. But you should expect increased thrust lever activity, and airspeed excursions of 10 to 15 knots are normal. That's just the system working to hold speed in bumpy air. However, the autothrottle is not to be used in severe turbulence. That's a hard limit. Now, the big one: FADEC. This stands for full authority digital engine control. The example we're using comes from the Airbus series of aircraft. FADEC provides complete engine management throughout all phases of flight. Let me break down its functions. First, gas generation control. This covers fuel flow, acceleration and deceleration, variable bleed valve and variable stator vane schedules, turbine clearance control, and idle setting. These are all the things that manage how the engine breathes and burns fuel. Second, engine limit protection — specifically over-speed protection for both N1 and N2. N2 is the high-pressure compressor speed. So FADEC prevents the engine from spinning too fast. Third, power management. This includes control of engine thrust rating, computation of thrust parameter limits, auto-thrust system demand, and thrust lever position manual demand. So it manages both what the autothrottle asks for and what you ask for with the levers. Fourth, automatic engine starting sequence. FADEC controls the start valve, fuel, and ignition, while monitoring N1, N2, FF — that's fuel flow — and EGT, exhaust gas temperature. Fifth, manual engine starting sequence. Here FADEC passively monitors the same things: start valve, fuel, ignition, N1, N2, FF, and EGT. So in manual start, you're driving, but FADEC is watching. In performing all these functions, FADEC takes into account variables like power demanded, air bleed for air conditioning and de-icing, temperature, static pressure, and engine accessory selection. Now, the advantages. FADEC reduces crew workload, provides engine limit protection, improves engine life, and saves fuel and maintenance down time. That's a big selling point. Finally, the components. FADEC consists of an electronic engine control — that's the EEC — plus a fuel metering unit, sensors, and peripheral units. There are suitable interface circuits between the EEC and its peripheral units. And critically, there are 2 FADEC channels per engine — one in control and one in standby for redundancy. So if the active channel fails, the standby takes over seamlessly. That's the core of the autothrottle and FADEC introduction. Let's keep going when you're ready.