Autothrottle — Page 401, Lesson 495

Let’s start with the Flexible Take-off mode, because it’s a great example of how the autothrottle can be used to protect the engines. In situations where take-off can be executed without the need for full engine power — for example, a light weight take-off from a long runway — reduced power may be used. This reduces engine wear and increases engine life. This is called the Flexible Take-off mode. In Airbus aircraft, there is a detent position on the thrust levers labelled FLEX TO — that’s the physical notch where you set the levers for this mode. The most basic way to achieve the reduced thrust is to manually set a lower rpm setting. But with the autothrottle, we can do it differently: we select a temperature on the control panel that is higher than the ambient airfield temperature. This causes the thrust computation system to calculate a lower limiting EPR or N1. EPR is engine pressure ratio, and N1 is the low-pressure compressor speed — either way, the system computes a lower limit, which produces reduced power for the take-off. So by telling the system it’s hotter outside than it really is, we trick it into derating the thrust. Now, turbulence. When operating in light to moderate turbulence, the autothrottle can stay engaged unless performance is poor. You should expect increased thrust lever activity, and airspeed excursions of 10 to 15 knots are normal. But the autothrottle is not to be used in severe turbulence — that’s a hard limit. Next, FADEC. The system example comes from the Airbus series of aircraft. FADEC stands for full authority digital engine control. It provides complete engine management throughout all phases of flight. Let me walk you through its functions. First, gas generation control — that covers fuel flow, acceleration and deceleration, variable bleed valve and variable stator vane schedules, turbine clearance control, and idle setting. Second, engine limit protection — specifically over-speed protection for both N1 and N2, the low- and high-pressure spool speeds. Third, power management — control of engine thrust rating, computation of thrust parameter limits, auto-thrust system demand, and thrust lever position manual demand. Fourth, automatic engine starting sequence — control of the start valve, fuel, ignition, and monitoring of N1, N2, FF (fuel flow), and EGT (exhaust gas temperature). And fifth, manual engine starting sequence — here it does passive monitoring of the same parameters: start valve, fuel, ignition, N1, N2, FF, and EGT. In performing these functions, FADEC takes into account variables such as power demanded, air bleed for air conditioning and de-icing, temperature, static pressure, and engine accessory selection. The advantages: FADEC reduces crew workload, provides engine limit protection, improves engine life, and saves fuel and maintenance down time. Now the components. FADEC consists of an electronic engine control — 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 full picture: Flexible Take-off for derating, the turbulence limits, and the FADEC system with its functions, advantages, and dual-channel redundancy.