The goal of any engine control system is to allow the engine to perform at maximum efficiency for a given condition. The complexity of this task is proportional to the complexity of the engine. Originally, engine control systems comprised simple mechanical linkages controlled by the pilot. By moving throttle levers directly connected to the engine, the pilot could control fuel flow, power output, and many other engine parameters.
Following mechanical means of engine control came the introduction of analog electronic engine control. Analog electronic control varies an electrical signal to communicate the desired engine settings. The system was an evident improvement over mechanical control but had its drawbacks including common electronic noise interference. This system was pioneered in the 1960s and first introduced as a component of the Rolls Royce Olympus 593 engine. The 593 was the engine of the supersonic transport aircraft Concorde.
Following analog electronic control, the clear path was digital electronic control. Later in the 1970s NASA and Pratt and Whitney experimented with the first experimental FADEC, first flown on an F-111 fitted with a highly modified Pratt & Whitney TF30 left engine. The experiments led to Pratt & Whitney F100 and Pratt & Whitney PW2000 being the first military and civil engines respectively fitted with FADEC and later the Pratt & Whitney PW4000 as the first commercial "dual FADEC" engine.
True full authority digital engine controls have no form of manual override available, placing full authority over the operating parameters of the engine in the hands of the computer. If a total FADEC failure occurs, the engine fails. If the engine is controlled digitally and electronically but allows for manual override, it is considered solely an Electronic Engine Control or Electronic Control Unit. An EEC, though a component of a FADEC, is not by itself FADEC. When standing alone, the EEC makes all of the decisions until the pilot wishes to intervene. FADEC works by receiving multiple input variables of the current flight condition including air density, throttle lever position, engine temperatures, engine pressures, and many others. The inputs are received by the EEC and analyzed up to 70 times per second. Engine operating parameters such as fuel flow, stator vane position, bleed valve position, and others are computed from this data and applied as appropriate. FADEC also controls engine starting and restarting. The FADEC's basic purpose is to provide optimum engine efficiency for a given flight condition.
FADEC not only provides for efficient engine operation, it also allows the manufacturer to program engine limitations and receive engine health and maintenance reports. For example, to avoid exceeding a certain engine temperature, the FADEC can be programmed to automatically take the necessary measures without pilot intervention.
With the operation of the engines so heavily relying on automation, safety is a great concern. Redundancy is provided in the form of two or more, separate identical digital channels. Each channel may provide all engine functions without restriction. FADEC also monitors a variety of analog, digital and discrete data coming from the engine subsystems and related aircraft systems, providing for fault tolerant engine control.
To perhaps more clearly illustrate the function of a FADEC, explore a typical civilian transport aircraft flight. The flight crew first enters the data appropriate to the day’s flight in the flight management system or FMS. The FMS takes environmental data such as temperature, wind, runway length, runway condition, cruise altitude etc. and calculates power settings for different phases of flight. For takeoff, the flight crew advances the throttle (which contains no mechanical linkage to the engine) to a takeoff detent or opts for an auto-throttle takeoff if available. The FADECs know the calculated takeoff thrust setting and apply it. The flight crew notes that they have merely sent an electronic signal to the engines, no direct linkage has been moved to open fuel flow. This procedure is the same for climb, cruise, and all phases of flight. The FADECs compute the appropriate thrust settings and apply them. During flight, small changes in operation are constantly being made to maintain efficiency. Maximum thrust is available for emergency situations if the throttle is advanced to full, but remember, limitations can’t be exceeded. The flight crew has no means of manually overriding the FADECs, so if they make a decision the crew doesn’t like, it must be accepted.
In piston-engine powered aircraft, the system replaces both magnetos, making obsolete repetitive and costly magneto maintenance, and eliminates carburetor heat, mixture controls and engine priming. By controlling each cylinder of the engine independently for optimum fuel injection and spark timing, the need for the pilot to monitor and control mixture is eliminated. Because imprecise mixture operation can affect engine life, the FADEC has the potential to reduce operating costs and increase engine life for the average General Aviation pilot. Tests have also shown significant fuel savings potential. FADEC paid for itself in reduced operating costs.