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'FADEC' is the acronym for 'Full Authority Digital Engine Control' (sometimes incorrectly interpreted as Full Authority Digital Electronics Control). It is a system consisting of a digital computer (called EEC /Electronic Engine Control/ or ECU /Electronic Control Unit/) and its related accessories which control all aspects of aircraft engine performance. FADECs have been produced for both piston engines and jet engines, their primary difference due to the different ways of controlling the engines.

Contents

History

Function

Safety

Application

Advantages

Disadvantages

References

History

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. To accurately explore the roots of today’s FADEC, one should first understand the evolution of the control interface of an aircraft engine. The original engine control system is 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 was the introduction of analog electronic engine control. Analog electronic control varies an electronic signal to communicate the desired engine settings. The system was an obvious improvement over mechanical control but had its drawbacks including the 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 is the choice engine for the famous 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.

Function

To be a true, 100%, Full Authority Digital Engine Control, there must not be any form of manual override available. This literally places full authority to 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.

Safety

With the operation of the engines so heavily relying on automation, safety is a great concern. Redundancy is provided in the form of two, 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.

Application

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 will have to be accepted.
FADECs today are employed by almost all current generation jet engines and increasingly in newer piston engines, on fixed-wing aircraft and helicopters.

Advantages

★ Better fuel efficiency

★ Automatic engine protection against out-of-tolerance operations

★ Safer as the multiple channel FADEC computer provides redundancy in case of failure

★ Care-free engine handling, with guaranteed thrust settings

★ Ability to use single engine type for wide thrust requirements by just reprogramming the FADECs

★ Provides semi-automatic engine starting

★ Better systems integration with engine and aircraft systems

★ Can provide engine long-term health monitoring and diagnostics

★ Number of external and internal parameters used in the control processes increases by one order of magnitude

★ Reduces the number of parameters to be monitored by flight crews

★ Due to the high number of parameters monitored, the FADEC makes possible "Fault Tolerant Systems" (where a system can operate within required reliability and safety limitation with certain fault configurations)

★ Can support automatic aircraft and engine emergency responses (e.g. in case of aircraft stall, engines increase thrust automatically).

Disadvantages

★ Engineering processes used to design, manufacture, install and maintain the sensors which measure and report flight and engine parameters to the control system itself

★ Integrity and reliability of the materials and the path over which this data flows

★ Software engineering processes used in the design, implementation and testing of the software used in these safety-critical control systems. This led to the development and use of specialized software such as SCADE.

★ Inability of the display subsystem to provide clear and unambiguous information to the crew, under conditions of high stress and intensive cockpit workload (for example, in an emergency)

★ Responsiveness of both the FADEC software and its acceptance of "human input" under dangerous flight envelopes, for instance at low airspeed, close to terrain, high gross weight, low fuel state, unusual airframe attitude, other systems reporting "anomalous behaviour" (typically, after combat damage or other component failure)

★ Completeness of the flight simulations and parameters used to populate the rulebase against which some FADEC systems compare for "valid" control inputs in prevailing flight conditions.

References

★ Hispano-Suiza: Digital Engine Control

★ Moren, Chuck. Interview with student. FADEC. Embry-Riddle Aeronautical University, Daytona Beach. 2007-03-13.

★ Title 14 CFR: Federal Aviation Regulations, , , , FAA, ,