FADEC

FADEC is the acronym for Full Authority Digital Engine 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.

Electronics' superior accuracy led to early generation analogue electronic control first used in Concorde's Rolls-Royce Olympus 593 in the 1960s. 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 TF-30 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.

The aircraft's thrust lever sends electrical signals (pilot's command, may also be the autothrottle) to the FADEC. The FADEC digitally calculates and precisely controls the fuel flow rate to the engines giving precise thrust. In addition to the fuel metering function, the FADEC performs numerous other control and monitoring functions such as Variable Stator Vanes (VSV's) and Variable Bleed Valves (VBV's) control, cabin bleeds and power off-takes control, control of starting and re-starting, turbine blade and vane cooling and blade tip clearance control, thrust reversers control, engine health monitoring, oil debris monitoring and vibration monitoring. The inputs come from various aircraft and engine sensors. Apart from the key parameters that are monitored for a safe thrust control (shaft rotational speeds, pressures and temperatures at various points along the gas path) the FADEC also monitors hundreds of various analog, digital and discrete data coming from the engine subsystems and related aircraft systems, providing a fully redundant and fault tolerant engine control.

This digital computer has to be considered an essential part of the engine as a complete failure of the FADEC would cause the complete loss of engine thrust. For redundancy reasons the FADEC incorporate two separate identical digital channels. Each channel may provide all engine functions without restrictions. The FADEC may be powered by the aircraft electrical systems, and in most modern aircraft it uses power from a separate generator connected to the related engine.

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
• the 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)
• the FADEC can support automatic aircraft and engine emergency responses (e.g. in case of aircraft stall, engines increase thrust automatically).

Disadvantages

• the engineering processes used to design, manufacture, install and maintain the sensors which measure and report flight and engine parameters to the control system itself
• the integrity and reliability of the materials and the path over which this data flows
• the 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.
• the ability 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)
• the 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 all-up-weight, low fuel state, unusual airframe attitude, other systems reporting "anomalous behaviour" (typically, after combat damage or other component failure)
• the 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.