1 Full Authority Digital Electronic Controller - FADEC
FADEC is an engine control and monitoring system. EEC is the brain of FADEC that computes and commands the control functions. It contains one or more microprocessors and read data from sensors, control actuators, valves and other aircraft systems. Using these readings together with the pilot's input EEC computes the position of actuators. EEC operates in a closed loop system. EEC also gather and transmit information about engine condition data and faults to the cockpit. A serial data bus is used to exchange information between FADEC and aircraft systems, such as FMS and ADS.
- ADC - Air Data Computer
- ADIRU - Air Data Inertial Reference Units
- ADS - Air Data System
- ARINC - Aeronautical Incorporated
- CMS - Central Maintenance System
- CWS - Central Warning System
- ECU - Engine Control Unit
- EEC - Electronic Engine Control
- EGT - Exhaust Gas Temperature
- EMH - Engine Health Monitoring
- EMI - Electromagnetic Interference
- FADEC - Full Authority Digital Electronic Controller or Full Authority Digital Engine Control
- FCU - Fuel Control Unit
- FMC - Flight Management Computer
- FMU - Fuel Metering Unit
- FOHE - Fuel Oil Heat Exchanger
- HMU - Hydro-mechanical Unit
- HP - High Pressure
- I/O - Input/output
- LP - Low Pressure
- LVDT - Linear Variable Differential Transformer
- N1 - LP compressor speed
- N2 - HP compressor speed
- RVDT - Rotary Variable Differential Transformer
- VBV - Variable Bleed Valve
- VSV - Variable Stator Vane
4 History and Introduction
Engine is the most expensive single component in an aircraft. Engine used to be controlled and monitored by the pilot or flight engineer, via a mechanical link to fuel valve. As the technology advanced, use of electronic engine control emerged. This decreased the workload on the flight crew, eventually the flight engineer disappeared from the cockpit. Before FADEC was invented, EEC which has a supervisory role in engine control was used. As the trust in technology grew to acceptable level, full authority was given to microprocessor, leading to the development of FADEC.
5 Function of FADEC
As the aircraft take-off and climb to higher altitude, the surrounding environmental conditions: temperature, pressure and, density changes. Since gas turbine engine operate by accelerating mass of air, as these factors change, the thrust produced by the engine also change. Pilots' main job is to fly the aircraft safely. To allow this FADEC take over the engine monitoring and control based on pilots' or FMS inputs. Controlling is mainly achieved by varying fuel flow and other parameters. Monitoring function of FADEC diagnoses engine systems and alert the pilot, ensuring that engine operates within the set limits. FADEC monitor and/or control the following typical functions:
- Fuel flow to the fuel injectors
- N1 and N2 speed
- Variable stator vane scheduling
- Bleed valve operation
- Turbine blade and vane cooling
- Turbine case cooling
- Oil temperature
- Fuel temperature
- Valves, Solenoids and actuators
- Starting and restarting
- Thrust reverser
FADEC also provide the engine parameters, status messages and fault messages to aircraft systems to display in the flight deck. In Boeing this information is displayed in EICAS displays and in Airbus it is displayed in ECAM displays. Some FADEC systems may integrate an Engine Health Monitoring (EMH) system, to provide data about engines' health to support maintenance scheduling (1 p. 199). Thus it can help an airline to reduce cost associated with scheduled and unscheduled maintenance. Figure below show a picture of FADEC.
FADEC monitor and control various components. However, the following components are considered as the basic components of FADEC:
- Electronic controller (EEC or ECU)
- Engine driven generator
- Sensors and transducers
- Data bus
EEC is enclosed in an aluminium chassis and mounted on the fan case with shock absorbers to provide protection against shock and vibration. The housing protects EEC from the hostile environment, where it is subjected to high temperature and Electromagnetic Induction (EMI) from airport radar and lightning. Similarly the wiring harnesses that connect EEC to other components (such as: sensors, actuators, etc) have metal braiding to shield from EMI. These wiring harnesses are usually designed for easy replacement, i.e. they are line replaceable. To prevent overheating of the EEC, ambient air from air scoop is used for cooling.
Unlike supervisory EEC, FADEC has the full authority over engine control, which means there is no manual override. Failure of FADEC results in engine failure. Therefore, many safety features are built into the system to prevent total system failure. FADEC has two independent EEC channels (A and B as shown in the above figure). Two channels are identical and each has its own processor, power supply, memory, sensors, wiring harnesses, and electrical parts of transducers. A cross link allow data to be transferred between channels without allowing fault to propagate from one channel to the other. Arrangement of a typical dual-channel EEC is depicted in figure below. To ensure that both channel remains serviceable, alternate selection of channels are employed; provided condition of both channels are equal. Selection occurs at engine start. However, manual selection can be made via the control panel in the flight deck (3 pp. 7-19).
To ensure uninterrupted power supply, two channels of EEC are powered by multiple independent sources. Aircraft normal bus, emergency bus and engine driven generator (alternator) provides AC power to EEC. In the EEC, the current is rectified and the voltage is regulated. Aircraft power supply is used to power the EEC for engine starting. However, upon N1 reaching certain speed, power supply is switched to alternator, which is the primary source of electrical power (3 pp. 7-22). There are two windings within the same alternator to provide independent power supply to two channels of EEC. In the event of failure of primary power supply, EEC automatically switches to an alternative power supply.
A data bus is employed at the FADEC-Aircraft interface to exchange information. EEC communicates with other aircraft systems such as: FMS, ADIRU, ECAM/EICAS and CMS, via this data bus to obtain necessary information. EEC also sends engine parameter data and associated messages to other aircraft systems via this data bus. Most aircraft use unidirectional ARINC 429 data bus.
In order for EEC to understand the inputs from various systems, transducers are incorporated. Transducers convert energy from one form to the other. For instance: rotational energy of compressor (N1) is converted into electrical signals by an alternator. Similarly electrical commands generated by the EEC are converted to mechanical energy by solenoids or transformers.
In order for EEC to function, it requires lot of input signals. These are provided by aircraft systems or from sensors directly connected to EEC. Sensors provide information about engine condition, pilot demand and feedback from actuators. Engine parameters monitored and/or controlled include:
- Engine temperatures (EGT, etc)
- Engine pressures (P1, P2 etc)
- Engine RPM (N1 and N2)
- Fuel flow
- Variable stator vane position
- Airplane bleed status
Additional sensors may be used to monitor aircrafts' immediate environmental conditions: such as temperature, pressure and air density. Alternatively, this information may be obtained from ADC or FMC. A RVDT, LVDT or another form of transducer is used to determine the throttle lever position. Sensors and actuators are described in details in another section.
EEC is the brain of FADEC. It is programmed to follow certain instructions when particular conditions are met, for instance: if some inputs fail, it switches mode or shutdown. These instructions are called control laws (1 p. 194). They determine the behaviour of the EEC.
Two channels of EEC are always operating, but there is only one channel in command called the active channel. If the active channel fails, EEC automatically switches to alternative channel. In other words, EEC always select most healthy channel. If both channels are unable to control an engine function, it is set to a predetermined failsafe position, which protects the engine (3 pp. 7-20). In some cases this may result in total shutdown of the engine. Each channel receives two signals of the same parameter. One signal is received from the hardwired sensors, and the other signal over the cross link. A validation test is carried out to assess the validity of the signals before they are processed. Input parameters are used to calculate the command functions for servos which control actuators.
EEC operates in a closed loop system (3 pp. 7-20). Feedback of the command is received by the EEC to ensure correct execution of command. EEC compares the command to feedback and sends the signal until the command is fulfilled. Actuators are controlled as a function of various factors (example: altitude, temperature, pressure, etc) that affect the flight condition. For example: EEC calculates the thrust settings based on the data entered into flight management system. This includes, take off thrust, cruise thrust etc. Once the throttle is set to the desired position (automatic throttle position) the EEC will maintain it throughout the flight by controlling factors such as: fuel flow, bleed air etc. Similar principle applies to other controlled functions.
There are two EEC control modes: EPR mode and N1 mode. EPR mode is the normal mode, in which the EEC maintains a commanded EPR by altering the fuel flow. N1 mode is automatically selected upon failure of EPR mode. In this mode fuel flow is controlled as a function of N1. Selection of N1 mode will trigger an advisory message in the flight deck display. If the analysis of an input, which is important to engine control, proves the data incorrect or if the input is not available, EEC will change to N1 mode or synthesise the required data using other available parameters. For example: if actual ambient pressure is not found, the EEC will change to N1 mode and synthesise this value from the ADC data.
EECs' monitoring function can be regarded as a BITE function. This is a continuous process in which EEC checks the separate components of the system for malfunctioning. Any fault detected in sensors, harness, etc is transmitted via a data bus to CWS and/or CMS. Depending on the seriousness of the fault, the message may be displayed as advisory, caution or warning message. Some information which is not critical may not be displayed to the pilot, but they are stored in the EEC and CMS memory and can be manually called up.
Performance of the engines from same manufacturer is not always same. There might be slight variations. EEC stores information for all engine configurations. In order to control engine most efficiently, unique configurations of each engine are programmed on to an EEC programming plug. This plug provides engines' EPR/thrust relationship data to EEC. It is permanently attached to the engine by a lanyard and it stays with the engine even if EEC is replaced. EEC uses the value from its memory for the previous plug configuration, if there is a fault with EEC plug or if the plug is missing (3 pp. 7-21).
FADEC carry out the tedious task of controlling and monitoring the engine parameters. It protects engine from exceeding set limits and alerts pilot only to those actions which requires pilots' attention. Thus reduces the workload of flight crew. Additionally it allows protection against surges to prevent catastrophic engine damage (1 p. 194).
FADEC also allows repeatable and efficient fuel scheduling, hence increase the fuel efficiency of the engine. If integrated with EHM, FADEC can help to reduce cost associated with scheduled and unscheduled maintenance by providing important supporting data (1 p. 199).
7 Works Cited
1. Rolls-Royce. The Jet Engine. London: Rolls-Royce plc, 2005. 0902121235.
2. Electronic Concepts & Engineering, Inc. FADEC - Full Authority Digital Engine Controls for Turbine Engines. Electronic Concepts & Engineering, Inc. [Online] [Cited: 24 April 2010.] http://www.eceinc.com/images/rc_fadec.jpg.
3. Otis, Charles E and Vosbury, Peter A. Aircraft Gas Turbine Powerplants. Colorado: Jeppesen Sanderson Inc, 2002. 0-88487-311-0.
1. Moir, Ian and Seabridge, Allan. Aircraft Systems. 3rd. Chichester: John Wiley & Sons, Ltd, 2008. 978-0-470-05996-8.
2. FAA. Pilot's Handbook of Aeronautical Knowledge: Chapter 6 - Aircraft Systems. FAA. [Online] 2008. [Cited: 28 February 2010.] http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2006.pdf.