FLUG REVUE April 2005: FADEC is vital for engine control

April 2005

FADEC TECHNOLOGY ADVANCES

By Patrick Hoeveler

The housing of the digital control system of the GP7200 engine for the giant Airbus A380 feels really hot. The box has the dimensions of a large shoebox, yet the output that it holds in check is up to 340kN The Full-Authority Digital Engine Control (FADEC) is the brain of every modern propulsion unit and ensures that the aircraft is flown both safely and efficiently. In the electronics laboratory of MTU Aero Engines in Munich this inconspicuous device is used to create a simulation model for the control of the most powerful turboprop in the Western world, the TP400-D6 for the A400M (see FR 2/2005). “The TP400 is more complex to control than the EJ200 with afterburner. The propeller control in particular is proving very challenging,” says Werner Wunderlich, head of Control, Monitoring, Equipment at MTU. On every variant, the main task remains the same: the FADEC controls the output of the propulsion unit, taking due account of the operational limits, so as to achieve optimal performance data at every phase of the flight. There is no longer any mechanical connection between throttle lever and engine. Depending on the type of engine, there are different design parameters. On helicopters, control is oriented towards the load on the driveshaft. On fighter aircraft, it is the rotational speed, as the central focus is on thrust, whereas on passenger aircraft where economy is the priority the pressure ratios are the key parameters.

On the TP400, for example, the sensors continuously pick up around 25 input parameters such as rotational speed, pressures, temperatures, position transducers of the adjustable parts and communication with the airframe. Inside the housings, a total of up to six processors in two identical channels which could be switched over in the event of problems process the data and work out the relevant control signals. The monitoring software distinguishes between seven levels of error handling both in the engine as well as in the FADEC. Only from the third level is the channel switched over, unremarked by the pilot in the cockpit. The system also limits the temperatures or rotational speed where necessary. Once again, there is no way of influencing this from the cockpit. Detection of the final stage results in shutdown of the engine. On MTU's Digital Engine Control and Monitoring Unit (DECMU) for the EJ200, both channels are constantly live so that they can switch over smoothly.

“One of the peculiarities of FADEC systems is the harsh environment in which they are installed,” says Jamil Dirani, Vice President Sales, Support and Services at Hispano-Suiza, “on the engine itself.” The sensitive electronics must be capable of withstanding interference from lightning strike or radar emissions, strong vibration and contamination by extremely corrosive fluids, and this is in a temperature spectrum of between -63ºC and +120ºC. On fighter aircraft, there is an additional requirement for protection against electromagnetic pulse after a nuclear explosion. In the DECMU of the EJ200, a sensors measures the radiation and switches the control system off quick as a flash.

Naturally there is an additional requirement that the FADEC should not be too big or too heavy, weighing no more than 15kg to 27kg in civil applications. In fighter aircraft such as the Eurofighter, it must weigh less than 13kg. “We need highly complex circuit boards with up to 20 layers so as to accommodate everything in the smallest possible space,” says Wunderlich. “Memory and processor are no longer an issue. What was a problem ten years ago we can solve today three to four times over.” Since the first civil FADEC systems entered into service in the mid-1980s, they have changed enormously. “There has been an overall growth in the ability to controlling an engine in a much more complex and efficient way,” says Joe Triompo, President Engine & Control Systems at Hamilton Sundstrand. In the early 1980s, the upper limit on processor speed was 0.5 to 1 million instructions per second (MIPS). Today processors achieve over 1000MIPS, which is roughly the equivalent of a modern Pentium PC. Thanks to the high computing speed, the system reacts dynamically and approaches closer to the limits, as Werner Wunderlich explains. “Today, with intelligent control we can utilise potential from the engine that no one dreamed was possible before.” However, the top priority continues to be safety. “The entire authority of the engine lies in the hands of the digital control system.” The probability of a catastrophic error should not exceed the design parameter of one failure per one billion flight hours. The software plays an important role here and must not be allowed to fail. As a result, the requirements specification that the software has to satisfy on the EJ200 extends to 800 pages.

Another problem lies in the rapid obsolescence of electronic components. “Obsolescence certainly is a challenge to our industry,” says Joe Triompo. “If you choose a suitable and available processor, it is quite possible that it may have been discontinued two years later,” Werner Wunderlich adds. This can become tricky when one considers that the average production run stretches to ten years and there is a requirement for a maintenance capability for up to 25 years beyond that. “It is very difficult to identify special problematic parts: it may be a matter of processors but sometimes it is just a matter of small resistors,” says Jamil Dirani. Manufacturers like Hamilton Sundstrand, Hispano-Suiza and MTU Aero Engines pursue similar strategies for solving this problem. They have specialist departments devoted entirely to the subject of obsolescence. Electronics manufacturers now announce when they plan to discontinue production so that customers have the opportunity to buy in a large stock of parts in the so called “last-time buy”. On the other hand, holding such large supplies is expensive, and no empirical data is yet available as to how the items function after they have been held in store for a long time. Commercial components from the automotive and entertainment industries are increasingly used, as these have better availability and are also cheaper. In addition, reliance is placed on family concepts with the same processor systems so as to reduce the risk. However, there should be a new platform every five to ten years at the latest, if the higher costs associated with retrofits are to be avoided. These upgrades could entail the replacement of circuit boards while the functionality has to be retained at the interface. Finally, a high degree of reliability is expected of FADECs, which do not undergo scheduled maintenance. “We have already achieved 40,000 hours on wing,” says Dirani.

Overall, the control units are growing in importance. The concept of the “more electric engine” provides for sophisticated prognosis and diagnosis methods so that errors can be detected earlier or even avoided. “Over the next few years further increases in computing power will enable smart pumps and actuators to be controlled, amongst other things,” Joe Triompo predicts. According to Werner Wunderlich, there is also an inclination to position these smart actuators locally and thus to distribute the intelligence. Another new direction is the use of model-based control on which MTU is currently working. Values such as the stall margin in the compressor or the turbine inlet temperature cannot be measured or only with difficulty. An engine simulation running in parallel to the real propulsive unit in real-time could produce the missing data that flows into the control system. This would then be in a position to optimise operating performance accordingly so that one could approach closer to the limits. To avoid simulation errors, the two engines are constantly compared during operation and the model is continuously adjusted to the real engine. On the other hand, such sophisticated methods are unlikely to come cheap. Development and production of the FADEC already accounts for between 12 and 16 percent of the total cost of an engine.

From page 82 of FLUG REVUE 4/2005

Last updated 15 March 2005

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