Specific testing of FADEC systems

FADEC systems are becoming increasingly prevalent in new helicopters and under­standing how to test them is an increasingly important skill for the rotary wing test pilot and flight test engineer. Essentially the testing of these systems involves the same techniques that have already been described, however, there are some areas where extra attention is required.

A key area of renewed interest is the instrumentation requirements. As FADEC systems are inherently of high bandwidth it is necessary to have a high sampling rate (>100 Hz) for data acquisition. In some systems this high bandwidth has caused the FADEC to generate damaging resonances within the drivetrain. Therefore modern instrumentation systems are designed with this possibility in mind. Extensive sets of transducers are fitted at key areas within the transmission system and there is often a capability to present the data in real time through telemetry. As the airborne environment is different from the bench, FADEC performance is checked against the results of bench tests at an early stage. To test failure modes of both hardware and

Collective pitch (deg)

software it is often necessary to build and test a fault injection system to simulate a range of failure modes.

Areas of specific concern for FADEC testing include: [15]

• Torque spikes. The rapid response characteristics of FADEC systems can eliminate problems with transient droop but may introduce problems with torque spikes if the gain is set too high.

• Training mode. Most FADEC systems incorporate a training mode to provide safe and realistic engine failure training. This mode is evaluated to ensure it meets its design aims. Many of these systems will restore the ‘failed’ engine to full power if Nr drops below a threshold value. The operation of this part of the system is checked with care to ensure that rapid acceleration of the engine does not lead to an overtorque.

• Failure testing. For correct operation a digital engine control system relies on software, processors and information supplied by a large number of sensors. Very extensive testing is usually required to determine the effect on the system of a failure or malfunction of any of these.

A final point to stress about FADEC testing is the need to have good configuration control over the software. During the development and testing phase a number of different standards of software may be in existence at different stages of testing. It is imperative that adequate control methods are in place so that the software version installed in the test aircraft can be tracked.