Helicopter OEI Training

One emergency condition pilots train for is loss of engine power. For a single engine helicopter, this requires the pilot to autorotate to the ground. For a multi-engine helicopter, this requires the pilot to fly with one engine inoperative (OEI).

In OEI flight, less power is available. Attempts to go beyond this power (or other engine limitations such as MGT) will cause rotor speed to decrease, which can be catastrophic. For this reason, pilots must be aware of OEI limitations and train for OEI flight.

While training for OEI could be done by shutting down one engine, this is often hazardous to pilot safety and/or the remaining engine. For this reason, many helicopters have a simulated “OEI training mode” that instructors and students can enable in flight. This mode limits power/MGT/… mimicking a real engine failure, but actually keeps all engines balanced and running at the equivalent reduced power.

In this mode, the cockpit displays for one of the two pilots are rigged to simulate real OEI engine parameters—it shows one engine going to 0 torque, NG, … while the remaining engine spool up to take on as much of the load as possible. Typically this will be done so that the student pilot sees the simulated OEI engine values, while the instructor sees the real engine values.

FAA Guidance

Because OEI training is so important, the FAA provides guidance for airworthiness approval of OEI training modes on helicopters. This is given in advisory circular 29-2C MG 22.

Safeguards

Of course, real helicopter malfunctions may occur while in OEI training mode. OEI training software, typically within the engine control unit, must be capable of detecting such malfunctions and providing a safe response. For example, let’s say a dual engine helicopter is in OEI training mode and then really loses an engine. Without safeguards, the helicopter would now be limited to one engine which is, due to the training mode, providing half its available power (just one quarter of the total power normally available). OEI training software should terminate here, so that the working engine can provide full power (half total power).

Other safeguards are present for high oil temperature, low oil pressure, low NP/NR, accidental change of input throttle or engine mode. Some exceptional behavior like low NR is tolerated to some extent for student training, but may disable the OEI training mode before flight is overly hazardous.

Validation

To be useful, OEI training mode must be representative of an actual engine failure. This includes the dynamic response immediately after engine failure and steady limitations thereafter. To prove this, helicopter manufacturers perform experiments comparing training mode behavior to real and expected behavior. Some tests are done with the helicopter restrained on the ground for safety.