Engine failures in multi-engine helicopters

7.6.5.1 Conduct of the test programme

A great deal of preparation is needed before the trials programme can start. A detailed knowledge of the engines and transmission system is essential. This should allow appropriate testing without unnecessary damage to the aircraft and powerplants. It may be necessary to approach the engine and airframe manufacturers for concessions to the normal limitations. Some thought has to be given to planning how the power failure is to be simulated and, most importantly, how the power output of the engine (or engines) which is not being ‘failed’ is restricted so that it is representative of a minimum specification engine. For hydro-mechanical fuel control systems this may be achieved by manually restricting the fuel flow using the normal engine controls in the cockpit or it may require modifications to the engine governing system itself. With FADEC systems it is often necessary for the manufacturer to provide a software ‘patch’ which causes the engine to operate at minimum specification power. Whichever way the required power output is achieved, trials risk is minimized if there is a method of rapidly restoring the power to its maximum value. It is vital to monitor the power output of the ‘good’ engine during the trial, particularly during the flyaway/vertical reject phase as it has been known for vibration to cause the cockpit controls to drift and reduce the available power.

The choice of which engine to ‘fail’ is an interesting question. During OEI testing on the triple engined Westland/Agusta EH 101 the choice was between the No. 1 and No. 3 engines as the central No. 2 engine suffered an extra 2% installation power loss in the hover under sea-level-ISA conditions. The No. 1 engine was eventually chosen when it was discovered that there was a further power loss on the No. 2 engine as the No. 1 ran down through ground idle and its exhaust gases were ingested through the No. 2 engine intake.

7.6.5.2 Post-failure performance and handling

The flight trials commence with an evaluation of the aircraft performance and handling with one engine inoperative (OEI). Initially one engine is throttled back in level flight at the speed for minimum power and the aircraft handling is checked. The next stage is to shut down one engine. This should take place at VMP just in case there is any effect on the power available from the other engine(s). The minimum and maximum speeds in level flight that can be sustained on the good engine(s) are found together with the maximum sustainable angle of bank. It may be necessary to conduct a full level flight performance test to determine the power required and fuel flow for a range of airspeeds. This information is then often incorporated into the performance manual for use by operational crews. Once the performance has been established a series of gentle manoeuvres is made to determine if there are any handling problems. This whole process is then repeated at higher altitudes.

7.6.5.3 Engine re-lights

To achieve a successful engine start the mass flow of air through the compressor must be sufficient to achieve self-sustaining operation whilst preventing excessive turbine entry temperatures. At high altitudes the air density may be too low for a start to be achieved. This usually results in a re-light envelope that is smaller than the operational envelope of the helicopter. Since the air temperature affects density the re-light envelope is often expressed in terms of density altitude along with airspeed as this can affect conditions at the engine inlet. Testing of engine re-lights commences at low altitude and a mid-speed range and will then go on to expand the envelope in terms of density altitude and airspeed. In addition to establishing the envelope the testing also encompasses the effect that engine starting has on other aircraft systems. For example, the load on the electrical system may be such that the AFCS is affected or navigation equipment disrupted. Clearly if engine start is achieved using bleed air from another engine the effect of a start on the functioning engine requires assessment.