Total loss of power

For obvious reasons it is always necessary to test the characteristics of a helicopter following a complete loss of power. The amount of testing required depends on the likelihood of such a complete loss occurring and also on the role of the aircraft. Clearly single-engine helicopters are more likely to find themselves in this condition than multi-engine ones and require extensive testing, however, it is still necessary to demonstrate that any rotorcraft can enter autorotation and perform a power-off landing. It has been known for twin-engined helicopters to run out of fuel. Equally in the event of tail rotor drive failure it is common practice to shut down all the powerplants. If the aircraft is being procured for the training role that involves entries into autorotation and engine-off landings then its qualities during power-off manoeuvres are of particular importance.

7.6.4.1 Conduct of the test programme

The testing of flight regimes associated with total loss of power clearly carries with it a significant degree of risk. To mitigate this risk the test programme is usually structured in such a way that before an area of testing is attempted all the preparatory tests have been completed. Thus tests start by investigating flight idle glide before going on to autorotation. The aircraft characteristics in FIG are assessed before rapid entries into descending flight and recovery to climbing flight are made. Of course the completion of engine and rotor governing tests such as engine acceleration, transient overswing and combustion stability are a prerequisite. Once the full range of FIG and autorotative tests has been performed the engine-off landing tests are conducted before going on to look at in-flight relights.

7.6.4.2 Autorotation and flight idle glide

The behaviour of the aircraft in autorotation is the first area to investigate and this involves the two main areas of handling and performance. Turning first to handling tests the flight control positions in trimmed autorotative flight are documented at a range of airspeeds. Of particular note is the pedal margin available at low speed, which may not be adequate to ensure yaw control during the engine-off landing (EOL) particularly if it involves a gusty crosswind. The aft cyclic margin is another area that has proved unsatisfactory on some helicopters being insufficient to allow adequate manoeuvrability in the flare. The behaviour of the rotor is also investigated to determine how easy it is for the pilot to control rotor speed and also how often intervention by the pilot will be needed to respect limits. The cues to high and low Nr , together with the effectiveness of any warning devices are evaluated. Manoeuvres that are representative of those a pilot would make during the approach to a forced landing are conducted to ensure that adequate control is available and that the workload required to manage rotor speed is not excessive. This usually involves determining the effect on the rotor speed of a range of gentle manoeuvres such as turns, flares and bunts. Areas of particular interest are the handling qualities at low values of rotor speed where problems with control margins and effectiveness are more likely. Clearly all low rotor speed testing is approached with great care.

Typical autorotative performance testing involves a measurement of the rate of descent and distance covered with various combinations of airspeed and rotor speed. From these results recommendations are made to the operational pilot concerning the airspeed for minimum rate of descent and the combination of airspeed and rotor speed that gives the maximum range. Pressure errors are often significant during autorotation and are documented as they can significantly increase the difficulty of manoeuvring the aircraft during a forced landing. For example, a pilot will attempt to maintain a set airspeed such as VMP when manoeuvring in autorotation, however, if he is unaware of the presence of large ASI PEs he may make unnecessarily large pitch attitude changes as he attempts to keep the indicated airspeed under control. Much of the performance information gained here is employed later in the trials programme for determining the initial airspeed for testing engine-off landings and for estimating the height loss during re-light tests.

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