Chinook - Still Hitting Back 3 (Merged)

[ShyTorque]

I would like to make a few things clear at this point because I believe one or two people posting here, with little or no helicopter experience and no real idea of the implications of certain engine emergencies, don't seem to have grasped some highly relevant points.

Firstly, helicopters do not follow the same aerodynamic laws as fixed wing. Because of the complexities of control and stability (lack of natural stability, cross-coupling etc), a small movement of any flight control can result in a very large aircraft flight path change. Similarly, to keep a helicopter on a steady flight path, depending on the regime of flight, a surprisingly large flying control input (or small one) may be required when compared to a fixed wing aircraft.

The term "rudder" is misleading because helicopters do not have a rudder. The correct term for the flying controls affecting yaw is "yaw pedals". The way they cause yaw is NOT the same as FW rudder pedal input, especially on the Chinook.

A helicopter with a mechanical flying control problem / failure may be affected in all axes, not just the one axis that might be expected by trying to relate to the fixed wing scenario of elevator/aileron/rudder. In this situation it may appear that the helicopter had responded to a pilot input in a conventional ("fixed wing") fashion, but it may not have done at all. The pilot may have operated one flying control in a particular way but the aircraft may have responded in an unexpected way. Alternatively, the aircraft may have "done its own thing" in one or more, or all axes, when it may have appeared that the pilot was flying the aircraft under control.

Captain Nick Lappos can undoubtedly talk at great length about this. For example on the S-76, in certain situations how yaw / collective interaction can cause the collective to rise and fall significantly when yaw pedal is applied. A pilot input on the yaw pedals can drive the collective up or down, resulting in the aircraft climbing or descending. An input of collective may drive the yaw pedals, resulting in a large yawing action. Use of one control may jam the other. There is a procedure for this in the Emergencies Checklist.

This is by no means unique in the helicopter world. There is currently an ongoing investigation into a fatal accident involving an S-76 where the aircraft may (or may not) have responded incorrectly to pilot inputs and spiralled rapidly downwards into the sea. Not a usual flight regime. A mystery, even with a CVR.

John Purdey, regarding your last paragraph; like it or not, flying in marginal VMC is how SH often gets the job done, especially if there is no IFR contingency, as in this case. To go over old ground yet again; Mr. Holbrook, the yachtsman who was an eye-witness to the aircraft, gave evidence that the helicopter was flying slowly enough over the sea for him to initially think it was carrying out a search in his vicinity. This indicates that the handling pilot had slowed the aircraft down in marginal weather conditions. This is completely normal - that is how the SH job gets done.

Gp Capt. Peter Crawford, like many of us, could not, at the the time of giving his evidence, understand how the aircraft had rapidly accelerated and climbed until it hit the hillside. However, an engine runaway up would require immediate rapid pilot intervention to contain the Rotor(s) RPM (usually known as NR). The only way to do this is to apply a large amount of collective pitch. This WOULD result in a climb and/or a rapid acceleration of the aircraft, no doubt about that. RAF helicopter pilots are trained, in the event of engine malfunctions (notice I didn't say failures) to concentrate on NR and contain it. Only then can the crew diagnose the problem and take the necessary further actions to control or shut down the faulty engine (handling pilot flies, non-handling pilot operates engine controls, having consulted the emergency checklist and confirmed his actions in advance with the handling pilot). In a twin engined helicopter it is critical to carry out these actions correctly and in the correct order to safeguard the NR, the transmission system and the integrity of the airframe.

IF an engine runaway up occurred in this particular case, (I'm not saying it did, but it was possible) at this critical time, causing the aircraft to enter IMC, coupled with the crew not being able to speak across the cockpit efficiently, because of a faulty intercom, a few seconds of miscommunication could easily have resulted in the accident.