AS332L2 Ditching off Shetland: 23rd August 2013

[obnoxio f*ckwit]

Lonewolf,

It seems, from the report, that the crew had not "selected" an airspeed, and were operating with just VS and Localiser coupled. In this instance the vertical mode is being controlled by cyclic, snd airspeed is being controlled manually by collective. If the collective is set in the right position airspeed is maintained but if not the airspeed will change as it is the only parameter available to the ac to maintain the chosen VS (in this case, the same happens in level flight if Alt is engaged).

If the collective is set too low, ie not producing enough 'power', then the aircraft will reduce airspeed to try and maintain the desired VS. If you are only slightly out then it will stabilise at lower speed. If you're a bit more than slightly too low the airspeed will keep reducing as the system continues to try to give you what you've asked for. However, once the speed passes Vy (which is usually around 68/70kts) the rate of speed drop can be very quick, until at an airspeed none of us can remember the autopilot gives up the ghost and the upper modes drop out.

In a 225, there are protections built in such that, should for some reason you elect to fly it 3-axis, at 60(?) kts the IAS upper mode automatically kicks in and the ac becomes 4 axis coupled.

Bit long winded but I hope that helps.

{Only intended as a description of the autopilot, not as any indication that this is what happened to G-WNSB!}