Originally Posted by
cyclic
It was common practice to fly the L2 in 3 axis as the IAS hold was quite poor. To fly 4 axis, you had to be very gentle with any speed changes or the aircraft would exchange altitude for speed. During a 200’ ARA this could be quite exciting. There’s no issue flying 3 axis as long as you monitor and apply power as the aircraft levels. Have I ever had to remind the PF to raise the lever in 3 axis...yes.
Not wanting to dispute the above but the AS332L2 deserves a bit more credit than this. ALL helicopters have a the same Power-Required for Level Flight Curve. This curve reverses as we pass through Vy!
If flying in 3 axis, with ALT hold engaged and the power setting is insufficient for level flight at Vy, the venerable AP will continue to raise the nose, speed decaying, to maintain the commanded altitude ([ALT]
If the power has been set low enough (paradoxically), the speed will decay through Vy. Now things get really exciting as the power demand for level flight increase exponentially as each knot of IAS is lost.
The AP keeps raising the nose in a departed effort to maintain the commanded ALT. Eventually a ROD will set in, now at low IAS, and if the crew do not intervene quickly enough and/or the surface is not far below the helicopter, a powered recovery may simply not be possible in time to prevent impact with the surface.
NOTE, the IAS and the ALT hold on the AS332 is more than competent. (as actually demonstrated by this accident). However, in 3 axis mode, power set below that required for level flight at Vy in any part of the envelope will result in a descent, required or otherwise.
This problem should be the No1 lesson for all 3 axis systems and 4 axis systems when operated in 3 axis.
As an aside, AIRBUS APM 2000 (EC225/175/145/135) incorporates flight envelope protections to "save" the crew/helicopter from these potentially disastrous mistakes.