The problem is rather neatly summed up by this rather technical explanation of the previous 139 accident in Kenya
(that makes 50% of their fleet destroyed?)
"
To avoid the bird, the pilot tried to manually make the chopper rise using the lever known as the collective. However, this was impossible in autopilot mode where the aircraft cannot move up and down. It reportedly turned sideways and stalled.
A vortex was created, sucking the chopper in and down."Hovering out of ground effect, or HOGE, is an aviation term describing when a chopper appears stationary in the air. It generally requires more power than if it is on the ground. It is the act of the pilot pulling the collective that led the helicopter to lose power, making it fall to the ground," the source said."In technical terms, the rotors should spin at 110 percent its normal rotations per minute. If it drops to 98 percent,alarms will go off, saying 'rotor low, rotor low'. When this helicopter crashed, it was down to 96 percent RPMs. You could clearly hear the warnings in the voice recorder and the panicked engineer directing Oduk on what to do," the source said."