On most helis it's normally airspeed, sometimes tied in with radalt as on the S-76.
Collective position isn't too useful or there would be a false warning on entry to autorotation. Also the position of the collective is raised as translational lift is lost on the approach so it wouldn't necessarily trigger just where and when it was needed.
[ 15 January 2002: Message edited by: ShyTorque ]</p>
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