If Low RPM has to be a factor and it is in the requirements, how do we get to it.
Robinson tell us that the magic figure to avoid Rotor stall in the R22 is 80% [+1% per 1000ft] so to get the Low RPM at impact has the rotor system stalled ? Low RPM coud be anything out of the Green Arc, so the question seems a bit ambiguous at this point.
Like has been said before
Carb Ice causing the engine to stop is the obvious favourite.
A couple of new ideas.
1] Unlikely as you would hope this would be mentioned in the relevant fact but a drive Belt Failure, at the moment of failure this would lead the engine to overspeed (some will say the governor will contain it, but I am lead to believe it wont) but the rotors will react as though the engine has failed as they are no longer being driven.
2] Governor switched off (or failed) and and an abrupt (probably) forward cyclic input made, which in some (not all) phases of flight will cause an abrupt drop in RRPM.
3] A hovering demonstration at altitude (which would be consistent with the low forward speed at impact), with a high power setting overpitching (or a genuine engine problem) at this point could stall the rotor with the given outcome.
4] An Autorotation demonstration with a faulty Sprag Clutch, would prevent the rotor system from freewheeling.
Any of these would be componded by an inexperienced instructor (inexperienced as an instructor) who will have his attention divided between the act of flying, allowing his student to take the controls and talking about it all at the same time, this takes a while to get used to (longer that the course normally takes).
Initial or Demo Flights can be fatal
Have a look at
Robinson Helicopter Company Service Bulletins and Letters Page
Helicopter Safety | Publications in particular the link to the NTSB reports on loss of R22 Main Rotor control.
and
Helicopter Safety | Carb Icing the CASA carb ice link is broken but will be fixed by about 1000Z today.