AAIB Observations:
1. The helicopter struck the flat roof of the single story building with a high rate of descent and low/negligible forward speed.
2. At the time of the impact [..] neither the main rotor nor the fenestron tail rotor were rotating.
3. Initial assessment [..] indicated that the main rotor gearbox was capable of providing drive [..] to the main rotor and to the fenestron drive shaft.
The bulletin also states that the no.2 engine was capable of delivering power and that this may have been the case for the no.1 engine but that this has not yet been verified due to impact damage.
So, if the main gearbox and engine or engines were capable of functioning, just what could cause such a rapid depletion of main rotor rpm?
