Fenestron
ShyTorque's point about control method reminds me of a phenomena Gazelle operators experienced and it was debated at great length. Not sure if the theory was ever proved true or false but here goes. There were a number of incidents/accidents? in military service were the Gazelle Fenestron was thought to have "stalled." IIRC - if the aircraft was yawed at such a rate that the airflow to the fenestron/fan overwhelmed the normal flow = no flow and no effective control of yaw. I think the incidents were mostly in hover with strongish wind and the yaw could not be stopped by opposite pedal input. Pilot diagnoses tail rotor/yaw control failure, but there was no mechanical failure of the fenestron or drive train. I think the incidents may have been put down to pilot mishandling but there were some emergency drills developed subsequently.
So - EC135 has a fenestron. Have there ever been incidents similar to those of the Gazelle? Was that Gazelle phenomena ever resolved? Is it possible the EC135 was in a high hover with a wind in the critical direction and loss of yaw control occurred? Diagnosis by pilot = mechanical yaw control failure and actions taken to shutdown engines and execute EOL from a far from ideal situation. Low NR after first impact, pause, roof collapse by which time rotors actually stopped = no obvious evidence of failure of mechanics and no evidence of NR at point when aircraft broke through roof.
Sorry it is all speculative thinking but fenestron stall does not seem to have been discussed on the thread.
Klingonbc