SAS The justification for reference to Nr as a priority after an OEI event is born from the AAIB report para below. If the collective is not lowered quickly enough and the NR decays to the figure indicated with a High ROD, a deep stall occurs and the rotor stops very quickly. The OEM testing indicated this can happen in around 2 seconds when the conditions have been met. Its scary and it is the major lesson from this accident. Our teaching practices are expressly aimed at providing a strategy to prevent this from happening whilst at the same time building resilience to overcome the startle effect. The simpler the response the more likely the pilot will follow. "OEI - Look at the Nr, if it remains stable after 1 minute, you can start to breathe again. If it remains stable after 5 minutes, you can allow you attention to go elsewhere momentarily"
Main and tail rotor system
Total loss of rotational speed of the rotor system during the subsequent unpowered descent is consistent with a higher than optimum main rotor blade pitch being applied and maintained during some part of the descent.
The physical damage identified on the main rotor blades was typical of lead-lag resonance. This occurs when the main rotor speed decays through the region of 60%-70% Nr. and can be brought about when significant main rotor blade pitch is applied to an unpowered rotor system.
Evidence of blade coning during lead-lag resonance also confirmed that the latter occurred before impact. Ear-witness evidence was also consistent with lead-lag resonance occurring whilst the helicopter was still airborne. Lead-lag resonance in flight occurs at a rotor speed below the minimum Nr (75% Nr) from which it is irrecoverable in autorotation.