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Factual Information

Summary

At about 1315 hours Eastern Daylight-saving Time on 7 December 2005, a Kawasaki Heavy Industries BK 117 B-2 (BK 117 B-2) helicopter, registered VH-IME, was being operated on a medical flight at 7,000 ft above mean sea level, in moderate to severe turbulence and in visual meteorological conditions (VMC), when the helicopter sustained an uncommanded nose-up pitch of 40° to 45°. The pilot attempted to counter the nose-up pitch by applying full forward cyclic control, but without effect. The pilot then lowered the collective control, producing a nose-down pitching moment, before recovery to normal level flight could be achieved. The Mast Moment advisory light illuminate 1 and the pilot continued the flight to the destination at reduced airspeed.

The pilot reported that, immediately prior to the nose-up pitch, the helicopter's indicated airspeed was about 120 kts, and the outside air temperature was 20° C. He indicated that the torque 2 setting was about 68 % and within the helicopter's published limits at that time, and that he felt the position of the cyclic control may have been further forward than normal.

The helicopter's estimated all up weight (AUW) was 3,200 kg and its Velocity Never Exceed 3 was estimated to be 125.5 kts. The Eurocopter Deutschland GmbH 4 (ECD) Flight Safety Department advised that that speed should have only been possible in the BK 117 B-2 helicopter when in a descent.

The pilot reported that he flew through severe turbulence just prior to the incident, but that he felt no abnormal vibrations prior to the nose-up pitch. He also stated that, during an after-flight crew de-brief, one crew member reported that the helicopter rolled right after the nose-up pitch. The pilot could not recall that roll.

The incident was subsequently investigated by an ECD test pilot and engineer, in consultation with representatives of the helicopter's Japanese manufacturer. That investigation revealed that the published procedure for setting autorotation rotor RPM in the BK 117 B-2 maintenance manual did not include for aircraft operations at high gross weight, and that the application of that procedure in the incident helicopter had resulted in an inappropriately high collective pitch setting.

The effect of the helicopter's collective pitch stop is to limit the collective travel and, in turn, establish a maximum horizontal speed (VH 5) for the ambient conditions and AUW of the helicopter. The ECD Flight Safety Department advised that the helicopter's abnormally high collective pitch setting meant that the pilot would require increased forward displacement of the cyclic control in order to maintain level flight. That would result in a reduction in the longitudinal cyclic control authority 6 that was available to the pilot.

The ECD Flight Safety Department indicated that the severe turbulence reported by the pilot could have precipitated the helicopter's nose-up pitch in this incident, and that the reduced longitudinal cyclic control authority would have restricted the pilot's ability to recover the changing pitch. Retreating blade stall 7 was considered by the ECD Flight Safety Department as an unlikely contributory factor in this instance, as it was only considered possible when the helicopter was in a descent profile.


  1. The illumination of the Mast Moment advisory light indicated that the rotor mast had exceeded its normal limits.
  2. A measure of the power provided by the helicopter's engines to overcome the resistance of the rotor blades to rotation, and maintain constant main rotor RPM.
  3. Never-exceed speed that was specified by the helicopter's manufacturer.
  4. Co-designed with Kawasaki Heavy Industries. The incident helicopter was manufactured by Kawasaki Heavy Industries.
  5. The maximum possible sustained airspeed in level flight at the helicopter's continuous engine power rating.
  6. The forward and aft limits of travel of the cyclic control.
  7. Stall of the retreating blades at high helicopter forward speeds. Occurs when the angle of the attack of the retreating blades becomes excessive, especially towards the tip of the retreating blades.
 
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