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Summary

Summary

On 7 February, 2013 at about 1655 Eastern Daylight-saving Time a Eurocopter AS350 B2 helicopter, registered VH-EWM (EWM) was conducting water-bombing operations near Hobart, Tasmania, when it collided with terrain. The pilot, the sole person on board, suffered minor injuries and the helicopter sustained substantial damage.

The spot fire EWM was working on was not particularly large, but was on a downhill slope and in a gully. The pilot reported that the overall wind was north-north-westerly, but the fire created a localised westerly in-draft, within the gully. The pilot slowed EWM in preparation of making a water drop. Approaching the hover at about 80 ft above ground level, and immediately following the loss of translational lift (TL), the helicopter suddenly commenced an uncommanded left yaw and descent. Without any warnings or alarms, the helicopter rotated rapidly 2-3 times to the left. The pilot raised the collective to decrease the rate of descent, and countered the yaw with anti-torque pedal input; however the rate of yaw increased. The pilot reported that “in a very short period of time” the helicopter was in the trees. The pilot received minor injuries and the helicopter was substantially damaged.

As the ATSB did not attend the accident site, or examine the helicopter, the reason for the accident could not be conclusively established.  The described behaviour of the helicopter by the pilot was consistent with Loss of tail rotor Effectiveness (LTE). In this condition of flight, the tail rotor loses aerodynamic efficiency. Factors which contribute are:

  • Low airspeed
  • High power
  • An adverse relative wind

Water-bombing helicopters operate at very low altitudes, in very challenging and often rapidly changing conditions. Any sudden onset of an abnormal condition of flight presents negligible time for recovery.

Eurocopter circulated Service Letter No 1673-67-04 in 2005 regarding the yaw axis control features for all helicopters under certain flight conditions.

 

Aviation Short Investigation Bulletin Issue 20

 
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