On June 20, 1989, a DE Havilland DHC-3 (registration N41755) operated by Woods Air Service was involved in an aviation accident near Palmer, AK. Investigators recorded the probable cause as: Inadequate amount of lubricating grease and excessive part wear in the horizontal stabilizer's trim actuator jack screw. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
An aircraft experienced an abrupt nose-down pitch after an inflight failure of the horizontal stabilizer's trim actuator jack screw. The crew landed successfully by applying back pressure and nearly full engine power. Investigation revealed inadequate lubrication and excessive part wear.
Event
An airplane was in flight when it experienced an abrupt nose-down pitch. This was caused by an inflight failure of the horizontal stabilizer's trim actuator jack screw. The crew responded by exerting a large amount of back pressure on the control wheel and adding nearly full engine power. Through these actions, the crew was able to maintain control of the airplane and make a successful landing.
Examination
Following the incident, an examination of the actuator was conducted. The examination revealed an inadequate amount of lubricating grease and excessive part wear. The jack screw, a component of the actuator, had suffered from insufficient lubrication and subsequent wear.
Crew Actions
The crew's prompt and effective actions were key to the safe outcome. They applied sustained back pressure on the control wheel and increased engine power to nearly full, which compensated for the loss of trim authority and allowed them to fly the aircraft to a landing.
Probable cause
Inadequate amount of lubricating grease and excessive part wear in the horizontal stabilizer's trim actuator jack screw.