On November 3, 1988, a Bell 47G2 (registration N5163B) operated by Pacific Aero Services was involved in an aviation accident near Phoenix, AZ. Investigators recorded the probable cause as: The pilot's inadvertent control of the helicopter during engine run-ups, which resulted in an unintended ascent and subsequent collision with electrical cables. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter inadvertently lifted off the ground and struck electrical cables while maneuvering at low altitude during post-maintenance engine run-ups.
What happened
The incident occurred while a mechanic was conducting engine run-ups on a helicopter that had recently undergone major maintenance. During this phase of the operation, the aircraft inadvertently became airborne, ascending to an altitude of approximately 100 feet above the ground. While at this height, the helicopter executed two turns utilizing a sideward skid maneuver. As the aircraft descended back toward the surface, it collided with electrical cables before impacting the terrain.
The investigation
A thorough examination was conducted to determine the cause of the accident. Investigators found no evidence of any mechanical failure during the inspection process. The absence of mechanical defects suggests that the loss of control or unintended ascent was not due to a pre-existing fault in the aircraft's systems prior to the run-up.
Findings
The primary factor contributing to this event was the inadvertent lift-off during ground operations. The combination of engine power application and control inputs resulted in an uncommanded ascent. The subsequent collision with overhead electrical infrastructure highlights the risks associated with low-altitude maneuvers near obstacles during maintenance activities.
Probable cause
The pilot's inadvertent control of the helicopter during engine run-ups, which resulted in an unintended ascent and subsequent collision with electrical cables.