What happened
During a heli-logging operation near the Stave Lake airstrip in British Columbia, a Sikorsky S-61N helicopter experienced a series of uncommanded nose-down attitude changes. The incident began while the crew was maneuvering over a logging area. The pilot, flying from the left seat, found that rearward cyclic control was ineffective during the initial upset. Although control briefly returned, a subsequent attempt at a different approach resulted in a more pronounced loss of control, accompanied by a loud, distinct bang heard by the crew.
Seeking to avoid the unstable flight regime, the pilot attempted a slow, shallow running landing at the airstrip. During the final approach, the aircraft's nose pitched down rapidly despite the pilot's efforts to apply collective pitch and rearward cyclic. The helicopter descended into trees at the end of the airstrip and rolled onto its left side. The crash resulted in one fatality and one serious injury.
The investigation
The investigation examined the aircraft's mechanical systems, the crew's experience, and the operating environment. Investigators found that the helicopter's weight and center of gravity were within permissible limits and that the weather conditions were favorable for the operation. The pilots were highly experienced, with thousands of hours in helicopters and significant experience in S-61 operations.
Technical examinations of the engines, flight controls, and hydraulic systems revealed no pre-existing mechanical defects or malfunctions that could have caused the loss of control. While a broken wire was found in a pressure switch, it was determined that this did not affect the primary hydraulic system's operation. Additionally, an incorrectly installed seal in an auxiliary hydraulic pump was identified, but it was not considered a contributing factor to the accident.
Findings
- The pilots experienced a sudden loss of rearward cyclic pitch control at an altitude too low to allow for recovery.
- The specific cause for the uncommanded nose-down attitude change could not be determined.
- The co-pilot sustained fatal injuries due to being crushed during the rollover.
- The practice of stowing shoulder harnesses to allow for better visibility during vertical reference flying was noted as a safety concern, as it limits the effectiveness of seat restraints during impacts.