What happened
During a training flight near Pitt Meadows Airport, British Columbia, a Eurocopter AS 350-B3 (registration N530NA) was performing longline operations near the top of a tree. The pilot-in-command, seated in the left seat, was demonstrating maneuvers to a trainee pilot. While hovering, the helicopter began to descend. The pilot attempted to increase power by raising the collective, but the engine failed to provide the necessary thrust.
As the aircraft continued to lose altitude, the pilot maneuvered toward a clearing and attempted to slow the descent by pulling the collective up. However, the helicopter struck the ground heavily. The impact caused damage to the main rotor blades, the skid gear, and the belly panels, and resulted in the collapse of the right-side pilot seat base. The trainee pilot sustained a minor injury, while the pilot-in-command was uninjured. There was no fire following the impact.
The investigation
The investigation focused on why the engine could not meet the power demand and why the crew was unable to mitigate the descent. Investigators examined the engine's digital engine control unit (DECU) and found evidence of a major governor failure, specifically a fuel valve or stepper motor failure. A sliver of O-ring material was discovered in the hydro-mechanical metering unit, which may have obstructed the fuel metering valve and caused a fixed fuel flow condition.
Furthermore, the investigation looked into the cockpit configuration. The aircraft was equipped with an intercom system where the audio warning for a red governor light was not enabled for the left seat. Additionally, the left-seat controls lacked the specific throttle lock release mechanism found on the right-seat controls, which is required to manually operate the throttle during such an emergency.
Findings
- The primary cause of the descent was the fuel control unit's inability to deliver increased fuel flow, likely due to an O-ring fragment obstructing the fuel metering valve.
- The pilot-in-command was unable to initiate manual throttle control because the left-seat controls lacked a release mechanism for the throttle lock.
- The crew's ability to respond was further hindered because both pilots were looking outside to monitor the longline and did not observe the visual warning light, and the pilot-in-command did not receive the accompanying aural warning due to the cockpit's audio configuration.
- The crew had not conducted a detailed briefing regarding specific duties and coordination in the event of an emergency.