What happened
Shortly after departing the Nova Scotia Department of Natural Resources helicopter base, a helicopter carrying a pilot and three passengers experienced a sudden engine failure. Approximately two minutes into the flight, while at an altitude of 300 feet, a loud bang was heard, followed by the activation of the engine out warning systems.
In an attempt to reach a road through a forested area, the pilot initiated an autorotation and extended the glide. During this maneuver, the main rotor RPM decayed. The helicopter struck a road with significant force, bounced, and drifted slightly before coming to rest. The impact caused the tail boom to be severed by the main rotor blades. There were no injuries to the four occupants.
The investigation
The investigation focused on the maintenance history of the aircraft, which was undergoing a 300-hour inspection. A change in maintenance personnel occurred mid-inspection when the original engineer was reassigned to forest fire duties. The replacement engineer completed the remaining tasks, including inspecting the engine inlet area.
Investigators discovered that a cardboard tube, used as a makeshift aid to prop open the engine by-pass door, had been left inside the engine inlet. This tube was found lodged against the compressor inlet following the accident. The investigation also looked into the lack of formal procedures for tool accountability and the low altitude of the test flight.
Findings
- The engine flameout was caused by the cardboard tube partially blocking the compressor inlet.
- The inspection aid used was a non-standard cardboard tube that lacked any bright flagging to make it visible.
- A change in the maintenance engineer assigned to the task contributed to the oversight.
- The second engineer failed to detect the tube during a visual foreign object inspection.
- The operator lacked a formal procedure to ensure all tools and aids were accounted for after an inspection.
- The low altitude of the flight (300 feet) reduced the pilot's options for a safe landing after the power loss.
- The pilot's decision to extend the glide to reach a road led to the decay of rotor RPM, resulting in the hard landing.
Safety action
Following the investigation, the operator implemented several safety improvements, including:
- Manufacturing a specialized tool with a 5-foot red flag to hold the by-pass door open.
- Amending the Maintenance Control Manual to require independent foreign object inspections by two engineers after maintenance actions.
- Updating work sheets to include specific sign-off sections for foreign object inspections.
- Prioritizing the creation of a tool shadow board to improve tool control.
- Briefing all maintenance staff on the importance of attentiveness and encouraging pilots to perform their own inspections.