What happened
On a visual flight rules mission from a hunting camp on the South Nahanni River to Fort Simpson, Northwest Territories, a McDonnell Douglas 369HS helicopter, registration C-FCVV, experienced a critical engine failure. During the flight, the pilot noticed the fuel quantity gauge was displaying a much higher level than expected for the duration of the flight. To ensure a safe landing area was available, the pilot began following roads and cut lines toward the destination.
As the aircraft approached the Fort Simpson landing pad at approximately 1900 local time, the pilot initiated a left turn at an altitude of 100 to 200 feet. During this maneuver, the engine flamed out. The pilot attempted an autorotation to reach a secondary road, but the helicopter struck trees and crashed. The accident resulted in one fatality and one serious injury.
The investigation
Investigators examined the wreckage and the aircraft's fuel systems. While the main fuel cells were found to be wrinkled and partially collapsed, the auxiliary fuel cell still contained 132.5 pounds of fuel. The investigation focused on why the engine lost power despite the presence of fuel in the auxiliary system.
Technical analysis revealed that the aircraft's fuel vent system had been partially modified following the installation of an external cargo pod. The original vent fairing had been removed and replaced with a drain spigot that reduced the vent's diameter. Crucially, the required alternate vent system had not been installed. Furthermore, the pilot had not activated the auxiliary fuel valve to transfer fuel into the main cells as recommended by the flight manual when the main tank reached 200 pounds.
Findings
- The engine flamed out due to fuel starvation because the pilot did not open the auxiliary fuel valve to transfer fuel into the main cells during the flight.
- A partial collapse of the left fuel cell, likely caused by negative pressure due to an incomplete vent modification, restricted the movement of the fuel sensor arm.
- This restriction caused the fuel quantity gauge to show an erroneously high reading and prevented the FUEL LOW caution light from illuminating.
- The auxiliary fuel cell lacked its own quantity indicator, preventing the pilot from verifying the actual fuel level in flight.
- The aircraft's weight and balance records had not been updated to account for a 30-pound external cargo rack.