What happened
On the evening of the accident, an MD 369D helicopter was performing a transport mission in the Jordan River area of British Columbia. After a day of moving cedar shake blocks, the pilot departed for Lake Cowichan at approximately 1715 local time. During the flight, the aircraft struck mountainous terrain in the Mt. Modeste area, resulting in one fatality.
Following the failure to close the flight plan by 1745, a search was initiated. An emergency locator transmitter signal led rescuers to the wreckage, which was found in a heavily forested, mountainous region. The aircraft's tail section had separated from the main fuselage during the flight. The impact was severe enough to crush the fuselage and scatter debris up to 500 feet from the main site, though no fire occurred at the scene.
The investigation
Investigators examined the wreckage and the condition of the five main-rotor blades. They observed significant damage, including a clean break on the red blade approximately one-third of the distance from its root. The white blade was entirely missing, and the yellow blade showed damage from striking the tail section. The damage to the lead and lag dampers and the distribution of debris suggested the aircraft experienced extreme vibrations and lost attitude control in flight.
Technical analysis focused on the structural integrity of the rotor blades. It was determined that the loss of the tail section likely occurred because the extreme vibrations from the unbalanced rotor system either compromised the tail boom's strength or caused the blades to strike the tail section directly. The investigation also looked into the manufacturing history of the blades, noting that certain older blades had not undergone tap inspections.
Findings
- The primary cause of the accident was the in-flight failure of a main-rotor blade due to fatigue.
- A lack of proper bonding between the blade skin and the spar allowed corrosion pits to develop.
- A fatigue crack originated from one of these pits and propagated through the blade, leading to the structural failure.
- The crack was not visible during a standard visual inspection because it remained hidden beneath the blade skin.
- The resulting imbalance caused the loss of the tail section and rendered the aircraft uncontrollable.