What happened
While operating near Port Hardy, British Columbia, the pilot of an MD Helicopters 369D, registration C-GHFL, noticed significant vibrations and a loss of rotor track. The condition worsened until one of the five main-rotor blades appeared to be approximately six inches out of track, at which point the pilot performed an emergency landing. The aircraft had been stationed at the operator's Eclipse Camp, where it had recently been subjected to high winds. Due to these weather conditions, the blades had been removed and reinstalled shortly before the flight.
The investigation
Investigators examined the damaged rotor blade and identified a manufacturing defect located within the C-channel at the radius of a lightening hole. This defect, an inclusion or nick, acted as a stress riser that initiated a fatigue crack. The crack propagated through the blade's lower skin and trailing edge, eventually reaching the top skin near the blade spar.
Further investigation into maintenance practices revealed that a 300-hour inspection performed two days prior to the event had failed to detect the damage. The investigation also looked into the tracking of "torque events"—high-stress maneuvers such as transitions from forward flight to a hover. It was noted that the frequency of these events is difficult to track accurately because it relies on manual pilot logs and varies significantly based on the type of operation performed.
Findings
- The structural integrity of the main-rotor blade was compromised by a manufacturing defect at the edge of an internal lightening hole.
- This defect caused a fatigue crack to spread through the blade's internal structure, lower skin, and trailing edge.
- The accumulation of fatigue damage may be difficult to monitor because the number of high-stress torque events is not precisely recorded by all operators.
- Existing inspection intervals may be insufficient to detect rapidly propagating cracks before they lead to component failure.