What happened
While performing heli-logging support functions near Adams Lake, British Columbia, the pilot of an MD 369D helicopter, registration C-GHFA, noticed resistance in the collective at high torque settings. This was accompanied by significant vertical vibration and a noticeable misalignment in the main rotor blade track. The pilot successfully completed the scheduled crew and material transport tasks and landed the aircraft safely.
Upon inspection by company maintenance personnel, a crack was discovered on the underside of one of the five main rotor blades. The damaged blade had accumulated 2981.1 total hours since new.
The investigation
The investigation examined the structural integrity of the rotor blades and the maintenance history of the aircraft. It was noted that the blades had been experiencing tracking issues for some time, requiring frequent adjustments by maintenance personnel.
Technical analysis of the failed blade revealed a 4.5-inch crack on the exterior lower skin, which ran from the aft edge of the spar to the trailing edge. Further sectioning of the blade showed a longitudinal crack in the "V" channel at the trailing edge. The investigation also looked into the manufacturing and inspection processes, noting that the manufacturer's recommended method for inspecting the "C" channel lightening holes involved a manual finger swipe, which may be insufficient for detecting small surface irregularities.
Findings
- The main rotor blade failed due to metal fatigue originating from surface pits or imperfections on the "C" channel lightening hole.
- These surface imperfections acted as stress risers, causing a crack that eventually propagated through to the blade's lower skin.
- The structural integrity of the blade was compromised by these pits even before the part was surface treated.
- The existing inspection method—a manual finger swipe—was inadequate for detecting the small imperfections that led to the fatigue.
- The aircraft had been parked with only one blade secured by a tie-down cord approximately one week prior, which may have subjected the unsecured blades to unbalanced bending loads due to wind.
- There were no established documented standards for accepting material flaws in this specific part of the blade, meaning the crack rendered the component unairworthy.