What happened
On March 27, 2001, a Eurocopter SA 315B "Lama" was performing external load operations for forest liming near Birkenfeld. During the fourth flight of the day, while the pilot was lifting a lime container approximately 2 meters off the ground, a sudden vibration was felt through the tail rotor pedals. Immediately following this sensation, the tail rotor controls became ineffective, causing the helicopter to rotate uncontrollably around its vertical axis in a counter-clockwise direction. After several rotations, the main rotor struck the tops of several trees, causing the aircraft to strike the ground and come to rest on its right side. The pilot sustained light injuries, but the helicopter was destroyed, and damage was caused to the surrounding terrain.
The investigation
The BFU investigation focused on the mechanical state of the tail rotor and main gearbox. Investigators found that two of the three tail rotor blades had broken laterally, while the third was bent. The tail rotor drive shaft had buckled just behind the cabin, though no signs of high torsional stress were present. While the tail rotor gearbox was functional and contained oil, the main gearbox showed significant issues. Most of the oil from the main gearbox had leaked at the crash site and was notably discolored to a rusty brown.
Upon disassembly of the main gearbox, investigators found the interior coated in a reddish-brown sludge. The spline connection on the main rotor shaft, which drives the tail rotor, had failed; the teeth on the shaft were so heavily worn that they no longer provided a positive mechanical connection to the pinion. Notably, plastic particles, identified as a type of thread-locking adhesive (similar to Loctite), were found within the remaining splines. The maintenance facility had used this adhesive to compensate for a loose fit between the components, a practice not authorized by the manufacturer.
Findings
- The primary cause of the accident was the failure of the spline connection within the main gearbox, which led to a total loss of tail rotor drive.
- The failure was driven by wear caused by friction and likely fretting corrosion, resulting from minute relative movements between the mating surfaces.
- The use of an unauthorized adhesive to secure a loose-fitting spline connection likely contributed to the development of axial play over time.
- The design of the spline and pinion allows for a loose fit within manufacturer tolerances, which can lead to instability if the preload from the shaft nut is insufficient or compromised by contaminants.
Safety action
Following the investigation, the manufacturer issued Service Alert Bulletin No. 05.99, establishing a program for the continuous monitoring of the gearbox through radial play inspections and regular oil checks. The BFU noted that daily oil inspections remain the most effective method for detecting early signs of such wear.