What happened
On February 7, 2017, a Schweizer Aircraft Corp. 269C helicopter, registration HB-XFQ, was conducting a mountain flight training mission near Locarno, Switzerland. The flight, operated by a flight instructor and a student, involved several approaches to various landing sites. During the second leg of the training, while cruising at approximately 3,500 ft AMSL near Losone, the student pilot noticed a faint smell of oil.
Upon inspection, the instructor observed that the engine oil temperature had reached its maximum reading while the oil pressure had dropped to its minimum. Recognizing the imminent risk of engine failure, the instructor took control of the aircraft and initiated an autorotative descent. The pilot successfully performed a precautionary landing on the paved runway of the former Ascona airfield at 11:16 AM. There were no injuries to the two occupants, though the engine sustained damage.
The investigation
SUST investigators examined the engine and the aircraft's maintenance history. The investigation revealed that the engine's lubrication system was heavily contaminated with metal particles. Specifically, the oil suction screen and filter contained significant amounts of metal debris, along with a piece of gasket material.
Detailed teardown of the engine showed that piston #3 was severely damaged. The area around the piston pin had suffered significant erosion and breakage of the piston rings and the piston web. Investigators found that the piston pin had shifted 10 mm from its original position. Furthermore, the investigation noted that the engine had been stationary for seven months without being preserved, contrary to manufacturer recommendations.
Findings
- The primary cause of the engine malfunction was corrosion-related pre-damage to the engine, which went undetected during routine maintenance.
- The lack of engine preservation during a seven-month period of inactivity led to microscopic material breakouts (silicon and aluminum oxide) within the cylinder.
- These particles mixed with the lubricating oil to create an abrasive sludge, which increased friction and temperature, eventually causing the destruction of the piston pin end plugs and the piston rings.
- Maintenance gaps were identified, including the failure to perform recommended engine preservation and the lack of recent oil analysis or compression tests that might have detected the degradation.
- The engine's technical records were noted to be incomplete, making it difficult to track certain maintenance details and parts lists.