What happened
On February 4, 1998, a Schweizer Aircraft Corp. 269C helicopter, registration HB-XJD, was performing a pilot check flight at the Gruyères aerodrome. The flight involved an instructor and a student pilot. Following an initial takeoff, the instructor instructed the pilot to land a short distance away because the intake pressure had not been checked. During a subsequent takeoff, while climbing at approximately 100 meters above ground level and maintaining a speed of 50 knots, a heavy hammering noise was heard from the engine. Upon checking the instruments, the instructor discovered that the oil pressure had dropped to zero.
As the instructor attempted to turn the aircraft away from sloping terrain, the engine failed. The crew initiated an autorotation at approximately 45 knots. The instructor managed to execute a landing in a skid across a flat, snow-covered area. However, the right skid struck a molehill, causing the aircraft to pitch forward and capsize.
The investigation
The investigation focused on the maintenance performed earlier that morning. A mechanic had been tasked with a 25-hour inspection following an engine break-in period, which included changing the engine oil and oil filters. While the mechanic confirmed that the new filters were installed, they admitted that the required post-maintenance leak run was omitted due to time constraints. The flight crew was not notified that this check had been skipped.
Physical evidence at the scene included an oil puddle at the site of the first takeoff and oil traces in the snow near the landing site. Inspection of the wreckage revealed that while the oil filter was correctly positioned with its safety wire, it had not been tightened to the prescribed torque.
Findings
- The primary cause of the engine failure was a lack of lubrication resulting from an oil leak.
- The mechanic failed to tighten the oil filter to the required torque during the 25-hour inspection.
- The maintenance procedure was left incomplete as the required leak run was not performed.
- The flight crew had no way of detecting the leak before the oil pressure reached zero.
- The engine seized within seconds of the pressure drop due to the lack of lubrication while operating at full power.
Safety action
- The investigation noted that the mechanic's failure to tighten the filter was likely due to routine-induced error.
- Two opportunities to prevent the accident were missed: the failure to complete the leak run and the failure to inform the crew that the maintenance task was incomplete.