Accident Overview
On 4 February 1998, a Schweizer Aircraft Corp. 269C helicopter, registration HB-XJD, was involved in an accident at Gruyères aerodrome during a pilot check flight. The accident occurred at approximately 1615 local time. The helicopter sustained substantial damage after an engine failure and subsequent nose-over.
Aircraft and Crew
The helicopter was privately owned and operated. The crew consisted of an instructor pilot, a Swiss national born in 1956, who held a professional helicopter pilot license with 7766 total flight hours (242 hours on type). The other pilot, a Swiss national born in 1973, held a private pilot license with 72 total flight hours (53 hours on type). Both pilots were uninjured or slightly injured in the accident.
Sequence of Events
Earlier that morning, a mechanic performed the 25-hour inspection after a new engine's break-in period, which included an oil change and filter replacement. The mechanic drained the oil, removed the filters, and installed new ones. He was convinced he tightened the oil filter using a torque wrench, but later inspection showed the filter was not torqued to the prescribed value. The mechanic omitted the prescribed leak-run (ground engine test to check for leaks) due to time constraints and did not inform the pilots.
The instructor intended to conduct a check flight with the other pilot. The pilots performed pre-flight checks, including a positive oil pressure check during engine start. After an initial takeoff and landing, they took off again. At about 100 meters above ground and 50 knots, a hammering noise from the engine became audible. The instructor removed his headset to identify the source and noted the oil pressure gauge indicated zero.
The instructor took control, deciding to turn back due to sloped terrain ahead. At that moment, the engine stopped. The helicopter entered autorotation at approximately 45 knots. The instructor flared and landed the helicopter in a slide on snow-covered ground. The right skid struck a molehill, causing the skid to break and the helicopter to nose over.
Maintenance Findings
Investigation revealed that the oil filter had been installed with the correct safety wire but was not torqued to specification. Oil stains were found at the initial takeoff point and at the landing/takeoff site in the snow, indicating a leak during ground operations. The mechanic's failure to perform the leak-run and to inform the crew of this omission prevented detection of the leak before flight.
Probable Cause
The accident was due to an engine failure at low altitude resulting from an incomplete maintenance operation, followed by an autorotation and a sliding landing on uneven, snow-covered terrain.