Introduction
On 22 August 2002 at 0830 UTC, a Hoac Flugzeugwerke DV20 Katana, registration G-BWLV, was conducting a training flight near Cranfield Airport. The aircraft was powered by a single Rotax 912-A3 piston engine and had been manufactured in 1996. The flight had two crew members on board, no passengers, and no injuries were reported. The aircraft sustained a collapsed undercarriage and significant wing damage.
Accident Details
The instructor reported that three circuits were completed without problems. During the climb-out for a fourth circuit at approximately 400 feet QFE, a smell of smoke was noticed and initially attributed to a nearby brick-works. White smoke then filled the cockpit as engine power reduced and began to fluctuate. The instructor took control, transmitted a MAYDAY call, and selected a grass field for a forced landing. A successful touchdown with one stage of flap was made at an estimated speed of 65 kts, 20 to 30 metres into the field. Braking was ineffective due to dew, so the instructor aimed for a gap in the fence. The right wing tip struck a fence-post, spinning the aircraft 140 degrees, after which it ran backwards through a wire fence and came to rest in a ditch alongside a cornfield.
Two days earlier, the instructor had rejected the aircraft after finding it covered in oil with no oil visible on the dipstick. A local maintenance company cleaned the external oil and replenished the engine contents. A subsequent circuit flight reported no problems. On the day of the accident, the instructor performed an 'A' check and noted an oil level of 2 3/4 quarts, ensuring no oil was on the underside. After the accident, the dipstick showed no oil, and an oily film emanated from the engine cowling vents.
Examination
The aircraft was dismantled and transported back to the airfield. After oil replenishment, an engine run was attempted. The engine started and ran, but oil leaked rapidly from the base of the No 3 cylinder push-rod tube. Removal of the cylinder revealed a damaged rubber O-ring seal. Detailed examination found that an area of the seal had been cut away, consistent with being trapped by the sharp lip of the lower edge of the push-rod tube during installation. A tensile failure had occurred across the reduced cross section. The No 3 cylinder head was one of three removed and refitted during recent maintenance.
Maintenance Practices
The Maintenance Manual emphasizes care when inserting the push-rod tube with the O-ring seal. Information from an overhaulier indicated that these seals are soft and easily cut if the tube is not installed accurately. The O-ring is compressed between a ridge on the tube and a ledge in the crankcase recess, with limited support length, making it easily dislodged during installation. Once installed, damage to the O-ring is not visible.
Seal Behaviour Assessment
The extent of leakage from the damaged seal would depend on residual tightness, end loading from the push-rod tube ridge, and whether the tensile failure occurred at the time of leakage. Expansion and contraction of engine metal components during operating cycles could alter fit and end-load, potentially opening or closing the gap from the failure. Consequently, a damaged seal that functioned under one condition could leak when engine temperature and temperature distribution changed.
Reason for Power Loss
It was not possible to determine why the engine lost power in flight yet ran satisfactorily on the ground after oil replenishment (albeit with a severe oil leak). One possibility is that reduced oil contents caused local overheating leading to partial seizure, which may have eased after cooling and correct lubrication. Without direct evidence, this could not be confirmed.
Discussion
Although the precise mechanism for the power reduction could not be determined, the AAIB concluded that it was most probably a consequence of oil loss caused by the O-ring seal being damaged during maintenance.