What happened
On 22 August 2002, a Hoac Flugzeugwerke DV20 Katana, registration G-BWLV, was conducting a training flight near Cranfield Airport. After completing three successful circuits, the aircraft was on the climb-out for a fourth circuit at approximately 400 feet when white smoke began to fill the cockpit. Simultaneously, the engine power fluctuated and decreased.
Upon noticing the smoke and power loss, the instructor took control and declared a MAYDAY. Unable to return to the airfield, the instructor identified a nearby grass field for an emergency landing. The aircraft touched down approximately 20 to 30 metres into the field at an estimated speed of 65 knots. During the landing roll, the instructor attempted to brake hard, but the presence of dew made the brakes ineffective. While attempting to steer through a gap in a fence to reach a cornfield, the right wing tip struck a fence post. This impact caused the aircraft to spin 140 degrees and run backwards through a wire fence before coming to rest in a ditch. There were no injuries to the two crew members on board.
The investigation
The investigation focused on the engine's failure and the source of the smoke. Prior to the accident, the aircraft had been flagged for an oil leak two days earlier, which had been addressed by a maintenance company. During the post-accident examination, the engine was found to be leaking oil rapidly from the base of a No 3 cylinder push-rod tube.
Upon removing the cylinder, investigators discovered that the flexible rubber O-ring seal on the push-rod tube had been damaged. A portion of the seal had been cut away, which was consistent with the seal being pinched by the sharp lower edge of the push-rod tube during installation. It was noted that the No 3 cylinder head had been one of three recently removed and refitted during maintenance.
Findings
- The engine power loss was most likely caused by oil loss resulting from a damaged O-ring seal.
- The damage to the rubber seal occurred when the push-rod tube was installed, as the seal was caught by a sharp metal edge.
- The investigation could not definitively confirm the exact mechanism of power loss, but suggested that reduced oil levels may have caused local overheating or a partial seizure of internal components.
- The maintenance manual specifically warns that these seals are soft and easily damaged during the installation of the push-rod tube.