What happened
During the flight, the pilot heard a muffled explosion originating from the engine compartment. Following this sound, the engine ceased operation and flames began emitting from the engine cowling. The cabin subsequently filled with smoke, and oil began spraying onto the windscreen.
The pilot successfully executed the required procedures for managing an in-flight fire and cockpit smoke. To reach a suitable landing area and avoid obstructions, the pilot delayed the extension of the landing gear. Because the gear did not have sufficient time to fully extend before touchdown, the landing gear collapsed upon landing in a field. The pilot was not injured.
The investigation
An examination of the aircraft revealed that the dual magneto had separated from the engine. Investigators located the bottom magneto hold down clip within the cowling; however, the nut and lockwasher were not recovered.
Inspection of the bottom surface of the magneto housing showed two longitudinal markings where paint had chipped. The spacing of these marks matched the position of the bottom clip, suggesting the clip had moved longitudinally relative to the housing. Additionally, an examination of the top magneto housing flange, which had separated from the magneto, revealed fracture surface markings indicating fatigue cracking occurred prior to the overstress separation.