What happened
While climbing through 3,500 feet following departure, the pilot of the aircraft heard a series of loud bangs. This was immediately followed by significant vibrations throughout the airframe. In response to the engine malfunction, the pilot retarded the throttle to idle and initiated a return to the departure airport.
Unable to reach the active runway, the pilot elected to perform an emergency landing on a grass strip located within the airport grounds. During the landing sequence, the pilot attempted to decelerate the aircraft before impacting a ditch. The aircraft nosed over at 40 knots and came to rest in an inverted position. There were no injuries reported in the accident.
The investigation
Mechanical examination of the engine focused on the cylinder assembly. Investigators discovered that the #4 cylinder head had separated from its cylinder barrel. This separation was caused by fatigue cracking that originated within the cylinder head threads.
The engine had undergone an overhaul 111.5 hours prior to the accident, during which time 6-channel chromed overhauled cylinders were installed. Due to a lack of regulatory requirements for tracking specific cylinder hours or overhaul frequencies, investigators could not determine the total accumulated hours or the previous overhaul history of the cylinders involved.