What happened
The incident occurred during an engine start operation when the pilot reported overpriming the right engine. This action led to an accumulation of fuel vapor in the area outboard of the right engine. The accumulated vapor subsequently exploded, resulting in significant structural damage to the aircraft's wing.
Specifically, the force of the explosion ripped open a portion of the right wing's upper skin panels that was approximately one yard in length. Evidence indicated that there was an uncorrected preexisting fuel leak located near the right engine's auxiliary fuel pump, which likely contributed to the presence of excess fuel vapor.
The investigation
Investigation findings confirmed the physical damage described above. Investigators observed clear signs of the explosion on the upper surface of the right wing. Additionally, evidence was found pointing to a preexisting fuel leak in the vicinity of the right engine's auxiliary fuel pump. This leak had not been corrected prior to the incident.
Findings
The pilot reported that he held an A and P mechanic certificate. He also stated that he owned, operated, and maintained the airplane himself. The combination of the uncorrected fuel leak and the overpriming during start-up created the conditions for the vapor explosion.