Amateur-built Lightning N62JV accident near Marana, Arizona

Probable cause

The insufficient clamping force resulting in a fatigue fracture of the propeller extension attachment bolts and subsequent separation of the propeller assembly in flight. Contributing to the accident was the pilot's failure to maintain sufficient airspeed to avoid an inadvertent stall while maneuvering during the emergency approach to the airport.

NTSB analysis

While maneuvering adjacent to a private airstrip at a low altitude, witnesses reported observing the propeller assembly separate from the airplane. The pilot maneuvered towards the airstrip and leveled out on runway heading about mid-length of the runway. As the airplane crossed over the end of the runway, it banked to the left and descended into terrain, impacting an open desert field adjacent to several residential homes. Examination of the airplane revealed that the airframe and both wings were structurally damaged. The propeller assembly, including the propeller flange extension were found separated from the engine crankshaft. No further anomalies were noted with the airframe and engine that would have precluded normal operation. The attachment bolts were installed on a painted surface of the propeller flange extension. The paint surrounding this area was flaking away from the surface. Evidence of thread locking material was observed in the threaded areas consistent with installation instructions. Examination of the attachment bolts revealed that four of the six bolts exhibited fracture surfaces consistent with fatigue. One of the four bolts exhibited a multifaceted fracture surface with multiple origins around the circumference. Hardness of this bolt was checked and found to be within the specified hardness range.

Text from the NTSB. Open the NTSB docket for SEA08LA149 for the full record.