Amateur-built RV-7 N17NK accident near Morgan, Utah

Probable cause

The pilot/builder improperly torqued the propeller attachment bolts while reinstalling the propeller, which resulted in the fatigue failure of the bolts and the separation of the propeller assembly.

NTSB analysis

The pilot reported he noticed an odor of burning oil shortly after takeoff. About a minute later, the airplane began to shake, oil covered the windscreen, and the propeller separated from the crankshaft. The pilot performed a forced landing in a snowy field, and during the landing roll the airplane nosed over and came to rest inverted. The propeller assembly was found in a field about two miles away.

The most recent condition inspection was completed by the pilot one week before the accident flight, and the airplane had accrued three flight hours since the inspection. Although brass flakes were observed in the engine oil during the condition inspection, engine data recovered from the airplane’s multi-function display was consistent with normal engine operation.

Five of the six propeller bolts were recovered and examined after the accident. The examination revealed fatigue cracks in multiple areas on each bolt that progressed until each bolt failed in overstress. Additionally, two of the recovered bolts displayed more extensive fatigue features than the other three bolts that were examined.

The pilot documented that he had removed, inspected, and reinstalled the propeller during the condition inspection. Given this information, it is likely that the pilot/builder had not torqued all the propeller attachment bolts to the correct value, resulting in the progressive fatigue failure of the bolts.

Text from the NTSB. Open the NTSB docket for WPR23LA149 for the full record.