Piper PA 18-150 N162SC accident near La Quinta, California

Probable cause

Maintenance personnel’s failure to properly repair a propeller deformation, which resulted in a fatigue crack, separation of a propeller blade tip, and forced landing onto uneven terrain.

NTSB analysis

The commercial pilot reported that, while orbiting during a banner tow flight, the airplane began to shake violently and lose altitude. The pilot reduced power to idle, which alleviated the violent shaking, and released the banner over an empty fairway at a golf course. While advancing the throttle after dropping the banner, he noted that the severe vibration returned, and the airplane was unable to maintain altitude. The pilot selected the next empty fairway as a forced landing site, but the landing area had significant elevation changes. The airplane touched down on a plateau and could not be stopped before reaching a large drop-off. The airplane bounced before contacting the ground sideways and came to rest inverted.

Postaccident examination of the airplane revealed that a piece of one propeller blade tip had separated; this piece was not located. The fracture likely originated from a fatigue crack in a deformed area on the propeller blade. Marks consistent with use of an orbital grinder overlapped hand finishing lines in the deformed material, suggesting multiple machining operations were employed during overhaul/repair of the propeller, none of which fully removed the deformed material. The airplane had flown about 66 hours since the last recorded inspection; no inspection or overhaul entry specified any work on a propeller blade deformation.

Text from the NTSB. Open the NTSB docket for WPR15LA149 for the full record.