Gulfstream American Corp AA 5B N28005 accident near Carbondale, Illinois

Probable cause

The failure of the propeller due to fatigue as a result of improper installation by unknown maintenance personnel at some point during the life of the propeller.

NTSB analysis

The private pilot reported that, about 1 1/2 hours after departure and in cruise flight about 6,700 ft mean sea level, he felt a "jolt" as if the airplane had impacted an object. The pilot realized that the propeller was missing, initiated an emergency descent, and attempted to land at a nearby airport. Unable to glide to the airport, he decided to perform a forced landing to a field. During the forced landing, the airplane impacted trees and terrain and came to rest upright. Propeller sections and components were located about 9 miles from the accident site.

Material examination of the propeller revealed that the propeller fractured due to fatigue that initiated adjacent to two of the propeller attachment bolt holes. Fretting contact damage was observed at the primary fatigue initiation location. The contact damage appeared to be associated with the movement between the forward bulkhead and the propeller at the edges of the attachment bolt clamping locations, and some of the fretting contact damage likely preceded crack initiation in the propeller. Fatigue cracks were present in the forward bulkhead outboard of each of the propeller attachment holes. The symmetric nature of the cracks at all 6 holes and the locations of fatigue initiation at the forward and aft sides of the bulkhead suggest that the fatigue in the forward bulkhead could be associated with a rotational torque vibration in the spinner. Possible sources for high rotational torque vibration include looseness in the spinner attachment at the aft bulkhead or an engine issue that produced excessive rotational impulses.

Excessive clamping force from the attachment bolts may have also contributed to the initiation of fatigue cracks in the forward bulkhead. Excessive clamping force could have warped the forward bulkhead adjacent to the clamped area, leading to higher stresses in areas around the attachment holes where fatigue cracking was observed. The analysis of deposits from the threaded area within the crankshaft lug pieces suggested lubricant may have been present on the attachment bolt threads that should have been installed dry. The use of lubricant at installation would have reduced the frictional resistance to the applied torque, leading to higher than expected clamping force on the attachment bolts.

Crack arrest marks that appeared to coincide with flight cycles were observed on the fracture surface. One of the crack arrest marks would have a very high probability of detection by fluorescent penetrant inspection (FPI); however, the presence of red deposits in the attachment holes and dowel holes suggest that red dye (visible) penetrant was used at some time on the propeller. The manufacturer recommends against using visible dye penetrant and indicates that the use of visible dye penetrant could prevent crack detection during subsequent FPI inspections.

Text from the NTSB. Open the NTSB docket for CEN17LA333 for the full record.