Nanchang China CJ-6A N4182C accident near Peoria, Arizona

Probable cause

The partial loss of engine power during cruise flight due to the improper installation of the master connecting rod main bushing set screws, which backed off and allowed the main bushing to shift, resulting in scoring damage on the crankshaft that led to the initiation of a fatigue crack and separation of the forward crankshaft.

NTSB analysis

The pilot reported that he was conducting a formation flight with another airplane when the engine began making “clicking” and “banging” sounds, surging in speed, and losing power. The pilot attempted to troubleshoot the partial loss of engine power but was unsuccessful. Unable to maintain altitude, the pilot performed a forced landing at a closed gliderport but was unable to stop the airplane on the runway. During the landing sequence, the airplane subsequently crossed over a road and struck an unoccupied camper and the ground, sustaining substantial damage to the left wing and engine mount.

A postaccident examination of the engine revealed that the crankshaft front section was separated (fractured through) at the master connecting rod journal and crankpin areas. The crankshaft fracture surfaces showed features consistent with a fatigue fracture initiating from an area of substantial scoring damage on the journal radius. The master rod main bushing (bearing) was found rotated within its pressed fitting about 90° from its original position and also showed substantial scoring damage and wear. The two main bushing set screws (which held the master rod main bearing in place) were not present and were not identified during the examination.

Based on the mechanical damage and deformation observed on the main bushing set screw holes, the forward side of the main bushing edge (positioned nearest to the journal radius), and the connecting rod ends near the set screw areas, the set screws likely loosened and backed out, then remained in the small cavity between the connecting rod ends and the master rod until they sustained enough mechanical deformation to exit the area.

The main bushing set screw holes showed three installation chiseled strike marks each, indicating that the main bushing was likely previously replaced two times on the master rod. It is likely that the set screws were not properly installed with the appropriate strike marks, which allowed them to loosen.

Text from the NTSB. Open the NTSB docket for WPR24LA159 for the full record.