Rockwell International S-2R N8520V accident near Brawley, California

Probable cause

The partial loss of engine power shortly after takeoff, which likely resulted from the failure of the piston connecting rod in cylinder number 3 due to hydraulic lock in that cylinder.

NTSB analysis

The pilot was en route to perform an aerial application pass when the airplane’s engine lost partial power. He made a forced landing in a nearby pasture; however, the airplane struck a railroad track during the landing roll, and a postcrash fire ensued. Examination of the radial engine revealed that the connecting rods for the pistons in cylinders 3 and 8 had failed, resulting in significant internal engine damage and the partial loss of power. Examination of the remains of the connecting rods for both cylinders revealed wear signatures that were mostly consistent with engine operation after failure. Because the engine continued operating after the failure, most features on the connecting rods fracture surfaces were obliterated, which made it difficult to identify the initial failure. However, further examination of the inside diameter surfaces of the wrist pin bearing of the number 3 cylinder connecting rod revealed wear signatures consistent with normal engine operation. Based on this finding, it is most likely that the event was initiated by the failure of the number 3 piston rod because it was not affected by the continued operation of the engine.

The design of the engine's fuel priming system was such that cylinder number 3 was prone to piston hydraulic lock due to its low position within the priming system. Hydraulic lock is caused by excessive fuel being injected into a cylinder prior to engine start, and can be caused by either excessive priming, or a failure in the plunger or check valve assemblies, either of which could result in fuel trickling from the lowest nozzle within the system during non-operation. Postaccident fire damage precluded a thorough examination of the plunger and check valve assemblies. Hydraulic lock can result in a bent connecting rod and subsequent rod failure. The airplane was a low wing type, with integral wing fuel tanks located below the engine, and as such, it is unlikely that fuel had leaked into cylinder number 3 during non-operation.

Text from the NTSB. Open the NTSB docket for WPR11LA279 for the full record.