Cessna 210 5(205) N8296Z accident near Sussex, New Jersey

Probable cause

Maintenance personnel’s inadequate inspection of the No. 2 cylinder valve area during the most recent annual inspection, which resulted in the in-flight failure of the intake valve springs due to rust on the spring surfaces and subsequent fatigue cracking.

NTSB analysis

The pilot reported that, shortly after the skydiving flight departed, the engine experienced a "mechanical failure" and that he then executed a forced landing in a farm field south of the airport. The airplane nosed over in the mud, which resulted in structural damage to the airframe.

During a postaccident test run of the engine on the airframe, lower-than-normal exhaust gas temperature indications were observed on the engine's left-side (Nos. 2, 4, and 6) cylinders. Excessive soot and smoke were also observed on the engine's left side. During a subsequent test run, the engine initially did not achieve full power. Further examination revealed that both of the No. 2 cylinder intake valve springs were fractured, and visible rust was observed on the surfaces of the springs. The springs showed evidence of fatigue fractures that had originated from rust pits on the fracture surfaces. After the valve springs were replaced, the engine was capable of operating normally at full power.

An annual inspection was completed on the engine less than 2 months (13 engine operating hours) before the accident. As part of the annual inspection, the engine manufacturer's operating manual required the removal of the cylinder rocker covers and inspection of the valve area for breakage and proper lubrication. It is likely that maintenance personnel did not adequately inspect the No. 2 cylinder valve area during the annual inspection, which allowed the rust to go undetected and resulted in the in-flight failure of the No. 2 cylinder valve springs.

Text from the NTSB. Open the NTSB docket for ERA15LA071 for the full record.