North American NAVION N91137 accident near Alamogordo, New Mexico

Probable cause

The failure of the No. 5 exhaust valve key spring, which resulted in a total loss of engine power.

NTSB analysis

The commercial pilot stated that, while in cruise flight, the engine began to run rough and that he noted that the Nos. 1 and 5 cylinders were indicating significantly cooler temperatures than the other cylinders. About 12 minutes later, the pilot noticed an "acrid metallic smell" in the cabin. The pilot turned the airplane toward the closest airport as the engine continued to run rough and produce a "metallic clanking" sound. Unable to reach the airport, the pilot made a forced landing in a field about 100 yards short of a road. The airplane continued into a drainage culvert and came to rest on the road.

A postaccident examination of the engine revealed that the engine crankcase was cracked at the base of the No. 5 cylinder and the cylinder's rocker box cover exhibited multiple holes. The oil sump contained a large amount of metal debris and damaged components from the No. 5 cylinder. The No. 5 cylinder exhaust valve spring key was worn and damaged. The exhaust valve guide was fragmented and the valve guide bore was worn beyond the specified size, which indicated the valve guide was moving inside the guide bore. The No. 5 piston and cylinder damage were a result of the valve being released into the cylinder during engine operation. It is likely that the valve spring key failed and allowed the valve to release into the cylinder.

Text from the NTSB. Open the NTSB docket for CEN16LA234 for the full record.