The commercial pilot stated that, while in cruise flight, the Navion’s engine began to run rough. He noted that the Nos. 1 and 5 cylinders were indicating significantly cooler temperatures than the other cylinders.
About 12 minutes later, he noticed an “acrid metallic smell” in the cabin.
The pilot turned the plane toward the closest airport as the engine continued to run rough and produce a “metallic clanking” sound.
Unable to reach the airport, he made a forced landing in a field about 100 yards short of a road near Alamogordo, N.M.
The plane continued into a drainage culvert and came to rest on the road.
A post-accident examination 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.
Probable cause: The failure of the No. 5 exhaust valve key spring, which resulted in a total loss of engine power.
NTSB Identification: CEN16LA234
This June 2016 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.