Piper PA-28-140 N665FL accident near Yacolt, Washington

Probable cause

A partial loss of power caused by carbon build up in the cylinders, which prevented the exhaust valves from seating properly. Contributing to the accident was flying in moderate icing conditions which resulted in reduced aircraft performance and vibrations.

NTSB analysis

The airplane was the subject of a Federal Aviation Administration Alert Notification (ALNOT) when the pilot reported a rough running engine and then radio contact was lost with the airplane. A few hours later, the airplane’s wreckage was located by Navy search and rescue in heavily forested terrain.

The pilot reported that while in cruise flight at 8,000 ft mean sea level (msl), and in instrument meteorological conditions (IMC), he felt a vibration in the airplane and thought it was from airframe icing, however he soon realized that the engine was running rough. He applied carburetor heat and requested a lower altitude. However, the engine seemed to run worse and lose power, so the pilot requested vectors to the nearest airport. While being vectored and after exiting IMC, he elected to make a forced landing in a nearby clearing.

While the pilot reported no airframe icing, a review of weather conditions in the area and a sounding model performed over the accident site revealed that the freezing level was about 3,227 ft mean sea level (msl) and supported broken to overcast clouds from about 3,000 ft to 8,000 ft msl and icing in the clouds. Additionally, AIRMET Zulu was current over the area for occasional moderate icing conditions from the freezing level, estimated at 3,000 ft, to 12,000 ft msl. It is likely that airplane experienced moderate icing conditions that contributed to the decrease in the airplane’s performance and some of the vibrations felt by the pilot.

Postaccident examination of the engine revealed that during manual rotation, thumb compression was not obtained on the Nos. 1 and 3 cylinders. Borescope examination of the cylinder’s interior revealed carbon deposits lodged in the area where the exhaust valve seats, preventing proper sealing.

Text from the NTSB. Open the NTSB docket for WPR21LA151 for the full record.