Accident Overview
On March 29, 2021, at approximately 1547 Pacific daylight time, a Piper PA-28-140 airplane, registration N665FL, sustained substantial damage during a forced landing near Yacolt, Washington. The pilot and a pilot-rated passenger received minor injuries. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
The airplane became the subject of a Federal Aviation Administration Alert Notification (ALNOT) after the pilot reported a rough running engine and subsequent loss of radio contact. The wreckage was located several hours later by Navy search and rescue in heavily forested terrain approximately 5 miles east of Yacolt.
Pilot Report
While cruising at about 8,000 ft mean sea level in instrument meteorological conditions (IMC), the pilot felt a vibration initially attributed to airframe icing. However, no icing was observed, and the engine began running rough. The pilot applied carburetor heat and requested a lower altitude. The engine performance worsened, and the pilot requested vectors to the nearest airport. After exiting IMC, the pilot attempted a forced landing in a nearby clearing. During the landing, the airplane struck trees, resulting in substantial damage.
Weather Conditions
A National Transportation Safety Board senior meteorologist reviewed the weather. A low-pressure trough extended from northern Idaho across Washington at the time of the accident. The accident site was ahead of the trough in an area of westerly winds and overcast clouds. Continuous snow was depicted north of the site near Tacoma. A sounding model indicated the freezing level at about 3,227 ft msl, with broken to overcast clouds from about 3,000 to 8,000 ft msl and icing in the clouds. AIRMET Zulu was current for occasional moderate icing from the freezing level (estimated at 3,000 ft) to 12,000 ft msl. Two jet aircraft reported moderate icing below 8,000 ft. The source notes that flying in moderate icing can decrease airplane performance and potentially cause vibrations.
Postaccident Examination
Examination of the engine revealed that during manual rotation, thumb compression was not obtained on cylinders Nos. 1 and 3. Leaking air was heard from the exhaust piping. A compression check showed No. 1 cylinder at 5 psi and No. 3 cylinder at 0 psi. Borescope inspection of cylinder interiors revealed scoring on the walls of No. 3 cylinder and carbon deposits lodged where the exhaust valve seats. Similar carbon buildup was observed in other cylinders, though No. 2 cylinder had substantially less buildup. No other anomalies that would have precluded normal operation were found.
Maintenance History
The pilot reported the last annual inspection occurred about six months before the accident, with 25 flight hours accumulated since. The engine had accumulated about 1,929 hours since overhaul.