What happened
On August 30, 2019, a float-equipped Piper PA-18 was in cruise flight near Fairbanks, Alaska, when the engine began making unusual noises. The pilot, operating the aircraft under Part 91 personal flight regulations, decided to return to Chena Marina Airport (AK28). Approximately 16 minutes later, while 13 miles west of the airport, the engine suffered a total loss of power.
The pilot performed an emergency landing in a heavily forested area. The aircraft, registration N1124, sustained substantial damage to its wings. The pilot was not injured.
The investigation
An examination of the Lycoming O-320-A1H engine revealed that the No. 3 cylinder had separated from the engine case. While the two bottom hold-down studs for the No. 3 cylinder remained intact with relatively good threads, the remaining cylinder hold-down studs were fractured and showed evidence of fretting on the engine case.
Investigators found chipped and broken black paint around the cylinder hold-down nuts. Additionally, paint was present inside the cylinder hold-down stud holes on the cylinder's flange, whereas the backside of the flange was free of paint. According to the Lycoming overhaul manual, machined bosses should be masked before painting, and paint should not be applied under hold-down nuts where torque is required.
Findings
- The No. 3 cylinder separated from the engine case due to the failure of the hold-down studs.
- Inappropriate paint application under the cylinder hold-down nuts likely caused inaccurate torque readings during assembly.
- This improper torque allowed the nuts to back off over time, leading to the fracture of the other studs and the eventual separation of the cylinder.