ANC19LA048

The commercial pilot reported that while in cruise flight in her float-equipped airplane, the engine began to sound "funny," so she elected to return to her destination airport rather than selecting a suitable nearby lake for landing. About 16 minutes later, the engine lost all power and she conducted an emergency landing to a heavily forested area. The airplane sustained substantial damage to the wings. Examination of the engine revealed that the No. 3 cylinder had separated from the engine's case, but the two No. 3 cylinder hold-down studs were intact and the studs’ threads were in relatively good condition; the other cylinder studs were fractured with fretting present on the engine's case. In addition, paint was chipped and broken around the cylinder hold-down nuts and paint was present inside the cylinder stud holes located on the cylinder's flange. The Lycoming overhaul manual states that "All machined bosses should be masked before painting. Do not paint areas under hold-down nuts where torque is required." Due to the presence of paint inside the cylinder stud holes and broken and chipped paint around the hold-down nuts, it is likely that paint was applied inappropriately under the cylinder hold-down nuts, which would have led to an inaccurate torque reading during assembly, which allowed them to back off over time, resulting in overload failure of the other nuts and separation of the No. 3 cylinder.

On August 30, 2019, at about 1310 Alaska daylight time, a float-equipped Piper PA-18 airplane, N1124, sustained substantial damage during a forced landing following a total loss of engine power while in cruise flight near Fairbanks, Alaska. The commercial pilot was not injured. The airplane was privately owned and operated by the pilot, under the provisions of Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed and a flight plan had been filed. The flight departed Chena Marina Airport (AK28), Fairbanks, Alaska. According to the pilot, she had been airborne for about 2 1/2 hrs when the engine began to sound "funny", and she decided to return to AK28. About 16 minutes later and 13 miles west of AK28 the engine lost all power, and she conducted an emergency landing to a heavily forested area. The airplane sustained substantial damage to the wings. An initial examination of the engine revealed that the No. 3 cylinder had separated from the engine's case. The airplane was equipped with a narrow deck Lycoming O-320-A1H engine. A detailed engine examination revealed that the bottom two cylinder hold down studs for the No. 3 cylinder were intact, and the stud's threads were in relatively good condition. The remaining cylinder hold down studs were fractured, with fretting present on the engine's case. In addition, black paint was chipped and broken around the cylinder's hold down nuts, and paint was present inside the cylinder hold down stud holes located on the cylinder's flange. Paint was not present on the backside of the cylinder's flange. Figure 1 – Remnants of black paint visible in one of the bottom of the No. 3 cylinder stud holes. Figure 2 - Black paint visible in a cylinder stud hole on the No. 3 cylinder. Figure 3 - No. 3 cylinder bottom two cylinder studs with fretting visible on engine's case. Figure 4 - Back side of No. 3 cylinder's flange. Maintenance records revealed that the airplane engine's cylinders had been replaced on July 24, 2003, about 815.7 hours before the accident with overhauled cylinder assemblies. The most recent annual inspection of the airframe and engine was completed on September 11, 2018. Lycoming's Overhaul Manual – Lycoming Direct Drive Aircraft Engines, Section 3, General Overhaul Procedures stated in part: Note All machined bosses should be masked before painting. Do not paint areas under hold down nuts where torque is required. The closest weather reporting facility is Fairbanks International Airport, (PAFA), Fairbanks, Alaska. At 1353, a METAR from PAFA was reporting in part: wind, 020 at 4 knots; visibility, 10 statute miles; sky condition, few clouds 6,000 ft, few clouds 10,000 ft; temperature, 63° F; dew point 43° F; and an altimeter setting of 30.04 inches of mercury. In the recommendation section of the National Transportation Safety Board Accident/Incident Reporting Form 6120.1, the pilot stated that when she made the decision to return to AK28 nosuitable landing areas were close by; however, if she would have proceeded directly to a suitable lake for landing rather than attempting to return to her departure airport, the accident may have been avoided.