Introduction
On November 22, 2016, at approximately 1400 Alaska standard time, a Piper PA-18-150 (Super Cub) airplane, registration N83641, sustained substantial damage during an emergency landing on tundra-covered terrain near Bethel, Alaska. The aircraft was operated by Brooks Flyers LLC as a visual flight rules (VFR) aerial observation flight under 14 Code of Federal Regulations Part 91. The commercial pilot and one passenger were uninjured. Visual meteorological conditions prevailed, and company flight following procedures were in effect.
Narrative
The flight departed Bethel at about 1050, with an intermediate stop at a remote unimproved landing site. According to the pilot's statement to the NTSB investigator-in-charge, the flight was conducting wildlife surveys for the Alaska Department of Fish and Game. About 2 hours and 40 minutes into the flight, the pilot landed on a gravel bar for a break. A few minutes later, during the climb-out, the pilot observed that the oil pressure had redlined at 100 psi, with an oil temperature of 138°F. He reduced power to 2150 RPM, bringing the oil pressure down to 90 psi while other engine instruments remained normal. He adjusted course toward Bethel, slowly climbing to about 1,000 feet above ground level, and applied carburetor heat. Shortly thereafter, smoke began filling the cockpit. The pilot turned into the wind, applied full flaps, reduced power to idle, and selected a small frozen lake as an emergency landing site. While maneuvering, the engine lost all power, necessitating a forced landing on tundra-covered terrain. During the landing, the left wing sustained substantial damage.
Examination
On February 24, 2017, the NTSB investigator-in-charge and another NTSB investigator examined the engine and propeller at Alaska Claims Services, Inc., in Wasilla, Alaska. The propeller remained attached to the engine crankshaft and exhibited torsional "S" twisting and minor chordwise scratching. Examination of the Lycoming O-360-A1A engine revealed no anomalies, contamination, or evidence of malfunction in any accessories. Cylinders, pistons, valve train, crankshaft, and other internal components were without anomaly or malfunction. However, no oil was present on the engine oil dipstick, and the crankshaft oil seal (nose seal) was displaced, with evidence of oil under and around the displaced seal. Further examination showed that the engine breather tube was not insulated and lacked a "whistle slot" located in a warm area on the tube.
During a telephone conversation on May 12, 2017, the mechanic who maintained the airplane stated that the breather tube had been replaced about two years prior during an engine conversion, and he forgot to install the whistle slot at that time.
According to Lycoming Engines' knowledge base, "How to Avoid a Nose Seal Leak," a restricted breather tube can cause pressure buildup in the crankcase, leading to a crankshaft oil seal leak. Methods to prevent freeze-up include insulation, placing the tube end in a hot area, an electric heater, or a whistle slot in a warm area. The source recommended operators ensure the configuration is maintained as specified by the airframe manufacturer.
Weather
The closest weather reporting facility was Bethel Airport, about 45 miles south of the accident site. At 1353, an Aviation Routine Weather Report (METAR) reported wind from 020 degrees at 10 knots, visibility 6 statute miles with mist, few clouds at 15,000 feet and 25,000 feet, temperature 0°F, dew point -2°F, and an altimeter of 29.41 inHg.