Bethel, AK, USA
N83641
PIPER PA-18
The commercial pilot reported that, about 2 hours 40 minutes into the aerial observation flight, he landed on a remote gravel bar to take a break; he departed a few minutes later. During the climb, he noticed the oil pressure had redlined. To correct for the high oil pressure, he reduced the engine power while slowly climbing the airplane to about 1,000 ft above ground level and applied the carburetor heat. Shortly thereafter, smoke began filling the cockpit. The pilot indicated that, while maneuvering for an emergency landing to a frozen lake, the engine lost total power, and the pilot made a forced landing in an area of tundra-covered terrain, during which the airplane sustained substantial damage to the left wing. A detailed examination of the propeller revealed torsional "S" twisting and minor chordwise scratching on both propeller blades, consistent with the engine producing power at the time of impact. An examination of the engine revealed a displaced crankshaft oil seal and an engine breather tube that was not insulated and lacked a whistle slot. The examination did not reveal evidence of any other engine malfunctions or failures that would have precluded normal operation. The mechanic who maintained the airplane stated that the engine breather tube had been replaced about 2 years before the accident during an engine conversion and that he must have forgotten to put the hole (whistle slot) in the tube at that time. The engine manufacturer's website indicates that a whistle slot is one method that is often used to prevent freeze-up of the crankcase breather. Thus, it is likely that the engine breather tube froze, which pressurized the crankcase and caused the crankshaft oil seal to be displaced, which resulted in a loss of oil, smoke in the cockpit, and the pilot's incorrect belief that the engine had lost total power, which led to his selection of a less-than-ideal landing surface for the forced landing.
On November 22, 2016, about 1400 Alaska standard time, a Piper PA-18-150 (Super Cub) airplane, N83641, sustained substantial damage during an emergency landing near Bethel, Alaska. The airplane was registered to and operated by Brooks Flyers LLC, as a visual flight rules (VFR) aerial observation flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91 when the accident occurred. The certificated commercial pilot and one passenger were uninjured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight departed Bethel, at about 1050 with an intermediate stop at a remote unimproved landing site. In a statement provided to the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot stated that the purpose of the flight was to conduct wildlife surveys for the Alaska Department of Fish and Game. He stated that about 2 hours and 40 minutes into the flight they landed on a remote gravel bar to take a break, followed by departure and climb-out a few minutes later. During the climb, he noticed his oil pressure had redlined at 100 pounds per square inch (psi) with an oil temperature of 138 degrees Fahrenheit. In an effort to correct for the high oil pressure, he reduced his power to 2150 RPM and the oil pressure came down to 90 psi, with all other engine instruments in the normal range. He adjusted his course for Bethel while slowly climbing the airplane to about 1,000 feet above ground level, and applied the carburetor heat. Shortly thereafter, smoke began filling the cockpit. He turned into the wind, applied full flaps, reduced the power to idle, and selected a small frozen lake as an emergency landing site. While maneuvering for the emergency landing the engine lost all the power, and he made a forced landing in an area of tundra covered terrain. During the forced landing the airplane sustained substantial damage to the left wing. On February 24, 2017, the NTSB IIC, along with another NTSB investigator examined the engine and propeller at the facilities of Alaska Claims Services, Inc., Wasilla, Alaska. The propeller remained attached to the engine crankshaft. Both of the propeller blades 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 of the engine accessories. The cylinders, pistons, valve train, crankshaft, and other internal components were all without evidence of anomaly or malfunction. However, no oil was present on the engine oil dipstick, and the crankshaft oil seal or nose seal was displaced, and evidence of oil was apparent under and around the displaced seal. Further examination revealed that the engine breather tube was not insulated, and did not have a "whistle slot" located in a warm area on the breather tube. During a telephone conversation on May 12, 2017 the mechanic who maintained the airplane stated that the engine breather tube had been replaced about 2 years prior to the accident during an engine conversion, and he must have forgot to put the hole (whistle slot) in the tube at that time. Lycoming Engines website, Contact and Support, Knowledge Base, Tips, "How to Avoid a Nose Seal Leak" states in part: "To avoid the problem of oil leakage at the crankshaft oil seal because of an engine breather restriction, examination of the breather tube to determine its condition is an excellent idea. If the tube is in good condition, also remember that the engine expels moisture through the tube. Under freezing conditions, there is some possibility that the moisture may freeze at the end of the tube and ice will build up until the tube is completely restricted. Should this happen, pressure may build up in the crankcase until something gives – usually the Crankshaft Oil Seal. Since the airframe manufacturers know this is a possibility, and since they design with the intention of preventing engine-related problems of this kind, some means of preventing freeze-up of the crankcase breather is usually a part of the aircraft design. The breather tube may be insulated, it may be designed so the end is located in a hot area, it may be equipped with an electric heater, or it may incorporate a hole, notch or slot which is often called a "whistle slot." Because of its simplicity, the whistle slot is often used and is located in a warm area near the engine where it will not freeze. Aircraft operators should know which method of preventing freeze-up is used and then ensure that the configuration is maintained as specified by the airframe manufacturer." The closest weather reporting facility was Bethel Airport, Bethel, Alaska, about 45 miles south of the accident site. At 1353, an Aviation Routine Weather Report (METAR) from Bethel Airport was reporting, in part: wind from 020 degrees at 10 knots; visibility, 6 statute miles, mist; clouds and sky condition, few clouds at 15,000 feet, few clouds at 25,000 feet; temperature, 0 degrees F; dew point -2 degrees F; altimeter, 29.41 inHG.
The mechanic's failure to install a whistle slot in the engine breather tube, which resulted in a frozen tube, displacement of the crankshaft oil seal, a loss of oil, and smoke in the cockpit, which led to a forced landing.
Source: NTSB Aviation Accident Database
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