Piper PA-18 N83641 accident near Bethel, Alaska

Probable cause

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ANC17LA007 for the full record.