Piper PA-18 N83641

Summary

On 22 November 2016 at about 23:00 local time, a Piper PA-18 registered N83641, operated by Brooks Flyers LLC, was involved in an accident near Bethel, Alaska, United States. 2 people were on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

22 November 2016 at 23:00

Classification

Accident

Location

Bethel, Alaska, United States

Coordinates

61.4147, -161.9581

Aircraft

Piper PA-18

Registration

N83641

Category

Airplane

Year built

1977

Engines

1

Operator

Brooks Flyers LLC

Operating rule

Part 91: General Aviation

Purpose of flight

Aerial Observation

Phase of flight

Not recorded

Route

Bethel → Bethel

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

ANC17LA007

Nobody was injured.

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.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.