Accident Details
A float-equipped airplane departed from a remote lake while a pilot-witness observed its westerly takeoff run from the shoreline. The witness reported that during the takeoff run, the nose appeared abnormally high as the airplane lifted off. Subsequently, "the wings began to wobble" as the airplane disappeared behind an area of hilly, tree-covered terrain at the west end of the lake. The airplane then collided with trees, which severed the right wing. It became inverted before colliding with an area of tundra-covered rock. The wreckage was located about 800 feet from the west shoreline.
Investigation Findings
During the National Transportation Safety Board's on-scene investigation, the pilot's seat was found in the full aft position. Postaccident inspections revealed no evidence of mechanical problems with the airplane's engine or flight controls.
An FAA airworthiness directive (AD) 87-20-03 R2 defines maximum acceptable wear limits on the seat locking mechanism and seat tracks. The AD states, in part: "...If the wear dimension across any hole exceeds 0.36 inches but does not exceed 0.42 inches, continue to measure each hole every 100 hours for excessive wear. ...If the wear dimension across any hole exceeds 0.42 inches, prior to further flight, replace the seat track."
The NTSB investigator-in-charge measured each of the 17 seat rail holes using a McFarlane seat rail wear gauge. The measurements revealed that 4 of the 17 holes were in excess of 0.42 inches. The four excessively worn holes were the 8th through 11th holes from front to back.
Seat Track Condition
Using an exemplar Cessna 180 and a pilot of the same height as the accident pilot, it was discovered that the 9th seat track rail hole was selected after the seat was adjusted to comfortably operate the flight controls. Cessna had issued service bulletin SEB07-5, which provides for the installation of a secondary seat stop kit for the pilot seat, free of charge. Compliance with the bulletin was mandatory within the next 200 hours of operation or 12 months, whichever occurred first. The bulletin was issued about 4 months before the accident, and the airplane had flown about 22.4 hours since its issuance. The accident airplane was not equipped with the secondary seat stop kit.
Given the lack of mechanical deficiencies with the airplane's engine or flight controls, and the discovery of the worn seat rail holes, it is likely that the pilot's seat moved aft during the takeoff run, making it difficult for him to properly operate the airplane's controls.