Cessna 170A N1742D accident near Wasilla, Alaska

Probable cause

The pilot's loss of directional control during landing, which resulted in a runway excursion. Contributing to the accident were the effects of multiple modifications to the main and tailwheel landing gear through the supplemental type certificate and FAA field approval process, which adversely changed their geometry, and resulted in degraded controllability during landing.

NTSB analysis

The pilot said that, when the tailwheel-equipped airplane touched down during a three-point landing, it immediately veered sharply to the left and exited the runway. She was unable to regain control before the airplane collided with a ditch.

The airplane had recently been purchased, and the pilot did not have any flight time in the make and model prior to the purchase. The airplane had been modified with larger/taller main landing gears, larger diameter tires, and a tailwheel assembly from another model of airplane. The pilot said that when the previous owner demonstrated the airplane, he used heavy braking and excessive thrust to get the tailwheel to straighten out. He told her he always did wheel landings as opposed to three-point full stall landings.

A postaccident examination of the airplane revealed that the rotational plane of the tailwheel was not horizontal/parallel to the ground. There was a pronounced negative camber (aft downward tilt), which caused the tailwheel to rotate forward when weight was applied. The tailwheel spring had been extended aft from its normal position, which added additional negative camber. An engineer from the Federal Aviation Administration aircraft certification office said adjustments to the tailwheel should have been made to make the tailwheel's plane of rotation horizontal.

An examination of the airplane's airframe logbook revealed that the airplane's landing gear had been modified under a supplemental type certificate and multiple field approvals. The airplane had been inspected for maintenance conformity, but not engineering.

Due to the multiple landing gear modifications, it is likely that the airplane's landing characteristics were degraded, requiring greater pilot vigilance and skill, particularly during landing.

Text from the NTSB. Open the NTSB docket for ANC11LA039 for the full record.