Accident Overview
On May 27, 2011, about 1620 Alaska daylight time, a tailwheel-equipped Cessna 170A airplane, registration N1742D, sustained substantial damage when it veered off the runway during landing at the Wasilla Airport in Wasilla, Alaska. The airplane was operated by the pilot as a visual flight rules (VFR) personal local flight under 14 Code of Federal Regulations Part 91. The private pilot and sole passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed.
Pilot Account
During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge on May 29, the pilot reported that when the airplane touched down during a three-point landing, it immediately veered sharply to the left and exited the runway. She stated she was unable to regain control before the airplane collided with a ditch. The left wing and fuselage received substantial damage during the collision.
Pilot Experience and Aircraft Modifications
The pilot had recently purchased the airplane and had no prior experience in a Cessna 170, accumulating about 29 hours of flight time in this model since purchase. The airplane had been modified with Cessna 180 main landing gear and larger diameter tires. Additionally, a "bird dog" tailwheel had been installed. The previous owner demonstrated the airplane using heavy braking and excessive thrust to straighten the tailwheel and always performed wheel landings rather than three-point full stall landings. After purchase, the original tailwheel was reinstalled to improve controllability.
Tailwheel Assembly Examination
On July 8, 2011, the tailwheel assembly was examined by the NTSB IIC. The rotational plane of the tailwheel was not horizontal/parallel to the ground; there was a pronounced negative camber (aft tilt downward), causing the tailwheel to rotate forward when weight was applied. The increased height of the Cessna 180 gear and taller tires contributed to the negative camber. The tailwheel spring had been extended aft from its normal position, as indicated by old clamp marks, adding additional negative camber.
Airframe Logbook and Modifications
Review of the airframe logbooks showed four recent major modifications: 1. Installation of a main landing gear reinforcement kit under supplemental type certificate SA2918NM. 2. Replacement of main landing gear spring legs and axles with Cessna 180 components under FAA field approval. Instructions for continued airworthiness included wheel alignment per Cessna Service Letter S.L.N.-56. 3. Replacement of stock 6.00 x 6 tires with 8.50 x 6 tires under FAA field approval. A tundra tire worksheet and test flight were completed, but no flight test or return to service was recorded in the logbook. 4. Replacement of the stock tailwheel spring with a "Bird Dog" spring under FAA field approval, with installation per the Cessna 170 manual.
Expert Comments
A Cessna accident investigator stated that Cessna does not encourage component exchange between differing models and noted that due to landing gear geometry differences, airplane-specific guidance would not guarantee airworthiness for a mixed configuration. The Cessna 170 manual does not include installation of the "bird dog" tailwheel on a 170 model.
An engineer from the FAA Anchorage Aircraft Certification Office concurred with Cessna, stating that due to changes in landing gear geometry, each airplane would need to be test flown or taxied for adverse effects and specific alignment determined, with proper alignment annotated in the logbook. He noted that the tailwheel plane of rotation should be approximately horizontal for proper operation.
The aircraft mechanic's company, responsible for the main landing gear change, stated they align gear per the maintenance manual without regard for dimensional changes. The FAA inspector who signed the FAA 337 form for the gear exchange said the inspection was for conformity to best maintenance practices, not engineering, and noted that the FAA does not provide guidance for cumulative effects of multiple modifications.
The previous owner, who had about 300 hours in the 170 over 7 years, stated he did not notice any difference in controllability after the modifications.
Regulatory References
Civil Air Regulation 3.106 requires that airplanes be satisfactorily controllable and maneuverable during landing, and that smooth transitions be possible without exceptional pilot skill. FAA Advisory Circular 43-210 provides guidance on alterations, including compliance statements, considering previous alterations, and instructions for continued airworthiness.