Accident Overview
On December 23, 2016, at approximately 1618 eastern standard time, an Aeronca model 15AC airplane, registration N1337H, collided with a snowbank and nosed over while landing at Oakland County International Airport (PTK) in Pontiac, Michigan. The private pilot was not injured, and the airplane sustained substantial damage. The airplane was registered to Samcran LLC and operated under 14 CFR Part 91 without a flight plan. Day visual meteorological conditions prevailed for the local flight, which had departed PTK about 1500.
Pilot Experience and Flight Details
The pilot reported that the purpose of the flight was to practice landings in the tailwheel-equipped airplane. After departure, he remained in the airport traffic pattern and completed 14 uneventful touch-and-go landings before deciding to conclude the flight with a full stop landing on runway 27R. He reported an uneventful full stall (three-point) landing on the runway centerline. However, shortly after touchdown, the airplane suddenly swerved to the left. He attempted to regain directional control with right rudder and increased engine power for an aborted landing. The airplane then swerved to the right as he simultaneously applied forward stick pressure to lift the tail. The airplane continued to swerve right until it collided with a 3-foot-high snowbank alongside the runway, then nosed over and came to rest in the snow-covered area off the right side of the runway. The pilot exited the airplane uninjured after releasing his lap belt.
At the time of the accident, the pilot had accumulated 205 hours in the airplane since purchasing it in May 2015. He noted that most of his flight experience in the airplane, 172.9 hours, had been flown with the airplane equipped with landing floats instead of conventional tailwheel landing gear. He had logged 32.1 hours of tailwheel time, all in the accident airplane. The pilot received his tailwheel endorsement on March 11, 2016.
Aircraft Maintenance and STC Modifications
According to maintenance documentation, on January 27, 2016, the airplane had been modified through the implementation of STC No. SA1114NW with Cleveland 8.00 x 6.00 main wheels, tires, and 6-inch external, single-cylinder, dual-piston brake assemblies. The landing floats were reinstalled on an unknown date during 2016. On December 17, 2016, the landing floats were removed, and the main landing gear and wheels were reinstalled to convert the airplane back to conventional landing gear. The accident occurred during the second flight after the airplane was reequipped with the conventional landing gear.
The installation instructions for STC No. SA1114NW specified that the stock Cleveland brake torque plates be modified per accompanying drawing No. 1200-4 and that 0.375-inch outside diameter (OD) to 0.25-inch inside diameter (ID) bushings be used. The drawing specified that each torque plate required two 0.25-inch holes to be marked and drilled for proper alignment with their respective landing gear axle endplate. Each torque plate was then attached to its axle endplate with four 0.25-inch anchor bolts and washers. The two 0.25-inch drilled holes did not require bushings, but the remaining two 0.375-inch holes required 0.375 OD to 0.25 ID bushings to properly support the bolts.
Postaccident Examination
A postaccident examination by a Federal Aviation Administration (FAA) airworthiness inspector revealed substantial damage to both main wing spars and the rudder. The inspector reported that the four 0.25-inch anchor bolts used to attach the left brake torque plate to its respective landing gear axle endplate had sheared. The unrestrained torque plate allowed the brake assembly to rotate with the brake disk attached to the wheel. With forward rotation, the brake line was pulled around the gear leg until the 90° aluminum fitting connecting the brake line to the caliper assembly sheared. The tire inner sidewall exhibited a gash that partially exposed the inner tube; the tube did not rupture but protruded from the gash. The sheared anchor bolts had worn a groove on the inboard surface of the left brake disk.
Further examination revealed that both torque plates were not modified and installed in accordance with the STC No. SA1114NW installation instructions. Specifically, the supplied brake torque plates were not modified with the two additional 0.25-inch holes to ensure proper alignment with the landing gear axle endplate. Additionally, both torque plates were installed with improperly sized bushings in multiple holes. The left torque plate, which had separated from the left landing gear axle endplate, had two bushings remaining: one bushing had the correct 0.25-inch ID, while the other had a larger 0.3125-inch ID. The right torque plate, which remained attached to the right landing gear axle endplate, was incorrectly installed with four 0.3125-inch ID bushings.