Incident Overview
On July 16, 2012, at about 1855 eastern daylight time, a Flight Design GmbH CTLS, registration N566FD, sustained minor damage during a runway excursion on takeoff at Union County Airport (MRT) in Marysville, Ohio. The aircraft, registered to B S Aviation LLC and operated by New Flyers Association, was conducting a solo instructional flight under 14 CFR Part 91. Visual meteorological conditions prevailed, and the flight operated without a flight plan. The flight had originated from the Ohio State University Airport (OSU) at about 1805.
Sequence of Events
The pilot reported that after departing OSU, he flew to a local practice area and completed several training maneuvers before proceeding to MRT to practice takeoffs and landings. After entering the traffic pattern, he landed on runway 27 (4,218 feet by 75 feet, asphalt) without incident and taxied back for departure. During the subsequent takeoff roll, the pilot felt a "jolt" and heard a "thumping" sound from the left main wheel. The airplane immediately veered left and departed the runway pavement, coming to rest about 20 yards from the runway edge after striking a Precision Approach Path Indicator light. The left main landing gear collapsed.
Post-Incident Examination
A post-incident examination revealed that the outer half of the left main wheel assembly had failed. Damage was limited to the left main landing gear, engine cowling, and propeller assembly. The operator reported that the right main wheel had failed on this airplane in a similar manner on December 6, 2011. Both wheel components were submitted for metallurgical examination.
Examination of the left and right outboard wheel halves revealed a crack adjacent to the rim, extending about three-quarters of the circumference on the left wheel half and about two-thirds on the right. Fracture surface features were consistent with fatigue cracking extending through the component thickness, with multiple origins along the outer surface. No unusual features, such as deep machining marks, were observed at the fatigue origin areas. The material thickness varied over the cross-section of both wheel halves, measuring about 0.083 inch at the formed radius adjacent to the rim, while the specified material thickness was 0.118 inch. Measured material properties, including conductivity, hardness, and composition, were consistent with the specified material.
Maintenance History
At the time of the incident, the airframe had accumulated 1,396 hours total time (980.3 hours tachometer time). The most recent annual inspection was completed on May 30, 2012, at 907.6 hours tachometer time. Records indicated that the left landing gear leg had been replaced after repair on November 4, 2011, at 702.1 hours tachometer time, following a hard landing inspection. The right main wheel outer half was replaced on December 7, 2011; no airframe maintenance time was recorded with that logbook entry.
Weather Conditions
Weather conditions recorded by the MRT Automated Weather Observing System (AWOS) at 1853 were: calm wind, visibility 10 miles, clear sky, temperature 33 degrees Celsius, dew point 12 degrees Celsius, altimeter 29.93 inches of mercury.