SEA98LA087

The pilot, flying from the front seat, was aware of a tailwheel shimmy condition prior to the flight. Upon touchdown, the aircraft's tailwheel began to oscillate. Marks on the runway showed tailwheel tire scuffing from the touchdown area and beyond. A second solid scuff mark associated with the left wheel was observed as both scuff marks diverged left of the runway centerline. Post-crash examination revealed that the rear right brake-pedal had disconnected from the connecting rod to the forward right brake-pedal and the disconnection rendered the forward right brake-pedal unusable. The disconnection occurred at the rear right brake-pedal attach eye, and one separation surface was observed to be clean whereas the opposing side was dirty. Cracks were observed inside the attach eye radii, as well as the opposing end of the pedal. The right-main landing gear separated. Examination of the tailwheel assembly revealed that the hexagonal nut which holds the inner lock ring in place had become loose allowing the lock ring to shift. This allowed the two lock ring pins to shear, thus allowing the tailwheel to oscillate or 'shimmy' during roll.

On June 4, 1998, at 1633 mountain daylight time, a Champion 7GCB, N912CP, being flown by an airline transport pilot, was substantially damaged during a loss of control on landing roll when the right main landing gear collapsed, at the Friedman Memorial airport, Hailey, Idaho. The pilot and a co-owner (a passenger in the rear seat), were uninjured. Visual meteorological conditions with variable winds existed at the time, and no flight plan had been filed. The flight originated from Boise, Idaho, approximately 1530.

The pilot reported flying the aircraft from Sun Valley to Boise earlier in the day with no abnormalities until after the landing at Boise. He indicated that while taxiing after the landing at a speed of 15-20 mph the tailwheel shuddered and that when he reduced power and decelerated to approximately 10 mph the shuddering stopped. The aircraft was taxied at slow speed to a maintenance facility and the tailwheel was inspected.

The pilot reported the inspection revealed "nothing amiss" but that the tailwheel "seemed a bit loose in the vertical plane, but generally looked OK." And that "when readying the plane for takeoff (on the return leg from Boise to Sun Valley) I again looked at the tailwheel and checked the locking mechanism to make sure it did lock." He made no reference to the shuddering during the taxi out on departure from Boise but did remark that while taxiing for departure at Boise he "had to use full right rudder and very heavy breaking on the right wheel to keep the plane centered on the taxiway."

The takeoff from Boise, and flight to Sun Valley were uneventful, and upon arrival at the Friedman airport a downwind to runway 13 was set up. The pilot reported conflicting winds from the multiple windsocks at the Friedman airport. He then executed a stabilized, full flap approach, and touched down in the vicinity of the fixed distance markers on the centerline.

The aircraft began to shudder again and a left drift developed which the pilot was unable to arrest with right rudder, aileron and brake. The aircraft departed the east side of the runway during which the right main landing gear collapsed and the right wingtip struck the ground (refer to photograph 1).

Post-crash examination of the runway revealed a periodic alternating, black smearing on the asphalt which first appeared slightly south of the fixed distance markers and near the runway centerline. These marks tracked down the runway centerline and then gradually diverged off the east side of the runway (refer to photograph 2). A second black smearing, continuous rather than alternating, was observed on the left side of the alternating marks (when looking south). This smearing appeared shortly after the alternating marks began to veer east and continued until reaching the edge of the runway (refer to photograph 3).

Post-crash examination of the aircraft revealed that the rear right brake-pedal (refer to photograph 4) detached from the push rod connecting it to the forward brake-pedal. The detachment would render the right front brake-pedal unable to actuate the aircraft's right-main brake master cylinder. If both brakes were being applied simultaneously when this occurred, it would be equivalent to an immediate release of the right brake. The rear right brake-pedal attach eye and its separated semi-circular portion were examined. The forward separation surface for both the pedal and the remaining piece were found to be somewhat shinier looking than the aft separation surfaces (refer to photographs 5 and 6). Additionally, a small crack was observed within the inside radii of the same separation area (refer to photograph 7), as well as the forward pivot end of the pedal piece (refer to photograph 8).

Disassembly of the aircraft's tailwheel assembly revealed that the hexagonal nut which holds the inner lock ring in place had become loose, allowing the lock ring to shift. Both lock-ring pins were found to be sheared perpendicular to their longitudinal axis (refer to DIAGRAM I). Shearing of the lock-ring pins would allow the tailwheel to oscillate or "shimmy" during ground roll.