NTSB Identification: SEA04LA117.
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Accident occurred Thursday, June 24, 2004 in Bozeman, MT
Probable Cause Approval Date: 09/13/2005
Aircraft: Cessna 185F, registration: N4585F
Injuries: 2 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

During the landing roll, the pilot was applying heavy braking in order to turn off the runway at taxiway Echo, when the aircraft suddenly and rapidly ground-looped to the left. An initial post-accident inspection of the aircraft revealed that the right main gear brake rotor assembly had torn lose from the wheel itself, rendering the right wheel brake totally inoperative. The wheel hub and brake disk assembly were shipped to the NTSB Materials Laboratory, where it was determined that the brake disk attachment flange failed in fatigue in the bend around the attachment flange. The brake disk showed a substantial amount of general corrosion, and although the attachment flange met the minimum thickness requirements, most of its surface was covered by corrosion and corrosion pitting. The fatigue crack propagation that eventually lead to the failure had originated in one of these corrosion pits.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The failure of the right main gear brake disk attach flange during the landing roll, due to fatigue cracks emanating from numerous corrosion pits present on the surface of the disk, leading to the right brake becoming totally inoperative, and resulting in a rapid inadvertent ground-loop. Factors include hard braking applied by the pilot in order to turn off the active runway at an early taxiway, and considerable corrosion on the surface of the brake disk flange.

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