F4U 5 N179PT accident near East Troy, Wisconsin

Probable cause

A failure of the O-ring in the left brake master cylinder for reasons that could not be determined, which resulted in a loss of directional control during landing.

NTSB analysis

Before the accident flight, the airplane's brakes were replaced with a custom brake system. Testing of the brake system after installation resulted in a failure of the right master cylinder. The cylinder was disassembled and the O-ring was found cut. The mechanic could not find any reason for the cut O-ring, so the O-rings on both master cylinders were replaced. The next brake test resulted in a brake fluid boil, and the brake builder informed the mechanic to change the type of hydraulic fluid. A subsequent ground brake test produced "no issues or hesitation with the brakes at all," to include "a full pressure pedal push to simulate a full locked brake to pressure test [the] system prior to taxi test." A maintenance flight was then conducted, and, during landing, the airplane began to drift to the right. The commercial pilot applied the left brake; however, the brake failed and the pedal "went to the floor." The airplane departed the runway and collided with a wind sock structure. Postaccident examination revealed that the left brake master cylinder O-ring was cut; however, the reason for the cut could not be determined.

After the accident, the mechanic contacted the master cylinder manufacturer for guidance. The company replaced the master cylinders with an upgraded model. The new cylinders were installed on the airplane and the mechanic, with guidance from the custom brake manufacturer, conducted more testing. A second airplane flew with the newer brake system without issue.

Text from the NTSB. Open the NTSB docket for CEN16LA274 for the full record.