What happened
On November 25, 2012, a privately owned Fuji Heavy Industries FA-200-180, registration JA3689, was conducting a sightseeing flight from Yakushima airport to Satsuma-Iojima airfield. The aircraft, carrying the pilot and three passengers, approached runway 36 at approximately 70 knots under clear skies and calm winds.
Immediately following touchdown, the pilot experienced a sudden failure of the left brake, which became spongy and lost all effectiveness. To prevent the aircraft from veering off the runway centerline, the pilot attempted to balance the deceleration by applying the right brake. However, the intense application of the right brake caused the aircraft to veer right, exiting the runway into a soft meadow. As the aircraft traveled through the field, the nose gear struck the terrain, causing the aircraft to tumble and eventually come to rest in an upside-down position. The incident resulted in one injury (a minor head bruise to a passenger) and moderate damage to the aircraft, including a bent propeller blade and a broken windshield.
The investigation
The investigation focused on the sudden loss of braking pressure in the left system. Upon inspection of the brake master cylinders, investigators identified a hydraulic fluid leak in the left unit. Disassembly of the cylinder revealed that the rubber O-ring on the piston, responsible for maintaining the seal, was significantly worn.
Review of maintenance records from the aircraft's 1,000-hour inspection in October 2011 showed that while the inspection checklist was marked to indicate the O-ring had been checked, there was no specific record of the part being replaced. Investigators noted that the level of wear on the O-ring was too advanced to have occurred in only 46 flight hours since the last major check, suggesting the component had likely not been replaced during that interval.