HAMILTON, MT, USA
N900CS
Dassault MYSTERE FALCON 900
The pilot reported that, after a normal approach and touchdown, he applied the brakes and thrust reversers. He stated that as the aircraft slowed to 60 to 70 knots, it veered to the right and exited the side of the runway. The pilot regained directional control after the aircraft exited the runway. Before the aircraft came to a stop, the right wing collided with the VASI lights for the opposite runway. Skid marks leading to the exit point on the runway, indicated that the right-side tire marks displayed heavier braking action than on the left side. Post-accident testing performed on the anti-skid generators revealed that the left-side inboard and outboard generators failed the manufacturer's specifications. The right-side inboard and outboard generators passed all manufacturers specifications. The left-side generators were disassembled. The inspection did not reveal any anomalies. A slight difference in the brush wear between the brush fittings was noted between the two generators. A short circuit between two adjacent collectors was found in the outboard generator rotor fitting. The manufacturer's representative reported that the short circuit was not due to a failure within a cell of the rotor, but was likely due to contamination present between the two collectors.
On July 2, 1998, at 1149 mountain daylight time, a Dassault Mystere Falcon 900, N900CS, registered to Charles Schwab and Company, operated by TAG Aviation USA as a 14 CFR Part 91 personal flight, veered off the side of the runway at Hamilton, Montana, and collided with VASI lights during the landing roll. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The aircraft was substantially damaged. The two airline transport pilots, the one flight attendant and the six passengers were not injured. The flight had departed from San Francisco, California, about one-hour and forty minutes prior to the accident. The pilot reported that a normal visual approach was accomplished to runway 34. The pilot stated that the aircraft touched down about 200 feet from the threshold. In one written statement, the pilot reported that a normal rollout with braking and thrust reverse application occurred, while in another statement the pilot reported that a rollout with heavier than normal braking and thrust reverse application occurred. The pilot stated that, as the aircraft slowed to approximately 60 to 70 knots, the aircraft veered to the right and exited the side of the runway. The pilot stated that directional control was regained after the aircraft exited the runway. Before the aircraft came to a stop, the right wing collided with the VASI lights for runway 16, damaging the leading edge high-lift device and the extended flap. The pilot reported that the pre-landing checklist items were accomplished. One of the checklist items is to push-to-test the anti-skid system. The pilot reported no warning malfunctions. The Cockpit Voice Recorder was read-out by the National Transportation Safety Board, Office of Research and Engineering, Vehicle Recorders Division. The recording verifies that the checklist procedures were accomplished prior to touchdown. Skid marks leading to the exit point on the runway indicate that the right-side tire marks displayed heavier braking action than the left-side tire marks. After the right-side high-lift device was replaced, the aircraft was flown to the operator's maintenance facility. The operator's mechanic reported that a functional test of the anti-skid system was found "okay" in accordance with the Falcon 900 Maintenance Manual procedures. Brake pressure was also okay. The anti-skid generators were functionally tested. The test found that the number 1 (left side outboard) tested with a low voltage output. Numbers two and four (left inboard and right outboard) tested with a high voltage output. Number three (right outboard) tested within normal voltage range, but high in accordance with the Falcon maintenance manual specifications. The anti-skid generators underwent further testing and subsequent tear-down inspection. The tachometer generators are manufactured by Messier-Hispano-Bugatti. The Component Maintenance Manual description and operation states "The tachometer generator, intended to measure the speed of rotation of a wheel, generates a voltage proportional to the latter." The manual further states, "This unit is a rotating assembly whose wound rotor moves in the magnetic field of the stator fitted with permanent magnets. The induced electromotive force is proportional to the speed of rotation of the rotor. Voltage is rectified by two brush assemblies rubbing against a seven-blade commutator and is delivered by the two output wires." Testing revealed that the left-side inboard and outboard generators failed the manufacturer's specifications. The right-side inboard and outboard passed all manufacturers specifications. The two left-side generators were torn down. During the inspection, it was found that no particular anomalies were noted. A slight difference in the brush wear between the brush fittings was noted between the two generators. A short circuit between two adjacent collectors was found in the outboard generator rotor fitting. The manufacturer's representative reported that the short circuit was not due to a failure within a cell of the rotor, but was likely due to contamination present between the two collectors. The Falcon 900 Maintenance Manual states that, "in the event of any problem related to the operation of the anti-skid system, or if a wheel has blocked during braking, check the tachometer-generators so as to check the voltage values in relation to the tachometer-generator rpm." The manual also states that when replacing tires, functionally test the tachometer generator. The manual describes the functional test procedures. The maintenance records indicate that the last functional tests were performed: 1. Right main gear inboard tachometer generator - December 5, 1995. Approximately 824 hours had been accumulated since the last test. 2. Right main gear outboard tachometer generator - July 21, 1995. Approximately 987 hours had been accumulated since the last test. 3. Left main gear inboard tachometer generator - August 1, 1994. Approximately 1,394 hours had been accumulated since the last test. 4. Left main gear outboard tachometer generator - August 9, 1995. Approximately 954 hours had been accumulated since the last test.
Partial failure of the anti-skid brake system due to contamination.
Source: NTSB Aviation Accident Database
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