SALINAS, CA, USA
N999
MASCARI DISCOVERY S-12
THE EXPERIMENTAL HOMEBUILT DISCOVERY S-12 AIRAILE INADVERTENTLY BECAME AIRBORNE DURING HIGH-SPEED TAXI TESTS. THE MECHANIC CONDUCTING THE TAXI TESTS WAS NOT A CERTIFICATED PILOT. HE CONTACTED THE TOWER AND INFORMED THEM OF HIS SUPRISE FLIGHT. WITHIN MOMENTS THE ENGINE FAILED, AND A FORCED LANDING WAS MADE IN A NEARBY FIELD. THE AIRPLANE COLLIDED WITH A STANDPIPE. WITNESSES REPORTED THAT THE PROPELLER, MUFFLER, AND REDUCTION GEARBOX HAD SEPARATED BEFORE THE AIRPLANE STRUCK THE STANDPIPE. EXAMINATION REVEALED THAT THE MUFFLER FRACTURED AT A FACTORY WELD; THE FRACTURE EXHIBITED EVIDENCE OF OXIDATION. THE MUFFLER THEN SEPARATED AS A RESULT OF NORMAL VIBRATION DURING OPERATION OF THE ENGINE. THE PROPELLER WAS DAMAGED AFTER THE MUFFLER STRUCK IT. THE DAMAGED PROPELLER WAS THEN OUT OF BALANCE, RESULTING IN THE REDUCTION GEARBOX SEPARATING FROM THE ENGINE.
On June 18, 1994, at 1604 hours Pacific daylight time, a homebuilt experimental Discovery S-12 Airaile, N999, collided with a standpipe during an off-airport forced landing near Salinas, California. The airplane was being operated by the noncertificated owner/pilot when it inadvertently became airborne during high-speed taxi tests at the Salinas Municipal Airport, Salinas, California. The airplane was destroyed. The pilot received serious injuries. Visual meteorological conditions prevailed. Witnesses reported the propeller, muffler, and reduction gearbox separated before the airplane struck the standpipe. Examination of the three-bladed propeller by the Federal Aviation Administration (FAA) revealed one of the propeller blades had separated several inches outboard of the hub. The airplane then glided into a lettuce field. During the force landing flare, the right wing struck a 6-inch-diameter standpipe separating the right wing tip. The muffler and dampener plate from the reduction gearbox was sent to the Safety Board's Material Laboratory for examination. The muffler was fractured in the area of a factory weld. The weld area was oxidized. Numerous fractures were found in the dampener. Microscopic examination of the fractures disclosed no evidence of progressive separation. A copy of the Safety Board's Materials Laboratory Report is attached.
THE FAILURE OF THE MUFFLER ASSEMBLY DUE TO PREEXISTING CRACKING IN THE OXIDIZED AREA OF THE WELD, WITH SUBSEQUENT DAMAGE AND SEPARATION OF THE PROPELLER AND REDUCTION GEAR BOX. A RELATED FACTOR WAS THE EXCESSIVE TAXISPEED WHICH RESULTED IN THE AIRPLANE BECOMING INADVERTENTLY AIRBORNE.
Source: NTSB Aviation Accident Database
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