NTSB Identification: MIA96FA168.
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Accident occurred Friday, June 28, 1996 in WEBSTER, FL
Probable Cause Approval Date: 10/31/1997
Aircraft: Hiller FH-1100, registration: N598F
Injuries: 2 Fatal.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The helicopter (N598F) was flying at about 750 to 1,000 ft, when witnesses heard a loud noise, then saw N598F descending & spinning to the right. N598F impacted the ground upright at the edge of a pond. Postcrash exam showed that a tail rotor blade had separated in flight, followed by the other tail rotor blade, hub, & gearbox. Metallurgical exam showed the tail rotor blade had separated due to fatigue fractures in 9 of 11 plates in the tail rotor blade torsion tension (TT) bar. Fatigue originated at machining discontinuities on the inner surfaces of eye holes at the bushing ends, which had been punched in the plates during the manufacturing process. In 1977, Airworthiness Directive (AD) 77-07-08 & Fairchild Hiller Service Bulletin (SB) FH1100-55-2A were issued to inspect for this problem. Records of N598F showed the AD & SB had been complied with on 3/9/77, but the tail rotor assembly had been removed & replaced with another assembly on 3/17/83. Records did not show if the AD & SB had been complied with on the replacement tail rotor assembly. Overhaul procedures did not allow disassembly of the TT bar to examine the holes. Weight & balance calculations showed that after tail rotor separated, the center of gravity moved about 2.1 inches forward of the forward limit. FH-1100 emergency procedure for tail rotor failure required that the pilot immediately reduce engine power to zero torque & perform an autorotative landing. Throttle was found in full power range. Helicopter was not equipped with shoulder harnesses.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: inadequate compliance with AD 77-07-08 (and SB FH1100-55-2A) by other maintenance personnel, and subsequent fatigue failure of plates in the tail rotor torsion tension bar. Factors related to the accident were: manufacturing discontinuities (scoring/indentations) in holes of plates of the tail rotor torsion tension bars, and failure of the pilot to follow emergency procedures (reduce power to attempt an autorotative landing, following loss of the tail rotor assembly). Full narrative available
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