NTSB Identification: MKC87IA034.
The docket is stored on NTSB microfiche number 34229.
Scheduled 14 CFR
Incident occurred Tuesday, December 23, 1986 in FORT SMITH, AR
Probable Cause Approval Date: 03/30/1988
Aircraft: EMBRAER BRASILIA EMB-120, registration: N223AS
Injuries: 6 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

THE FLIGHT CREW STATED THAT THEY WERE INITIATING A DESCENT INTO FINAL APPROACH FOR A LANDING WHEN THE RIGHT ENGINE GEARBOX CHIP DETECTOR LGITH ILLUMINATED FOLLOWED BY A LOUD BANG AND SEVERE VIBRATION.WHEN THE PROPELLER WAS FEATHERED AND THE ENGINE SHUT DOWN WITH THE CONDITION LEVER THE ROTATION AND VIBRATION STOPPED, THE CREW FURTHER STATED. THE AIRCRAFT LANDED WITHOUT FURTHER INCIDENT. A POST-INCIDENT TEARDOWN INSPECTION OF THE POWERPLANT AT THE FACTORY REVEALED A SUDDEN AND CATASTROPHIC FAILURE OF THE PROPELLER REDUCTION GEARBOX ASSEMBLY DUE TO A HIGH CYCLE, METAL FATIGUE FRACTURE ON ONE, OF TWO, HELICAL GEARS. TEARDOWN AND INSEPCTION OF THE PROPELLER ASSEMBLY REVEALED SEVERE DAMAGE TO THE COMPOSITE PLASTIC BLADE RETAINER RINGS AND METAL ACTUATOR YOKES DUE TO SUDDEN DECELERATION AND STOPPAGE.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

REDUCTION GEAR ASSY..FATIGUE



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