NTSB Identification: CHI89MA046.
The docket is stored on NTSB microfiche number 40161.
Scheduled 14 CFR PIEDMONT AIRLINES
Accident occurred Friday, January 20, 1989 in CHICAGO, IL
Probable Cause Approval Date: 06/30/1992
Aircraft: BOEING 737-201, registration: N242US
Injuries: 33 Uninjured.

NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft accident report.

AN IN-FLIGHT TEARAWAY OF THE RIGHT ENGINE (FROM THE RIGHT WING) OCCURRED AS THE AIRPLANE LIFTED OFF FROM THE RUNWAY. THE AIRCREW CONTINUED THE TAKEOFF, THEN RETURNED TO THE AIRPORT & LANDED SAFELY. THE PLANE WAS STOPPED & SHUTDOWN ON THE RUNWAY. AN EXAM OF ITS WING & SEPARATED ENGINE REVEALED THE AFT CONE (ENG MOUNTING) BOLT HAD FAILED FROM FATIGUE, THEN THE TWO FORWARD CONE BOLTS FAILED FROM DUCTILE OVERSTRESS. A RAISED MECHANICAL DEFORMATIVE WAS FOUND ON THE CONICAL SURFACE OF THE AFT BOLT. THE DEFORMITY WAS INDICATIVE OF DAMAGE PRODUCED PRIOR TO OR DURING ASSEMBLY OF THE CONE BOLT IN THE ISOLATION MOUNT. A MATCHING CAVITY WAS NOTED ON THE ISOLATION MOUNT. THERE WAS EVIDENCE THAT THE MECHANICAL IRREGULARITY RESULTED IN A NONUNIFORM FIT WHICH ALLOWED THE TORQUED FITTING TO LOOSEN DURING CYCLIC LOADING. AD 88-01-07 REQUIRED THAT ULTRASONIC INSPECTION OF THE CONE BOLTS BE PERFORMED AT INTERVALS NOT TO EXCEED 600 CYCLES. AN ULTRASONIC INSPECTION OF THE BOLTS HAD BEEN ACCOMPLISHED ABOUT 330 CYCLES BEFORE THE ENGINE SEPARATED.

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

PREVIOUS DAMAGE TO THE AFT CONE (ENGINE MOUNTING) BOLT, WHICH RESULTED IN MISMATCHED SURFACES BETWEEN THE BOLT AND ISOLATION MOUNT, LOSS OF TORQUE DURING CYCLIC LOADING OF THE MOUNTING BOLT, AND SUBSEQUENT FATIGUE FAILURE OF THE BOLT.

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