NTSB Identification: DEN93IA027.
The docket is stored on NTSB microfiche number 50557.
Scheduled 14 CFR operation of UNITED AIRLINES
Incident occurred Saturday, February 13, 1993 in GRAND JUNCTION, CO
Probable Cause Approval Date: 11/03/1993
Aircraft: BOEING 757-222, registration: N508UA
Injuries: 47 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.

WHILE CRUISING AT FL390, THE AUTOTHROTTLES AND AUTOPILOT DISENGAGED, THE CAPTAIN'S INSTRUMENTS FAILED, AND THE COCKPIT LIGHTING CYCLED ON AND OFF. THE LEFT GENERATOR LIGHT ILLUMINATED, THE EICAS FAILED, A CYCLIC BANGING WAS HEARD, AND AN ACRID ODOR WAS NOTICEABLE. THE LEFT AC BUS AND TRANSFER BUS WERE DEENERGIZED AND THE CAPTAIN ELECTED TO DIVERT TO THE NEAREST SUITABLE AIRPORT. POST INCIDENT INVESTIGATION DISCLOSED A SHORTED DIODE IS THE LEFT GCU (GENERATOR CONTROL UNIT), A DEFECTIVE IDG (INTEGRATED DRIVE GENERATOR), A SHORT IN THE INPUT POWER FILTER IN THE EICAS (ENGINE INDICATOR AND CREW ALERT SYSTEM), AND A DEFECTIVE BTB (BUS TIE BAR).

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

FAILURE OF THE LEFT GENERATOR CONTROL UNIT (GCU), INTEGRATED DRIVE GENERATOR (IDG), RIGHT (SIDE) ENGINE INDICATOR AND CREW ALERT SYSTEM (EICAS), AND BUS TIE BAR (BTB).

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