United Airlines Boeing 757-222 incident
Boeing 757-222 · Incident: airframe, component or system failure en route
From Philadelphia International Airport (PHL) to Los Angeles International Airport (LAX)
Event
- NTSB case
- DEN93IA027
- Event typeAccident: someone killed or seriously injured, or the aircraft substantially damaged (NTSB definition). Incident: an occurrence that affected or could have affected safety, short of that. Glossary
- Incident
- InvestigationHow far the investigation had got when the record was published: preliminary, ongoing, final or completed. Unknown where the source does not say. Glossary
- Completed
- API-reported fatalities
- 0 · all aircraft and ground
- Ground fatalities
- 0
- OperationWhat kind of flying it was, grouped by the rules it flew under: airline (US Part 121), air taxi and commuter (Part 135), general aviation (Part 91 and similar), military or government. Glossary
- Airline
- WeatherVisual meteorological conditions (VMC): good enough to fly by looking outside. Instrument conditions (IMC): cloud or low visibility, flying by instruments. Glossary
- Visual Meteorological Cond
Boeing 757-222
- Aircraft type
- Boeing 757
- Category
- Airplane
- RegistrationThe aircraft's tail number, such as N12345 or G-ABCD. Registrations are reissued, so the same one years apart can be a different aircraft. Glossary
- N508UA
- Operator
- United Airlines
- Onboard fatalities
- Unknown
- Route
- From Philadelphia International Airport (PHL), Philadelphia, PATo Los Angeles International Airport (LAX), Los Angeles, CA
- Aircraft age
- Not recorded
- Flight rulesThe regulations the flight operated under: in the US, Part 91 (general aviation), Part 121 (airlines), Part 135 (air taxi and commuter) and others; for flights abroad, the NTSB's coarser commercial or non-commercial code. Glossary
- Part 121: Air Carrier
- Phase of flightThe stage of the flight when things started to go wrong: standing, taxi, takeoff, initial climb, en route, maneuvering, approach or landing. Glossary
- En route · cruise
- First occurrenceThe single coded event the NTSB judges best describes what happened (records from 2008 on). Older records name the first occurrence in the sequence instead. Glossary
- Airframe, component or system failure
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- None
Cause areas
- Electrical system, generator › Shorted
- Warning system(other) › Shorted
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
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).
Probable cause
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).
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record DEN93IA027
- Event ID
- 20001211X11816
- Case number
- DEN93IA027
- Dataset
- historical-pre2008
- Source SHA-256
- 89e2df6d848e7ab3b42e7d7c7a03ff403303057a26ca04c95b3ea67201c3ce74
Source notes (4)
- Unreviewed is an editorial label, not an investigation status. API-sourced is not report-checked or human-reviewed. Explicit event totals are used without summing aircraft injury tables; onboard allocation is withheld. Unknown values remain unknown. The operation category is mapped from the NTSB-reported FAR part and has not been reviewed.
- Filled from the NTSB case API where the bulk record had no value: local date, investigation status, damage. Values present in the bulk record are kept.
- The date is the local date; the NTSB stores the UTC date 1993-02-14.
- API snapshot SHA-256: 60fee022b86effa76d8391b5fc900d8ca7f2ea27cbbf168656bc6444d21fd63f; retrieved 2026-09-18T18:40:59.091Z.