NTSB Identification: MIA98IA193.
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Scheduled 14 CFR SOUTHWEST AIRLINES COMPANY
Incident occurred Tuesday, July 07, 1998 in BIRMINGHAM, AL
Probable Cause Approval Date: 02/22/2001
Aircraft: Boeing 737-7H4, registration: N701GS
Injuries: 96 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 data recorder showed that as the flight descended through 2,875 feet pressure altitude on approach to land, the no. 2 engine fuel flow increased followed by the EGT. The engine N1 speed was at 25.6 percent and decreased to 12.8 percent. Engine cutoff was performed 74 seconds after fuel flow increase began. An uneventful landing was made. Examination showed a broken wire in the no. 2 engine electronic engine control, hydromechanical unit, fuel metering valve resolver Channel B. The broken wire was due to damage that was incurred during the manufacturing process and caused unstable Channel B sine out voltage when the resolver was heated above 160 degrees Fahrenheit. The fuel control computer software locked on Channel B, disabled Channel A, and due to the unstable sine out voltage, commanded the fuel metering valve to go to a full open position. The large increase in fuel flow with the engine at idle speed resulted in the engine core stalling and heat damage to the low pressure turbine.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The improper electronic control unit software logic that permitted the electronic control unit to lock on a failed hydromechanical resolver, which failed due to damage to a wire that was incurred during the manufacturing process, resulting in an uncommanded and sudden increase in fuel flow to the engine that damaged the turbine and caused the engine to lose power. Full narrative available
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