19980707026039I

On July 7, 1998, about 0640 central daylight time, a Boeing 737-7H4, N701GS, registered to and operated by southwest airlines company as flight 1565, a title 14 CFR part 121 scheduled domestic passenger flight from tampa, Florida, to Birmingham, Alabama, had failure of the number 2 engine during descent for landing at Birmingham. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The aircraft received minor damage. The airline transport-rated pilot, first officer, 3 flight attendants, and 91 passengers were not injuried. The flight originated from tampa, Florida, the same day, about 0549. The pilot reported to the operator that about 40 miles from Birmingham, while in descent for landing, the number 2 engine failed. Flight attendants reported to him that flame was visible coming from the number 2 engine exhaust and extending aft toward the tail of the aircraft. He did not receive a fire warning and did not activate the fire extinguishing system. He shut down the number 2 engine and made an uneventful landing at Birmingham. After landing, he stopped on the taxiway and shutdown the remaining engine. After fire department personnel on the ground examined the aircraft for fire and deemed it safe, the aircraft was towed to the gate where the passengers were deplaned via the jetway. Post crash examinaiton of the engine by an FAA inspector showed the tailpipe of the number 2 engine contained metal debris. The aircraft structure had no fire damage. Examination of the no. 2 engine was performed at general electric aircraft engines, strother, Kansas, following removal from N701GS at Birmingham, Alabama. Examination showed that the melted metal debris found in the tailpipe was from the low pressure turbine and that the N1 rotor was locked. All chip detectors were clean and there was no evidence of fuel in the engine oil. Diagnostic interrogation of the engine's electronic control unit (ecu) revealed the hydromechanical unit (hmu) had a fuel metering valve (fmv) feedback disagree fault on channel a and b and an fmv feedback out of range fault on channel b. the hmu was removed from the engine for testing. Testing of the hmu showed the resolver module's channel b sine output voltage became unstable when the hmu was tested on a flow bench with the test fluid heated to 180 degrees fahrenheit. The resolver modu le was removed from the hmu and tested separately. The tests confirmed that when the resolver module was heated above 160 degrees fahrenheit, the channel b sine output voltage became unstable. Disassembly of the resolver unit revealed a broken wire to the sine magnet stator. The ecu computer software logic allowed the unit to lock on the unstable channel b and lock out channel a. the unstable sine out voltage of channel b resulted in the ecu commanding the fmv to a full open position. With the sudden increase in fuel flow while the engine was at idle speed, the core of the engine stalled. Examination of the broken wire by the NTSB materials laboratory showed the wire had fracture features typical of ductile overstress separation and that the fracture surface was covered with lead-tin deposits. (See attached NTSB materials laboratory factual report) changes were made in the resolver unit design and manufacture process, the ecu computer software to improve handling of resolver failures, and a dispatch prohibition if a single channel resolver fault is indicated in the cockpit. (See attached powerplants group chairman report and cfm report-CFM56-7B uncommanded fuel flow increas e events). An additional party to the NTSB investigation was john h. middleton, kearfott guidance and navigation, black mountain, north carolina. *******the above narrative is from .4; incident was investigated by NTSB************