FTW97IA045

The airplane was departing when a thump was felt with a slight left nose pull, and the pilots aborted the takeoff. The pilots taxied clear of the runway and came to a stop. Several flight crews on the ground reported on the tower frequency that the incident airplane's left engine was on fire. The captain initiated evacuation of the airplane. The Airport Rescue and Fire Fighting (ARFF) personnel reported that the engine fire was confined to the tailpipe area. Turbine blades had separated from the number 4 turbine disk, penetrated the engine case, and subsequently damaged the engine pylon and empennage area. The teardown examination of the engine revealed that the Low Pressure Turbine (LPT) shaft was deformed and fractured into two pieces. Heavy coke deposits were identified on both the outer diameter of the LPT shaft and the inner diameter of the center tube. The two pieces of the LPT shaft were twisted and subjected to 'considerable' permanent deformation. The amount of plastic deformation was indicative of exposure to high temperatures. Additionally, the High Pressure Turbine shaft was found with a (one of three) 4 1/2 bearing stack retention pin missing.

History of flight

On November 23, 1996, at 1622 central standard time, an MD-82, N3507A, sustained an uncontained engine failure during takeoff roll at DFW Airport, Texas. The 2 pilots, 3 flight attendants, and 111 passengers were not injured; however, 3 passengers sustained minor injuries during the emergency evacuation of the airplane. The airplane was being operated by American Airlines as flight 1447 under Title 14 CFR Part 121. Visual meteorological conditions prevailed for the scheduled international passenger flight which was originating at the time of the incident. An IFR flight plan had been filed with Acapulco, Mexico, as the intended destination.

According to the captain, the airplane was departing on runway 17R when "on takeoff roll (approximately 120 knots), a thump was felt with a slight left nose pull." The onboard flight data recorder indicated that the pilots aborted the takeoff at 124.5 knots (V1 was 132 knots). The pilots taxied clear of the runway and came to a stop on taxiway L3. The flight attendants in the cabin area reported that there was "smoke and smoke odor" in the cabin. Several flight crews on the ground at DFW airport reported on the tower frequency that the incident airplane's left engine was on fire. The captain stated that, after he received these radio communications (about his left engine being on fire), he initiated evacuation of the airplane.

The flight attendants reported that the captain told them not to use the left aft emergency exit. The rear tail cone was deployed, the rear stairs were lowered, and a flight attendant estimated that approximately 40 to 60 individuals used them for evacuation. Another flight attendant reported that smoke was observed outside of the right over wing exits and she "commanded that the right window exits be blocked." She further reported that the left over wing exits were used to evacuate approximately 30 to 40 individuals. The third flight attendant deployed the left forward slide and she estimated that approximately 50 individuals evacuated through that exit.

A witness reported that "it took the fire equipment approximately 4 minutes to arrive at the airplane." The Airport Rescue and Fire Fighting (ARFF) personnel reported that the engine fire was confined to the tailpipe area and was extinguished in approximately 2 minutes.

Injuries to persons

After the evacuation was completed, three women requested medical assistance. Two of the women received treatment by paramedics and then continued their trip. The third woman was taken to a local hospital for x-rays and she continued her trip that evening.

Damage to airplane

Examination of the airplane revealed that all the turbine blade airfoils had separated from their roots in the number 3 and 4 turbine disks, and the engine case had one 12 inch by 3 inch hole in it on the inboard side of the turbine section. The cowling was punctured, the lower side of the engine pylon had sustained "impact damage," and the unpressurized empennage area had two puncture holes each approximately one quarter inch square.

The left engine was removed from the airplane and shipped to American Airlines Maintenance and Engineering Center in Tulsa, Oklahoma for disassembly and examination. The teardown examination of the engine revealed that the Low Pressure Turbine (LPT) shaft was deformed and fractured into two pieces. The adjacent 17 inch long High Pressure Turbine (HPT) center tube was found twisted and broken into several pieces. Extensive coke deposits were identified on both the outer diameter of the LPT shaft, and the front and aft ends of the inner diameter of the center tube. The designed clearance between the LPT shaft and the center tube is 93 mils; shaft variations due to vibrations and maneuvering loads will utilize 30 to 40 mils of that space (see the enclosed Powerplant Group Chairman's Report for details of the examination which was accomplished on December 4, 1996).

Aircraft information

The aircraft was built in 1989 and had two Pratt & Whitney JT8D-217C axial flow jet engines (21,000 pounds of thrust each) mounted on the empennage. The left engine had accrued a total of 25,072 hours of time, and the last major overhaul was performed 6,407 hours before the incident. Aircraft records indicated that the engine had no abnormal oil usage nor any unusual vibrations reported prior to the failure.

Tests and research

The NTSB Materials Laboratory examined the two pieces of the LPT shaft. According to the Materials Laboratory Report, the broken ends of the shaft were twisted and subjected to "considerable" permanent deformation. The amount of plastic deformation was indicative of exposure to high temperatures. The surfaces of the shaft adjacent to the fracture were dark blue to black and soot deposits were present. The fracture surface was also discolored dark blue (see the attached NTSB Metallurgist Group Chairman Report for details).

Pratt & Whitney requested that parts from the engine be sent to their facility in East Hartford, Connecticut, for further evaluation (see attached letter). Their staff did identify an anomaly; the HPT shaft was found with a (one of three) 4 1/2 bearing stack retention pin missing. Coke deposits were found inside and around the missing pin hole.

Additional data

Subsequent to this event, Pratt & Whitney initiated and is still involved (as this report is being written) in a study of the coke depositional processes, i.e., the temperature range, the oil particle size, and depositional surface characteristics. The project completion date is proposed for the end of 1998. Pratt & Whitney is also designing and testing a double-walled HPT center tube which engineers believe will reduce the coking on the LPT shaft.

The airplane was released to an American Airlines representative on November 24, 1996; the engine parts that were analyzed by Pratt & Whitney were released to American Airlines on November 17,1997.