WEST PALM BEACH, FL, USA
N17356
Boeing B737-300
During climb after takeoff, the flight had an uncontained failure of the No. 1 engine. The flight returned to the departure airport and landed without further incident. Examination of the airplane showed there was damage to the left wing high lift devices, fuselage, vertical and horizontal stabilizers, rudder, and tail cone. The No. 1 engine left thrust reverser had separated. Debris from the No.1 engine was located over an eight block long by three block wide city block area. Examination of the No. 1 engine revealed that it had a 360 degree separation in plane with the high pressure turbine (HPT) rotor and that the HPT shaft and the HPT forward rotating air seal had exited the engine. The HPT rotating air seal was recovered in the debris field. Metallurgical examination showed the rotating air seal had two radial fractures that passed through the bore and the HPT front shaft-to-HPT disk attachment bolt holes. Fatigue cracking was identified in one attachment bolt hole which initiated from a heat affected layer. The heat affected layer was caused by abusive machining during the manufacturing process due to loss of coolant during the drilling process. Based on striation count, the fatigue crack was at detectable length during two in-service inspections, but was not detected.
HISTORY OF FLIGHT On September 12, 1999, about 0711 eastern daylight time, a Boeing 737-300, N17356, registered to ICX Corporation, and operated by Continental Airlines, Inc., as flight 1933, a Title 14 CFR Part 121 scheduled domestic passenger flight from West Palm Beach, Florida, to Houston, Texas, had a failure of the No. 1 engine shortly after takeoff from West Palm Beach. Visual meteorological conditions prevailed at the time, and an instrument flight rules flight plan was filed. The airline transport-rated pilot, second pilot, 3 flight attendants, and 84 passengers were not injured. The flight was originating at the time of the accident. The captain stated he was in the right seat and acting as a check airman. The second pilot, who was flying the aircraft from the left seat, was receiving initial operating experience as a captain. Both pilots stated that after takeoff on runway 9 left, as they were climbing through about 1,000 feet, there was the sound of a loud bang, they felt a extremely violent jolt, and felt severe shaking of a short duration. The No. 1 engine gauges went to zero. There was no indication of fire. The second pilot continued to fly the airplane and the captain ensured the No. 2 engine was increased to full power. They entered a left downwind leg for runway 9 left and returned to West Palm Beach International Airport, where they landed without further incident. A witness stated he was working on the roof of the Palm Beach Post building. He was to the north of the airplanes flight path, and was looking at the left side of the airplane. He saw flames shoot from the left engine that lasted about 2 seconds, and then the engine exploded and the flames went out. A black smoke cloud remained at the point of the explosion. He then saw a large piece of the airplane falling from the sky. He did not see any more smoke or flame coming from the left engine and the airplane turned to the north and then to the west and was lost from sight. PERSONNEL INFORMATION The captain (who was acting as a check airman) holds a airline transport pilot certificate with a Boeing 737 type rating. Continental Airlines, Inc. reported that at the time of the accident he had accumulated 10,500 total flight hours with 6,500 flight hours in the Boeing 737. The second pilot (who was receiving initial operating experience as a captain) holds a airline transport pilot certificate with a Boeing 737 type rating. Continental Airlines, Inc. reported that at the time of the accident the second pilot had accumulated 10,000 total flight hours with 550 flight hours in the Boeing 737. AIRCRAFT INFORMATION The airplane is a Boeing 737-3TO, serial number 23942, registered to ICX Corporation. Continental Airlines, Inc. reported the airplane had accumulated 34,602 total flight hours at the time of the accident and 222 flight hours since the last inspection. The No. 1 engine is a CFM International, model CFM56-3-B1, SN 720957, that had accumulated 31,732 total flight hours time since new, 16,496 cycles since new, 5,604 hours time since overhaul, and 2,771 cycles since overhaul at the time of the accident. The last maintenance visit for the engine was at General Electric Engine Services Inc., Strother, Kansas, from July - October 1997. The engine was removed by Continental on July 27, 1997, and was sent to GE Strother because of damage to the high pressure compressor 6th-stage compressor blades. The engine was returned to service on October 27, 1997. Prior to the 1997 shop visit, the engine was overhauled by GE Strother in September 1996. The No. 1 engine forward rotating air seal was forged by Wyman-Gordon, Grafton, Massachusetts, and was shipped on January 24, 1986, to NCI Inc., Asheville, North Carolina, for finish machining. There was no maintenance review board (MRB) activity at Wyman-Gordan. The forward rotating air seal was finished machined as PN 1282M72P03, SN NCE24161, and was Fluorescent Penetrant Inspected (FPI). There was no MRB activity for the rotating air seal at NCI. NCI shipped the forward rotating air seal on July 31, 1986, and Continental installed it in CFM56-3-B1 engine, SN 721641, on January 1, 1987. According to the GE Strother's "Received Offlog Configuration" sheet, dated October 10, 1988, the HPT rotor, which includes the forward rotating air seal, was removed from engine SN 721641 and was not disassembled. A GE Strother's "Onlog-Offlog Comparison Final Report", dated April 10, 1989, indicated that the HPT rotor removed from SN 721641 was installed on CFM56-3-B1 engine SN 721729 where it remained until January 1993. The forward rotating air seal had accumulated 5,426 hours TSN and 2,222 CSN when installed in engine SN 721729. According to GE Strother's "Cycle Limited Parts Disposition Sheet", dated January 29, 1993, Work Order (W/O) 905422, the HPT rotor was removed from engine SN 721729 and the front shaft was replaced. Overhaul sheets, marked W/O 905422, indicated that HPT blades were replaced but that the forward rotating air seal, HPT disk, and HPT rear shaft were not disassembled and inspected. Final build sheets indicate that the HPT rotor was installed on CFM56-3-B1 engine, SN 720957, the accident engine, on April 23, 1993, where it remained until October 1996. The forward rotating air seal had accumulated 17,555 hours TSN and 8,289 CSN when installed in engine SN 720957. In October 1996, GE Strother removed the HPT rotor from engine SN 720957, disassembled it, and shipped the forward rotating air seal to GE Engine Services, Aviation Component Service Center (ACSC), Cincinnati, Ohio, for inspection and repair. ACSC paperwork (W/O 26092C) indicated that a visual and FPI was performed on the forward rotating air seal on October 25, 1996, with no nonconformances noted. The FPI was the first in-service inspection of the forward rotating air seal and the second inspection since it was manufactured. A seal teeth coating replacement repair, CFMI Engine Manual Task 72-52-03 was also performed. GE Engine Services ASCS issued a FAA Form 8130-3 Airworthiness Approval tag for the forward rotating air seal on November 1, 1996, and shipped it to GE Strother were it was reinstalled on engine SN 720957. Engine SN 720957 remained in operation until July 1997. The forward rotating air seal had accumulated 27,460 hours TSN and 14,102 CSN when repaired by ACSC. Continental removed engine SN 720957 and shipped it to GE Strother because of damage to the HPC 6th-stage blades. The HPT rotor was disassembled and the forward rotating air seal was FPI'd on September 16, 1997, with no nonconformances noted. The FPI was the second in-service inspection of the forward rotating air seal and the third inspection since it was manufactured. GE Strother's maintenance records also indicate that a part re-marking repair was performed because the part number had become illegible. The forward rotating air seal was reinstalled on engine SN 720957 where it remained until the accident. The configuration comparison final report for engine SN 720957 indicated that the forward rotating air seal had 29,362 hours TSN and 15,067 CSN when installed. The forward rotating air seal had accumulated 5,604.3 hours TSO, 2,771 CSO, 34,966 hours TSN, and 17,838 CSN at the time of the accident. The forward rotating air seal has a published service life limit of 20,000 cycles, and at the time of the accident, 94 percent of the service life had been utilized. METEOROLOGICAL INFORMATION The West Palm Beach International Airport 0653 surface weather observation was clouds 20,000 feet scattered, visibility 10 statute miles, altimeter setting 29.92 inches Hg., temperature 81 degrees F., dewpoint temperature 73 degrees F., wind from 030 at 3 knots. FLIGHT RECORDERS The cockpit voice recorder (CVR) was a Fairchild model A100, serial number 6670. The CVR was taken to the NTSB, Vehicle Recorder Laboratory, Washington, D.C., after the accident for readout. Information contained on the CVR was recorded while the aircraft was parked after returning to West Palm Beach International Airport after the failure of the No. 1 engine, and did not contain any information recorded at the time of the failure of the No. 1 engine. A transcript was not prepared. The digital flight data record (DFDR) was a Sunstrand model UFDR, serial number 6386. The DFDR was taken to the NTSB, Vehicle Recorder Laboratory, Washington, D.C., after the accident, for readout. Readout of the DFDR revealed that at FDR Subframe Reference Number 772 seconds, the airplane was at 167 knots airspeed, at an altitude of 1,196 feet. Engine No.1 and engine No. 2 N1 values were 89.5% and 90.9%, respectively. The following second, N1 for engine No. 1 dropped to 25.7% and airplane pitch angle dropped from 16.1 degrees nose up to 12.8 degrees nose up. Vertical acceleration dropped to its lowest recorded value at 0.574 G's. Over the next 17 seconds, N1 for engine No.1 decreased to approximately 12% and pitch angle decreased to approximately 4 degrees before leveling off at 6 degrees. Vertical acceleration values increased and remained at approximately 1 G. N1 for engine No. 2 decreased to 82.4% at 775 seconds before increasing to 97.5% at 788 seconds. Airplane altitude increased to a maximum value of 1,367 feet after the No. 1 engine failure and airspeed was maintained at approximately 160 knots prior to the airplane starting to accelerate 7 seconds after the No. 1 engine failure. WRECKAGE AND IMPACT INFORMATION Postcrash inspection of the airplane revealed there was an uncontained failure of the No. 1 engine, along with damage to the wing's high lift devices, the fuselage, vertical and horizontal stabilizers, and the tail cone. Debris from the engine fell over an 8 x 3 city block area off the departure end of runway 09L, and caused damage to vehicles and residences; however, no one on the ground was injured. Examination of the airplane and No. 1 engine nacelle revealed that the left-hand thrust reverser had departed the airplane intact, the vertical stabilizer had three major penetration wounds, the vertical fin front spar lower truss fitting left hand chord was severed, and the rudder skin was torn. The left-hand thrust reverser was recovered intact in the debris field along the flight path of the airplane. The left-hand thrust reverser exhibited a puncture wound in the upper hinge beam aft of the middle hinge. The No. 1 engine was severed 360 degrees circumferentially in plane with the HP rotor. The aft section of the engine was sagging downward from the rear engine mount about 10 degrees creating a 13-inches axial x 30 degrees circumferential gap at about the 5:00 to 6:30 o'clock positions. Everything forward of the combustion case was intact with no noticeable damage except for minor impact damage to external components and parts. Large pieces of metal were found in the engine tailpipe at the 6:00 o'clock position. The 4th-stage LPT blades were intact and exhibited minor damage when examined from the rear. The combustion case rear flange was torn from the rest of the case from about the 7:00 to 1:00 o'clock positions and was not recovered. The combustion case skin from the mid flange area on back to the rear flange was missing from about the 4:00 to 7:00 o'clock positions. The remaining part of the case skin was distorted, fragmented, and pushed outward and forward. A piece of the combustion case rear flange was still attached to the LPT case from about the 1:00 to 7:00 o'clock positions. An 18-inch piece of the combustion case mid flange area was recovered in the debris field along the airplane's flight path. The HPT nozzle inner support was fractured and pushed radially outward and forward about 6-inches at the 10:00 o'clock position. The integral HPT shroud/LPT nozzle support was almost completely fractured aft of the 5th-stage cooling air holes, and most of the support was completely separated from the engine. The remaining pieces were pushed outward and forward from the 3:00 to 12:00 o'clock positions creating an opening about 6 inches axially. Remaining sections of the HPT cooling air distributor located outboard of the HPT shroud were pushed outward through this opening. Five HPT nozzle segments were found still attached and in place from about the 6:00 to 9:00 o'clock positions. The nozzle segments were heavily impact damaged. Many nozzle segments were recovered in the debris field along the airplane's flight path and exhibited similar damage. The HPT inner and outer nozzle supports were also missing from the 9:00 to 6:00 o'clock positions. The HPT shrouds (outer air seal - stationary seal) from the 6:00 to 9:00 o'clock positions were installed. All the HPT blades were fractured transversely across the airfoil at various lengths and exhibited considerable leading edge and tip damage. The HPT blade front retainer was missing except for a 60 degree arc from about the 12:00 to 2:00 o'clock positions. The HPT disk was circumferentially fractured just forward of the HP disk-to-forward rotating air seal attachment flange transition radius, about 0.25 inches forward of the disk face. The HPT forward rotating air seal assembly (subsequently referred to as the forward rotating air seal), PN 1282M72P03, SN NCE2{4161} exited the engine. Two pieces of the forward rotating air seal were recovered in the debris field along the airplane's flight path. The smaller of the two pieces was an 12.5-inch long section of the 4-tooth seal. The entire piece was twisted about 90 degrees along its axis and the 4-tooth seal was heavily damaged with portions of the seal completed obliterated. The larger piece of the forward rotating air seal comprised about 2/3rds of the entire seal. This piece was heavily distorted, impact damaged, and exhibited two primary radial fractures that went through the bore. The two primary fracture surfaces bisected a bolt hole. The fracture surfaces were damaged and metal transfer was observed. The forward rotating air seal has two sets of knife edge seals: a 4-tooth seal that is located outboard of a 2-tooth seal. The entire 4-tooth seal was missing and a small piece of the 2-tooth seal remained and was rolled over. Pieces of the HPT shaft and the HPT disk attachment flange were still secured to the forward rotating air seal. The front HPT shaft fractured about 1-inch aft of the compressor discharge pressure (CDP) seal and just forward of the forward rotating air seal. The entire HP shaft exited the engine except for the pieces still attached to the CDP and the forward rotating air seals. Piece of the HPT shaft were recovered in the debris field along the airplane's flight path. The small center vent tube and the larger air duct were fractured 360 degrees in plane with the forward rotating air seal. MEDICAL AND PATHOLOGICAL INFORM,ATION The captain, first officer, three flight attendants, and 84 passengers were not injured. No toxicology testing was performed on specimens from the flightcrew. TESTS AND RESEARCH Pieces of the HPT shaft and the HPT forward rotating air seal were sent to the Safety Board's materials laboratory in Washington D.C. for initial metallurgical evaluation. The examination occurred September 21-23, 1999, with a representative from General Electric Aircraft Engines (GEAE) in attendance. After initial evaluation at the Safety Board, the forward rotating air seal was sent to GEAE Evandale, Ohio, for further evaluation. Nondestructive evaluation of the forward rotating air seal was conducted at the GEAE Quality Technology Center and destructive metallographic evaluation was conducted at the GEAE Materials Engineering and Process Department Metallography Laboratory. The examination at GEAE occurred September 27-30, 1999, with a representative of the Safety Board in attendance. Examination of the high pressure turbine shaft fracture surfaces with the aid of a binocular microscope revealed fracture features typical of overstress separations and areas obliterated by mechanical damage. No evidence of a pre-existing fracture was found. The recovered forward rotating air seal had two radial fractures that passed through the bore and the HPT front shaft-to-HPT disk attachment bolt holes. The thir
The fatigue fracture and separation of the high pressure turbine forward rotating air seal due to a manufacturing defect in a bolt hole that was not detected by the engine manufacturer due to inadequate and ineffective inspection techniques. Contributing to the accident was the engine manufacturers failure to provide adequate hole making requirements at the time the forward rotating air seal was manufactured and the engine manufacturers failure at the time of last inspection to require eddy current inspections for the high pressure turbine forward rotating air seal bolt holes.
Source: NTSB Aviation Accident Database
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