Thursday, June 25, 2009

Final report: 2007 Southwest Airlines Boeing 737 uncontained engine failure

Damage to No. 2 engineOn November 17, 2007, a Southwest Airlines Boeing 737-300 aircraft (registration N676SW) suffered an uncontained engine failure in flight, during which pieces of the fan blades and the spinner separated from the number two (right) engine. No one was injured, but the aircraft sustained substantial damage to the engine and its housing components, as well as some damage to the fuselage and wing. The U.S. National Transportation Safety Board (NTSB) has issued a final report on the accident, concluding that the engine failure occurred "as a result of an unidentified object striking the spinner, separating it from the fan disk and causing the spinner to be ingested into the fan blades." [Click on the photo for a larger view.]

The Event

On November 17, 2007 the Boeing B737-300 aircraft, operating as Southwest Flight 438, departed Dallas Love Field en route to Little Rock with five crew members and 133 passengers on board. The number two engine, a CFM International CFM56-3B1, failed while the aircraft was climbing through FL250 to FL330. According to the NTSB report:
The flight crew reported feeling severe vibration, pulled both throttles back to idle, declared an emergency, and started an air turn back to Love Field (DAL), Dallas, TX.

While heading back to DAL, the pilot reported seeing the following cockpit warnings: a No. 2 constant speed drive (CSD) low oil pressure, No. 2 engine low oil pressure, No. 2 generator bus OFF, No. 2 pack trip OFF lights, and No. 2 engine vibration meter at 5 units.

The pilot also reported that while heading back to DAL, the start lever on the No. 2 engine was CLOSED. An uneventful single engine landing was performed and no injuries were reported to any of the occupants.
Airframe Damage

Once the aircraft was back on the ground it was discovered that there were "impact marks along the fuselage from about 10 feet aft of the right-hand forward entry door to about 6 feet forward of the right-hand aft entry door and along almost the entire length of right wing leading edge. The right horizontal stabilizer also exhibited impact marks along almost the entire leading edge." The aircraft's pressure vessel was not punctured.

Engine Damage

The NTSB reports that all of the fan blades in the number two exhibited heavy airfoil damage.
...all the fan blade roots remained installed in the disk, and several fan blades fractured near the platform.

The forward and rear spinner cones were no longer attached to the fan disk and a large penetration hole was noted on the right-hand side of the fan cowl just forward of the engine fan case.

No breaches of any of the engine cases or signs of fire damage were noted.
The pieces of the forward and rear spinners cones that exited the engine were never recovered, despite an extensive search on the ground. The missing engine components presumably fell to the ground in a sparsely populated rural area of Texas.

NTSB Findings

According to the NTSB report:
  • Metallurgical examination of the fracture surfaces of the fan blades and the fragments of the rear spinner aft flange revealed no preexisting fatigue-type mechanism and all the fractures were consistent with overstress.
  • Four sequential fan blade spacers were distorted consistent with a severe axial load rearward applied to the spacers by the rear spinner cone prior to the spinner cone release; however, no determination could be made as to the initial failure or what may have impacted the spinner cone.
Probable cause: "A total loss of engine power due to the No. 2 engine experiencing a release of its fan spinner through the fan cowl as a result of an unidentified object striking the spinner, separating it from the fan disk and causing the spinner to be ingested into the fan blades."

Here is the link to the NTSB final report: NTSB ID ENG08IA002 - June 22, 2009