Southwest B733 at St. Louis on Dec 23rd 2013, bird strike causes uncontained engine failure

Last Update: January 13, 2018 / 19:50:33 GMT/Zulu time

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Incident Facts

Date of incident
Dec 23, 2013

Classification
Incident

Cause
Bird strike

Airline
Southwest Airlines

Flight number
WN-1091

Departure
Saint Louis, United States

Destination
Kansas City, United States

Aircraft Registration
N360SW

Aircraft Type
Boeing 737-300

ICAO Type Designator
B733

A Southwest Airlines Boeing 737-300, registration N360SW performing flight WN-1091 from Saint Louis,MO to Kansas City,MO (USA) with 110 passengers and 5 crew, was climbing out of Saint Louis' runway 29 when the aircraft received a bird strike and ingested birds into an engine (CFM56). The crew stopped the climb at 2500 feet, shut the engine down and returned to Saint Louis for a safe landing on runway 29 about 10 minutes after departure.

The occurrence became known only after a brief NTSB preliminary statement released on Apr 14th 2015 stating, that a post flight examination revealed a hole in the leading edge of the wing which was surrounded by red coloured organic debris as well as evidence that a piece of high speed debris had exited the engine through the outboard fan cowl near the fan plane. Two fan blades were found fractured below the mid span shroud, the debris ejected radially outboard, there was no damage to the fuselage. The NTSB rated the occurrence an incident and opened an investigation.

On Jan 11th 2018 the NTSB released their factual report reporting the aircraft was climbing through 1700 feet AGL when multiple birds impacted the right hand leading edge of the wing and the right hand engine. The crew declared emergency and returned to St. Louis.

The NTSB reported the birds were identified as male and female mallard ducks.

The NTSB wrote with respect to the damage:

On site examination of the airplane revealed a hole in the leading edge of the wing with its immediate surroundings splattered with red colored organic debris. The inner barrel of the RHE inlet cowl exhibited multiple impacts, gouges, and through-holes. There was no evidence of fuel or oil leaks from the engine. The engine and nacelle were removed from the airplane and sent to the Southwest Airlines Maintenance Training Building in Dallas, Texas for detailed examination.

Examination of the nacelle revealed that the inlet nose cowl had a dent at 5:30 o'clock location on the outer surface of the inlet lip, approximately 8 inches x 4 inches x 0.5 inches deep. The inlet inner barrel had multiple small punctures on the inner skin (airflow) side and two large thru-holes; the one at 2:30 o'clock location was approximately 3 inches x 2 inches in size and the other at 3:00 o'clock was approximately 5 inches x 4 inches in size. One fan blade fragment penetrated the outer skin of the inlet at the 3 o'clock location, creating a 7-inch long tear, in the shape of a fan blade chord, consistent with a piece of fan blade passing thru edgewise. The exiting direction of the uncontained fan blade particle was outboard, causing no damage to the fuselage.

Examination of the RHE revealed that all the fan blades were extensively damaged with two adjacent engine fan blades fractured transversely across the airfoil below the mid span shrouds. No penetration or breaches were observed in any of the engine cases, but the fan case exhibited several bulges that corresponded to hard impacts and missing fan blade rub strip material exposing the parent material below.

On Jan 13th 2018 the NTSB released their final report and docket with photos concluding the probable causes of the incident were:

The initial damage to the fan blades was caused by the ingestion of two mallard ducks that caused one or more fan blades to fracture, striking the fan shroud, as well as other passing fan blades, producing various sized blades fragments that created a cascading effect of collateral impact damage to the other fan blades, the fan case, and the inlet cowl. One large blade fragment was deflected out of the fan containment plane, which pierced and exited the inlet cowl in a benign direction.

The engine and airplane met the applicable bird and containment design standards since the engine did not catch on fire, no engine cases exhibited any penetrations, the engine was able to be shutdown normally, and the airplane damage did not impact the safe operation of the airplane or create a hazard to the persons on board.