Boeing 737 3H4 N360SW incident near St. Louis, Missouri

Probable cause

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.

NTSB analysis

On December 23, 2013, at approximately 1615 central standard time a Boeing 737-344, registration number N360SW, operated by SWA as flight 1091, and powered by two CFM56-3 turbofan engines, experienced a bird strike and ingestion on the No. 2 or right-hand engine (RHE) after takeoff from Lambert-St. Louis International Airport (STL), Missouri. As the airplane climbed through 1,700 feet, it impacted multiple birds causing damage to the RHE and wing. The pilot declared an emergency and returned to STL for an uneventful landing. There were no injuries reported to the 110 passengers, 2 flight crew and 3 flight attendants. The incident flight was a 14 CFR Part 121 domestic passenger flight from STL to Kansas City International Airport, Kansas City, Missouri (MCI). Day visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed.

Examination of the airplane revealed no damage to the fuselage. Examination of the engine revealed no penetration or breaches of the cases; however, the fan case exhibited several bulges that corresponded to hard impacts and missing fan blade rub strip material exposing the parent material below.

Examination of the RHE revealed that all the fan blades were extensively damaged exhibiting hard-body impact damage with significant material loss along the length of the leading edge. Two adjacent fan blades were fractured transversely across the airfoil near the one-third span, and exhibited leading edge soft-body impact damage. The remains of the two birds were identified as one male and one female mallard duck. The average weight of the male mallard is 1246 grams or 2.75 pounds; the average weight of the female mallard is 1095 grams or 2.4 pounds.

Comparing the airplane and engine damage to the requirements for bird ingestion and engine debris containment at the time the engine and airplane were both certificated revealed that the engine complied with the bird ingestion and containment requirements set forth in Parts 33.77 and 33.19 and the airplane complied with the containment requirements set forth in Parts 25.903.

Text from the NTSB. Open the NTSB docket for ENG14IA005 for the full record.