History of Flight
On December 23, 2013, at approximately 1615 central standard time, a Boeing 737-344, registration N360SW, operated by Southwest Airlines as flight 1091, struck multiple birds after takeoff from Lambert-St. Louis International Airport (STL), Missouri. The aircraft was climbing through 1,700 feet when the birds impacted the right-hand engine (RHE) and wing. The pilot declared an emergency and returned to STL for an uneventful landing. There were no injuries among the 110 passengers, 2 flight crew, and 3 flight attendants. The flight was a 14 CFR Part 121 domestic passenger flight to Kansas City International Airport (MCI). Day visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed.
Engine and Nacelle Damage Examination
On-site examination revealed a hole in the leading edge of the wing, with red organic debris splattered around it. The inner barrel of the RHE inlet cowl showed multiple impacts, gouges, and through-holes. No fuel or oil leaks were found. The engine and nacelle were removed for detailed examination at the Southwest Airlines Maintenance Training Building in Dallas, Texas.
The inlet nose cowl had an 8 x 4 x 0.5-inch dent on the outer surface at the 5:30 o'clock position. The inlet inner barrel had multiple small punctures and two large through-holes: one at 2:30 o'clock (3 x 2 inches) and another at 3:00 o'clock (5 x 4 inches). A fan blade fragment penetrated the outer skin at the 3 o'clock location, creating a 7-inch tear, and exited outboard without damaging the fuselage.
All fan blades of the RHE were extensively damaged; two adjacent blades were fractured transversely below the mid-span shrouds. The fan case exhibited bulges corresponding to hard impacts and missing rub strip material, but no penetrations or breaches of the engine cases were observed.
Tests and Research
A USDA Wildlife Biologist collected tissue and feathers from the leading edge of the wing. Analysis by the Smithsonian Institution's Feather Identification Laboratory identified the remains as both male and female mallard ducks. The average weight of a male mallard is 1,246 grams (2.75 pounds), and a female is 1,095 grams (2.4 pounds). No damage to the fuselage was reported.
Additional Information
The CFM56-3 engine was certified under Part 33 (Amendments 33-1 through 33-6), with bird ingestion requirements at that time (Part 33.77) specifying a 4-pound bird. The engine did not catch fire, no case penetrations occurred, the pilot shut down the engine normally, and calculated imbalance loads from lost fan blade material were within certified limits.
Engine containment standards (Part 33.19) require rotor cases to contain damage from blade failure. The fan case bulged but had no exit holes or penetrations. No requirements exist for airplane manufacturers to design inlets or nacelles to contain engine debris. However, Part 25.903(d)(1) requires design precautions to minimize hazards from uncontained engine debris. Examination of the airplane found minor superficial gouging of the fuselage, left-hand wing, and one passenger window, none posing a hazard.