Incident Overview
On August 27, 2016, about 0922 central daylight time, Southwest Airlines flight 3472, a Boeing 737-7H4 (registration N766SW), experienced a failure of the left engine while climbing through flight level 310 en route to its assigned cruise altitude. The flight, operating under Title 14 Code of Federal Regulations Part 121, was en route from Louis Armstrong New Orleans International Airport to Orlando International Airport with 2 pilots, 3 flight attendants, and 99 passengers. Portions of the left engine inlet departed the airplane, and fragments from the inlet struck the left side of the fuselage, creating a hole. The cabin depressurized, and the flight crew declared an emergency and diverted to Pensacola International Airport, where the airplane performed an uneventful single-engine landing. No occupants were injured, though the airplane sustained substantial damage.
Sequence of Events
The flight was the first of the day for both the airplane and crew. The left engine started at 0903:21, followed by the right engine one minute later. Air traffic control cleared the flight for takeoff at 0907:08. The climb was uneventful until 0921:45, when the cockpit voice recorder captured a "bang" sound and a decrease in engine rpm. Flight data recorder readings indicated the altitude was 31,259 ft at 0921:45, and the left engine fan speed dropped from 99% to 39% rpm by 0921:49. At 0921:52, the captain requested the first officer to declare an emergency; three seconds later, the first officer contacted ATC, reporting the emergency and that the aircraft was descending. At 0922:06, the crew completed the Engine Fire or Engine Severe Damage or Separation checklist. The cabin altitude warning horn activated at 0922:17. By 0922:18, left engine fan speed had decreased to 33% rpm.
ATC informed the crew that Pensacola was approximately 70 miles ahead. At 0922:51, sounds of oxygen mask use were recorded, and two seconds later, ATC cleared the airplane to flight level 180. At 0926:47, the captain communicated with cabin crew about the engine failure and instructed them to secure the cabin. At 0929:22, the crew noted high vibration levels and intended to maintain speed. The flight data recorder stopped at 0929:37 when the aircraft was about 17,400 ft and the left engine fan speed was 17% rpm.
At 0931:38, the crew advised cabin crew that no evacuation would be needed. Oxygen masks were removed at 0932:10. Further vibration comments were made at 0935:17. ATC assigned the ILS runway 17 approach at 0937:39. The crew selected flaps and autobrake as part of the approach. At 0939:10, ATC cleared the approach, and the crew completed the descent and approach checklists. The airport was sighted at 0940:01, gear was lowered at 0940:13, and vibration was again noted at 0941:40. The 500-ft automated callout occurred at 0941:51, and the airplane touched down at 0942:30, about 21 minutes after the engine failure.
Aircraft and Engine Information
The airplane was equipped with two CFM International CFM56-7B24 turbofan engines. The accident left engine (ESN 874112) was assembled by General Electric Aviation. It had accumulated 60,790 flight hours and 36,329 cycles since new parts; its last shop visit was in August 2007. The fan blades were part number 340-001-022-0 (later reworked to 340-001-027-0 per CFM Service Bulletin 72-0253). The engine had a history of removal and repair; the fan blade set had been reworked and reinstalled multiple times. The inlet, manufactured by UTC Aerospace Systems, attached to the engine at the fan case A1 flange with 24 bolted assemblies.
Certification and Testing Background
The CFM56-7B engine was certificated by the FAA and DGAC (later EASA) under 14 CFR Part 33. To demonstrate compliance with fan-blade-out (FBO) requirements, CFM conducted eight development rig tests and two engine certification tests. The tests showed that after a fan blade release, fragments could penetrate the fan case and inlet if not properly contained. Based on test results, CFM redesigned the fan case and strengthened the blade axial retention feature, while Boeing instructed the inlet manufacturer to add a containment shield. The Boeing 737-700 was certificated under Part 25 in December 1997, with the design intended to prevent inlet separation during an FBO event.