A US National Transportation Safety Bureau (NTSB) investigative hearing into the April inflight engine failure of a Southwest Airlines Boeing 737-700 on Nov. 14 honed in on the design and testing of the CFM56-7B engine fan blades and the containment capability of the engine inlet and fan cowl.
Questioning representatives of Boeing and engine manufacturer CFM International, NTSB investigators compared the April accident to an engine failure on a Southwest 737-700 in August 2016. Both were “fan blade out” (FBO) events stemming from cracked fan blades. Since the 2016 accident, inspections have detected eight cracked fan blades, four on Southwest aircraft, CFM reported.
Following the earlier accident, CFM, the 50/50 joint venture of GE Aviation and France’s Safran Aircraft Engines, recommended a series of corrective actions through service bulletins, but they did not apply to the fan blades in the later accident.
On April 17, the pilots of Southwest Airlines flight 1380 made an emergency landing at Philadelphia International Airport (PHL) after the left CFM56-7B engine of the 737 failed, blowing out a nearby window and causing the cabin to depressurize.
One of the 144 passengers on board was killed—the first fatality involving a US passenger airline since February 2009—and eight passengers suffered minor injuries. There were five crew aboard the planned flight from New York La Guardia Airport to Dallas Love Field.
NTSB investigators responding to the scene discovered that one of the 24 titanium alloy fan blades was missing from the damaged engine, and there was evidence of metal fatiguewhere the blade separated. The engine cowling was found in Bernville, Pennsylvania, about 70 mi. northwest of the airport.
The accident was reminiscent of the Aug. 27, 2016 uncontained engine failure on a Southwest 737-700 that forced pilots to divert to Pensacola International Airport, Florida, during a flight from New Orleans to Orlando. In that case, the NTSB found that one fan blade had separated from the fan disk because of fatigue cracking. Debris from the CFM56-7B engine inlet damaged the aircraft’s fuselage, wing and empennage.
Boeing developed the design requirements for the CFM56-7 engine inlet and fan cowl, which it provided to United Technologies Aerospace Systems (UTAS) to build the structure. FBO containment capability was not required in the initial inlet design, according to a UTAS presentation.
Based on a rig test conducted in 1995, Boeing issued a revised design specification requiring containment capability for a “forward-spiraling fan blade fragment.” The production inlet design that went through engine certification test in 1996 included an inlet containment shield that performed its intended function in two bird strikes.
During certification, the FAA would expect the airframe manufacturer “to show that anything that departed the engine during a fan blade out event wouldn’t impact the wing or the empennage,” said Victor Wicklund, representing the agency’s Transport Standards Branch. “They would have to demonstrate that whatever they lost wouldn’t jeopardize safe flight and landing.”
Following the Pensacola accident, CFM implemented a set of corrective measures, recommending eddy current, or electrical current, testing of fan blades when an engine is in the repair shop and ultrasonic fan-blade inspections for on-wing engines, said CFM International CFM56 engineering leader Mark Habedank. Previously, maintainers examined for cracks through fluorescent penetrant inspections.
The engine manufacturer issued the first related service bulletin in March 2017, which later was revised. Based on the most recent bulletin, ultrasonic inspections using a probe now are recommended every 1,600 engine cycles.
NTSB board member Bella Dinh-Zarr, who chaired the investigative panel, explained the hearing was a fact-finding exercise to assist the board in determining probable cause of the accident.