Internal cracks had developed in a fan blade that exploded out of a CFM International CFM56-7B powerplant on 17 April, severely damaged a Southwest Airlines Boeing 737-700 and killing one passenger, according to the National Transportation Safety Board.
"The crack was interior," says NTSB chair Robert Sumwalt. "It was on the interior part of the fan blade [and was] certainly not detectable from looking at it from the outside."
"We are very concerned about this particular event. Engine failures like this should not occur, obviously," Sumwalt says. "If we feel that this is a deeper issue, we have the capability to issue urgent safety recommendations."
A Southwest Airlines 737-700 CFM56 engine of the same model suffered an uncontained failure in 2016 over the Gulf of Mexico, leading the Federal Aviation Administration in 2017 to issue an airworthiness directive requiring ultrasonic inspections of certain CFM56 engines.
Boeing delivered that aircraft and the 737-700 involved in the 17 April incident to Southwest in mid-2000. Neither Southwest nor engine maker CFM has said if the failed engines were original to the aircraft.
Sumwalt says the NTSB remains uncertain if the blades from the failed engine were subject to the FAA's 2017 directive; determining that information can be more difficult than expected because part numbers can change, he says.
But, he says the CFM56 has a history of reliable service. The engine type has logged more than 350 million flight hours and powers some 6,700 aircraft worldwide, CFM says.
"To be willing to extrapolate [the failure] to the entire fleet – I'm not ready to do that," Sumwalt says.
The fan blade broke in two places: near where the blade meets the engine hub and halfway along the length of the blade, Sumwalt says. The internal crack likely caused the break near the hub, which then caused the mid-blade facture.
The NTSB has recovered the hub-end of the blade, but not the outer portion.
Sumwalt made his comments from Philadelphia at the end of the NTSB's second day investigating Southwest flight 1380, a LaGuardia-Dallas flight that diverted to Philadelphia after the left-side engine failed.
The left CFM56-7B22 exploded as the aircraft climbed through 32,500ft, flinging shrapnel against the wing and the fuselage and breaking a window in row 14.
The broken window caused a pressure differential that sucked one female passenger partially out of the aircraft, according to reports. The passenger was outside the aircraft from the waist up, other passengers have said.
Other passengers hauled her back in, but the woman died, marking the first fatality caused by an accident on a US airline in nine years.
Following the explosion, the aircraft rolled left 41.3°, significantly greater than a typical roll, which tends to be no more than 25°, Sumwalt says.
"It would be alarming, because it was a rapid roll," he says, adding that the pilots levelled the aircraft a few seconds later.
About that time the aircraft's cabin altitude warning horn activated, indicating the cabin pressure had exceeded pressure equal to 13,500-14,000ft altitude, says Sumwalt.
The pilots landed the aircraft at Philadelphia International airport 22min after the engine explosion. Concerned about their ability to control the aircraft, the pilots landed with 5° flaps and at 165kt. Pilots typically set flaps at 30° or 40° for landing and touchdown around 135kt, Sumwalt says.
Parts of the engine cowling were found about 65nm (120km) from Philadelphia, but the NTSB has not recovered internal engine components.
Investigators will complete transcription of the cockpit voice recorder in the next several days, and the NTSB's maintenance investigators are in Dallas at Southwest's headquarters.
"They will examine the inspection records of this engine," Sumwalt says.
Today investigators interviewed flight attendants. They have also studied the window to determine how it failed and have removed the plastic interior wall of the cabin to study the fuselage, Sumwalt says.
The NTSB's investigation team will leave Philadelphia tomorrow, concluding the on-site phase of their work.
"We've got a long way to go," says Sumwalt.
CFM, which is equally owned by General Electric and Safran Aircraft Engines, say it has a team assisting the NTSB. The company says laws prohibit it from commenting about the accident.
CFM says the CFM56-7 "has compiled an outstanding safety and reliability record since entering revenue service in 1997".