WASHINGTON — Federal authorities on Tuesday recommended a sweeping overhaul on some of the most commonly used aircraft in the world as a result of last year’s Southwest Airlines engine failure over Pennsylvania that caused a passenger’s death.
The National Transportation Safety Board recommended that the plane’s manufacturer, Boeing, redesign the fan cover for the CFM56-7B engine and install it in all new 737s — described by NTSB chairman Robert Sumwalt as “the most popular single model out there” — and retrofit older models. More than 7,000 planes in service use that engine.
The NTSB did not recommend that any planes be grounded.
The redesign recommendation must be adopted as a regulation by the Federal Aviation Administration to require Boeing to comply.
“It is not enough to just prevent the failure; we must also actively work to find ways to minimize the effects of a failure if one does occur,” Sumwalt said after Tuesday’s hearing.
Boeing issued a statement Tuesday, though, that it would implement changes to meet the NTSB’s recommendations and was already working to redesign parts to be more resilient to damage from a fan blade failure.
“Our common goal is to help prevent similar events from happening,” the company stated.
Boeing did not include an estimate on how long the work would take or how much it would cost.
The recommendations increase pressure on Boeing, already under fire over faulty flight-automation software, on its 737 Max 8 jets, that was blamed in two fatal crashes in October 2018 and March 2019.
Southwest Flight 1380, carrying 144 people and five crew, was forced to make an emergency landing at Philadelphia International Airport on April 17, 2018, after a fan blade in one of two engines snapped. Pieces shredded the casing around the engine, which then struck the fuselage and shattered a window in Row 14. The plane depressurized, causing passenger Jennifer Riordan, 43, to be partially pulled through the hole. Two passengers helped pull her back inside, but she died of her injuries.
“We want the FAA to move on these recommendations,” Sumwalt said. “We feel they’re very critical.”
In the aftermath of the failure at 32,000 feet, First Officer Darren Lee Ellisor described hearing a loud bang and feeling the plane immediately begin to vibrate. Within two minutes, Captain Tammie Jo Shults — one of the first female fighter pilots in the Navy — took control of the plane and was able to land about 20 minutes later. The plane had been en route from New York City’s LaGuardia Airport to Dallas Love Field Airport.
Interviews with the flight crew described sudden chaos, as flight attendants rushed to get portable oxygen bottles and struggled to help passengers. Riordan’s window shattered 3.2 seconds after the engine failure, and the cabin decompressed in five seconds.
The FAA must give a written response to the NTSB recommendations within 90 days, a spokesperson said.
A cascade of unanticipated failures led to tragedy on Flight 1380, the NTSB found. Repeatedly, pieces of the engine and the cowling surrounding it didn’t perform as expected, causing the federal investigative agency to recommend either new testing techniques or a return to the drawing board for key components.
The failures weren’t predicted in part due to the technology available when the plane and its engine were certified in the 1990s, NTSB officials said. Technology developed since is more precise. One investigator, Brian Murphy, noted the industry typically does not regularly retest certified parts with the most advanced tools.
“We don’t go backward unless we have an event like this,” he said.
The blade itself snapped due to a fatigue crack near where the blade fastened to the engine. That crack was likely present as early as 2012 but was about 1/16 of an inch, officials found, virtually impossible to see with the naked eye. Other testing techniques weren’t refined enough to spot it. A similar break happened on another Southwest flight over Pensacola, Fla., in 2016, though no deaths resulted from that incident.
The crack itself was a result of the normal operation of the engine producing higher-than-expected stress. When the break happened, the blade was pointed earthward, and testing of fan blade failures had accounted only for blades separating while facing upward, toward the wing.
The blade was thrown forward at up to a 30 degree angle, farther than had been predicted. That contributed to the engine’s inlet, which controls air flow into the engine, tearing away from the nacelle , or housing. Fragments of the blade also struck the fan case, causing a displacement wave that rippled outward at supersonic speed along the inner case and outer cover.
Larger aircraft have circular cowls, but the 737s sit low to the ground when their landing gear is extended, requiring the cowl be flattened on the bottom. A radial restraint fitting at the bottom of the cowl prevents it from bulging downward over time.
Tests had predicted that restraint would break away during a fan blade failure, but on Flight 1380 it didn’t. Instead, it conducted the displacement wave through the cowl to an aft latch fitting, which spiraled around the outside of the nacelle and slammed into the fuselage, shattering the window.
The plane’s windows are made of three panels, investigators explained. Two outer panels ensure that the cabin’s pressure is maintained, while the inner layer is cosmetic. Plane windows are not designed to withstand a direct impact.
The CFM56-7B engine is installed with 24 fan blades, and there are about 356,000 now in service. Since the Pensacola incident, inspections have found 23 failed or cracked fan blades using more precise inspection tools. The FAA requires new fan blades be inspected with more sensitive tools than those used before the Philadelphia episode, after their first 20,000 cycles, and then again every 3,000 cycles. In a statement Monday, Southwest reported that it is exceeding federal regulations by testing blades every 1,600 cycles.
The NTSB wants Boeing to work with engine manufacturers to look at whether other airframe components are susceptible to serious damage at all the engine’s operating conditions. The NTSB issued an identical recommendation to the European Aviation Safety Agency.
The NTSB recommendations also urged Southwest to improve training for its flight attendants, who should have been in their jump seats during the emergency landing, to avoid possible injury, but were instead sitting on the floor. The agency’s board members, though, mitigated that finding with praise for the flight attendants’ conduct.
“They were dealing with a horrific situation, and one that they are trained for, but you would never expect to see it,” Sumwalt said. “I think under the circumstances, they did a nice job.”