The National Transportation Safety Board will likely take more than a year to determine what caused the catastrophic failure of an engine on Southwest Flight 1380, rupturing the cabin and killing a passenger. No surprise, then, that nobody’s waiting for the final verdict to try to stop it from happening again.
The NTSB says the engine failed after one of the blades that make up the fan at the front of the CMF56-7B engine sheared off, at 32,500 feet. Investigators found signs of metal fatigue in the blade’s stumpy remains. Here, “fatigue” essentially means weakening—a possible result of subjecting metal alloys to the extreme temperatures and heavy loads that come with every flight. The regular expansion and contraction of the metal can exaggerate the smallest defects, like micro fractures, to the point where they become dangerous.
So the CMF56-7B, made by CFM International (a joint venture between Safran and General Electric) and bolted to 6,700 planes around the world, is getting a lot of extra attention. Southwest crews will spend the next 30 days inspecting hundreds of its CFM engines, according to Reuters. And the Federal Aviation Administration says it will issue an airworthiness directive within the next two weeks, requiring all airlines run an ultrasonic inspection of all 24 fan blades on every CFM56-7B they use, after it has been through a certain number of takeoff and landing cycles.
The ultrasonic bit is important, since the fatigue on the blown engine was on the interior of the snapped blade, according to the NTSB, where it would have been hard to spot in a visual inspection.
Much like a doctor inspecting an expectant mother, technicians go back and forth over each blade with a hand-held sensor, pulsing ultrasonic waves through the metal, looking for defects. The results don’t came back as an image, but more like an EKG graph, says Antonios Kontsos, an expert in structural fatigue and failure detection at Drexel University in Philadelphia. Cracks in the metal show up as an abnormal signal. It’s laborious, time-consuming, and the best way to see inside these all-important metals.
The FAA and NTSB are already investigating another Southwest flight, in August 2016, where this type of engine failed in midair. The plane made an emergency landing in Pensacola, Florida, without injuries. Afterward, the FAA proposed voluntary airworthiness inspections for the CMF56-7B. This time around, it’s making them mandatory.
Still, modern jet engines are a paragon of reliability. Failures—ie, In-flight Engine Shutdowns—cause less than 3 percent of flight diversions. That’s largely because airlines have robust inspection and maintenance programs. As engines cost up to $30 million and are the main thing keeping air between the plane and the ground, they’re worth looking after. KLM, for example, says the CF6-80E, which powers its Airbus A330s, needs major maintenance about every 7,300 takeoff and landing cycles, and minor maintenance every 200 to 400 cycles.
At Delta’s Atlanta maintenance facility—which is the size of 47 football fields—techs dismantle entire engines. They clean and inspect every part, from the albatross-like fan blades to the tiny component inside the fuel injector. It takes them 50 to 80 days to do that, replacing the worn out bits and putting everything back together. They then haul the refreshed engine into a bunker-like concrete cell, where they run it at speed to verify it is indeed good as new. Only then do they bolt the thing back onto a jet wing and let it return to work.
Long before they get to fly, new engine types go through a bruising array of tests—ingesting water, ice, sand from all over the world, and dead chickens. And when they are in service, airlines collect reams of data on vibration, temperature, and speed, hoping to spot problems before they become catastrophic.
In the future, a new generation of ultrasound and infrared sensors, built into the engines, could detect structural defects before they present any danger at all. “It would be a paradigm shift, integrating diagnostics and prognostics,” says Kontsos, who is working with military and commercial operators to develop such systems. “You could infer the engine health as you fly or operate the device.”
As with all such aviation advances, it will be years, at a minimum, before such sensors can make their way onto real, people-packed airplanes. Until then, we’ll have to rely on the men and women building—and rebuilding—those engines to keep us in the air.