The Blown Engine on Southwest Flight 1380 Suffered "Tooth Decay"

Mechanics say the engines wore down from sucking up debris on frequent takeoffs and landings.

A preliminary National Transportation Safety Board report on the fatal Southwest Airlines accident on April 17 has revealed that traditional engine-checking protocols didn’t go far enough to prevent a catastrophe, according to aviation experts. This could be a crucial development for airlines like Southwest that make frequent takeoffs and landings with their jets, because those extra cycles add to the wear and tear on engines taking in foreign material from runways.

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NTSB investigation
NTSB

“These engines are like big vacuum cleaners that suck this stuff off the ground,” says a former federal safety official who asked not to be named because he is not authorized to comment on this particular case. “Airline mechanics call it ‘tooth decay,’ and that fan section that is right up front is like your front teeth,” effectively Hoover-ing up detritus from the tarmac.

This engine had been rigorously inspected as recently as 2012. Still, nobody saw this disaster coming.

"Airline mechanics call it ‘tooth decay,’ and that fan section is like your front teeth."

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What They Missed

In its on the engine explosion aboard Southwest Flight 1380, the April incident in which debris punctured the cabin and one person was killed, the NTSB did not pin any blame on the airline industry, the engine-maker, or the FAA, all of whom had been aware of the risks of fatigue-related damage to engines, especially after an explosion aboard another Southwest 737-700 in 2016. This report was more of a “just the facts” outline, and the NTSB has said it will take up to 18 months to complete the investigation and determine a cause.

Damage to leading edge of left wing of Southwest 1380.
NTSB
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That said, the report did reveal more about the history of the engine that blew. It is one of the popular CFM56-7B models made by a consortium of GE and France’s Safran. This particular engine had last been through an extensive inspection in 2012, the NTSB said, using visual checks and a fluorescent penetration inspection, which means applying a dye designed to highlight small cracks that might not be visible to the naked eye. That check did not show any signs of cracks, the NTSB said. The plane made 10,712 additional takeoff and landing cycles between that 2012 inspection and last month’s deadly accident. (The engine has gone through more routine checks in the years since.)

The inspection methods used in 2012 met the requirements in place back then. But the most recent recommendations from the FAA and the engine-maker underscore the need for more sophisticated techniques to catch a sub-surface flaw like the one that caused this accident. An examination of the failed fan blade from Flight 1380 after the fact did confirm signs of fatigue, NTSB said.

Fracture surface with fatigue indications.
NTSB

“Now we know that the traditional way wasn’t adequate," our maintenance expert says. "So what do you do? If you are the NTSB, you say ‘kick it up a notch’ and use sub-surface techniques” such as ultrasound to probe for signs of damage.

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Airlines are already under an emergency order from the FAA and its European equivalent, EASA, to conduct these advanced checks on engines with more than 30,000 cycles within a few weeks. Carriers have until August to inspect engines that have logged up to 20,000 cycles.

A Tick-Tock of Tragedy

Picture of window 14 with portion of engine inboard fan cowl
NTSB

The report also laid out in detail the timeline of the accident, which forced the flight crew of Flight 1380 to make an emergency landing after the left engine exploded about a half hour into a flight from New York to Dallas. The engine blowout sprayed shrapnel into the fuselage. A piece of the engine cowling cracked a window, causing the cabin to depressurize. A woman sitting next to the window was partially sucked out of the plane and later died from injuries caused by “blunt force trauma,” likely from exposure to flying debris.

The report says the cockpit crew was aware of the dire situation and accelerated the approach for landing, even though this move made the plane harder to control. Right after the explosion, the plane “rolled left to about 40 degrees before the flight crew was able to counter the roll with control inputs,” the NTSB said, basing its account on interviews with the crew and information from the flight data recorders. “The flight crew reported that the airplane exhibited handling difficulties throughout the remainder of the flight.”

After captain Tammie Jo Shults took over flying duties from the first officer, the crew prepared for an emergency landing in Philadelphia, amid what they described as “ loud sounds,” and other distractions. “The captain initially was planning on a long final approach to make sure they completed all the checklists, but when they learned of the passenger injuries, she decided to shorten the approach and expedite landing,” the report says. For one passenger, at least, it was already too late.

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