Uncontained Engine Failure

UNCONTAINED ENGINE FAILURE is the term you’ve been hearing, and it aptly describes what befell Southwest Airlines flight 1380 on on a flight from New York’s La Guardia Airport to Dallas on April 17th.

Basically, a jet engine can fail one of two ways. The first and more innocuous way is that it simply shuts down and ceases producing thrust. This is more or less akin to switching off the ignition in your car. Of course, all commercial jetliners have at least two engines, and can fly just fine should this happen. In fact, per certification requirements, should an engine quit even at takeoff speed while still on the runway, a plane still has enough power to become airborne and climb safely away — a performance buffer that all pilots are intimately familiar with; the so-called “V-1 cut” being a maneuver we practice regularly in the simulator.

More dangerous is the uncontained engine failure. As the name implies, this type of failure involves the high-velocity ejection of an engine’s internal components. The moving parts of a jet engine consist of a series of shafts and discs — fans, compressors, and turbines — spinning at tremendous speeds. Should any of this machinery fracture or otherwise come apart, whether from an unseen crack or some immediate trauma, the extreme centrifugal forces can send bits of metal straight through the cowling and into the airframe, potentially penetrating the cabin or even the fuel tanks.

Luckily this almost never happens. Aircraft engines are incredibly reliable, and unconfined failures are among the rarest type of malfunction. But when they do happen, the results can be deadly — as was the case aboard the Southwest 737. One passenger, a 43 year-old woman from New Mexico, was killed and several others were injured after shrapnel pierced the cabin and caused a window to blow out. The crew made an emergency landing in Philadelphia.

Two years ago, an uncontained engine failure on an American Airlines 767 touched off a fire that destroyed the aircraft on the runway. A similar incident involving a British Airways 777 occurred in Las Vegas in 2015. And in 2010, shrapnel from a failed engine caused a cascade of dangerous system failures aboard Qantas flight QF32, an Airbus A380 flying between Singapore and Sydney.

Nobody was killed in those accidents, but two passengers died in 1996 when an engine turbine disc on a Delta Air Lines MD-88 came apart on a runway in Pensacola, Florida. And, most infamous of all, over a hundred people died in 1989 when United Airlines flight 232 crashed in Sioux City, Iowa, after a fractured engine disc took out all of the widebody DC-10’s hydraulic systems (a disaster due more to a design flaw in the hydraulics system than fault of the engine, but still).

Maybe that all sounds scary — and the media, predictably, is going cuckoo over Southwest. But we’re talking about a small handful of fatalities over more than a quarter of a century. The death of the passenger on Southwest 1380 is the first fatality involving a U.S. major air carrier since 2005, when a Southwest 737 slid from a snowy runway in Chicago and collided with a car, killing a young boy.

The fuselage breach also caused the cabin to rapidly decompress. No doubt you’ve seen the photos of passengers with the plastic oxygen masks clamped to their heads. But although the noise, the masks, and the whole sudden-ness of it was, I’m sure, a scary moment for those on board, this would have been a perfectly manageable secondary problem.

Some decompressions are more hazardous than others. Bombs, for example, can cause an entire fuselage to tear apart in seconds. Large-scale structural failure, like the fuselage burst of an Aloha Airlines 737 in 1988, can be similarly disastrous. But those are rare occurrences. The vast majority of decompressions are harmless. Even sudden decompressions — such as when engine parts tear through a window, as apparently happened on Tuesday — are pretty easy to deal with. The pilots don their oxygen masks and initiate a rapid descent to a safer altitude (normally ten-thousand feet). Passengers, meanwhile, have ample supplemental oxygen if need be. An emergency descent might feel very abrupt, but it’s well within the capabilities of the airplane.

The crew of flight 1380 was essentially dealing with three situations at once: a failed engine, a decompression, and serious injuries to multiple passengers. Compound emergencies are never fun, and the pilots certainly had their hands full. But none of this required any seat-of-the-pants heroics, and despite what you’ve seen online or on TV, the plane was never in any danger of crashing. What to do, and how to do it, would have been pretty straightforward. Put a thousand pilots in that situation and you’d likely have the same outcome each time. They did exactly what they were trained to do, and what they were expected to do.

And on that note, please be wary of passenger accounts cited in the media. Claims that the jet was in “free fall,” was “diving toward the ground,” or was in any way out of control are simply untrue. I don’t blame anybody on the plane for being frightened, but passenger accounts in situations like this — more specifically, the way they are packaged and presented by the media — are notoriously inaccurate and prone to exaggeration, almost to the point of being totally unreliable.

Some follow-up FAQs…

Q: I fail to understand how a passenger wearing a seat belt could have been sucked through the window.

It’s gruesome to think about, but even with your belt on, it’s still possible to be at least partly ejected. The windows are low, and on smaller jets like a 737, you are positioned very close to them. A person’s body can stretch quite a bit, and if the decompression is powerful enough, the head and shoulders (and maybe an arm) could easily be forced through the opening.

Q: I’m curious why they landed in Philadelphia. The flight was very close to Harrisburg, Pennsylvania, when the problem happened. There is a decent- sized airport at Harrisburg, so why continue all the way to Philadelphia?

From Harrisburg to Philadelphia is less than a hundred nautical miles. In aviation terms that’s nothing. And don’t forget, they needed to descend several thousand feet. They could lose that altitude by spiraling down over Harrisburg, or they could lose it en route to Philadelphia, just a few minutes further away. Meanwhile, PHL has better emergency equipment, better passenger handling facilities, full-time Southwest staff, etc. If the plane had been on fire, sure, the objective would’ve been to get on the ground immediately, regardless of anything else. But this wasn’t that dire an emergency — injuries to the passengers notwithstanding.

Q: The 737 has wing-mounted engines. Does this accident imply that planes with aft-mounted engines are safer?

No, not necessarily. In the story above I reference the two people killed when the engine of an MD-88 came apart. That’s a plane with aft-mounted engines. Even with this type of configuration, the last few rows of seats are often adjacent to or behind the engines’ fan and compressor sections (this is one of the reasons it’s so damn noisy sitting in the back rows on these planes). And although the engines are further from the fuel tanks, they’re closer to the rudder, horizontal stabilizers, elevators, and other important components. It was the tail engine that broke apart on that United DC-10 in 1989, and there were a couple of disasters involving Soviet-built planes with aft-mounted engines (an IL-62 and a Tu-154, I believe) in which uncontained engine failures fatally damaged the tail section.

Reprinted with permission from Patrick Smith.

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