Image: Southwest jet
Ross D. Franklin  /  AP
A Southwest Airlines plane is seen Monday in a remote area of Yuma International Airport in Arizona after the plane had a section of fuselage tear off during a flight on Friday.
updated 4/5/2011 12:06:14 PM ET 2011-04-05T16:06:14

A five-square-foot hole opened in the roof of a Southwest Airlines jet as it was cruising at 36,000 feet on Friday afternoon. Passengers reported hearing an explosive noise and feeling "air being sucked out" of the cabin, but the pilot was able to land the plane safely and there were no major injuries. Why wasn't anyone blown through the hole?

Seat belts and safe distance. As the Explainer described six years ago, the phenomenon of explosive decompression is very real: People and objects can indeed be forced out of an airliner at cruising altitude when a large hole opens in its fuselage. This results from the substantial pressure difference between the external atmosphere at high altitudes and the air inside a pressurized cabin: When the cabin is breached, air rushes outward all at once. While such an event can involve thousands of pounds of force, it's focused in the area immediately surrounding the hole. A passenger seated just a few rows from a five-square-foot hole could probably hold himself down without a seat belt. Anyone who was wearing his seat belt would be very unlikely to sail through the gap, regardless of location. (That is, assuming their seats remain bolted to the floor.) The few people who have slipped through holes not big enough to bring down a commercial aircraft — and there are a handful of famous examples — were located right next to the breach and weren't wearing a seat belt low and tight across the hips.

How risky might the latest incident have been for passengers? The pressure differential between the cabin and the outside was approximately 7.5 pounds per square inch, and the hole measured 720 square inches. That means the maximum force applied would have been around 5,400 pounds — more than enough to blow an unrestrained person out of the plane. But a passenger would only feel that much force if he were literally plugging the hole with his body.

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Keep in mind that the hole was not right next to any passengers or beneath their legs. It was in the ceiling of the cabin, which would have been at least two to three feet away from the heads of anyone sitting inside. At that distance, the force of an explosive decompression would be greatly dissipated. Air-flow patterns are complicated, and it's impossible to quantify this effect for any given passenger. But as a simple way to visualize the effect of distance, we might imagine the force spreading itself out across the surface of an expanding hemisphere centered on the hole. Using the formula for spherical surface area, we see that at a distance of three feet, the 5,400 pounds of force would be spread across an area of 8,143 square inches. Four feet from the hole, it would cover 14,476 square inches, and so on. As the force gets more and more stretched out, the proportion of it that would be working to push a body out of the plane diminishes. On top of that, the air would be free to flow around the passengers in their seats, which would make its impact still weaker. Since the hole was in the ceiling of the plane, any decompressive force would have had to act against the full body weight of any passengers, lifting them up and out of their seats before it could eject them in a gust of air.

Here's an experiment that illustrates the principle: Plug your sink drain with a rubber stopper, fill the sink with water, and then slowly remove the stopper. You'll notice that the stopper is much harder to budge when it's close to the hole and perfectly aligned with it. But tilt the plug slightly, or pull it out a little bit, and the force becomes barely noticeable. That's more or less analogous to what happens when a hole opens in a Boeing 737 at cruising altitude.

A brief review of similar incidents shows how unlikely it is for a passenger to fly out through a hole. In 1973, a window shattered (PDF) on a National Airlines DC-10, and the passenger sitting directly next to it had left eight inches of slack in his seat belt. He was blown out of the plane, but the person next to him didn't go anywhere.

In 1988, an enormous hole — at least 10 times bigger than the one from Friday — opened up in the roof of an Aloha Airlines jet. (The plane involved in that incident was, like the one from Friday, a Boeing 737 with metal fatigue caused by thousands of take-offs and landings.) While passengers seated directly beneath the hole suffered some cuts, concussions, and broken bones, only one person fell to her death — a flight attendant who happened to be standing in the aisle directly beneath the hole.

In 1990, one of the cockpit windows blew out of a British Airways BAC-111. The pilot, who had loosened his belt, was pushed halfway out the window. He saved himself by grabbing the controls with his feet, and the flight attendants managed to hold him steady during the 20-minute emergency landing. The co-pilot didn't budge.

Got a question about today's news?Ask the Explainer.

Explainer thanks George Bibel of the University of North Dakota, a Boeing consultant and author of Beyond the Black Box: The Forensics of Airplane Crashes.

That Didn't Suck

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Video: Southwest struggling to recover from air scare

  1. Closed captioning of: Southwest struggling to recover from air scare

    >>> good evening. while they are fortunate to have avoided a disaster in the air tonight southwest airlines has a big problem. so does the maker of the 737. so do thousands of people with plans to fly. the issue is cracks in the fuselage of some boeing 737s. most of the cracks too small to see, but one of them grew large enough to open up a hole in the roof of a southwest jet a few days back. that incident has now triggered the inspection of more 737s across the country. this is going to bring new attention to our short-haul aircraft in this country. in the air for multiple flights per day and carrying a lot of the passenger load. nbc's tom costello who covers aviation for us, starts us off from washington tonight. good evening.

    >> reporter: hi, brian. the f.a.a. is talking about more frequent inspections while southwest says of the 79 planes grounded over the weekend three were found to have small cracks and 64 have been returned to service already. nearly 72 hours after southwest flight 812 made the emergency landing in arizona the f.a.a. is ordering checks on specific groups of the 737 300s, 400s and 500s with at least 30,000 cycles. the plane involved in friday's emergency had nearly 40,000 cycles on it. investigators were surprised to find signs of pre-existinging cracking under a lap joint and along a rivet line. a large chunk of the fuselage will undergo metal fatigue tests at the lab in washington .

    >> it was not believed that this was an area that could fail until we see it now.

    >> reporter: today's f.a.a. inspection order affects 175 planes worldwide. 80 in the u.s. the vast majority at southwest . investigators are wondering whether planes used on short haul flights with multiple takeoffs and landings like southwest routes incur greater metal fatigue as the skin of the plane expands and contracts.

    >> i think part of this is not only inspecting the plane but looking at how often the planes are used, how often they take off, how often they land.

    >> reporter: the 737 involved had a 14-year maintenance overhaul in march of 2010 . those overhauls involve stripping the plane to its frame and going over every centimeter looking for corrosion or cracks but inspectors rarely look under the lap joints where this was found. a former chief for the f.a.a. says it's stunning how quickly this crack turned into an emergency.

    >> this is the worst decompression pilots plan for. getting down as quickly as you can.

    >> reporter: after cancelling several hundred flights over the weekend southwest cancelled another 70 today as it continued inspecting its planes. southwest says it hopes to be back to normal service by tomorrow and says the planes the f.a.a. today ordered to be inspected are the planes it has been inspecting, no more. brian?

    >> tom costello in washington tonight.


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