The Boeing 737–300 in question has been in service for 15 years. Is that a long time for this type of aircraft?
It is really not a question of age in years as much as it is a question of how many cycles the airplane has in 15 years. It appears that the business model that Southwest has is one in which some of their airplanes are already old and already have thousands of cycles when they acquire them, and they operate them so rapidly that they go up to this critical number of cycles faster perhaps than they would for a different airline. To give you an idea, a Boeing 737 might be designed for 70,000 cycles, something that might happen over 20 years of operation normally. Aloha Airlines Flight 243, which experienced explosive decompression in flight in 1988 that caused a piece of the roof to rupture, killing a flight attendant, was a Boeing 737–200 that had been through about 90,000 cycles when that incident happened.
What was it like for the passengers on board Southwest flight 812 experience when the cabin ruptured?
In these situations people who were sitting in an atmosphere that corresponds to about 5,000 feet [1,500 meters] above sea level all of a sudden are sitting in an atmosphere that corresponds to 30,000 or 35,000 feet [9,150 or 10,650 meters]. At that point, air in the body starts to escape, but the biggest terror for people would probably be the popping noise associated with the rupture, followed by a very rapid accumulation of condensation on the windows—which rapidly goes away.* Oxygen masks come down. It's an immediate procedure at that point for pilots, when they realize there's a rapid decompression in the airplane, to dive down to 14,000 feet [4,250 meters], because below that altitude is where almost any human being is capable of breathing. This type of dive occurs normally at about 4,000 feet [1,200 meters] per minute, and I guess that would be a terrifying experience also, because most of passengers are not going to realize that this is actually being done to save their lives. The pilot is quickly taking the airplane someplace where there is oxygen-rich air.
Would a five-foot by one-foot rupture such as the one Southwest flight 812 experienced greatly destabilize the aircraft for the pilot?
This was way too small a crack to be an issue in terms of the stability of the airplane.
The aircraft had been given its last "heavy check" in March 2010. How might the cause of the rupture have been detected ahead of time?
There are several techniques that are used, including the eddy-current technique and x-rays. These cracks do not just appear out of nowhere. It takes years before such cracks would cause a panel to fail. I cannot tell you why Southwest did not detect these cracks in their last major overhaul. An aircraft will fly for maybe 15,000 cycles before they start to inspect for cracks. Then, they will do it every 3,000 cycles, or something like that. Depending upon how quickly an aircraft racks up cycles, it may be anywhere from two years to six years between overhauls where they are actually looking for cracks. I have to say that I am surprised that they didn't catch these cracks in that particular airplane. Why? They will have to answer that.
What are they looking for during a normal preflight maintenance check?
During a preflight check, typically the co-pilot will do a walk around the airplane to check the airplane's wheels, sensors and external controls to make sure nothing is blocking them. A walk-around is not designed to catch cracks because these are usually microscopic.
*Editor's Note (4/08/11): Gudmundsson later clarified that there would have been a very rapid accumulation of condensation of humidity in the airplane's cabin that would appear like a fog (not clouding the windows, however) and then would quickly dissipate.