Why did the roof of a Boeing 737 rupture in mid-air?

The aluminum skin of a Southwest flight ruptured mid-air on Friday, April 1st. A hole formed in the roof, and caused the pilot to make an emergency landing in Yuma, Arizona.

First, listen to a Turlock couple tell the Modesto Bee what happened on the flight.

You must be wondering: Is it safe to fly? The Southwest Airlines Flight 812 incident is rare. But just how unusual is it? While cracking is a natural part of aging for an aircraft, this wasn't necessarily true in this case. In fact, the part of the aircraft that suffered structural damage was in an unlikely spot, according to The Seattle Times.

It turns out the 1-by-5 foot hole is a known phenomenon called fatigue cracks. The aluminum skin of an airplane is held together by lap joints. However, when cracks form underneath the surface of the fuselage on older planes, this can spell trouble. In this case, the fasteners holding the skin of the aircraft had fatigued and failed before they were supposed to. The plane only flew less than 40,000 times: It wasn't even due for inspection yet.

The Turlock folks were one of the first people to share their side of the story. In all, there were 118 passengers on the Southwest flight, all collectively experienced the explosive decompression at 34,000 feet. In general, the passengers on that flight were lucky. Only one flight attendant was injured, and more importantly, there were no fatalities.

However, previous accident reports tell another story about the metal failure problem that exists in the airline industry. According to USA Today, if the hole is big enough, people can go right through... they can literally get sucked away into the sky. In other cases, metal failure has been known to cause the plane to disintegrate in flight and the consequences have been deadly.

Following the Southwest incident, the Federal Aviation Administration issued an emergency directive that required inspections for early Boeing 737 models- 300/400/500 series airplanes. The thing is, not all damage can be seen visually. To spot cracking in a certain part of the aircraft, FAA requires electromagnetic inspections to check for damage to the metal skin.

The FAA, however, describes the damage a little more technically:

This emergency airworthiness directive was prompted by a report indicating that a Model 737-300 series airplane experienced a rapid decompression when the lap joint at stringer S-4L between the body station (BS) 644 and BS 727 cracked and opened up. Investigation showed that the cracking was located in the lower skin at the lower row of fasteners. The airplane had accumulated 39,781 total flight cycles and 48,740 total flight hours. This condition, if not corrected, could result in an uncontrolled decompression of the airplane.

In other words, the material cycled as the aircraft pressurized and depressurized during each takeoff and landing. A lifetime of pressurizing and depressurizing takes its toll in the form of cracks and ultimately cause the material to fail. After the FAA demanded emergency checks on passenger jets, five more planes also showed signs of cracks.

In a press conference, Chairman Deborah Hersman of the National Transportation Safety Board showed off parts of the plane that ruptured. Watch it in full here.

The key parts of the plane will be investigated further in a NTSB laboratory in Washington D.C. But so far, investigators say the 812 flight to Yuma had pre-existing fatigue cracking, according to CBS.

In light of the incidence, Boeing says its 737-Next Generation planes won't have the same issues as the older models.

Still... how exactly did the crack cause such a big hole in the roof in the first place? To find out more, I asked Robert Ritchie, a professor at the University of California at Berkeley, about the Southwest situation and the future of aviation.

SmartPlanet: Can you tell me what happened to the Southwest flight on Friday?

RR: The Southwest Airlines event last week was quite a surprise. The delamination of sections of the airframe is of course the so called "aging aircraft problem".

It arises from what's called "multisite damage" - not one propagating crack that could be readily detected but lots of tiny cracks which cumulatively reduce the strength of sections of the metallic airframe.

It arises primarily where the various aluminum alloy plates that make up the airframe are fastened together, specifically at the fastener holes (like rivet holes).

These are locations where there is minute relative motion between the adjoining sections of the airframe, which leads to fretting/fretting fatigue.

This in turn leads to tiny surface cracks that can grow by fatigue, exacerbated by the fact that moisture can collect in the crevices between the plates and in the fastener holes which results in local surface corrosion and faster (corrosion fatigue) damage rates.

SmartPlanet: But this isn't the first time we've seen this kind of fatigue damage. Can you tell me about what happened in the 1998 Aloha Airlines accident?

RR: After the Aloha Airlines incident in Hawaii in April 1998 - also a Boeing 737, which underwent explosive decompression after a hole formed in the roof of the aircraft - the FAA and Boeing sponsored extensive research into this problem, how to detect it and when it might become a problem.

The surprise with the Southwest Airlines incident is that it occurred in much younger planes. The cause is open to speculation - maybe we don't understand as much as we thought about multisite damage but it's more likely, in my opinion, that this occurred more rapidly in Southwest's fleet as they're a short haul airline with lots of take-offs and landings associated with their short flight profile.

SmartPlanet: What does this mean for the future of aviation?

RR: If Boeing doesn't fully understand the damage accumulation on metal airframes which they're been making for decades, how will they be able to manage the polymer-matrix composite airframes on the pending Dreamliner, is a much more complicated issue.

Although such polymer-matrix composite airframes have been used for some time in military aircraft and in the tail sections of current commercial planes, the Dreamliner is the first time that they have been used for the main fuselage in major commercial passenger planes.

SmartPlanet: Why does it take so long to change the skins and other materials?

RR: The aircraft industry is a very conservative industry. Because safety is of paramount importance, it has moved very slowly with the use of newer and hence less tried and tested materials. Boeing has been one of the most conservative players - which has served it well. Airbus has been far more innovative in its choice of airframe materials (such as the GLARE laminated aluminum composites in its latest planes). However, now Boeing has leapfrogged Airbus with the fiber composite airframe of the Dreamliner.

Time will tell if this is a prudent move, but the reality is that we definitely have far less understanding of the mechanisms of damage and their rates of evolution in these materials, compared to the current aluminum alloy airframes which have been the mainstay of the commercial aircraft industry for 50 years or so.

Updated on April 6, 2011.

Related on SmartPlanet:

• Metallic glass is stronger and tougher than steel (video)
• New metallic glass is stronger and tougher than steel

This post was originally published on Smartplanet.com