Posting in Science
This is the science behind the 812 flight to Yuma. Given the Southwest incident, what's the risk of flying?
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.
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- New metallic glass is stronger and tougher than steel
Apr 5, 2011
@kevinrs1 "Situations where a driver had no control or responsibility for what happened to them would actually be near 50%, as it was the other driver who was the cause." That's assuming 50% of drivers are causing serious accidents to other people which is a HUGE LIE! "I see your post as a kind of fear of flying post, like many of the others, " While you, are a hero, you don't care about gambling your life each time you fly and you're happy and grateful that companies don't give a s..t about you, because they don't even try to design a plane with protection features! Not only that, but you post against such a complain or possibility, rewarding the companies for their inhuman policies! Well, that's a case of extreme idiocy even if you work for such a company -congratulations!
@ Administrator. The problems with your arguments are, 1. you aren't the only one making the choices, the drunk driver/teenager on a joyride/unsafe driver/unsafe car is more likely to kill another person than the one that made the choice. Also, all of these choices have already been made for safety in airplanes. Your car is not required to have safety inspections of brakes etc periodically or be grounded, Drivers are mostly not highly trained and driving with copilots who could take over in an emergency, etc. Situations where a driver had no control or responsibility for what happened to them would actually be near 50%, as it was the other driver who was the cause. If 2 airplanes come anywhere near being close together, and INVESTIGATION happens, and either one or more people are going to be found to blame, and lose their livelyhood, or a rare flaw in procedures is found and changed. I see your post as a kind of fear of flying post, like many of the others, you would rather be in direct control, even though you don't have nearly the training, and are actually much safer with the experts.
@John T. Hill III "Statistically, being in a moving car in traffic is the most dangerous thing the great majority of us will ever do." Unfortunately "there are lies, damn lies and statistics". And unfortunately there are companies that promote them and naive people who believe them. The most important flaw in car accident statistics is that they include all driving conditions possible: The state of the driver, the car, the road, the weather, etc. This concludes that everyone has the same possibility to die in a car accident -which is a damn lie. The big difference between flying in commercial airlines and driving is choice. You only have a choice in the second case. Many choices actually: To drive slow or fast, carefully or carelessly, drive only in good shape, follow a safer path, avoid roads in bad condition, keep your vehicle in good condition, drive a safer vehicle, avoid bad weather etc. You also have a chance to react. The timing and quality of your reaction when something goes wrong, can make all the difference in many cases and depends mainly on how good a driver you are -your choice too. Situations where what happens to the driver is totally out of his responsibility and control are a small percentage -they are the exception, not the rule. On the contrary, you have no choice at all when flying in commercial airlines, no matter who you are or how able you are. And if something goes wrong, you can't react at all -you can't do anything about it!
I think you should be much more angered by the auto industry. Statistically, being in a moving car in traffic is the most dangerous thing the great majority of us will ever do. Compare the annual average numbers of deaths from auto accidents and airplane accidents and you'll see that flying is extremely safe. Factor in accidents per passenger mile for both transportation modes and air travel becomes even safer. Recall the Boeing airliner coming in for landing a good many years ago when a small plane flew right in front of it. The pilots pulled back on the yokes so far that the plane did a full barrel roll and then landed safely. And you say air travel isn't safe?
Blah, blah, blah, blah, blah, blah, blah, blah,.... Bottom line / answer: Because the planes are cr@p. Period. The aircraft companies always try to reduce the cost of the plane, the cost of the flight and so far they have reduced to zero the cost of the on board lives i.e they don't give a s..t about peoples' lives. Proof: They never tried to make a safe plane -ever. If something goes wrong, everybody dies. They are just satisfied with the statistics. "Shit happens" is their motto. Now if a company ever makes a really safe plane (a plane able to protect the lives of the crew and passengers in case of whatever mechanical failure) it will be instantly the #1 aircraft manufacturer in the world. No one would want to fly with a conventional death-jackpot plane anymore. This is 100% feasible. They just don't care.
@danny.sloop The "N" in NTSB stands for National, not Natural, not sure where you could have ever gotten that idea.
@colinnwn - "In short, I see no hard evidence for your allegation." -- You just did. Boeing can't explain the metal fatigue on these planes.
Don't you just love one media writer siting another media writer as an "expert" source when neither knows much about what they are writing about. The design may be flawed (a bit) in local areas but the overall airframe combined with a well trained crew brought everyone safely back to the ground in spite of gravity and a big hole in the cabin. YEA BOIBG!!
@dangnad You are completely wrong and misinformed. The 737 NG is a substantially redesigned, very different, and much safer aircraft than the legacy 737 models. The only substantial problem for the original variant was a weak rear wing spar, that was quickly remedied and I don't believe resulted in the loss of any life. In its day, the legacy models were on par with similar aircraft in the industry. Their hull loss and fatality records are normal for aircraft of that vintage. In short, I see no hard evidence for your allegation.
The U.S. should invest heavily in high-speed, low environmental impact trains. In this case, China may serve as a role model for us.
The Boeing 737 first flew in 1967. They've been "upgrading" that basic flawed design since then. They were hard-riding junkers when they first flew, still are. Now with their NG (Next Generation) models they just add rows of seats to the old design. I'm 6'1" and my back-to-knee length is longer than the back of the seat in front of me. The 767 was the next logical move forward after the 707, 727, 737 but they stopped at the 737. Beware! These piles of junk are going to continue to sprout skylights.
I remember an article in the news around 1988 titled "Aging Airline Fleets". After I read the article it convinced me not to fly again even with the statistical plus of the "safest mode of transportation". Now maintenance is done in many other countries with one licensed airline mechanic. Similar to the construction business having one licensed electrician, i.e., and the other people running wire using the one license. What are the requirements in other countries for inspection? Some maybe better and some worse than the U.S., and worse could be cheaper.
In my youth I had to fly and never thought twice about it. Then I married a guy who was an Air Inspector for the FAA. I never flew again. Nope, not happening.
@Willyvon4 No, it was mentioned 4 comments above yours. It was also pointed out here, and in various places, some of the lessons learned from Aloha prevented this incident from being worse, as well as the fact the Aloha plane had many more cycles on it than this aircraft. Fatigue in this area of the fuselage on this young of an aircraft was not expected. @16Tons Almost any aircraft with 30,000 cycles has some amount of cracking that has been reinforced or repaired. This area of the fuselage did not have previous cracking in the same region. When doubling (what you pejoratively refer to as band-aiding) is done, extensive engineering accompanies it to assure it is more than sufficient strength (and in many cases preferred over replacing the whole skin panel).
when you have the stress of pressure from air and the combined weight of the multi ton plane. you are asking for trouble when inspections are far and few between.
Am I the only one who remembers Aloha Flight 243 in 1988 wherein the whole center of the roof of the fuselage of a 737-200 ripped away in flight? Dejavu all over again? http://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243
Actually, djelliott & kmurphy, that was a close paraphrase of the USA Today article. In context, the meaning is that some incidences of metal failure have caused catastrophes rather than merely forced landings. If the 1st inspection for metal fatigue is at 40,000 flights, that would translate to over 18 years at 6 flights a day, 365 days a year.
Metal fatigue is nothing new, but since it had already been detected on this plane, it should have been 'properly' dealt with. Airlines should not be allowed to band-aid repair. That entire panel, and any other affected panels, should have been replaced. The frequencies that travel through the metal affect all that are conitguous.
Ditto on the writing. I had to read several sentences several times trying to find what I thought I was misreading. Like : "The plane only flew less than 40,000 times, even though it wasn?t due for inspection for another 20,000 flights." Huh?
The Aloha airlines accident brought to light a design component called a controlled breakaway zone. The design was intended to limit the size of the decompression damage. It was later determined a poor design actually contributed to the catastrophic failure of the entire top of the plane ripping off from the aloha Airlines plane from the back of the cockpit wall to above the wing. The one upside of this incident is at least it looks like design changes made after that accident worked and limited the damage.
yeah, the quality of the writing in these "smart-planet" articles cracks me up. I'm picturing some intern typing on an android phone during a break in a ESL class.
In other cases, metal failure has been known to cause the plane to disintegrate in flight and the consequences have been? deadly. This has to be the funniest line ever.
I think the NTSB is far from being the "Natural Transportation Safety Board ". Maybe the UTSB. "unnatural".