Southwest FLT 812 Decompression and diversion
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NASA Langley paper
The engineer types in the audience may enjoy this from NASA-Langley. It's directly applicable to the current unpleasantness.
Residual Strength Pressure Tests and Nonlinear Analyses of Stringer- and Frame-Stiffened Aluminum Fuselage Panels with Longitudinal Cracks (1998)
-drl
Residual Strength Pressure Tests and Nonlinear Analyses of Stringer- and Frame-Stiffened Aluminum Fuselage Panels with Longitudinal Cracks (1998)
-drl
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The tear along a riveted "lap joint" near the roof of the Boeing 737 above the midsection shows evidence of extensive cracking that hadn't been discovered during routine maintenance before the flight — and probably wouldn't have been unless mechanics specifically looked for it — officials said.
"What we saw with Flight 812 was a new and unknown issue," Mike Van de Ven, Southwest executive vice president and chief operating officer, said. "Prior to the event regarding Flight 812, we were in compliance with the FAA-mandated and Boeing-recommended structural inspection requirements for that aircraft."
[NTSB inspector] Sumwalt said that the rip was a foot wide, and that it started along a joint where two sections of the plane's skin are riveted together. An examination showed extensive pre-existing damage along the entire tear.
The riveted joints that run the length of the plane were previously not believed to be a fatigue problem and not normally subjected to extensive checks, Sumwalt said.
"Up to this point only visual inspections were required for 737s of this type because testing and analysis did not indicate that more extensive testing was necessary," Sumwalt said.
That will likely change after Friday's incident, he said.
"What we saw with Flight 812 was a new and unknown issue," Mike Van de Ven, Southwest executive vice president and chief operating officer, said. "Prior to the event regarding Flight 812, we were in compliance with the FAA-mandated and Boeing-recommended structural inspection requirements for that aircraft."
[NTSB inspector] Sumwalt said that the rip was a foot wide, and that it started along a joint where two sections of the plane's skin are riveted together. An examination showed extensive pre-existing damage along the entire tear.
The riveted joints that run the length of the plane were previously not believed to be a fatigue problem and not normally subjected to extensive checks, Sumwalt said.
"Up to this point only visual inspections were required for 737s of this type because testing and analysis did not indicate that more extensive testing was necessary," Sumwalt said.
That will likely change after Friday's incident, he said.
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You know I'm wondering if the 737's shape makes this problem worse. A plane with a mostly circular cross section will be less stiff to the sort of bending moment encountered in turbulence and during landings. The 737's football shape is certainly much more resistant to bending, meaning the energy does not go into deflection, rather, is absorbed internally by the structures.
-drl
-drl
DeSitter:
No it doesn't "make it worse". The fuselage cross section is one of the fundamental design decisions that establishes the performance and economics of an airliner. Furthermore a bending failure due to landing gear loads would have been lateral.
Like this one:
The 737 cross section is exactly the same as the B707 and B727 cross section (probably KC135 as well?), in other words exactly the same as the Thousands of other Boeing jets flying on any given day.
The fuselage failed longitudinally and the tear straps bonded into the structure functioned exactly as they were supposed to - stopping the crack from spreading.
The crack has nothing to do with the "age" of the aircraft and everything to do with the number of pressurisation cycles the fuselage has undergone during its life. We do not yet know if this failure was due to old age, a manufacturing defect, poor maintenance and overhaul procedures or a combination of all Three. Look up the term "hoop stress".
You know I'm wondering if the 737's shape makes this problem worse. A plane with a mostly circular cross section will be less stiff to the sort of bending moment encountered in turbulence and during landings. The 737's football shape is certainly much more resistant to bending, meaning the energy does not go into deflection, rather, is absorbed internally by the structures.
Like this one:
The 737 cross section is exactly the same as the B707 and B727 cross section (probably KC135 as well?), in other words exactly the same as the Thousands of other Boeing jets flying on any given day.
The fuselage failed longitudinally and the tear straps bonded into the structure functioned exactly as they were supposed to - stopping the crack from spreading.
The crack has nothing to do with the "age" of the aircraft and everything to do with the number of pressurisation cycles the fuselage has undergone during its life. We do not yet know if this failure was due to old age, a manufacturing defect, poor maintenance and overhaul procedures or a combination of all Three. Look up the term "hoop stress".
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Thanks for the lesson (didn't really need it), but I think you misunderstood. A cylindrical fuselage structure - as in the 757 and 777 - will be more easily bent, in a good way (one that dissipates energy) on landing. Any shape that is not precisely cylindrical will be much stiffer to bending moments as a whole structure. That means the energy will be internally absorbed into the structure as heat and lattice dislocations in the metal structure of the fuselage. That is what leads to cracks.
Now, I'm getting up in age, but I can still tell a cylinder from a football.
Now, I'm getting up in age, but I can still tell a cylinder from a football.
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Aircraft life
Lots of comments in this post are about OLD 10 to 15 year old jets.
10 to 15 years old IS NOT OLD. This sort of aircraft is designed for an economic life of at least 30 years 80000 hours and 80000 landings.
Lots of well financed or very credit worthy airlines rollover their fleet at between 10 and 20 years but this is purely a financial decision. It is not based of the aircraft being “worn out”.
10 to 15 years old IS NOT OLD. This sort of aircraft is designed for an economic life of at least 30 years 80000 hours and 80000 landings.
Lots of well financed or very credit worthy airlines rollover their fleet at between 10 and 20 years but this is purely a financial decision. It is not based of the aircraft being “worn out”.
De Sitter:
I'm afraid you misunderstand. The Boeing and Airbus engineers who design Aircraft cross sections are perfectly aware of the insignificant issue you raise.
What determines aircraft cross section is the internal dimensions necessary to provide the required seating layout and passenger space requirements. plus cargo containment, consistent with the available engine power to make the thing move. That is a function of the complex economic modelling that is required to maximise the profitability in service (and hence the saleability) of the finished product.
The fuselage shape has everything to do with economics. Third or fourth order metallurgical concerns such as those you mention are immaterial.
To put it another way, if the marketing departments and economic modellers decided a triangular fuselage cross section was optimum from a profitability in service point of view, then that is what the engineers will produce.
To put it yet another way, the 757 and 777 profiles are a result of economics, it has nothing to do with strength or fatigue. Those aircraft hulls will be designed to exactly the same limits (barring technology or regulatory change) as the B737 fuselage. This was all sorted out by about 1962.
Thanks for the lesson (didn't really need it), but I think you misunderstood. A cylindrical fuselage structure - as in the 757 and 777 - will be more easily bent, in a good way (one that dissipates energy) on landing. Any shape that is not precisely cylindrical will be much stiffer to bending moments as a whole structure. That means the energy will be internally absorbed into the structure as heat and lattice dislocations in the metal structure of the fuselage. That is what leads to cracks.
Now, I'm getting up in age, but I can still tell a cylinder from a football.
-drl
Now, I'm getting up in age, but I can still tell a cylinder from a football.
-drl
What determines aircraft cross section is the internal dimensions necessary to provide the required seating layout and passenger space requirements. plus cargo containment, consistent with the available engine power to make the thing move. That is a function of the complex economic modelling that is required to maximise the profitability in service (and hence the saleability) of the finished product.
The fuselage shape has everything to do with economics. Third or fourth order metallurgical concerns such as those you mention are immaterial.
To put it another way, if the marketing departments and economic modellers decided a triangular fuselage cross section was optimum from a profitability in service point of view, then that is what the engineers will produce.
To put it yet another way, the 757 and 777 profiles are a result of economics, it has nothing to do with strength or fatigue. Those aircraft hulls will be designed to exactly the same limits (barring technology or regulatory change) as the B737 fuselage. This was all sorted out by about 1962.
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From the AP:
It sounds like they had re-skinned about half their 737-300s, but not the one in question
Southwest operates about 170 of the 737-300s in its fleet of 548 planes, but it replaced the aluminum skin on many of the 300s in recent years, a spokeswoman said. The planes that were grounded over the weekend have not had their skin replaced.
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Maintenance Issues
Just finished reading a very interesting lead article in the MRO AMERICAS special report section of this weeks Flight International...interseting statistics, answers a lot of the questions posed by posters here and debunks a number of suggestions/implications including fleet ages fleet types maintenance quality and intervals...The Southwest comments are somewhat topical!
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I know boeing builds good planes. And builds them well. And I know the 737 has been flying since the middle 60's...first flight of type.
I flew the 737 and as many of you know, it pre presurizes slightly on the ground and fully depresurizes on landing.
I wonder if the plane might last a bit longer if it would simply wait till airborne to pressurize and completely depresurize at 200 feet agl or similiar altitude.
just a thought...just a pilot, not an engineer.
I flew the 737 and as many of you know, it pre presurizes slightly on the ground and fully depresurizes on landing.
I wonder if the plane might last a bit longer if it would simply wait till airborne to pressurize and completely depresurize at 200 feet agl or similiar altitude.
just a thought...just a pilot, not an engineer.
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I find this event a bit spooky ... I fly as pax on a particular 737-800 fleet very often and have formed the firm opinion that these things behave much as I might like ... except when I have heard k-nocking type sounds up behind the bins behind me a sort distance when I've sat in exit rows 16 or 17 over the wing.
It is only on one, two or three airframes that I have noticed it but I have noticed it 3 times in a couple of hundred flights. It was noticeable during the take off roll and maybe climb-out and in one case I think, on final approach or maybe it was on the landing roll, but not during the cruise. In fact I see I actually asked a question about it 3 years ago: What would cause a 738 to creak like a submarine at depth?
Of course I got over it then, and it might be nothing but this talk of missing rivets and engineers not discovering problems unless they go looking has as I say, spooked me a bit
It is only on one, two or three airframes that I have noticed it but I have noticed it 3 times in a couple of hundred flights. It was noticeable during the take off roll and maybe climb-out and in one case I think, on final approach or maybe it was on the landing roll, but not during the cruise. In fact I see I actually asked a question about it 3 years ago: What would cause a 738 to creak like a submarine at depth?
Of course I got over it then, and it might be nothing but this talk of missing rivets and engineers not discovering problems unless they go looking has as I say, spooked me a bit
Last edited by slip and turn; 4th Apr 2011 at 14:32. Reason: Link to my 3 y old thread: What would cause a 738 to creak like a submarine at depth?
I wonder if the plane might last a bit longer if .......
Whatever causes are found to be involved here would likley have still been present a year later in its life.
There likely is a light switch cause here and not an old age wear-out mode else there would be a lot more aircraft around the world with severe cracking at this number of flight cycles.
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From Olasek...
"I found discussion on another forum in relation to this accident that allegedly Southwest ask their pilots to do firm landings (hard landings?). I am not going to repeat the whole line of argument how this claim might relate to this accident but I would like to verify if there is any truth in it in case there are some SWA pilots around here."
Having flown the 737 since 1990 and for nearly 10 years now at Southwest, I can assure you that I have never seen or heard of any recommendation to perform a "firm" or "hard" landing. What is in our Flight Operation Manual is common language regarding putting the aircraft in the touchdown zone and words to the effect of "do not hold the aircraft off the runway in an attempt to make a smooth landing." I think we can pretty much rule out hard landings as the cause of the skin tear.
"I found discussion on another forum in relation to this accident that allegedly Southwest ask their pilots to do firm landings (hard landings?). I am not going to repeat the whole line of argument how this claim might relate to this accident but I would like to verify if there is any truth in it in case there are some SWA pilots around here."
Having flown the 737 since 1990 and for nearly 10 years now at Southwest, I can assure you that I have never seen or heard of any recommendation to perform a "firm" or "hard" landing. What is in our Flight Operation Manual is common language regarding putting the aircraft in the touchdown zone and words to the effect of "do not hold the aircraft off the runway in an attempt to make a smooth landing." I think we can pretty much rule out hard landings as the cause of the skin tear.
Slip and turn, the "creaking" noise you hear over the wing will be from the air conditioning ducts and/or filter/coalescers, not the fuselage. Someone just needs to tighten clamps or realign things a little.
For the non engineers here, the crack is a longitudinal crack, which means it is related to hoop stress caused by pressurisation. If it was anything to do with landing stresses which for those nautically knowledgeable induce "hogging" stresses, the crack would have been lateral, not longitudinal.
I am wondering if we may be seeing the results of a pattern of behaviour involving the supply and fitting of defective fuselage skins and doublers?
From 2005:
Frequent Flyer? Read this. Boeing Whistleblowers Say Planes Must Be Grounded
For the non engineers here, the crack is a longitudinal crack, which means it is related to hoop stress caused by pressurisation. If it was anything to do with landing stresses which for those nautically knowledgeable induce "hogging" stresses, the crack would have been lateral, not longitudinal.
I am wondering if we may be seeing the results of a pattern of behaviour involving the supply and fitting of defective fuselage skins and doublers?
From 2005:
Frequent Flyer? Read this. Boeing Whistleblowers Say Planes Must Be Grounded
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FOR IMMEDIATE RELEASE
Date: April 4, 2011
FAA Will Mandate Inspections for Early Models of 737 Aircraft
WASHINGTON, D.C. – The FAA will issue an emergency directive tomorrow that will require operators of specific early Boeing 737 models to conduct initial and repetitive electromagnetic inspections for fatigue damage. This action will initially apply to a total of approximately 175 aircraft worldwide, 80 of which are U.S.-registered aircraft. Most of the aircraft in the U.S. are operated by Southwest Airlines.
“Safety is our number one priority,” said Transportation Secretary Ray LaHood. “Last Friday’s incident was very serious and could result in additional action depending on the outcome of the investigation.”
“The FAA has comprehensive programs in place to protect commercial aircraft from structural damage as they age,” said FAA Administrator Randy Babbitt. “This action is designed to detect cracking in a specific part of the aircraft that cannot be spotted with visual inspection.”
The FAA airworthiness directive will require initial inspections using electromagnetic, or eddy-current, technology in specific areas of the aircraft fuselage on certain Boeing 737 aircraft in the -300, -400 and -500 series that have accumulated more than 30,000 flight cycles. It will then require repetitive inspections at regular intervals.
Last November, the FAA published a rule designed specifically to address widespread fatigue damage in aging aircraft. The rule requires aircraft manufacturers to establish a number of flight cycles or hours a plane can operate and be free from fatigue damage. The rule requires aircraft manufacturers to incorporate the limits into their maintenance programs.
Date: April 4, 2011
FAA Will Mandate Inspections for Early Models of 737 Aircraft
WASHINGTON, D.C. – The FAA will issue an emergency directive tomorrow that will require operators of specific early Boeing 737 models to conduct initial and repetitive electromagnetic inspections for fatigue damage. This action will initially apply to a total of approximately 175 aircraft worldwide, 80 of which are U.S.-registered aircraft. Most of the aircraft in the U.S. are operated by Southwest Airlines.
“Safety is our number one priority,” said Transportation Secretary Ray LaHood. “Last Friday’s incident was very serious and could result in additional action depending on the outcome of the investigation.”
“The FAA has comprehensive programs in place to protect commercial aircraft from structural damage as they age,” said FAA Administrator Randy Babbitt. “This action is designed to detect cracking in a specific part of the aircraft that cannot be spotted with visual inspection.”
The FAA airworthiness directive will require initial inspections using electromagnetic, or eddy-current, technology in specific areas of the aircraft fuselage on certain Boeing 737 aircraft in the -300, -400 and -500 series that have accumulated more than 30,000 flight cycles. It will then require repetitive inspections at regular intervals.
Last November, the FAA published a rule designed specifically to address widespread fatigue damage in aging aircraft. The rule requires aircraft manufacturers to establish a number of flight cycles or hours a plane can operate and be free from fatigue damage. The rule requires aircraft manufacturers to incorporate the limits into their maintenance programs.
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on my beloved DC9, douglas always kept an airframe in a pressure cycle tank that was double the number of the highest cycles of any active airframe.
does boeing do this for the 737?
does boeing do this for the 737?