Southwest FLT 812 Decompression and diversion

[grumpyoldgeek]

KCRA 3 Insider

YUMA, Ariz. -- A Southwest flight headed to Sacramento from Phoenix had to be diverted to Yuma, Ariz. on Friday because of rapid decompression caused by a hole in the plane. Sacramento International officials said that Flight 812, a Boeing 737, had 118 passengers aboard and landed safely at Yuma Marine Corps Air Station/International Airport at 4:07 p.m. The airport is a dual-use facility. The plane descended 16,000 feet in a minute, according to the flight-tracking website called flightaware.com.

One passenger said they heard an explosion during the flight and could see daylight coming through a hole in the plane. The KCRA viewer sent in a photo that also shows oxygen masks dropping inside the aircraft.

"The hole is about 6 feet, oxygen masks are down," passenger Brenda Reese told KCRA 3. Reese said she was asleep when the rapid descent took place but heard a jarring noise.

A separate picture sent to KCRA 3 shows the hole above a luggage compartment.

The FAA said they have not confirmed what caused the decompression.

Sacramento airline officials said they have been told that all passengers are safe, and Marine medical personnel went aboard the plane to check on the passengers once they landed in Yuma.

Flight 812 was scheduled to arrive at 5:30 p.m. in Sacramento and is now set to arrive at 8:45 p.m. An FAA inspector is headed from Phoenix to Yuma to conduct an investigation. In July 2009, a Southwest Boeing 737 going from Nashville to Baltimore was forced to make an emergency landing in West Virginia after a foot-long hole opened in the top of the plane. Federal investigators later said that metal fatigue caused the hole to rip open in the roof of the plane. Nobody was injured in that case.

Former United Airlines pilot Jerry Blalock said pilots are trained in rapid decompression cases, and that it appears the pilot of Flight 812 landed the plane in textbook fashion. Reese said passengers started clapping for the pilot when the plane landed.

###

Hope the crew is enjoying a well-deserved beer for a great effort.
The reporting wasn't all that bad either.

[lomapaseo]

I did a quick search and a pic I found showed a hole of several square ft.

If confirmed, that is a big hole and I would expect some luggage to have been ejected

[VJW]

Having flown a 737 800 for 3 years now, I'm not sure I believe it could do 16,000 feet per min in an emergency decent. Recon you'd have to drop the gear to even obtain half that!

[VFD]

"The hole is about 6 feet, oxygen masks are down," passenger Brenda Reese told KCRA 3. Reese said she was asleep when the rapid descent took place but heard a jarring noise.
I caught some pics on another site and from the inside it appears that the hole/tear is at least 4 ft long. The previous fuselage problems have been holes about 8" x 12". We will have to wait on this one to see if there was a large failure from the start or airflow over the fuselage increased the size of the hole.

[deSitter]

Registration N632SW, 737-300, 15 yrs old, probably a lot of cycles.

[Greytraveler]

http://www.nytimes.com/aponline/2011/04/01/us/AP-US-Southwest-Flight-Diverted.html?hp

"It dropped pretty quick" this report indicates 6 ft hole

[blueirishPDX]

As reported on another forum, N632SW currently at 39,768 cycles and 48,722 hours.

[sprocky_ger]

Three questions from a dumb guy:
1. did something like this happen outside US also in the past?
2. did something like this happen with an Airbus already?
3. if I had to pay $7.5 million two years back: wouldn't I take care of such inspections or is it like: "Uhm, it happened once and the lightning will not strike the same place again?"

[Mark in CA]

probably a lot of cycles
On Southwest routes, that's pretty much a certainty.

[BrissySparkyCoit]

sprocky, you are not dumb.
Your questions however, are dumb.
1) Do your research.
2) Don't try starting an Airbus vs Boeing argument. Once again, do your research
3) This one is just dumb. Reasearch once again.

[sprocky_ger]

1. I could not find any evidence for this happening outside US. I remember the famous happening with Aloha 1988. There were several accidents following decompression but they were attributed to other causes (cargo door failures: DC-10; design failure: Comet).

2. Did not want to start a fight Airbus vs. Boeing. Just wanted to know if this has happened to an Airbus before. I could not find evidence for this. Sure, Airbus is a much younger company with less sales in total. But in about 30 years there should be some aircraft 15 years old with about the same amount of cycles. Or do airlines trash Airbus aircraft after 10 years. I doubt that.

3. Please explain your statement. If this is something serious I'd definetely have a look at this. Or is it all about saving money? Squeeze the valets of pax as much as you can but don't care about their lives?

I am no expert. I only want to understand a bit more. The only major knowledge in aviation is related to the PA200 I was working on for 3.5 years. At that time there was still something installed called the "fatique counter". If there was anything suspicious the a/c was taken out of service for further investigation. Also the a/c fuselage, wings and tailerons were inspected after a certain amount of cycles. During those inspections the a/c was almost taken apart completely. I remember having seen a show on TV that this is also done on civil aircraft.

[StrongEagle]

39,768 cycles and 48,722 hours

As SLF, I find these numbers to be enlightening.

First, for a 15 year old plane, it is running 2,651 cycles per year, 7.26 cycles per day. That means this plane must have, on average, started out at 6 AM in the morning and hit the terminal at last flight at 9 or 10 PM at the earliest.

Second, the plane is building hours at 1.23 hrs per cycle. This suggests an awful lot of puddle jumps from Austin to Houston, to Dallas, and back again.

Seems like this plane has absolutely been beat to death, and makes me ask the question if there is some kind of "multiplier" to take into account cycles in gauging aging and airworthiness?

[boxmover]

The hours and landings and age are about half the life of a 737.

[vanHorck]

Some people shows signs of old age at 45, I guess the same applies to planes... Early retirement....

[Jetjock330]

Wow, pax must have gotten a big fright, along with the crew. Here is a picture from inside (http://www.bbc.co.uk/news/world-us-canada-12945453)

BBC News - Holed Southwest Airlines flight makes emergency landing (http://www.bbc.co.uk/news/world-us-canada-12945453)

[Eboy]

Regarding descent rate, from the NYT article linked above:

Ian Gregor, a Federal Aviation Administration spokesman in Los Angeles, said the pilot "made a rapid, controlled descent from 36,000 feet to 11,000 feet altitude."

Don Nelson, who was seated one row from the rupture, said it took about four noisy minutes for the plane to dip to less than 10,000 feet, which made him "lightheaded."

[VJW]

Eboy, that sounds much more realistic. That's about 6k feet a minute. The inital 16k in a minute sounds impossible. Don't even think you'd get that if you were shot down :ok:

[mono]

Another chem mill problem or something else I wonder?

[320DRIVER]

Interesting as Southwest may be considered as the Low Cost model pioneer. The financials look great but past and recent history of maintenance compliance isn't great... Wonder if the European LoCos have taken on this part of the model...? what you pay is what you get...

[glad rag]

Was there not a thread about a report on inspectors/subcontractors and stuff that didn't fit on &£&-* assembly a wee while back......<whistling>

[Piltdown Man]

Seems like this plane has absolutely been beat to death, and makes me ask the question if there is some kind of "multiplier" to take into account cycles in gauging aging and airworthiness?

An airliner is designed to fly, not be parked up. Being a shorthaul aircraft, this one has "only" managed to stay airborne for just under nine hours per day where as some long haul aircraft manage 16 hours per day, every day. This aircraft, like all others, has some components which are "lifed" by cycles, some which are "lifed" by hours and others which are calendar dependent and finally some that have no finite life as long as they meet certain inspection criteria. So there is already a very good "multiplier" in effect. From what I understand about Southwest, it will remain in service as long as it firstly serviceable and secondly it is economic to keep it there. They will be as keen as everyone else to find out what actually happened, just so they can stop it reoccurring. Unfortunately, I can also feel the legal scum looking information just so they can screw someone over.

PM

[me myself and fly]

Couple of new pics here

Large hole discovered after Southwest flight makes emergency landing - CNN.com (http://www.cnn.com/2011/US/04/02/arizona.flight.diverted/index.html?hpt=C1)

Tweets and twitpics from PAX

Twitter (http://twitter.com/#!/BluestMuse)
@BluestMuse

[ATPMBA]

SW claims to always make a profit every year.

We all know in the airline business this is an impossibility.
Something has to slide and it's usually maintenance.

[MidlandDeltic]

You have to wonder why the European equivalent of SWA, who operate the vast majority of their flights outside of Ireland, still have all aircraft operating under Irish registration? Whilst not inheritantly unsafe - I'm sure it has absolutely nothing whatsoever to do with the somewhat lax requirements of the Irish CAA to aircraft maintenance overhaul inspections, compared to other nations!!

First, someone tries to make it an Airbus v Boeing slanging match, now you try an anti-Irish, anti Ryanair rant. Par for the course.

IAA are subject to EASA rules, as are ALL EU CAAs. All EU registered airlines can operate anywhere within the EU without restriction - look at it as a Texan carrier being able to operate in New Jersey.

FR operate a robust maintenance regime - even arch cost cutter M O'L is on record many times as being very maintenance aware (read Siobhan Creatons book for references). FR do not operate old aircraft - they are moved on after no more than 10 years, as current disposals indicate. Certainly no second hand aircraft such as those SW use, and al to a standard spec at present.

MD

[Super VC-10]

N632SW was delivered new to Southwest on 13 June 1996.

AIRFRAMES.ORG - Aircraft Database - N632SW (http://www.airframes.org/reg/n632sw)

[MacDaddy]

SW claims to always make a profit every year.

We all know in the airline business this is an impossibility.
Something has to slide and it's usually maintenance.

Same story from you the ppune "expert pontificators"....it must be short cuts on the MX side...they must be hiding something. Y'all are full of SHIITE, these aircraft as are other models are on an expanded inspection schedule that is a product of BOEING and the regulators, fact is this happens to other airlines and aircraft as well. And who on this planet believes with the previous and current scrutiny on the aviaiton industry that SWA would attempt to hide anything relating to MX.

6 foot hole really??????Says who the investigators??? or Brenda Reese and the MEDIA machine. Glad Ms Reese and the 118 pax and 5 crew are safe thanks to the professional flight and cabin crew at Southwest Airlines.

Of course while Reese is making her rounds on the talking head programs this morning I have not heard her once utter a word of praise for the crew, just sensationalist BS like I 'll have to think more carefully about getting on a plane...fine good luck on the roads.

Do us all a favor, put a muzzle on it, let the investigation move forward and in the meantime SWA has taken the step of a precautionary safety standown of 81 aircraft that are on a similar inspection program to the event aircraft (notice I did not use incident or accident because the NTSB has not made that determination yet) in order to determine if further inspections beyond that currently required by AD are required..that to me does not sound like hiding something...

THAT SOUNDS LIKE PROACTIVE AIRLINE SAFETY...

[DX Wombat]

1. did something like this happen outside US also in the past?
You are probably thinking of the Aloha (http://www.aloha.net/~icarus/)accident. That was also a Boeing 737.

[PPRuNe Towers]

Could that be a bit of attack being the first line of defence McDaddy?

So skin problems, including aircraft that already have been reskinned once already, aren't the subject of conjecture and discussion in house, with the Feds and up in Seattle for a significant amount of time before this incident?

Is it possible that those in the know have been bracing themselves for an aging aircraft program to be developed and invoked for months now?

[Daysleeper]

notice I did not use incident or accident because the NTSB has not made that determination yet)
Lemme see... Failure in the structure requiring major repair. I think you can stop worrying, it will be accident.

[thcrozier]

Sprocky_

I don't know why some of the guys here can't be polite enough to just answer a simple question. There was a decompression incident in July of 2009 where a "football sized" hole opened in the top of the fuselage of a Southwest 737; that was inside the U.S.

Southwest inspects planes after emergency landing - USATODAY.com (http://www.usatoday.com/news/nation/2009-07-14-southwest-emergency-landing_N.htm)

The $7.5 million fine was agreed to in March 2009 as a settlement for violations which occurred in 2008.

[heavy.airbourne]

Over the years, a single airline seems to suffer increased numbers of inflight failures, fires etc. - Qantas. As per areas, this seems to be the USA, concerning pressurized structures failures. (Even when it happened in Taiwan, it was maintained by TBC in the US.) Once is coincidential, twice is a trend...

[pattern_is_full]

CNN is reporting, in interview with aviation "consultant", that SWA gets most of its "heavy maintenance" done in El Salvador.

a) Correct?
b) I though mechanics had to have FAA licenses same as pilots - is there some kind of recognition of non-US licensing that allows mechanics without FAA licenses to do work on aircraft owned and operated in US airspace by US operators?

[aterpster]

pattern is full:

CNN is reporting, in interview with aviation "consultant", that SWA gets most of its "heavy maintenance" done in El Salvador.

a) Correct?
b) I though mechanics had to have FAA licenses same as pilots - is there some kind of recognition of non-US licensing that allows mechanics without FAA licenses to do work on aircraft owned and operated in US airspace by US operators?
CNN is reporting, in interview with aviation "consultant", that SWA gets most of its "heavy maintenance" done in El Salvador.

a) Correct?

b) I though mechanics had to have FAA licenses same as pilots - is there some kind of recognition of non-US licensing that allows mechanics without FAA licenses to do work on aircraft owned and operated in US airspace by US operators?

Fox just interviewed a former NTSB board member. I recall the gentleman was a aircraft mechanic with US Air before he went with the Board.

He stated that SWA's heavy maintenance is done in El Salvador. He had serious questions about oversight. Further, he said FAA inspectors can't just show up at the facility as they do in the U.S. Instead, they have to phone ahead for an appointment.

Smells a tad, doesn't it.

[lomapaseo]

Way too much speculation to known fact ratio here.

Lots of questions need to be answered, but let's try and keep the questions simple rather than connect the dots with a broad paintbrush.

For starters look at how the questions and answers played out in the Aloha incident (accident) and the earlier SWA incident (accident) then let's see what lines up in this incident (accident)

[Machaca]

Another lap joint failure?

http://i337.photobucket.com/albums/n385/motidog/N632SWyuma3.jpg

[grounded27]

Some people shows signs of old age at 45, I guess the same applies to planes... Early retirement....

Well put, the same aircraft with the same hrs/cycles could have had less hard landings, turbulence etc. HRS/CYCLES do not equal wear and tear.

[grounded27]

CNN is reporting, in interview with aviation "consultant", that SWA gets most of its "heavy maintenance" done in El Salvador.

a) Correct?
b) I though mechanics had to have FAA licenses same as pilots - is there some kind of recognition of non-US licensing that allows mechanics without FAA licenses to do work on aircraft owned and operated in US airspace by US operators?

Can not confirm what MRO's SWA is using at this time. I can say though that foreign MRO's are inexpensive and they as domestic MRO's only need a ratio of FAA liscenced to non liscenced labor. Being in a foreign land there is much doubt that the FAA gives said MRO the attention they need to ensure their compliance.

Having worked in domestic MRO's I dislike them greatly as profit is a evil motivator. In my opinion, a tech at an airline unionized or not is less likely to rush a job, cut a corner (just do what their supervisor said for fear of losing their job).

[VFD]

Another lap joint failure?
There is just not enough resolution in your picture to tell. This is the first exterior shot I have seen. I know that the aircraft in question has been painted within the last year or so.
I remember some years back when a repainter was leaving scribe marks from sharp tools at lap joints from the paint removal process causing localized stress and failure.
I suspect that since SWA has grounded the 737-300 aircraft for inspections that they have a clue as to exactly what they are looking for. They just forgot to inform PPRuNe.

[techgeek]

I took a quick look at B737-300 ADs involving fuselage frame and skin crack inspections. AD-2010-25-06 requires inspection of frames at STA 616 and 639 which, I believe, is on either side of row 12 in a 737-300. This is where the decompression occurred (http://bit.ly/fYegoW) leading me to wonder if this aircraft had been inspected under this AD. The AD as issued allows the operator 4500 hours to perform the inspection. That's about a year and a half (using my fingers and toes) and the AD was issued 9 months ago. So, my guess is that SWA was operating the aircraft within the range of hours permitted by the FAA for the inspection and repair to be performed when this incident occurred. Go read the AD yourself here (http://bit.ly/e3Yvs1).

[sprocky_ger]

Some guys were so kind to have a serious look into my questions. Thanks.

DX Wombat
You are probably thinking of the Aloha (http://www.aloha.net/%7Eicarus/)accident. That was also a Boeing 737.
Maybe my question was a bit misleading and I hope to make it more clear with this one: Did something like this happened to any other carrier not running under the flag of the USA?

thcrozier
Does that mean the fine was not related to the decompression accident in 2009?

MidlandDeltic
There was no intention to start a Airbus vs. Boeing match. I only wanted to know if decompression happened to an Airbus already. You can also take Tupolev, Antonov, Embraer, Bombardier, etc. I picked Airbus because they are well-known and some of their aircraft might have the same duty as this B737 has.

[DX Wombat]

No problem Sprocky, that was the only one which I could think of. I'll let others more knowledgeable than I answer your questions. :)

[unmanned transport]

There is a lap joint at that location, so either it was corrosion, a substandard repair or a scrape in the surface of the aluminum which cracked from pressurization cycles. Missing rivets thru the aluminum skin to the underlying stringer is also another possibility.

Hope they find the root cause soon and get all of those craft flying again.

[BARKINGMAD]

How very inconvenient for this to happen to a -300, just before the Supreme Court case involving the firing of the 737NG assembly line inspectors who blew the whistle on the defective chord sections and "bear-straps" from a Boeing sub-contractor?!

What happens when the 1st NG made mid 90s to mid 2000s suffers a sardine can experience?

Put A H F Ducommun in your search engines, ignore the company puffs and look for the nasty bits if anyone fancies doing some research on this topic.

Then buy our ticket and pray. :confused:

[Alber Ratman]

CGs have a regular NDT inspection on this structure I believe. HFEC as well. Most NDT inspectors I know aren't into the sharp pen business or bowing to pressure from management. Of course I cannot speak for the inspectors that last checked the airframe in question.. Lap Joint?? More possibly a pocket IMHO, but what do I know?:E

I would like to hang anybody I see scraping lap joints with metallic or non approved scrappers.. they should be excuted..:E

[D120A]

Two observations on a brighter note:

1. This and previous happenings with happy outcomes do speak volumes for the damage-tolerance of the 737 fuselage design.

2. The flying public, seeing this on the TV news and hearing the testimony of witnesses, will be motivated to heed the safety advice and keep their seatbelts fastened at all times they are in their seats.

[ankh]

> Ducommun
Good pointer, thank you. Among the first page of results:
Washington Post Article | Political & Social Justice Project | Schuster Institute | Brandeis University (http://www.brandeis.edu/investigate/pol/post-article.html)

[Data Guy]

Mar 2009 thru Nov 2010 Service Difficulty Reports

For full text reports, see FAA’s SDR database at this Link > FAA :: SDR Reporting [Service Difficulty Report Query Page] (http://av-info.faa.gov/sdrx/Query.aspx) Enter SDR # into the “Operator Control #” field and run query.

Service Difficulty Report – Control Numbers >

SWAA107101 11/14/2010 Reg 773SA No data.
SWAA106876 10/28/2010. Reg 902WN At Fl 380
SWAA094502 07/13/2009. Reg 387SW At FL330
SWAA093897 06/15/2009. Reg 522SW At FL 230
SWAA092547 04/18/2009. Reg 654SW At Fl 290
SWAA091992 03/29/2009. Reg 358SW At Fl 260.

[thcrozier]

Sprocky_ger

That is correct. March 2009 $7.5 million settlement was for alleged maintenance violations which occurred in 2008. That figure was reached after months of negotiations - the FAA had originally demanded much more.

[unmanned transport]

I was inspecting an A320 fuselage during a heavy mtce check a few year ago and I found a row of missing rivets thru the skin and underlying stringer. So human factors play a major part with regards to mistakes.

[sevenstrokeroll]

more rivets ...the only answer!

[bubbers44]

I flew the Aloha 737 that became a convertible about 100 hours before our airline gave it to Aloha. We normally cruised at 35,000 ft. Aloha was only about 20,000 ft when the top blew off. We always train for emergency descents but have never had to do a real one. They did just fine at SWA getting it down. Good bunch of pilots there.

[Machinbird]

Techgeek may be on to something. Many of the recent commentators have apparently not read the link to the AD that he posted. Here it is
in summary form without the compliance time limits:


AD 2010-25-06

This AD is effective February 1, 2011.
BS 616 and BS 639 inspection/lower frame and stub beam
15 work hours
This AD applies to The Boeing Company Model 737-200, -300, -400, and -500 series airplanes ...
The Federal Aviation Administration is issuing this AD to detect and correct fatigue cracking of certain fuselage frames and stub beams and possible severed frames, which could result in reduced structural integrity of the frames. This reduced structural integrity can increase loading in the fuselage skin, which will accelerate skin crack growth and could result in rapid decompression of the fuselage.

It appears that the observed failure is in just that area. Severed frames would cause a longitudinal crack to develop.

[repariit]

AD 2010-25-06 could be involved here.

It looks like it was a bit away from the crown skin lap joints on this one.

[snolat]

that seems right

[StallBoy]

Two Questions:-
(1) Did the hole in the roof cause the decompression?????:confused:
(2) Do 737's have a pull top tab on the inside of the fuselage???:ok:

[techgeek]

Looking at the picture someone posted and a 737-300 Station Diagram it looks like the skin panel that opened up begins around STA 663.75 and continues past STA 706 and ending before STA 727. So, it is close but not exactly correlated with the locations cited in AD-2010-25-06. I suppose the NTSB will have something to say if it turns out this AD was involved.

[Passenger 389]

Regarding post # 8

<<< 1. did something like this happen outside US also in the past? >>>

It is difficult to say exactly what "this" is until we know more about what caused the event in question. (For instance, was there any prior damage near the site of the failure that was undetected or improperly repaired?)

With that caveat, my understanding is there have been some incidents outside the US involving explosive decompression attributed to undetected (or unrepaired) corrosion or cracking.

A Far Eastern Air Transport (Taiwan) 737 crashed on August 22, 1981, following an explosive decompression event and in-flight breakup. Probable cause was found to be extensive corrosion damage in the lower fuselage structures, probably exacerbated by what was considered to be a large number of flight cycles.

Two other events outside the US involved 747s: China Airlines 611 and JAL 123. My understanding is that in each instance, the plane had sustained a tail strike some years before (22 years for China 611, and approximately 10 years for JAL 123).

In both cases, repairs were not properly performed. Over the years (and thousands of cycles), small fatigue cracks developed, grew, and connected with other cracks. Inspections either were not performed or failed to detect/correct the problem. Both planes eventually sustained an in-flight explosive decompression culminating in catastrophic structural failure, at a total cost of over 700 lives.

The real pilots here undoubtedly know of other such incidents. (I posted this only because it did not appear that anyone had directly addressed the question you raised.)

[RogerClarence]

Ditto, the eldest a/c in the FR fleet EI-DAD! (no pun honestly) is about 4 years old from recollection.
The average fleet age is circ 2.5 years (airframe 300 came in Feb11!)
So with one of the youngest and most standardised fleets in the business, coupled with as strong a maintenance regime as any of the dinosaur airlines they are inherently safe.


If they are anywhere near as "pikey" as a lot of contributors try to imply, how come the only "decent" hiccup they have had was the multiple bridstrike in Ciampino? And whats that? oh yeah they did as good a job as the boys and girls on the BA 777 into LHR

Sorry ill take off my reality spec's now and look at the world in the rose tinted Legacy Is Best -The Rest Is Cowboys and Dangerous:ugh::}

Jeez some days if i had a subscription I'd cancel it
Unfortunatly im addicted to trashy comments as much as i am at throwing my shoes and houshold goods at the TV when ever Jeremy Kyle comes on

:}:}:}

[Shell Management]

The past history of SDRs as SWA is very concerning as it smacks of Aloha but on a far bigger scale.:\

Its another sign that airlines need to invest in SMS introduction to manage this sort of problem better.;)

I for one had not realised that they had not been investing in new aircraft like Ryanair.
:ok: to M O'L.

I wonder if the AD will now be revised.:confused:

[ian16th]

I took a quick look at B737-300 ADs involving fuselage frame and skin crack inspections.

Techgeek,

Whats all this doing research before posting?

Don't you realise this is a 'Rumour Network'?

You have confused the whole thread by introducing FACTS!!!

Well done that man :ok:

[aterpster]

Shell Management:

The past history of SDRs as SWA is very concerning as it smacks of Aloha but on a far bigger scale.:\

Its another sign that airlines need to invest in SMS introduction to manage this sort of problem better.;)

I for one had not realised that they had not been investing in new aircraft like Ryanair.

They spent a huge amount of money retrofitting all their aircraft to be able to do RNP AR. Then, they are pushing complex, multi-path RNP AR IAPs at all their airports to hopefully save a bit of fuel here and there.

This was not the original objective of RNP AR, but SWA has forced the issue at the highest levels of the FAA. They are a very tough player.

[cwatters]

Seems there might have been some warning?..

Southwest to ground 81 planes after hole prompts emergency landing - CNN.com (http://edition.cnn.com/2011/US/04/02/arizona.flight.diverted/index.html?hpt=C1)

I heard a loud popping sound about three or four minutes before it blew open on us,

[Shell Management]

aterpster - not sure why you think that investment in aging aircraft is a postive one.

..SWA has forced the issue at the highest levels of the FAA. They are a very tough player.

I wonder if that attitude has had a negative effect on safety.

[aterpster]

Shell Management:

aterpster - not sure why you think that investment in aging aircraft is a postive one.

Did I state or imply that? The only investment aging aircraft should get is increased maintenance, especially heavy maintenance

I wonder if that attitude has had a negative effect on safety.
Their flt ops culture seems to be quite good. But, overall, it is not the same spirited outfit it was when Herb ran the show.

[sevenstrokeroll]

lot of older planes out there...our B52's are in their 50's...but they constantly get rebuilt

[aterpster]

sevenstrokeroll:

lot of older planes out there...our B52's are in their 50's...but they constantly get rebuilt

The Air Force doesn't try to cut costs in the B52 program. Also, I suspect the cycles per flight hours is very low.

[deSitter]

Regarding the AD, what exactly is a severed frame? TIA

[deSitter]

The main issue with cycles is repeated pressurization/depressurization of the airplane. The B52 is not pressurized over the length of the fuselage.

[gofer]

Have 2 questions for U guys out there:

a) Where do you find the cycle and hours data for a given airframe ? If you feel uncomfortable posting the answer, a private message is fine.:ok:

b) Given that the low cost airlines will automatically end up with high cycles and relatively high hours on short haul aircraft how do the different airframe retention concepts of SW v. the 2 major european players affect safety and incident statistics ? (run forever to get as big a bang for your investment buck v. replace while still in prime of life and optimise on second hand value at times when new airframes are at a premium due to short supply).:ugh:

[cessnapete]

Ryanair still getting new airframes. 3 new 737 on the flightline at Seattle when I passed an hour ago!!

[Machaca]

AP reports (http://news.yahoo.com/s/ap/20110403/ap_on_re_us/us_southwest_flight_diverted):

Federal investigators say the entire length of a 5-foot-long tear in the skin of a Southwest Airlines Boeing 737 shows evidence of pre-existing fatigue cracking.

It started along a lap joint where two sections of the Boeing 737-300's skin are riveted together.

Southwest mechanics will cut the entire ripped section out of the plane on Sunday. Sumwalt says he expects a 9-foot by 3-foot section will be sent to Washington D.C., for analysis.

[lomapaseo]

Where do you find the cycle and hours data for a given airframe ? If you feel uncomfortable posting the answer, a private message is fine.:ok:

b) Given that the low cost airlines will automatically end up with high cycles and relatively high hours on short haul aircraft how do the different airframe retention concepts of SW v. the 2 major european players affect safety and incident statistics ? (run forever to get as big a bang for your investment buck v. replace while still in prime of life and optimise on second hand value at times when new airframes are at a premium due to short supply).:ugh:

I may be missing the crux of your question so feel free to accept other's responses.

The airframe hours and cycles are part of the required record keeping and of course one of the first things the NTSB will publized after they are vetted.

Fleet management has many considerations, commonality, market fit, repair and maintenance, etc. regardless of new or old safety management is a task shared by the operator, the manufacturer and the regulator. So in my mind there is no intent to see how far you can run an airplane to see if it will break catstrophicaly. Anywhere along the line, human error and/or ignorance (surprise factor) is present. No way would I predict anything else. I await some more specific investigative facts before delving more deeply into anything more.

[Machaca]

cessnapete:
Old Fleets
Ryanair still getting new airframes. 3 new 737 on the flightline at Seattle when I passed an hour ago!!



Correlation based on random observations is not useful.

737 deliveries since 1999:
Southwest = 320
Ryanair = 300

737 orders in last year:
Southwest = 28
Ryanair = 0

[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.

[RogerClarence]

You are neglecting the "options" that RYR turned into orders, no point in placing any more orders when you have the fleet size you want and new players offerring more attractive price/equipment propositions

[Machinbird]

deSitter

Regarding the AD, what exactly is a severed frame? TIA

Aluminum fuselage construction involves a series of hoop like frames that give shape to the fuselage. If fatigue cracking initiates on the inner or outer face of the hoop, it can progress until it cracks through all portions of the hoop, finally completely severing the hoop's continuity. At that point, all pressurization loads are being entirely handled by the skin in the area of the severed hoop (frame).
One can envision the entire fuselage tube bending down aft of the wing during turbulence and during firm landings. The tube bends by flattening its outside radius in the area of the bend. The frames resist this flattening which is how they become exposed to fatigue.

Note: Those who design aircraft structure are welcome to improve on this description--Not my area of specialty.

[sevenstrokeroll]

I don't fly for southwest...but landings are like sandpaper...use the rougher grades in certain circumstances, the smoother grades in other situations.

plus, it is a bit of luck getting a smooth landing.

actually, firm landings in some circumstances are the safest ones to do.

[Mr @ Spotty M]

Machaca, Your post was of no use to anyone.
However to add to the pointless posts, with regards to SWA & RYR.
Fact, RYR only have 737NG aircraft and are retiring the oldest airframes, SWA still have B737 classics.
One other pointless fact, an MRO that has FAA approval, does not need to have Mechanics or Inspectors with an FAA licence, thank God.
If you have seen some of the FAA registered aircraft that have gone through overseas MRO's, then you would all change your opinion on how good FAA operators are.

[Snow_Owl]

In the UK, look at BMI Baby...

I cant remember if theyve got rid of all their 300's but I'm sure there are still some around and even their 500 series 737's are getting old.
Some used to be ex BMI and I believe they had an ex BA one too.

Some are over 10 years old. Most of them 15 years or more.

I like BMIBaby but dont you think it is time they renewed some of their aircraft.

[Alber Ratman]

Try NAS...:E

[sevenstrokeroll]

is it true that this plane was maintained/D check whatever in el salvador?

[deSitter]

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) (http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=8E656C47649D700D25DC97ECE4541989?doi=10. 1.1.31.4528&rep=rep1&type=pdf)

-drl

[Mark in CA]

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.

NTSB: Cracks found in 3 grounded Southwest planes - Yahoo! News (http://news.yahoo.com/s/ap/20110404/ap_on_re_us/us_southwest_flight_diverted)

[deSitter]

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

[Sunfish]

DeSitter:

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.


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:

YouTube - DC 9 80 Hard Landing

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".

[deSitter]

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.

[FlightCosting]

Small cracks found in three Southwest Airlines jets (http://www.bbc.co.uk/news/world-us-canada-12954335)

[boxmover]

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”.

[Sunfish]

De Sitter:

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

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.

[FlexibleResponse]

This might be somewhat related?

YouTube - People & Power - On a wing and a prayer

[seacue]

From the AP: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. It sounds like they had re-skinned about half their 737-300s, but not the one in question

[Cytherea]

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!

[sevenstrokeroll]

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.

[BOAC]

I doubt it - just a pilot, not an engineer. 0.125psi???

[slip and turn]

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? (http://www.pprune.org/tech-log/312821-what-would-cause-738-creak-like-submarine-depth.html?highlight=sound)

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 :}

[lomapaseo]

I wonder if the plane might last a bit longer if .......

That's not the issue here.

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.

[winddoggie]

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.

[Sunfish]

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 (http://www.godlikeproductions.com/forum1/message177268/pg1)

[forget]

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.

[sevenstrokeroll]

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?

[TabsAZ]

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.

As the moderator of the forum you saw that on (which is an FSX simming forum), I'm actually dumbfounded that it's getting posted here on a real aviation forum. The guy saying those things is obviously wrong and we all told him that in the thread. Seriously though, no one here should be looking to MSFS forums for information on a real life accident, sheesh.

Ryan

[grounded27]

Not a sheetmetal guy but that damb DC-9 had "finger dublers". Probably lots of extra weight but internal strapping of seams made this bird tough. To pose the question if a butt joint is enough with a standard rivit pattern? Inspections of the crown of an aircraft are not very strong to date, we usually look at the floor to bildge as corrosion causes the most damage.

[deSitter]

I think we can pretty much rule out hard landings as the cause of the skin tear.

Yes, the bear is repeated pressurization - but the structure of the fuselage and the amount of beating the plane takes has a direct bearing on the initial formation of cracks. Once started, repeated pressurization and depressurization propagates them.

-drl

[sevenstrokeroll]

grounded 27

I read somewhere that douglas used lots of rivets...more than boeing did. dc9 was slightly less fuel efficient but a heckuva lot stronger...wish I had the real data

[Flightmech]

Looks like lots of Southwest jets arriving back at Boeing for inspection. 11 since the weekend.

[TURIN]

I read somewhere that douglas used lots of rivets...more than boeing did. dc9 was slightly less fuel efficient but a heckuva lot stronger...




Was this before or after they added the extra rivets?

http://www.youtube.com/watch?v=QIsbSz03WdU
Sorry about the thread drift but I couldn't resist. :E

[repariit]

"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?"

There are two tests done (I believe by both Boeing and Douglas in that era) that you may be referencing.

A proof pressure test is done on a completed airplane. In this test the cabin is pressurized to a much greater differential that would ever be experienced in service, and it is held for many hours. This is followed by an extensive inspection of the structure. This test is done once to show that the structure holds pressure without failures.

Fatigue testing is done in what Boeing staff called the "Iron Bird". A completed primary airframe structure is placed in a heavy steel frame and many hydraulic actuators flex the airframe to simulate flight operations. Such testing goes on for years. Pressurization is not involved in it.

[sevenstrokeroll]

dear you tube poster...I can't see it due to old technology. if you are talking about the plane that the FAA inspector cracked in half, that' is a bit different, don't you think?

[Mr @ Spotty M]

The jets are not arriving back at Boeing, l think you will find that the inspections are being carried out by ATS at PAE.

[Sunfish]

It is not "Hard landings" it is not "Not enough rivets" that has caused this. There are well over a Thousand B737 being "Hard landed" and "Pressurised" every hour of every day.

The aircraft is a long proven design. It was designed to "fail safe" principles which is exactly what it did in this case. We know that cracks will propagate along riveted joints. That is why tear straps are installed, and they appear to have worked exactly as designed, stopping the crack propagating beyond design limits.

By "fail safe" we mean that no single point of structural failure will be catastrophic and the failure will announce itself so that it cannot go unnoticed. Modern practice is a little different - the structures are designed to be "damage tolerant".

The issue for Southwest is not that the structure cracked. All aircraft crack, all the time.

The first issue is why this part of the structure cracked? As far as I know, this is not a known location with a history of cracking. It may be that this aircraft is a fleet leader in terms of the number of pressurisation cycles in which case other B737s may exhibit this behaviour at around that age, but my personal opinion is that this is unlikely, there may instead be a latent manufacturing defect or a botched modification or repair of previous damage that started the process. The NTSB will work it out.

The second issue is why was it not picked up? What inspections are supposed to be done? Were they done? Is the nominated inspection method adequate to detect this type of damage?

[FougaMagister]

Considering that airframe is 15 years old and has only limited residual value, it will probably be struck off by Southwest, sold on by the insurers, patched up, and converted into a freighter!

Cheers :cool:

[sevenstrokeroll]

fouga...not likely...it looks like an easy fix. and I really don't see too many 737 freighters, do you?

[sevenstrokeroll]

I would like to know where the work on the ruptured plane was done one year ago. Fox implied it was outside of the US.

Just wondering if anyone knows where the last heavy mx was done. not current inspections ordered in last few hours.

[Mr @ Spotty M]

Its last big check in early 2010 was done by SWA, at its own base in TX.
So you can not try and shift any blame to overseas MRO's. :=

[lomapaseo]

Sunfish

good points:ok:


It is not "Hard landings" it is not "Not enough rivets" that has caused this. There are well over a Thousand B737 being "Hard landed" and "Pressurised" every hour of every day.

The aircraft is a long proven design. It was designed to "fail safe" principles which is exactly what it did in this case. We know that cracks will propagate along riveted joints. That is why tear straps are installed, and they appear to have worked exactly as designed, stopping the crack propagating beyond design limits.

By "fail safe" we mean that no single point of structural failure will be catastrophic and the failure will announce itself so that it cannot go unnoticed. Modern practice is a little different - the structures are designed to be "damage tolerant".

The issue for Southwest is not that the structure cracked. All aircraft crack, all the time.

The first issue is why this part of the structure cracked? As far as I know, this is not a known location with a history of cracking. It may be that this aircraft is a fleet leader in terms of the number of pressurisation cycles in which case other B737s may exhibit this behaviour at around that age, but my personal opinion is that this is unlikely, there may instead be a latent manufacturing defect or a botched modification or repair of previous damage that started the process. The NTSB will work it out.

The second issue is why was it not picked up? What inspections are supposed to be done? Were they done? Is the nominated inspection method adequate to detect this type of damage?


Just to add

The latest news says the Boeing didn't expect cracks at this location and therfore only suggested a visual inspection. Of course Sothwest offers that they followed the original Boeing advice so no error on their part.

as I suggested earlier I would look for a lightswitch error in thinking about the fleet management.

possible manufacture problem

possible hard useage problem (too many cracks for too long elsewhere etc. ?)

Possible maintenace problem e.g. the AA fork lift etc.

The Boeing folks and the NTSB will likely get to this pretty quick, if it's a light switch problem. All aircraft fleets await the answer

[VFD]

The first issue is why this part of the structure cracked? As far as I know, this is not a known location with a history of cracking. It may be that this aircraft is a fleet leader in terms of the number of pressurisation cycles in which case other B737s may exhibit this behaviour at around that age, but my personal opinion is that this is unlikely, there may instead be a latent manufacturing defect or a botched modification or repair of previous damage that started the process. The NTSB will work it out

I would not go out on a limb and say a botched modification but maybe the previous AD's to repair/strengthen cracking in other areas have caused the stresses to transfer and cause the cracking in the location involved with this incident. According to the FAA and I assume Boeing this area of the aircraft was not known to be an issue. There were I believe 3 other airframes that were starting to display this issue as well in the Southwest fleet. It will be interesting the findings when they inspect the other 100+ high cycle frames in other fleets.

[techgeek]

You can read the new AD in response to this incident here (http://bit.ly/hiuUGy). The AD requires eddy current testing to find and confirm cracks in the lower skin at the lap join lower fastener row at 2 specific stringers along a rather long section of the aircraft fuselage in aircraft having over 30000 cycles. For those of you concerned about airframe age, note there is nothing in the AD about how old the aircraft is - just how many cycles. Also of note, SWA aggressively keeps their planes in the air (and making money) and has many short routes. IMHO the combination of these factors likely results in SWA having a higher # of cycles for a given age airframe than other carriers. It would be most interesting to know how this ratio varies across carriers. I wonder if the FAA has that stored away in a database somewhere.

[Mark in CA]

Boeing never expected failures in the riveted skin joints running along the top of the 737-300, 737-400 and 737-500 models until the planes were much older, said Paul Richter, Boeing's top engineer for older 737s.

Richter said Boeing also didn't anticipate the need to inspect for cracking on the redesigned lap joints — where two pieces of the fuselage skin overlap — until it had reached 60,000 pressurization cycles, the number of takeoffs and landings.

And it certainly didn't expect such a dramatic failure, he said.

Boeing didn't expect 737 cracks so soon - Yahoo! News (http://news.yahoo.com/s/ap/20110405/ap_on_re_us/us_southwest_flight_diverted)

[boxmover]

AS I said before this is not an old aircraft. It is about half its life.

It will not be the fleet leader as the first 300s came on line in the mid 80s.

Southwest have lots of quite short flights for the US but I bet the average cycles per hour are higher in some european airlines.

A big part of the concern is that this is a big problem for a mid life aircraft used in a normal way.

[crippen]

I think this it.http://i.dailymail.co.uk/i/pix/2011/04/05/article-1373701-0B7E6BCB00000578-613_634x363.jpg

http://i.dailymail.co.uk/i/pix/2011/04/05/article-1373701-0B7E733900000578-740_634x401.jpg

http://i.dailymail.co.uk/i/pix/2011/04/05/article-1373701-0B7E750000000578-258_306x423.jpg

Southwest airlines forced to land another flight as cracks found in 5 more planes | Mail Online (http://www.dailymail.co.uk/news/article-1373701/Southwest-airlines-forced-land-flight-cracks-5-planes.html)

[ozaub]

I have no inside knowledge of the SWA failure but was heavily involved in corrective action after Aloha accident; see [/FONT]http://avstop.com/stories/aloha.html (http://avstop.com/stories/aloha.html)[FONT=Arial] for that story. So I'll make an informed guess of what happened.



Hopefully the problem is confined to Boeing 737s up to LN 288, because these used a bonding process that was found to be defective; the process was relied upon to reduce fatigue stresses across longitudinal lap joints in the fuselage. Aloha happened after the bond failed and skin cracked at the top row of rivets; which were critical because they were countersunk. Fix was to replace all those rivets with button heads. BTW you can spot an old, suspect aircraft by the prominent rivets. This did nothing to aleviate criticality of bottom row. Just postponed the evil day and my understanding is that it is the bottom row that failed on SWA.


For aircraft structures, bonding has special attraction of minimizing stress concentrations and associated fatigue failures that seem inevitable with rivets or welding. [/COLOR][COLOR=#000000]However adhesive bonding is never as easy as it seems. A major reason is difficulty in establishing, substantiating and maintaining process specifications that ensure bond reliability and durability. Not only is the adhesive prone to variability depending on temperature cycles, pressure etc. Perhaps more so, durability depends on surface preparation.

Boeing learnt these lessons the hard way, with a series of embarrassing failures. The worst was the Aloha B737 fuselage failure in 1988. In (Australian) Bureau of Air Safety Investigation Journal, June 1992 I wrote:

“(The fuselage) is made in panels which are typically about 4 metres long and 2 metres wide. These are joined together with rows of rivets. Obviously the skin is weaker where it is drilled for the rivets, so on early B737's Boeing engineers tried to reinforce the joints with epoxy adhesive. It was these joints which failed first and let the skins rip away from the aircraft.
The first Boeing 737 was delivered in 1967, the Aloha aircraft was delivered in May 1969 and by about then Boeing became aware of problems with the adhesive bonding process. The adhesive worked much like a two tube mix used by a home handyman except that the glue was premixed and held on a scrim tape. By keeping it refrigerated the adhesive reaction was suspended. At the right time in construction the adhesive tape was laid between the skins, these were riveted together and the glue then cured as it warmed up to room temperature. That was the theory. In practice the adhesive did not really bond to the aluminium skin, it only bonded to the very thin layer of oxide on the surface of the aluminium. Attachment of the oxide film to the metal underneath was dangerously variable. Also if the scrim was too cold when it was applied it attracted condensation which prevented proper adhesion. If the scrim got too warm it partially cured before it was in place and again adhesion failed. Whenever adhesion failed the rivets and surrounding skin were overloaded and the skin began to crack.

Boeing was not secretive about the problem. The bonding deficiencies and their rectification were discussed in many technical papers in the early 1970's. The whole U.S. industry was embarrassed because the Europeans had been successfully bonding aircraft for 30 years. In 1975 the U.S.A.F. stepped in with a large contract to catch up with the Europeans. It went to Boeing's arch rival, Douglas.

Boeing progressively improved the design of the skin joints and hoped that for aircraft already in service the problem could be controlled with enhanced inspections. From May 1970 onwards Boeing sent the airlines a series of bulletins recommending inspection and sealing of the joints. Cracking and corrosion still went on. Diligent airlines found it and fixed it. Less diligent ones stayed lucky.”Of the tech papers mentioned above specific mention needs be made of “Surface Preparation – the key to bond durability” by Boeing Chief Chemist Corey McMillan. It spells out the evolution of surface preparation and failures along the way.

Problem is that many aircraft are still flying which were built using surface preparations of dubious repute!

Apart from the faulty cold bond technique that led to Aloha, we knew at the time of Aloha that Boeings hot bond technique was better but not completely reliable. A year earlier in 1987 hot bond failures on B747 were addressed by SB 747-53A2279, FAA AD 87-16-13 and Australian AD/B747/59.

At the time, Australia tried but never really got Boeing or FAA to acknowledge the risk of hot bond failures on B737; that hazard still lurks!

[captplaystation]

Ironically the problem this time has been caused by a mod to the design designed to reduce the risk caused by cracking.

Distance between tear strips was increased from 10 to 20 inches to reduce possibility of cracking propogating from frame to frame as is believed to have happened in the Aloha incident.
This redesign is believed to have led to this problem, hence why only 579 aircraft manufactured between 1993/2000, of which 175 are believed to be above the 30,000 cycle trigger are affected.

Best laid plans of mice & men :rolleyes:

[Flightmech]

Mr @ Spotty M

I stand corrected. I always associate KPAE with Boeing. Forgot about ATS! Cheers:ok:

[readywhenreaching]

there is a list of all affected airlines and aircraft on jacdec.de (http://www.jacdec.de)

[Sunfish]

Ozaub, thank you for your illuminating and concise post.

[blakmax]

Ozaub

Your assessment of Aloha 243 is close. The problem with the bonding was not the mixing process, because they did not mix the adhesive. They used a film adhesive, similar to double sided tape. The root of the problem was that the material they used was a room temperature curing system, so that they did not have to heat the structure (as the Europeans always did). That saved complex heating procedures for large structures and was much quicker.

The essence of the problem is the requirement to cure at room temperature. Because the material is a pre-mixed tape, it must be stored frozen, or the material will cure off prematurely. When the material was cut is was frozen and when it was applied to the structure it was frozen. As a consequence, atmospheric moisture condensed on the cold surface. Now adhesive bonds depend upon chemical bonds formed at the interface during the curing process. These chemical bonds give the adhesive strength and also durability. In the case of the early 737s and some 727s and 747s as well, the moisture inhibited adequate contact between the ahdesive and the metal, resulting in weak bonds which later failed in service. The disbonding led to higher than expected loads at the fasteners, which when combinded with knife-edge countersinks led to the fatigue cracking at multiple sites, and then to the failure once the small cracks linked up.

Adhesive bonded structures can be very reliable when the processes are correctly validated and implemented. In this case they were not.

[martinprice]

If I may be permitted a daft SLF question - I only drive little airplanes with the small wheel at the back - I'm not sure really sure what correction Blakmax is making here. Ozaub described the bonding process in exactly the same way and listed condensation as one of the possible causes of the bonds failing. What'd I miss?

Regardless, I'm getting a great education here - thanks, folks.

[grounded27]

Fact is despite the cycles and hrs, none of us can compile the data that resulted in this failure. Starting from the last rivits holding the skin, unknown "g" stress from hard/semi hard landings to turbulence (largest unmeasurable factor), the environment over 15 years "hot/cold/wet/dry". Just to speculate that this piece of skin of this aircraft had a poor run of luck based on the factors above is probably in atleast in one way the truth.

Cycles and hrs are not a factor given this type aircraft. Allthough the crown is least inspected and incinedence like this will likely prove for for more detailed inspection.

Human factors, pilots are often on the fence to report a (semi hard landing), this area of the aircraft is not scrutinized by maintenance and difficult to inspect at the gate. These are factors that will change.

[Sunfish]

Grounded:

Cycles and hrs are not a factor given this type aircraft.

Rubbish. Why do you think we maintain them at set time intervals and life a lot of components in terms of hours or cycles? Didn't you know that the Aloha aircraft was the cycle leader of the fleet?

Did I waste Six years of my life looking at cycles and hours flown then making judgements about component reliability?

[captplaystation]

readywhenreaching,

I am guessing that list is merely compiled from production line number, the affected batch were specified on the AD, but takes no account of cycles.

Of 579 aircraft in the batch, only 175 are thought to have reached the 30,000 cycle trigger point.

[blakmax]

For martinprice. The point is that it was an active decision by the manufacturer to choose a specific form of adhesive which was highly susceptible to moisture problems. It had nothing to do with the mixing of the adhesive. It was therefore not a quality control problem, it was a design/material selection issue.

Hope that point is clearer now. :) Thanks for the interest.

[pattern_is_full]

The second piece of skin in those pictures seems to have a "hump" or bulge to the middle section of skin between the longitudinal rows of rivets.

Is that likely just due to tension when the adjoining piece tore loose? Or from the disassembly procedure? Or something else?

[GroundedSpanner]

The fuselage is circular. The 'hump' Is the natural shape of the skin.

[Sunfish]

The fuselage is not circular, it is ovoid. :ugh:

There are bloody great beams running across each frame at at deck level that support the floor. They are in tension when the aircraft is pressurised and keep it in its shape.

This is the B707 profile it is exactly the same as the 727 and 737.

http://www.lx97.com/FUSELAGE339582.jpg

[GroundedSpanner]

Sunfish
Yes I am aware of the true shape of a B737 fuselage. In much greater detail than I would care to.
I was sure when I posted my hastily written explanation of the 'bulge' in the cutout lap section that someone would correct me, and you have excelled with that. ;)
Thanks for the detailed post.

[grounded27]

Rubbish. Why do you think we maintain them at set time intervals and life a lot of components in terms of hours or cycles? Didn't you know that the Aloha aircraft was the cycle leader of the fleet?

Did I waste Six years of my life looking at cycles and hours flown then making judgements about component reliability? We use flight hrs and cycles for the bean counters. EG: 26 people show up to work and x amount of work is performed, 13 of them do 125% of the work the other 13 only actually complete 75%, this is a common factor in most workplaces, they all get paid the same and all the bean counters know is that it takes 26 people to accomplish the workload they have.

Don't get me wrong cycles and HRS are a solid rule of measure but not accurate, not absolute.

The answer to your second question is hard to answer. Did you enjoy your job, were you compensated well, were you appreciated for your skills? I am sure you count beans very well, not to belittle your task, just saying that the bottom line is you were payed to SPECULATE based on statistics.

[pattern_is_full]

@ grounded spanner and sunfish

Sure, I know the aircraft skin is curved. But to my eye (trained in photography, photo analysis and graphics arts), the curvature is different in that particular piece of metal in the area between the rows of rivets, than it is overall from edge to edge.

I.E., I see, in small scale, the same kind of change in curvature radius as at the "top of floor beam" in sunfish's X-section.

(And, yes, I know the piece of metal pictured does not come from the area of the floor beam :hmm: - which is why I find the change in what should be a smooth, constant-radius rooftop curve interesting.)

In the first picture, the reflection of the overhead lights makes a sharp change in shape as it crosses the lower rivet line. Which indicates a change in the curvature at that point - a distortion of the smooth curve.

In the third picture, end-on, there is also a change in curvature visible between the rivet lines.

I'd diagram what I'm seeing, but pprune doesn't allowed for direct uploading of images. :hmm:

[GroundedSpanner]

pattern_is_full

Now that I look at the photos on something bigger than an iPhone, and with your detailed description, I do see what you are talking about.

The change in curvature in the first photo (something akin to a slight crease) is very normal with a removed skin section across a rivet line. It is sometimes called 'pillowing'. You can often see on an aircraft fuselage that the skin changes profile at a rivet line. This is because as the aircraft is pressurised, it stretches ever so slightly, and when unpressurised, the stringers behind (only slightly) return to a smaller diameter circle. Nothing to be concerned about as long as it is not pronounced and/or there are signs of distress (cracks, pulled rivet heads etc). That photo looks very normal (ignoring the torn section next to it!!).

Whether there is any extra curvature is impossible to tell. If there were any extra curvature in the skin section it would have to be material that had come from the lap joint. I cannot see any signs of paint distress at the lap joint, which means that the skin will not have moved by more than 1/16 to 1/8 of an inch. Any more than that and the lap would have let go anyway.

I would lay money that that unbroken section of skin contains cracks (significant ones) in the lower row of fasteners, just due to its proximity to the torn section, but that the cracks are not sufficiently large to continue to link up yet. The NTSB will be very interested in how much longer that section had before it would let go.

[Sunfish]

The stupidity is coming thick and fast now.

Grounded27:

Don't get me wrong cycles and HRS are a solid rule of measure but not accurate, not absolute.

The answer to your second question is hard to answer. Did you enjoy your job, were you compensated well, were you appreciated for your skills? I am sure you count beans very well, not to belittle your task, just saying that the bottom line is you were payed to SPECULATE based on statistics.


I'm an engineer not a bean counter. The reason we use cycles and hours has nothing to do with bean counting. Those measures are surrogates for the strain history of the part in question and in metals that don't have a yield point (ie Everything except steel) the stress history determines when the component will fracture.

We make estimates based on experiment as to how long a component will last, then we apply safety factors to that estimate. When the aircraft is in service, we monitor all failures and continue testing to refine those estimates based on experience. In more than one case I dealt with, we actually reduced the number of failures by extending the time in service limits for an assembly, see if you can work out why.

In other words, try telling a turbine disk or a chunk of aluminium that its life in service is really infinite, it's just that bean counters make us change them.


Ozaub has already explained that Boeing had a problem with quality control in relation to the adhesive system it once used. That does not mean that the process was defective, it means that Boeing discovered after the fact that it could not precisely control the manufacturing conditions tightly enough to ensure a sufficiently reliable bonding system.


Before you yappers now scream for more of Boeing s blood, by "sufficiently", I do not mean 100% guarantee, I mean to an extent to where any defects are small enough to not result in stress concentration likely to cause severe crack growth during the expected service life of the aircraft which is god knows how many thousand hours and cycles (60,000 hrs? 60,000 cycles?).

The bloody aircraft has not done badly considering, and the safety measures worked as advertised. The only issue for Boeing is that the problem surfaced considerably earlier than expected.

The question is now to work out how extensive the problem is and determine what the best inspection methods are and what the repair schemes are.

I'm also getting fed up with what I term "The pprune effect" whereby every self proclaimed expert offers a solution from within their own expertise.

By that I mean; when there is an aircraft accident, Prune attracts the computer expert who posits the cause as a software problem. The chemist suggests the fuel was faulty. The teacher wonders about the pilot training and the lawyer blames it all on criminal negligence by the designer.

If you have never worked with aircraft, I wish some of you might be a little more tentative in coming to conclusions.

[grounded27]

We make estimates based on experiment as to how long a component will last, then we apply safety factors to that estimate. When the aircraft is in service, we monitor all failures and continue testing to refine those estimates based on experience. In more than one case I dealt with, we actually reduced the number of failures by extending the time in service limits for an assembly, see if you can work out why.


You work with statics, not based on experience but from actual, factual failures. Your experience is 2nd hand to these factors . Granted it's the best we will do from a cost v/s liability factor.

the stress history determines when the component will fracture Really now, sounds like you are in vegas betting against the house, your odds are worse than someone skilled at counting cards, now that guy has experience, a mathimatical formula that leaves less liability other than getting busted up by a bunch of thugs.

Inspection processes will improve as will manufacturing processes, at the same time manufacturers will develope products that push limits, use new alloys that will fail as they push limits. Over time we develope new inspections and servicable on wing times.

[pattern_is_full]

Grounded Spanner - roger that.

I thought that perhaps when the main torn section cut loose, it applied a little extra tension ("tug") to its neighbor before breaking free, causing the hump. But normal pillowing makes equal sense. In any case, obviously a section of interest to the investigators, if only because it is the other half of a joint.

[ian16th]

grounded27

You just might be confusing two different types of 'stress testing'.

See:
1.Stress testing - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Stress_testing)
2.Stress Rupture & Creep Testing at WMT&R, Inc. (http://www.wmtr.com/Content/creep_testing.htm)

[Shell Management]

By that I mean; when there is an aircraft accident, Prune attracts the computer expert who posits the cause as a software problem. The chemist suggests the fuel was faulty. The teacher wonders about the pilot training and the lawyer blames it all on criminal negligence by the designer.


Very true sunfish.

However, if Boeing had implemented a proper safety case, they may well have avoided this mistep.

Aircraft structure is not subject to anykind of risk assesment.:ugh:

[RatherBeFlying]

Having built a couple musical instruments, I came to appreciate that gluing is a complex operation. It took hundreds of years to develop bonding technology for wood -- and today the proliferation of new adhesives is truly amazing.

I do love tape adhesives -- but they don't always work;)

Bottom line -- all adhesives have their particular quirks, some of which take decades to manifest themselves.

Yes, pulling out adhesive tape from the freezer and uncoiling invites condensation.

The amount of condensation depends on the ambient temperature and humidity of the day.

I doubt the Boeing factory floor is an environment where temperature and humidity are precisely controlled.

It may be useful to relate the discovered bond failures to the day they were performed and the temperature and humidity at the time. Are there environmental records of temperature and humidity on the factory floor? We may find ourselves limited to public weather records and then infer from building management procedures what the HVAC would have produced. My experience of building management is that HVAC is managed much more to the dollar sign than anything else:}

[lomapaseo]

Aircraft structure is not subject to anykind of risk assesment

Are you kidding?

Everything that we do in designing and certifying aircraft has risk assesment built into it. That's why things break, including acts of God. We do a pretty good job of balancing risks, that's why it is safe to fly.

[Shell Management]

I'm 100% serious (and correct;)).

A probabalistic, risk based approach is only required during certification of systems and equipment. Structure and handling for example are exempt. If they wernet the 2001 AA A300 accident would have been prevented.

[grounded27]

Depending on type. During heavy inspection lower skins are common to replace, if there are enough skins that need replacement you have 2 options... Replace only a few at a time as not to allow for the hull to torque or often secondary procedure for additional shoring and the hull is shot with a transit allowing for many skins at the same time.

Now if something goes wrong and an aircraft comes off jacks after a multiple skin change, the skin seams could be subject to abnormal stresses. The whole frame being torqued.

[Shell Management]

That is needless and irrelavent speculation.

[lomapaseo]

I'm 100% serious (and correct;)).

A probabalistic, risk based approach is only required during certification of systems and equipment. Structure and handling for example are exempt. If they wernet the 2001 AA A300 accident would have been prevented.


You really are serious! but not 100% correct :)

I really can't find fault in your words above, it was only your original statement that I challenged


Aircraft structure is not subject to anykind of risk assesment.:ugh:

The idea behind the cert standards is to require a specification of conformance within a specified range. Anything outside of that range is expected to be minimized to practical extent.

Thus a structure may be specified to a stress limit but therafter "managed" to a life limit.

[Shell Management]

OK, I'll cut you some slack;), but just tell me old boy what the probability of this piece of structure reaching design life target Boeing had to work to?

Can you answer that? Can you?;)

The other advantage of apply an SMS philosphy would be that the earlier failures would have resulted in earlier Boeing action.:ok:

[Machaca]

Shell Management -- since you are the one swinging your ISO 31000 hammer at everything, why don't you contact Boeing and then report back to us the answers to your questions.

[Shell Management]

I didn't mention ISO 31000:2009 old bean.:= That a pretty poor Antipodean drafted standard that is compromised by its application beyond safety.:ugh:

If you were an aviator, you would know that it is ICAO that defines tsndards in aviation not ISO.:=

[Fargoo]

The affected aircraft gained winglets some time in 2009, is there a possibility that the extra weight of these winglets causes more wing flex which may stress the skin above the original design spec??

[lomapaseo]

Shell Management

OK, I'll cut you some slack;), but just tell me old boy what the probability of this piece of structure reaching design life target Boeing had to work to?

Can you answer that? Can you?;)



Given that they have conducted lots of fatigue tests on similar designs, they would probably statiscicaly expect cracking at about half their target life. Such cracks would be inspectable and a confirmation of their model. Now if the cracks are not easily inspectable then they better half the life expectancy even more and continually adjust their mainteance program based on findings.

Sounds to me like that's where they are.

[Sunfish]

This thread is deteriorating rather quickly.

Aircraft are designed and built under massive risk management programs that are driven by parameters set by the insurance industry starting with the hull loss probability. That then gets driven down into the design through Failure mode effects analysis (FMEA) right into the design MTBF's of the components - including the hull.

Those programs do not stop when the aircraft design is frozen, but continue for the life of the aircraft in service or until the company goes under, after which someone else has to be found to assume design authority if the aircraft is to remain in service.

We have one person here (grounded) that confuses bean counting with statistical analysis of failures and doesn't understand what a strain history is either.

To put it another way Grounded, why do you think an automobile manufacturer sets service intervals and component replacement schedules for the cars they make and how do they do it?

P.S. If you ever worked on aircraft replacing lower skins, then they must have been pretty old and crappy aircraft because Boeing started paying attention to bilge corrosion - sealing faying surfaces and filleting with PRC 1422G8 for example, around 1975.

[grounded27]

Aircraft are designed and built under massive risk management programs that are driven by parameters set by the insurance industry starting with the hull loss probability. That then gets driven down into the design through Failure mode effects analysis (FMEA) right into the design MTBF's of the components - including the hull.



Really, I was under the impression that it was as simple as keeping a clean driving record. Do you guys get a safe driver discount down under. Quantas has a wonderful record, shame they do not get a discount for it.



Sorry to have offended your profession. I have in depth knowledge of modern corossion preventative programs. CPCP goes way beyond using a variation of tank sealant in joints, I have had factory training by Dynatrol and yes have worked around some old aircraft "quick go wiki it and come back with more slander".

All your statistical analysis is great in general. BUT it is absolutely useless when something fails before you predict it.

To word it another way HRS/CYCLES/ANALYSIS are not worth a damb to the crew and passengers of an aircraft diving for breathable air.

Also events like this allow tools like yourself to change the shelf life of a product, initiate an inspection at an earlier date. I am sure it made you feel like you were doing the world a grand favor. Sir by mentioning you as a tool it is with the utmost respect, persons like yourself create job security for people like myself, we are truely a breed apart.

The only thing exciting in this business are things that can not be predicted, the unpredictable changes the industry.

[daikilo]

Winglets: I would look more at the additional lift on the wing in critical conditions, which could be transmitted as bending load into the fuselage. That said, given the speed with which Boeing revised the inspection interval without any reference to modifications, I doubt they are relevant.

Structure life: I am intrigued by the suggestions that structures are not subject to risk assessment. This may have been the case decades ago but I thought all recent new aircraft designs had to be tested to show compliance, the so-called fatigue tests. Maybe the 737-300s, being derived from the 1960s -100/200 didn't need such a demonstration, or service experience was used.

[Sunfish]

Grounded:

All your statistical analysis is great in general. BUT it is absolutely useless when something fails before you predict it.

To word it another way HRS/CYCLES/ANALYSIS are not worth a damb to the crew and passengers of an aircraft diving for breathable air.

Also events like this allow tools like yourself to change the shelf life of a product, initiate an inspection at an earlier date. I am sure it made you feel like you were doing the world a grand favor. Sir by mentioning you as a tool it is with the utmost respect, persons like yourself create job security for people like myself, we are truely a breed apart.

The only thing exciting in this business are things that can not be predicted, the unpredictable changes the industry.


Have you told the aircraft manufacturers, operators and the worlds regulators the wonderful news that all those extremely costly and carefully constructed aircraft maintenance programs they engage in can now be abandoned because failures "just happen"?

Why do you think we bring a particular aircraft in for you to maintain? Do you think we just pick its number out of a hat?


Shell Management, I was taught risk management by Exxon. The starting point for the airline, air traffic control and aircraft manufacturer risk management programs are the acceptable passenger death rates per million miles travelled as well as the calculated cost to the insurers of a fully loaded aircraft crashing in the middle of a block in central London or downtown Manhattan.

[lomapaseo]

Shell Management, I was taught risk management by Exxon. The starting point for the airline, air traffic control and aircraft manufacturer risk management programs are the acceptable passenger death rates per million miles travelled as well as the calculated cost to the insurers of a fully loaded aircraft crashing in the middle of a block in central London or downtown Manhattan.

Leave the air-insurers consortium out of it := they don't have a hand in managing safety. They enter into it to gauge what the overall statistical risk is and what level of their money to set aside for the aftermath.

And nobody in regulated aviation goes around with managing a known problem against the number of dead bodies.

Risk management can address known problems that are understood well enough (all the potential causal chains in a link) such that the risk of all the links coming together within the time frame that corrective actions are implemented are far less than 1 single catastrophic event.

It's the unknowns that catch us by surprise and this thread subject is just another example of a surprise. But now that it's no longer an unknown a corrective action will be implemented (closing out the risk) in a period of time where only a surprise (unforeseen combination) will result in a catastrophe.

If critical combination in the future turn out to still be unknowns after an investigation then expect forced inspection and limitations forever. If this can't be reliably performed then expect the limitations in operations to be severe.

This is nothing more than managed risk at least far better than the average person freely lives their own life.

[grounded27]

Risk Management is based on a theory of what should happen, it is nothing more than financial liability. Reality changes your factors and the variable factor of events we can not predict is reality. I do not understand the argument here? Our environment our reality is certainly unpredictable, thus my point. What should be is a big part of society, what is, is real.

[Sunfish]

lomapaseo, I'm afraid it all starts with the insurers and actuaries. I know its a gloomy thought, but there you are.

Contrary to public perceptions, a value is put on human life. Actuaries also put assign an acceptable probability of death to any endeavour. From memory when I started my Engineering degree, the figure was about Four million to one.

Basically, what risk management is about is multiplying the probability by the cost. When you then multiply that number by the activity e.g. (million flights), you get a big number. If you can then take some safety action that reduces the probability of a given accident, you can then quantify what the savings will be in the long term, if the cost of taking action is less than the savings you are in the money and you take the safety action.

This concept of "affordable safety" underpins all engineering. This is why your aircraft does have non inflammable furnishing materials but doesn't have individual parachute equipped escape pods.

[Volume]

the stress history determines when the component will fractureTrue statement. However, as the longitudinal lap joint is mainly loaded by cabin pressure and cabin pressure (hence stress levels) are closely controlled by a dedicated system, stress history for this component is almost 100% determined by flight cycles.
For other componens (e.g. wing lower skin) a lot of statistics is needed to define the life of the component.

[Sunfish]

Volume:

True statement. However, as the longitudinal lap joint is mainly loaded by cabin pressure and cabin pressure (hence stress levels) are closely controlled by a dedicated system, stress history for this component is almost 100% determined by flight cycles.
For other componens (e.g. wing lower skin) a lot of statistics is needed to define the life of the component.

Agree 100%. As for wings, I assume these days what with data storage, miniature accelerometers, etc. it is possible to integrate strain (or an analogue of strain) with respect to time and store the results, thus being able to distinguish between aircraft with different duty cycles and environments.

I think engine rpm and T's and P's are already recorded for disk life limit calculations, but I'm getting out of my depth here.

[lomapaseo]

I think engine rpm and T's and P's are already recorded for disk life limit calculations, but I'm getting out of my depth here. I Agree 100%. As for wings, I assume these days what with data storage, miniature accelerometers, etc. it is possible to integrate strain (or an analogue of strain) with respect to time and store the results, thus being able to distinguish between aircraft with different duty cycles and environments.

Time to come up and take a deep breath of reality :)

Nobody's got the time to records and calculate the infinite variations in duty cycles and stress.

The cyclic life is typically predicated on a statistical stress level and duty cycle. Sometimes even an overstress condition (above the typical stress) can lengthen the cycle life considerably. confusing isn't it???

sorry about the rambling ... but its cocktail time here and I'm feeling it :)

[Alber Ratman]

Or can gauge what idiot is going to use an unapproved scrapper/knife on a lap joint.. or the unscruplous inspector that pens it off without carrying out the prescribed measurement / correct rectification..:ugh:

Wouldn't surprise me if a scribe line was the stress raiser that induced this skin failure.

[FlightPathOBN]

stress history for this component is almost 100% determined by flight cycles.

Flight 'cycles' is a factor that needs to evolve with relation to temperatures, aerodrome elevation, and air quality conditions.

If the aircraft routinely 'cycles' between low and high alt conditions, the model busts.

Another missed component is with the aircraft parked in the southwest, power off for months, cycling cold, condensation, hot weather conditions.

[Pugilistic Animus]

Don't confuse testing with the assesment of probability and severity of failures.:)I agree, everybody knows much more than Boeing-and you seem to know much more than the one person who knows more than everybody else who knows more than Boeing:rolleyes:

[MountainBear]

Reality changes your factors and the variable factor of events we can not predict is reality.

There is no such thing as reality, just various descriptions of it.

The problem when dealing with extremely long odds is that we never can tell what happened, in reality. If I tell you the odds of getting hit by a car crossing the road are a trillion to one and tomorrow you go out and get hit by a car crossing the road, what just happened? Were you just amazingly unlucky? Was my probability calculation wrong? It's impossible to say with certainty.

If the odds of an airliner spontaneously combusting inflight is a trillion trillion to one it's still remains a possibility that you and I could see three of them do it tomorrow. If time is infinite, then anything whose odds of occurring is less than infinite will happen sooner or later.

Personally, I don't think that either the FAA or Boeing knows what just happened in reality. Nor do they care. They are politicians and businessmen and not metaphysical philosophers. They just want to cover their ass and make sure they have someone to blame reality on, whatever it is.

[Sunfish]

Mountainbear:

Personally, I don't think that either the FAA or Boeing knows what just happened in reality. Nor do they care. They are politicians and businessmen and not metaphysical philosophers. They just want to cover their ass and make sure they have someone to blame reality on, whatever it is.

It isn't as bad as that. There is a small element of backside covering, but the reality is that if people perceive air travel to be unsafe, then they aren't going to use it. Bear in mind that before stratospheric jet travel was available, air travel was a lot more dangerous than it is today.

What we strive for is reliability what that means is that the bits that make up an airliner perform consistently exactly the same way all the time. It matters not how long they last, although the bean counters care about that, what matters is that they all behave predictably.

You are right to a point when you say that we cannot predict "reality" for an individual aircraft. We don't even try as far as i know.

However what we actually do is some what different; what we say is as follows:

For critical components whose failure is catastrophic; "based on our experience with thousands of aircraft flying for Tens of thousands of hours and landings and takeoffs, we are 99.999 percent sure that such and such a component is going to last at least this long" before we change it. (hard time)

For components that we have designed to be fail safe or damage tolerant, like the skin of the aircraft in question; "based on tests and long experience of failures, if this part fails we will know about it because it will announce itself in plenty of time for us to land and take corrective action". (on condition)

For components that wear out; "we will measure this, and if it is worn out we will replace it before it causes harm." (condition monitored)

For components that don't have any effect on airworthiness - seat jacks, interior lights and in flight entertainment systems they are "as required".

Don't nit pick, it's a simplified description.

[Mr @ Spotty M]

I am surprised no one today has picked up on what was said yesterday,
This is a passage from flightglobal
Speaking at the MRO Americas conference in Miami, FAA administrator Randy Babbitt said: "People have leaped to the conclusion that it was fatigue. The airplane didn't have that many cycles on it so we're looking at other things. The manufacturing techniques. Boeing is very interested too. This is not good for anybody's business." :uhoh:

[Sunfish]

Shell Management:

Clearly if you were taught by Exxon and think that risk management is about insurance you had a poor teacher who doesn't under stand the concept (or you have the IQ of a hermit crab). Insurance is one mitigation but not a very effective one. I suggest you read this:

If you don't think risk management is about money, then you are deluded. I never said that insurance is a mitigator, what I said was that insurance requirements ultimately drive risk management programs.

To put it another way; Insurers will make you do these things, or no insurance at any price.

Why do you think the technical and standards arms of Lloyds, Det Norsk Veritas, etc. exist otherwise?

[captplaystation]

22 years ago when my base became a 737 base and I stopped flying the lovely DC9 (Thank God mind you, how many jobs I have had subsequently thanks to that rating) I remember feeling a bit underwhelmed by the perceived "quality" of my new steed.
I remember being horrified at what I was looking at whilst performing external inspections/walk-rounds . . . whatever we call it these days.
"For christs sake" I used to muse," do Boeing make aircraft or ships ?"
Don't know if the worst was the "slap it together with rivets and never mind the overlapping joints" or the " orange peel is not only for middle aged womens legs , it is fine for lower aft fuselage panels" that bothered me the most.
If you were used to the smooth flawless surface, that one rather expected on a high speed vessel & certainly found on a DC9 , the 737 was a bit of an eye opener.
Maybe my initial impressions were right . . . quality product ? ? well, maybe not, but has the "quality" been enough, that may prove to be THE question.

[lomapaseo]

If you don't think risk management is about money, then you are deluded. I never said that insurance is a mitigator, what I said was that insurance requirements ultimately drive risk management programs.

To put it another way; Insurers will make you do these things, or no insurance at any price.

Why do you think the technical and standards arms of Lloyds, Det Norsk Veritas, etc. exist otherwise?

All true, but the insurrer doesn't set the standards. they can only monitor that you are working to a standard acceptable to a regulator, FAA, ISO, ICAO etc. If the Insurer were to set a standard then the lawyers would have a field day sueing against them for extra cause.

In all my dealings with the consortiums, they simply conducted audits against my process and compared it to their overall risk. I then justified my on-the-job performance against the relative cost of this insurance to my own (secret) standard.

[Alber Ratman]

Seen the photos of the removed section for the first time.. If you were looking for scribe lines, that is the typical area of a lap where you look for them. :E

[glad rag]

Doesn't have to be scribed, a lead (proper one) pencil has caused the same before, many years after the job was signed off.:hmm:

[Alber Ratman]

Yeah, pencil marks can do the same.

[sevenstrokeroll]

I wish to echo captainplaystation's feelings on the difference between the DC9 and the 737.

First, one must understand that these two planes were built (original versions) around the same time, to do the same job.

The Douglas design is simply more robust in structure and its systems are the definition of KISS (keep it simple, stupid)..

Given a choice, if someone were to offer me a thirty year old DC9-30, a brand new 737ng, or a brand new Airbus 320 series, I would take the DC9 if it was properly maintained .

And I would bet that in 20 years, the other two planes would have been retired and my 9 would be fine.

[twochai]

[The Douglas design is simply more robust in structure]

If I'm not mistaken the DC-9, like all DC airplanes certificated to CAR 4b, was designed with robust alloys, with high copper content and sufficient structural reserves such that they were certificated to FAA standards of the day without fatigue testing.

[RWEDAREYET]

Haven't read the entire thread...so it might have already been said....but I agree with the couple posts about the 9....completely different philosophy on aircraft design...

Main problem is...each airline deals with aircraft "fatigue" in different ways...depends on the airlines "authority" and "recommended" practices from the manufacturer.....This is a slippery slope.

Airline A could check the structural integrity of the 737 every 5000 hours and Airline B could do the same check every 20,000 hours and it's all legal....

It's only when the sunroof occurs....that the "authority" decides that we must look into the fatigue issue....

As far as I know....Southwest with about 80 aircraft and Alaska with about 2 where the only two airlines in the USA that needed to have their aircraft inspected and were grounded till inspected....I could be wrong, but that is what I have read...

That being said, what else is Southwest or Alaska not doing that the other airlines in the USA or Worldwide are?? Not pointing fingers, and not saying they did anything illegal....I do not think they did anything wrong, they just went by Boeing and their FSDO's rules...

So the question is....what happens with the rest of the operators of the 737's around the world???

Last, I miss the Douglas products.....what great machines!!!!:ok:

[RWEDAREYET]

Shell,

I understand that and I never said that either SWA or Alaska did not use Boeings guidance.

I said other airlines in the USA must have gone above and beyond the guidance that both SWA and Alaska had been following for those particular airframes. Not trying to pick on either airline, they did nothing wrong. But, USAir and United which both have some older 300's of the same era did not have any issues after inspections were complete. (that I know of anyway)

The real issue is....when these older 300's find there way to a foreign nation, who's guidance will they be using??

[sevenstrokeroll]

rwedareyet

I agree with you. It is odd that United and USAIR didn't have the same problem. I do know that USAIR had the best ageing aircraft program in the country.

[sb_sfo]

The reason that UA didn't have any trouble is that they have none on the property as of 2009, at least according to Wikipedia?

United Airlines - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/United_Airlines#Fleet)

[Tanker]

Southwest with about 80 aircraft and Alaska with about 2 where the only two airlines in the USA that needed to have their aircraft inspected and were grounded till inspected....
The inspection program didn't cover all of the Classic 737s, only the most recent that Boeing had built. This was due to a change that Boeing had made in the manufacture of the Classic 737s, a change that Boeing thought would make the aircraft last 60,000 cycles before inspection. Instead this aircraft lasted 30,000 cycles. Southwest has 172 737-300s and 25 737-500s and only 78 737-300s required this inspection. It is sort of hard to plan to inspect for a failure when the aircraft is only halfway to the point where the manufacture said the failure may occur, and you have aircraft that are way older being inspected that show no sign of this failure.

[dfstrottersfan]

seven......

I agree with you. It is odd that United and USAIR didn't have the same problem. I do know that USAIR had the best ageing aircraft program in the country.

United Airlines Fleet | Airfleets aviation (http://www.airfleets.net/flottecie/United%20Airlines.htm)

[Data Guy]

Nothing new about these fuselage cracks, but rather that they aren’t being caught in time.

A quick and dirty search of the Service Difficulty reports for users like Continental, United, US Air, American and Southwest shows over 260 SDRs of fuselage cracks from 2000 through this year.

Over 14 ADs have been issued since 2002 including; 2002-07-08, 2002-07-10, 2002-07-11, 2003-08-15, 2003-23-03, 2004-18-06, 2004-23-10, 2005-13-27, 2005-13-30, 2007-26-04, 2008-12-04, 2008-19-03, 2009-21-01 and 2010-01-09 affecting every model of the 737s from the 200s to the 900s (2008-12-04).

Worst yet, is the appearance that the chem.- milled sheet step-cracking problem has carried forward to the 757s with AD-2011-01-15.

Just plug in the AD numbers above into this AD search query for full details >
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/MainFrame?OpenFrameSet (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/MainFrame?OpenFrameSet)

[alisoncc]

News reporting here in Oz that the problem has been traced to a manufacturing defect. Apparently the section rivets were incorrectly fitted - the drilled holes were too large, leading to movement in the joint.

[Data Guy]

Regarding the above post Permlink #198)- “News reporting here in Oz that the problem has been traced to a manufacturing defect. Apparently the section rivets were incorrectly fitted - the drilled holes were too large, leading to movement in the joint.”

This doesn’t make sense. Here is why.


Rivets or Hy-Locs ?

If the fasteners were rivets – then the rivets would assume the precise size of the holes drilled as they swell. We are talking in a few thousands of an inch and assuming they didn’t abandon all common sense (if it wiggles, something is wrong).

More likely - they were Hy-Locs which come in graduated oversizes to accommodate hole sizes including those that have been re-worked or enlarged in the removal of any earlier fasteners (rivets or Hy-Locs). For each specific oversized Hy-Loc, there is a specific oversized drill. Hy-Locs come in two components; the Hy-Loc itself, and a threaded collar incorporating a hex/nut portion to allow the collar to be driven by a wrench, a socket, or more commonly, an air powered driven socket wrench. Once proper tension is achieved, the ‘nut’ portion snaps away and leaves a small collar.

Hy-Locs won’t install in an oversized hole.
More importantly, there is a built-in feature to prevent wrong sizing with every Hy-Loc installation. The ‘nut’ can only be driven if the Hy-Loc does not rotate in the hole.
You can not drive the nut to the required tension permitting the ‘nut’ portion to snap away with a spinning Hy-Loc !

It is this interference fit – the ‘grip’ – that assures the hole sizing, and the Hy-Loc size selected are proper.

Drilling out fasteners changes the original hole sizes

Lastly, you can’t determine the original hole size after having drilled out a rivet. When it was driven, it may have swelled and expanded that hole, or more likely, by drilling it out, some hole material was also taken out as well and thus you have a ‘double hole’, one that is enlarged or ‘out-of-round’.

So, in sum, wrong sized holes at the manufacturing level where thousands of fasteners are installed a day seems highly improbable where not only are the installers involved, but also the Q.C. guys. It doesn’t make sense. Boeing has had a number of controversies but the very basics of fastening shouldn’t be one of them.
---------------

[VFD]

Doesn't make sense
Well right now that is what the NTSB saying. Oversized holes and elongated holes or double holes. Apparently the overlap joint has been loose at least since the last repaint as the paint (Blue in this case) had seeped between the lap joint.
So, in sum, wrong sized holes at the manufacturing level where thousands of fasteners are installed a day seems highly improbable where not only are the installers involved, but also the Q.C. guys. It doesn’t make sense. Boeing has had a number of controversies but the very basics of fastening shouldn’t be one of them
This appears to be the direction that NTSB will be looking. Something wrong in the manufacture of the lap joint. As you suspect Data Guy that should be basic operation. Something is not passing the smell test here.

VFD

[IGh]

NTSB Press Release (http://www.ntsb.gov/Pressrel/2011/110425.html)

"... NTSB Materials Laboratory, microscope examination of the fracture faces of the ruptured skin revealed fatigue cracks emanating from at least 42 of the 58 rivet holes connected by the fracture. Electrical conductivity measurements, hardness tests, and X-ray energy dispersive spectroscopy elemental analysis of the skin in the area of the fracture revealed that the aluminum skin material was consistent with the specified material. The skin was the specified thickness.

"Non-destructive eddy current inspections conducted around intact rivets on the removed skin section forward of the rupture revealed crack indications at nine rivet holes in the lower rivet row of the lap joint. To assess the condition of the intact rivets and the skin rivet holes, X-ray inspections were performed on the skin located forward of the rupture location. This inspection revealed gaps between the shank portions of several rivets and the corresponding rivet holes for many rivets associated with S-4L. Upon removing selected rivets, the holes in the upper and lower skin were found to be slightly offset relative to each other and many of the holes on the lower skin were out of round.

"In this ongoing investigation, the NTSB Materials Laboratory work is actively conducting additional inspections and examinations in the following areas:


Removal of rivets and examination of rivet hole dimensions, rivet dimensions, and rivet hole alignment between upper and lower skins.
Detailed fractographic analysis of the skin fractures emanating from the rivet holes using optical and scanning electron microscopes.
Fatigue striation analysis using a scanning electron microscope of specific skin fractures to determine the rate of crack propagation.
Additional portions of the lap joints from the accident aircraft...."

[Sunfish]

We seem to have a manufacturing or repair defect here.

Off topic, as for DC9 vs. B737, there were different design philosophies at work. Douglas was more elegant than Boeing, or thought they were, as anyone who has examined the pilots opening windows on both aircraft will know.

However Boeing didn't design that elegant tracery of linkages for the DC10 cargo door either.

My experience of dealing with Douglas was always: "Fukc you! What would you know? You are just an airline engineer! We built the DC3!" Boeing actually had a certain humility in dealing with its customers, at least on an engineering level.

[lomapaseo]

This isn't a customer relations problem.

From my read so far

Fatigue outside of typical reliable lifing expectations

something wrong with the rivet holes

Repair-rework or something else at play here (my light switch on-off theory)

Population suspect associated with a manufacturing time frame rather than basic design philosophy

Happy to be corrected if I misread something.

[BARKINGMAD]

Will the NTSB report of the first NGs to suffer this problem take into account the reported defective structural parts supplied to and fitted by Boeing viz. the chord sections and the "bear-straps", which needed a hammer and/or some extra hand-drilled holes out of tolerance, in order to make them fit on the company's assembly line????

Yet again take a look at " A H F DUCOMMUN " in your search engine and see how the sacked whistleblowers are faring. Then thank the lord that we've got such strict oversight of our major aircraft manufacturers......

Sorry, I made that last part up, only joking!:eek:

[Data Guy]

Some Briefs From the Federal Register and ADs. See source Links for full texts.

2002. Inadequate Level Of Safety - lap Joints.
AD- 2002-07-08. 737-200, -200C, -300, -400, and -500 series airplanes having line numbers 292 through 2565 inclusive …… that currently requires repetitive inspections to find cracking of the lower skin at the lower row of fasteners in the lap joints of the fuselage …also requires modification of the fuselage lap joints at certain locations,… . This amendment is prompted by the FAA's determination that, in light of additional crack findings, certain modifications of the fuselage lap joints do not provide an adequate level of safety. Compliance; before the accumulation of 50,000 total flight cycles or within 2,250 flight cycles after the effective date of this AD, whichever comes later. Locations; lower skin at the lower row of fasteners in the lap joints of the fuselage, - stringers 4R and 10R
Source; http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/7ffda03569290c6e86256b99005a278a/$FILE/020708.pdf

2005. Fuselage Structure Tests Failures at 21,000 cycles.
AD- 2005-13-27. 737-300, -400, and -500 series airplanes. …. repetitive inspections for cracking of the crown area of the fuselage skin, ….. prompted by a Model 737 fuselage structure test and fatigue analysis that indicate fuselage skin cracking could occur between 21,000 and 42,000 total flight cycles.. . Within 4,500 cycles. Effective August 1, 2005.
Source; http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/e89e6471bfd6d8908625702d005c4b90/$FILE/2005-13-27.pdf

2006. 757s. Countersinks Too Deep.
2006-20-11. 757-200, -200PF, and -200CBs. ….requires doing initial and repetitive detailed or high frequency eddy current inspections for cracks around the rivets at the upper fastener row of the skin lap splice of the fuselage, and repairing any crack found. This AD results from a report indicating that certain rivets were incorrectly installed in some areas of the skin lap splices during production because they were drilled with a countersink that was too deep. Boeing states that the production rivets are commonly referred to as ''Briles'' rivets, and are manufactured with a 120-degree, modified shear head. Effective 11/8/2006.
Source; http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/abc1cd0c338beefd862571fd0053120d/$FILE/2006-20-11.pdf

2008. Missing Or Loose Fasteners or Milled Steps.
AD- 2008-12-04. 737- 600, -700, -700C, -800, and -900 series airplanes. ….requires various repetitive inspections to detect cracks along the chemically milled steps of the fuselage skin or missing or loose fasteners in the area of the preventative modification or repairs, …. This AD results from a fatigue test that revealed numerous cracks in the upper skin panel at the chemically milled step above the lap joint. Compliance per the Service Bulletin. Within 36 months. Effective July 16, 2008.
Source; http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/0/3db5e1f9035bc25e8625746500523451/$FILE/2008-12-04.pdf

2011. 100 ADs - Widespread Fatigue Damage .
“Since 1988, the FAA has issued approximately 100 airworthiness directives to address WFD
Widespread Fatigue Damage in airplanes. Approximately 25 percent of these airworthiness directives were too urgent to allow the public an opportunity to comment in advance. These airworthiness directives required inspections, and the FAA later superseded the majority of them to expand the inspections or require modifications because inspections were not enough to preclude WFD. Effective January 14, 2011.” Source; Federal Register; http://www.regulations.gov/#!documentDetail;D=FAA-2006-24281-0088 (http://www.regulations.gov/#%21documentDetail;D=FAA-2006-24281-0088).

[heavy.airbourne]

sideline:

However Boeing didn't design that elegant tracery of linkages for the DC10 cargo door either.

Those were designed and manufactured by Convair.

[forget]

Those (DC10 door latches) were designed and manufactured by Convair.

Manufactured, yes; designed, no. Didn't Convair warn Douglas of shortcomings in the Douglas design before the events?

[jxk]

By chance have just been (re) reading the 'The Comet Riddle' and although there were other defects found by flight testing the main research was concentrated around the tests performed in a water tank. The test was conducted on a whole airframe; the wings subjected to typical flight loads and the pressure vessel (ie the fuselage) was pressurized to simulate the constant ascents and descents. And as we know it was eventually discovered that there was a fatigue crack emanating from the ADF window.
As new technologies come into existence, for instance the Boeing 787, are similar 'Comet' tests conducted to ascertain how many cycles the airframe can withstand? Or is it all computer simulated?

[ozaub]

In answer to jxk, yes new aircraft types must be fatigue tested though not now in a water tank. However B737 was never properly fatigue tested relying instead on read over from 707 and 727.
Further to my post #126, I wanted to better explain how the lap joint evolved, by posting some illustrations from NTSB report on Aloha accident. That doesn’t seem possible on pprune so those who are interested should refer to pg 15 and on of the report at: http://www.airdisaster.com/reports/ntsb/AAR89-03.pdf.
Prior to LSN 292 the outer skin at the lap was fatigue critical at top row of rivets (the most heavily stressed row), due to stress concentrations at the countersink and likely failure of the cold bond process. After LSN 292 the outer skin was locally reinforced with a hot bonded doubler. In both cases the next weakest link is fatigue of inner skin at bottom row of rivets. That's what failed on SWA aircraft. And if reports of loose rivets are true then premature failure is not surprising. What is alarming is the much greater difficulty of detecting cracks in inner skin compared with outer. There’s a further hazard in that when the inner skin starts to crack it is initially braced by the outer skin and cracks become excessively long before controlled decompression happens.