737NGs have cracked 'pickle forks' after finding several in the jets.

[Fly Aiprt]

In a clamped joint with high tensile bolts, the bolts are not (supposed to be) subjected to shear loads, which are carried by friction between the surfaces which are clamped together by the bolts. Holes have clearance and individual bolts are not subjected to load in sequence, failing before the next bolt takes load.

That's correct.
Maybe the reason why those cracks are taken so seriously : the joints have slipped.

[Aihkio]

A slip can usually be seen. Usually the problem is incorrect tightening moment or procedure depending on the fastener type.

[lc3m4n23]

Wow, if that is true, there is a serious manufacturing deficiency. Is there any source reference that you can share?
the source comes from what Boeing and spirit told our facility during their visits this week. Though the company I work for is not the cause of the problem forms, we took over the pickleforks parts in 2018. Spirit and Boeing have visited and seen our entire procedure from start to finish. They are very happy with our production of the pickleforks for the max 737 and the replacement forks we are building for the 737ng. We are the sole company in charge of making the forks now and the info I have about the grossly oversized holes is straight from Boeing and spirit themselves.

[Pilot DAR]

In a clamped joint with high tensile bolts, the bolts are not (supposed to be) subjected to shear loads, which are carried by friction between the surfaces which are clamped together by the bolts.

In a standard structural lap joint in an aircraft, the fastener shear load capacity is the predominant load path. There is no conventional methodology to calculate, and then later control the factors of friction for the purpose of carrying a structural load across a lap joint. So bolts and rivets are primarily subjected to the shear loads, which is why they, the hole preparation, and their installation is such a controlled process. If joint friction were to be a factor in lap joint structural capacity, that would be akin to bonding a joint. Bonding a joint in a lap joint is certainly done, and very common in composite structure, though less common in metal structure.

An important factor in bonding, beyond the capacity of the bonded joint itself, is that the bonding excludes contaminates from the joint. If the primary structural lap joint were to be depending upon friction for load carrying, what would happen when oil seeped into it? The friction would go away! You'd sure hope then that the fasteners would carry the load in shear! This can be better understood in real life, in that friction is a factor when properly torquing nuts and bolts, to assure that the designed torque is not exceeded.

In most cases, torquing specifies "dry threads", because the many different types of lubricants, if applied to the threads of the fastener being torqued, could dramatically change the achieved torque, and thus tension applied to the bolt while torquing. When experimenting, I have managed to snap off 125ksi aircraft bolts in their normal torquing range, by applying really good lubricants to the threads. Friction is very hard to predict and calculate, but the lack of friction is really easy to figure out, and quite achievable with a good lubricant. My structural designs rely on standard methodology fastener shear allowables calculated to carry the entire load of a structural lap joint in shear.

[Aihkio]

It is a bit more complicated. In practice the load is carried (almost all) by friction, it is calculated to be ably to carry the load by bolt shear too.

If it were bolt shear only the tightening torques would be quite low, no need for high ones.

With pre drilled holes the load carrying capacity would be a lot lower than the theoretical one with in place reamed holes.

I remember one composite-metal connection where it was dimensioned for both bonded and bolts. In practice the load is/was carried by the adhesive.

[IFMU]

In a clamped joint with high tensile bolts, the bolts are not (supposed to be) subjected to shear loads, which are carried by friction between the surfaces which are clamped together by the bolts. Holes have clearance and individual bolts are not subjected to load in sequence, failing before the next bolt takes load.
That sounds like nonsense to me! At the airframers I worked with bolts were always loaded in shear and for critical joints special attention was paid to bolt fit in both design and manufacturing.

[Grebe]

That sounds like nonsense to me! At the airframers I worked with bolts were always loaded in shear and for critical joints special attention was paid to bolt fit in both design and manufacturing.

Agree re sounding like nonsense- he/she is mostly- completely wrong. In aircraft design and construction for at least 60 years, most- if not all fasteners are designed to be used in shear. true there are some tension applications - but structural rivets, bolts, etc are designed and installed to reasaonably assure the hole is filled.

One of the earliest versions of a tight hole filled fastener is the ' taperlok'- used extensively/ on the B-52. Then came close fit lockbolts, hi shear and similar. Then came things like rivbolt- interference fit - 'cold expansion' ( AKA coldwork ) in the 60s. Rivets were designed to expand in then hole via both squeeze and squeeze vibrate- and for some critical areas ONE SHOT installation and hole filling was controlled by die shape. ( I'm talking aeospace - since until the 60's and 70s, hot riveting was used on buildings and bridges ) and hot riveting was tried - tested ( electrical heating ) for aerospace but had other problems. One shot riveting is now done by ' electro- magnetic riveting ' and the major firm is known as electro-impact. Turns out that properly done, one shot riveting does have significant fatigue improvement.

ALL of which to say is the major- most common installation of fasteners in aerospace is to produce close or interference fit to ensure fasteners are uniformly loaded in SHEAR.

stepping down from soapbox :rolleyes:
BTW- electro-magnetic- riveting ( one shot ) was developed and patented by Boeing in the 70's- actually developed and tested in the late 60's and used on early 747's. Electro impact came along later ( long story ) with a low voltage version and thecompany was founded on that modification.

In the late 60's, coldworking was used as a field ' repair ' on some fastener holes in high strength steel on lufthansa 707- by sending an AOG crew there with a few driils, reamers, and sleeves and mandrel. I turned down that particular trip ..

[lc3m4n23]

IF- BIG IF the claim that holes are-were oversize as claimed ( some worker- bee who is involved in repair-replace would know ) then ther problem is almost exactly as I postulated back on sept 11 - see below. depending on the defined process - which SHOULD have required the holes on final assembly in that area be properly drilled- reamed- inspected as a minimum ( and skipped or pencil whipped ) - or IMHO been properly coldworked, then the problem could be fleetwide. Although it would be awkward - it **may** be possible to schedule lower cycle planes to have those holes in that area reamed, coldworked, and proper interference fit fasteners installed before say 15,000 to 20,000 cycles.

So much for faster - cheaper- - by god we didn't do that on the Boeing- westervelt B-1, and its just an extra time consuming thing to do on assembly and meet or beat the BAR ( chart ). Just a SWAG- but I'll bet that process IF done- specified up thru about 1998-99 may have been dropped. :\


As I said before the ng pickleforks were built prior to 2018. By a company. The company I work for took over the pickleforks process and only built 200 of the ng style pickleforks. The forks we built are not problematic because we never had oversized holes. We then changed over to the pickleforks for the max 737. There are no issues fleetwide because when we took over we have had numerous inspections and have never had an oversized hole. You are correct that somewhere someone was rushing and not doing proper inspections of the parts, however this was in the beginning by a smaller company that didn’t have the engineering and quality we currently have on the max pickleforks. We’re fixing their mess ups.

[Aihkio]

Plate rivet joints are a different design case. And expanding rivets were/are used for the reason mentioned.

Steel/steel joints are mostly designed both for friction and bolt shear. Friction caries the normal loads but if something goes wrong in the chain of events the bolts still have the capacity.

​​​​In the pickle fork case at least the fork is Al, don't know about the center wing box. I have never designed a steel/Al joint but might be that the three fold difference in elastic moduli could cause some trouble. Al's low bearing strength would be a problem with that large steel bolts. I still think friction is included.

[ST Dog]

ALL of which to say is the major- most common installation of fasteners in aerospace is to produce close or interference fit to ensure fasteners are uniformly loaded in SHEAR.


Agree and matches my experience.

The actual cause of the pickle fork cracks is because of the the holes being over drilled by around 6 thousands.

6 thou? How could such a gross oversize get through? I'd expect an interference fit and 6 thou would have been obvious. Even close fit is on 5-15 ten thou and 6 thou should have been clearly too loose when installing.

[Vendee]

Talking of oversize holes, I used to work for a helicopter manufacturer who designed and embodied a flotation bag modification for the British Army. They used the forward undercarriage mountings as the main attachment points for the flotation bags. Unfortunately they carried out the modification at a time when the aircraft was stripped down and jacked up for heavy maintenance. When the aircraft was rebuilt, lowered off jacks and fuelled up, The 8mm holes didn't even come close to lining up. The solution was to drill out one set of holes to 12mm and the other set of holes to 13mm then use the original 8mm bolts with very big washers to clamp the thing up. Cringeworthy

https://i.postimg.cc/yYf3DwQp/holes1.jpg (https://postimg.cc/4m9xMFTp)

[Grebe]

6 thou? How could such a gross oversize get through? I'd expect an interference fit and 6 thou would have been obvious. Even close fit is on 5-15 ten thou and 6 thou should have been clearly too loose when installing.


Yes it would be obvious if the hole in ONE piece was perfectly in line with the hole in the other piece. But my **GUESS **is that the hole in the pickle fork was drilled in a jig, and the hole in the Fail safe strap was drilled/enlarged on final assembly, and most probably never reamed as an assembly. Hand drilling a hole can easily result in an out of round hole or slightly oversize. An we do not know the exact assembly process. Or IF interference holes/fasteners were specified. trying to hammer in an supposed light interference hole during assembly creates other problems. For example one could coldwork a hole, and not ream i giving an elongated hole which partially defeats the whole purpose. There is of course the incorrect ' logic ' that reaming a coldworked hole destroys the advantage of prestress around the hole.


Properly done, with correct size mandrel, the stressed area of significance typically exists for one radius external to the origional hole eg start with a 1/4 nominal hole, radius = 1/8 inch. = giving a radial stressed area equal to about 1/2 inch diameter. And usually due to how the aluminum part was processed/rolled, the resulting hole will be slightly elongated ( ellipitical ) by as much as a few thousandths, again depending on material and hole size. Thus to mmaintain a proper interference fit, a follow up ream is required. The preceeding is an overly simplIfied explantion of course.

Sort of a side note- if you want to raise the hair on your friendly stress person, talk about vertical scratches in a structural hole... which then leads to trying to make a sleeve with a spiral opening. Which made early attempts to make a practical sleeve a real problem. After much soul searching, a simple vertical split in the coldworking sleeve was tried, worked great. But still the issue was a major discussion of what about- what if.... etc. So a significant test program was initiated in 60's-70's , with groups of holes and individual holes coldworked with various sleeve split orientations to see if eventual failure was along the vertical split opening or somehow related. Bottom line- end result- no significant relationship re sleeve orientation and eventual overload- fatigue failure.

[B727223Fan]

Agree and matches my experience.



6 thou? How could such a gross oversize get through? I'd expect an interference fit and 6 thou would have been obvious. Even close fit is on 5-15 ten thou and 6 thou should have been clearly too loose when installing.

You are right on.
Most Hi-Lok installations are interference fit and we use a Rivet Gun [Variable Trigger] with a Flush Set with Brass Face to drive the Hi-Lok into the structure.
.006 O/S Hole the Hi-Lok would fall into the hole or be easily pushed into the hole with the fingers.

[Uwe_]

newly enlisted, Hello all around.

Boeing seems to have handed out information that
Classic, NG and MAX do not show the same design.
(i.e. pickle fork issue is deemed to be a distinct NG problem)

What differentiates the NG and MAX pickle fork designs?
Or lies the diference just in having changed suppliers?

[ST Dog]

Yes it would be obvious if the hole in ONE piece was perfectly in line with the hole in the other piece. But my **GUESS **is that the hole in the pickle fork was drilled in a jig, and the hole in the Fail safe strap was drilled/enlarged on final assembly, and most probably never reamed as an assembly.

If the full stack wasn't drilled, cold worked and then final reamed that's a process problem.
I've been involved with similar stack ups and they were always clamped up then final work was done on the assembly.

[B727223Fan]

The Boeing 737 - 800 has cracking issues all over the aircraft including the Aft Pressure Bulkhead - Main Frames in main cabin where the overhead bins attach to the frame. Numerous cracks on the -800 main cabin Frames at the overhead bin lower mounts were discovered during the first Major Check.
[After 5 years in service]
The NG and later aircraft structures - in my opinion - do not even compare to the early generation 707-727-737-747. Lots of the major structure parts on NG's were made with lighter gage materials including frames- stingers- skins- bulkheads in an effort to save weight.

The skin and stringers above the main cabin floor line required bonded doublers called "acoustic dampeners" to reduce the vibration and buzzing of the airframe on the -800.
The ones on the stringer hats look like railroad tracks. The ones on the skin has a corrugated look.

I expect for many more cracking issues to arise as the NG Fleets age.

[Webby737]

I agree, in the quest to save weight there are a lot of thinner gauge parts. The cargo bays also suffer from the cracking of minor parts.
The Aft Pressure Bulkhead inspection is a hangover from the Classics, these were known to crack in the "Y" Chord area. I'm not aware of any cracks being found on the NGs.

[Grebe]

I agree, in the quest to save weight there are a lot of thinner gauge parts. The cargo bays also suffer from the cracking of minor parts.
The Aft Pressure Bulkhead inspection is a hangover from the Classics, these were known to crack in the "Y" Chord area. I'm not aware of any cracks being found on the NGs.

Of note,- the first 737 NG flew in 1996- just before theMdc buyout and the intallation of the MDC management in various levels on mahogony row.

Then started the major change in 737 production and the emphasis on ' shareholder value " - the faster- cheaper game. All sorts of inspection and procedures were ' trimmed ' and now the rest is history. Name a major program in commercial that since that date has not had late deliveries, over budget, criminal activity resulting in club fed time, shareholder lawsuits for ' lack of candor ' screwing over of unions, moving corporate HQ, etc.

MDC has been more than a " cancer" its been a plague. And apparently the worst is yet to come . .

[clearedtocross]

9 out of 42 at Korean Air and others airlines in South Korea 737NG's grounded!
https://simpleflying.com/south-korea-grounds-9-boeing-737s/

[jmmoric]

MDC has been more than a " cancer" its been a plague. And apparently the worst is yet to come . .

That is not only an aviation thing.... it's called "capitalism" or "liberalism" at it's best. The thing we all swear is the saviour of this world, is beginning to show the signs of "old age"....

Maybe it's time someone sit down, look at how things work in the world, and see if it's possible to come up with a new philosophy that can actually handle the "problems" all the old ones generate...

[etudiant]

That is not only an aviation thing.... it's called "capitalism" or "liberalism" at it's best. The thing we all swear is the saviour of this world, is beginning to show the signs of "old age"....

Maybe it's time someone sit down, look at how things work in the world, and see if it's possible to come up with a new philosophy that can actually handle the "problems" all the old ones generate...

Don't think capitalism is the issue, the problem is when short term financial metrics dominate all other considerations.
It is almost always a lot easier and more lucrative for management to goose the share price with a big stock repurchase program that to risk a new investment that may not pay off.
The pernicious longer term effect include a surrender of market position, but the culprits will likely have retired by then. Nearer term, the short term mind set drives efforts to minimize all costs, including worker compensation, development efforts and compliance standards.
Seen that firms that did take investment risks, such as IBM with the S360 or Boeing with the 747, were hugely rewarded, one must assume that there has been a loss of courage in the industrial management mindset, perhaps driven by pressures generated by 'shareholder activists'.
Perhaps punitive short term capital gains taxes might help return the focus to longer term investment, or incentives for longer term returns such as extended vesting schedules, but no consensus has yet developed on this.

[Takwis]

Don't think capitalism is the issue...

It is almost always a lot easier and more lucrative for management to goose the share price with a big stock repurchase program that to risk a new investment that may not pay off.

... the short term mind set drives efforts to minimize all costs, including worker compensation, development efforts and compliance standards.

That's the stage of capitalism we are in, now.

[Grebe]

Any more news on picklefork issue from workers or BA AOG crew ?

[nerf]

Anybody got the total number at this point? With the 9 Korean NGs and the Ryanair, we gotta be pushing 50

[Big Pistons Forever]

So how much money did the new style pickle fork save Boeing ? How much are they going to be on the hook for the repair ?

[FlexibleResponse]

Wiki says that 7,031 B737 NGs have been delivered.

Only those NGs that have exceeded 30,000 cycles need to be checked within 7 days, (or 22,600 cycles within + 1000 cycles).

FAA estimate that only 1911 (US aircraft) are affected of the 7,031 that have been delivered so far (presumably because most haven't yet reached 30,000 cycles).

This will be an ongoing problem for many years to come as each and every one of the 7,031 delivered air frames reaches the AD cycle inspection requirements.

Say 5% of inspected aircraft fail the AD at 30,000+ cycles, what percentage of the remainder will subsequently fail the AD Inspection at say 40,000, 50,000, 60,000 cycles etc?

[The AvgasDinosaur]

Just curious how many airframes have ‘failed’ so far ?
Thanks
David

[DaveReidUK]

FAA estimate that only 1911 (US aircraft) are affected of the 7,031 that have been delivered so far (presumably because most haven't yet reached 30,000 cycles).

The FAA's determination is simply the number of NGs on the N register (i.e. around 27% of the world fleet), not based on individual aircraft cycles.

[bud leon]

Qantas will inspect 33 of its Boeing 737 aircraft by the end of this week after it discovered structural cracks on one of the short-haul jets that are the backbone of its domestic operations.

https://www.smh.com.au/business/companies/qantas-launches-inspection-blitz-after-cracks-found-on-boeing-737s-20191030-p535xo.html

[zukini]

Guardian says of 810 jets inspected so far 5% have failed

[rattman]

Qantas will inspect 33 of its Boeing 737 aircraft by the end of this week after it discovered structural cracks on one of the short-haul jets that are the backbone of its domestic operations.

https://www.smh.com.au/business/companies/qantas-launches-inspection-blitz-after-cracks-found-on-boeing-737s-20191030-p535xo.html


2 have been grounded with more inspections still to come

[kiwi grey]

2 have been grounded with more inspections still to come

"Boeing had previous thought the cracks were only occurring on aircraft with over 35,000 landings, the issue has now been identified on two Qantas aircraft with as few as 27,000 landings and Qantas are yet to inspect the majority of its 737 fleet." (emphasis added)
Steve Purvinas, Federal Secretary of the Australian Licensed Aircraft Engineers Association

[UltraFan]

"Boeing had previous thought the cracks were only occurring on aircraft with over 35,000 landings, the issue has now been identified on two Qantas aircraft with as few as 27,000 landings and Qantas are yet to inspect the majority of its 737 fleet." (emphasis added)
Steve Purvinas, Federal Secretary of the Australian Licensed Aircraft Engineers Association

Oh, it gets better. MUCH better.
https://www.flightglobal.com/news/articles/qantas-dismisses-alarmist-union-call-to-ground-737-461920/

Quntas's response (quote): The Oneworld carrier says it "rejects the alarmist claims" for grounding made by the engineers' association which, it argues, are "irresponsible and completely inconsistent with advice from regulators and the manufacturer".

So Qantas trusts the "regulators" and "manufacturer" who both have JUST been caught cooking the books and making Powerpoint presentations to rectify the death of 346 people, but dismisses their own country's engineers who actually know what they are saying, as "irresponsible".

The tone of this statement itself is simply outrageous. Even if Mr. Whoever doesn't like unions, he should've chosen his words A LOT more carefully. I'm an adventurous person and I like risky bets. But not for any money in the world would I post a response like that just a few months after two deadly disasters that were caused by managers not listening to engineers. If something happens to any of Qantas's 737s (and I sure hope it doesn't), this is not just a PR disaster, this is a guaranteed social death. The events of the past few months got me thinking that maybe Qantas is not led by as smart people as they thought they were.

[physicus]

Do we know which airframes QF has grounded?

[DaveReidUK]

Only!? That's 27% of the fleet. If 27% of the doctors in a hospital failed their medical school exams, would you still be going there for heart bypass surgery?

Read my previous post. It's not an estimate of affected airframes, just a count of the number of the world's NGs that fall under the FAA's jurisdiction.

[rattman]

So a 3rd qantas plane has been a grounded for cracking, but all the inspections have been carried out according to qantas statement. So for them 3 of 33 airframes have been grounded

[Fly Aiprt]

So a 3rd qantas plane has been a grounded for cracking, but all the inspections have been carried out according to qantas statement. So for them 3 of 33 airframes have been grounded

That's 9%...

[MickG0105]

Do we know which airframes QF has grounded?

This is most assuredly not definite but based on aircraft movements it looks like it's VH-VXA (their oldest B737-800), -VXF and -VXM.

[kiwi grey]

So a 3rd qantas plane has been a grounded for cracking, but all the inspections have been carried out according to qantas statement. So for them 3 of 33 airframes have been grounded

That's 9%...

9% pickle fork cracking in the fleet of an airline with a reputation for good maintenance and well-trained pilots, and a fleet that by and large flies in very benign weather conditions. This should be one of the 'best' fleets from the wear and tear point of view, but 9% have cracks in their pickle forks. :eek:

This is really quite alarming

[rattman]

9% pickle fork cracking in the fleet of an airline with a reputation for good maintenance and well-trained pilots, and a fleet that by and large flies in very benign weather conditions. This should be one of the 'best' fleets from the wear and tear point of view, but 9% have cracks in their pickle forks. :eek:

This is really quite alarming

Virgin australia has also inspected their planes, dunno how many, but they found no issues

[Water pilot]

That is quite a few planes, what percent have they missed? Since the check is allegedly quite easy to do, perhaps it should be added to regular maintenance.

[rattman]

That is quite a few planes, what percent have they missed? Since the check is allegedly quite easy to do, perhaps it should be added to regular maintenance.

What do you mean what % have they missed. They have checked all the planes they were required to. They have a fleet of 75 737's of which they were required to check 33 of them. They actually checked them all way before the a due date. The others have not hit the a inspection cycles.

[Dave Therhino]

... Since the check is allegedly quite easy to do, perhaps it should be added to regular maintenance.

The AD requires a repetitive inspection at 3500 cycle intervals.

[wdew]

Al Joyce must be apprehensive as this may result in a chunk out of his share pocket if Qantas has to bring forward his postponed fleet upgrade /renewal...

[MickG0105]

There is also the possibility/probability that the addition of the Split Scimitar winglets has had an adverse effect on the wing bending loads carried by the pickle forks.

Apparently the spanwise lift distribution has shifted the centre of lift further outboard on the wings which would necessarily give rise to greater wing bending loads at the wing attachment points from both static flight and gust loads.

Possibly those B737s that were designed from the outset to have Split Scimitar winglets might have strengthened pickleforks to account for the increase in bending loads.

But those B737s (eg, 800, NG) that were fitted with the after-market mod Scimitar installation (without beefing up the wing attach load path design) would necessarily suffer a reduced wing attachment fatigue life due to the higher loads imposed by the addition of the Split Scimitar winglets.

Perhaps more engineering learned members than I might like to comment?

https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/741x731/atwinglets_diagram_5120ddbd0171259ee5aa19d76174c19bcdb96c53. jpg

I've been trying to get a line on the LSNs of the grounded aircraft and noticed something with regards to the Korean Air aircraft that have stopped flying recently - none of them were fitted with winglets. On the flip-side it looks like all Korean Air B737s that are fitted with winglets are still flying.

I know that it's never say never when it comes to this sort of thing but I'm thinking that the pickle-fork issue is probably not related to the winglets.

[DaveReidUK]

I know that it's never say never when it comes to this sort of thing but I'm thinking that the pickle-fork issue is probably not related to the winglets.

I'd be wary about reaching any conclusion based on such a small sample size. It's still possible that the incidence of cracking is higher on winglet-equipped aircraft.

[Aihkio]

As the pickle fork does not carry any primary wing bending loads thast is an unlikely alternative. Spanwise change slightly towards the wing tips would put the rear fork into more compression which is not compatible with the seen cracks.

[MickG0105]

I'd be wary about reaching any conclusion based on such a small sample size. It's still possible that the incidence of cracking is higher on winglet-equipped aircraft.
I don't disagree but what the Korean Air sample shows is that the problem is pervasive, it's impacting a range of B737s regardless of whether they had a traditional wing or whether winglets were fitted.

[ManaAdaSystem]

SAS found 2 737-700’s with cracks, one with winglets and one without. They have been flying 737’s for 20 years in some of the worst conditions in the world and probably have some of the oldest frames in the industry flying. A good mix of -700 and -800 aircraft with and without winglets. None with scimitars.
Yet, only two of 50 - 60 (?) had cracked pickle forks.

I struggle to see any pattern with this issue. I think they need to look at where the aircraft were made, time period, pickle fork batches and production methodes.

[Bend alot]

SAS found 2 737-700’s with cracks, one with winglets and one without. They have been flying 737’s for 20 years in some of the worst conditions in the world and probably have some of the oldest frames in the industry flying. A good mix of -700 and -800 aircraft with and without winglets. None with scimitars.
Yet, only two of 50 - 60 (?) had cracked pickle forks.

I struggle to see any pattern with this issue. I think they need to look at where the aircraft were made, time period, pickle fork batches and production methodes.
Not finding a pattern makes for a very big problem - design!! and strict compliance with!!.

A thing I would not have considered a year ago - but now my first thought.

No problem, Denis owns it.

Any news on availability of all the required pickle forks?

[cappt]

When I was in the tech side we had a certain NDI inspector who could ground any aircraft for a crack if he was in the mood. Our job was to prep the areas that needed inspection by removing whatever was in the way, ( landing gear, hyd lines, etc.) then he’d set up and spend the rest of the day with his eddy current or penetrating dye, man we would groan when he rolled up because the jet was likely going to fail. Any jet has cracksif you look long enough, some obvious and some require a microscope, just an observation.

[UAV689]

A few have been found cracked at Europe's favourite low cost airline.

One has gone to the states for a repair, granted a 1 time ferry flight, another is under preservation status in STN and can been seen with all the engines/tyres etc all wrapped up outside the hanger until they work out what to do with it. I have been told it will take up to 6 weeks to repair, and they do not have the spare hangar bays available to repair it as yet.

[sprite1]

I think a couple of aircraft flew to Victorville, US, in the past few days due to maintenance capacity constraints around Europe/Seattle.

[rog747]

Anyone know if TUI has found any cracks?

[OliTom]

No. and adding words for a 10 character reply.

[Yeehaw22]

Anyone know if TUI has found any cracks?

I doubt it as their oldest is only 12 years old. Even doing 4 sectors a day every single day for 12 years still leaves them well under the inspection threshold.

[Pilot DAR]

No. and adding words for a 10 character reply.

The extra characters may have been better devoted to elaborating on whether no one knows, or someone knows, and no cracks were found...

[YRP]

How does one repair these cracks?

Anyone heard where the cracks are being found? At the fastener fittings seems likely?

[ST Dog]

How does one repair these cracks?

Anyone heard where the cracks are being found? At the fastener fittings seems likely?

The repair seems to be a replacement of the fork and the strap behind it.

As for the location, the Boeing instructions, reference by the FAA AC includes photos of the cracks.
Go back to the earlier posts for links to both documents.

The cracks are through fasteners to the forward edge of the part.

[Fly Aiprt]

How does one repair these cracks?

Anyone heard where the cracks are being found? At the fastener fittings seems likely?


Go up the thread, there are plenty of technical documents, pictures, etc.
The cracked pickle forks have to be replaced.

[MickG0105]

SAS found 2 737-700’s with cracks, one with winglets and one without.

Would you happen to know what the regos are for those two SAS aircraft?

[Pearly White]

Yes, it was only a matter of time.
Boeing is going to find this a tough PR exercise.
Got to wonder whether Boeing's engineering of the 737 is all it's cracked up to be.

[Australopithecus]

Someone’s always gotta make a wisecrack.

D’oh!

On a serious note: spoke to an engineer today who opined about five weeks work to repair, but there are no parts yet apparently, Plus the aircraft has to be put in jigs which are uncommon and not very portable and of course one set of jigs might do one repair per month. My employer apparently has a set, but several aircraft to repair.

These cracks appearing so much earlier than the AD contemplates would indicate that eventually almost every NG will need at least one new set of forgings. Perhaps replacement will become a mandatory part of heavy overhaul.

I read that these forgings may have 0.008” oversized bolt holes. I wonder if tighter tolerances on the replacement parts will be a permanent fix?

[FlexibleResponse]

Australopithecus

I read that these forgings may have 0.008” oversized bolt holes. I wonder if tighter tolerances on the replacement parts will be a permanent fix?

And also, would reworking of the so far, uncracked pickleforks in situ, be feasible, effective in preventing future cracking and still be economical?

Otherwise, one would assume that the pickleforks on all B737 NG aircraft will eventually crack when reaching say, 35,000 to 45,000 cycles, unless some other close-out action is taken.

[Australopithecus]

Australopithecus



And also, would reworking of the so far, uncracked pickleforks in situ, be feasible, effective in preventing future cracking and still be economical?

Otherwise, one would assume that the pickleforks on all B737 NG aircraft will eventually crack when reaching say, 35,000 to 45,000 cycles, unless some other close-out action is taken.

maybe not in situ, as the stress relieving process involves drawing a mandrel through an interference-fit sleeve, thus work hardening the perimeter of the bolt hole. I don’t know the ramifications of revisiting a component that has already suffered the tribulations of line service let alone the utility of not just replacing the component, since removal would seem to be required for any retrospective fatigue relief anyway.

[Bueno Hombre]

Boeing 737 - Wikipedia (https://en.wikipedia.org/wiki/Boeing_737)https://en.wikipedia.org/wiki/Boeing_737The 737 was originally envisioned in 1964. The initial 737-100 made its first flight in April 1967, and entered airline service in February 1968 with Lufthans


Surely computers can now design better airframe to accommodate modern engines .

[cats_five]

Boeing 737 - Wikipedia (https://en.wikipedia.org/wiki/Boeing_737)https://en.wikipedia.org (https://)/wiki/Boeing_737The 737 was originally envisioned in 1964. The initial 737-100 made its first flight in April 1967, and entered airline service in February 1968 with Lufthans


Surely computers can now design better airframe to accommodate modern engines .

Better airframes can and have been designed, but they are not type-compatible with older 737s.

[Bueno Hombre]

737 airframe is ancient airframe .and should be scrapped because computers these days can design airframe, available engine, compatability.

[Sunfish]

Not sure there is a repair scheme other than complete replacement.

[Maninthebar]

737 airframe is ancient airframe .and should be scrapped because computers these days can design airframe, available engine, compatability.

The number currently flying says otherwise.

Boeing may NOW consider that they should have put their considerable money into the development of a fresh frame perhaps 5 years ago but the 737 family is so popular that the incentive was not there

[Aihkio]

At their present level computers still cannot design anything even close to the complexity level of an airliner. On the other hand without computer analysis the now used structures could not be designed.

[UltraFan]

Does Aeroflot have any affected 737s? I just don't know what to look for on all those "fleets" websites. Could anyone help, please?

[Momoe]

Bueno Hombre,

The airframe isn't ancient, the design is which is the problem. Boeing stuck with the 737 due to it's popularity and to avoid a full re-certification associated with a new design.

As the well worn Irish saying goes, 'If you're going there, I wouldn't start from here',

[Chu Chu]

At their present level computers still cannot design anything even close to the complexity level of an airliner.

Much less eliminate the possibility of unanticipated fatigue cracks, which may relate the the interaction of design and fabrication techniques.

Of course, larger engines and increased weights on an old-design airframe can expose new issues. But if anything, that seems less likely to happen than with a wholly new design (assuming the appropriate analysis is done when the upgrades are made, of course). There are lots of possible advantages to starting with a clean slate, but this doesn't seem like one of them.

[Pilot DAR]

Though I am not expert in B 737 structure, I know of other types which have had vulnerable fastener holes reworked in place to prevent anticipated cracking based upon experience with that type. Twin Otter wing strut attach structure in the wing is an example. In some cases, a crack or other defect can be allowed to continue in service with inspection. I have approved two aircraft this way. Analysis of the defect showed that the load could be carried in the structure anyway, and inspection for further crack growth was easy. I'm working on such an approval right now on a smaller GA type. In the worst case, allowing the defect to remain in service unrepaired could involve a small gross weight, or maximum landing weight penalty. Part replacement is a great idea, and the best solution, if it is practical, and the replacement parts are available. If not, more lateral thinking may be appropriate.

I was a part of an inspection team for a prepurchase inspection of an Air New Zealand DC-8 30 years ago, which had a crack in a landing gear trunnion. The crack was monitored using a Douglas approved inspection program, and the plane remained in service. Air New Zealand had the replacement part in stock, but it would be a huge task to install it.

I have no idea what Boeing will propose for this defect, and in the present climate, Boeing will have to be very robust in their proposal. But I'm not imagining a wholesale "replace every pickle fork out there" program.

[Dave Therhino]

The type of activity you described led 35 years ago to formally issued FAA policy that prohibits approval of revenue "flight with cracks" for primary structure or fail safe features, although some inspectors or offices may not consistently follow the formal policy. The AD program for the pickle fork fitting undoubtedly will continue to involve monitoring for cracks beginning at some threshold, with repetitive inspections, with the intent of providing multiple opportunities to detect the cracks before they can go critical. However, once any cracking is found an appropriate repair or replacement will be required. In this case nobody has yet identified a practical and effective repair scheme, so replacement for now is the corrective action once a crack is detected. Since the NG is out of production and the cracks seem to appear later in the life of the airplane, Boeing may just stick with the existing design and live with the potential for one replacement required in the life of an airplane. I have not heard anything about development of a redesigned fitting, but it's possible one is in work.

I'm hearing rumors that cracks have been found in adjacent locations on a few airplanes ...

[ozaub]

Courtesy of Grandfather, 737 is only significant airliner still in service that has not been subject to a proper full scale fatigue test. Fail-safe fatigue integrity was originally certificated on basis of similarity with 707 and 727. There was also a fatigue test of rear fuselage to prove integrity of rear pressure bulkhead, which was misapplied to prove damage tolerance of fuselage lap splices. All of which ended in tears (pun!!). See LESSONS FROM ALOHA (http://avstop.com/stories/aloha.html) or http://www.iasa.com.au/folders/Safety_Issues/others/lessonsfromaloha.html

Aloha story mentions Boeing’s untimely letter immediately before accident - saying nothing could go wrong! I’ll share it with anyone interested; just PM me. No, I didn’t steal from Australian Government files. Got mine from Boeing’s file copy tabled at NTSB inquiry. Letter is dated two weeks before accident, so in days of snail mail its arrival was even closer.

Apparently 737 NG had a better fatigue test of fuselage. See https://www.newsweek.com/boeings-737-airplane-prone-problems-63629 including:“The Aviation Safety Institute's Pat Duggins told me that 38 changes were made to the fuselage before the NG went into production. And then, to make sure that no critical weaknesses remained, Boeing took a standard 737NG fuselage off the 737 assembly line at Wichita, Kans., and tested it to the breaking point. The airframe was pushed through the equivalent of 225,000 cycles (three times the assumed safe life of 75,000 cycles for the NG series) on short duration flights—exactly the way Southwest, for example, uses the 737.The problem, Duggins says, was that the test fuselage did not represent the realities of everyday flight. It lacked a wing box, the core load-bearing part of the wings where they meet the fuselage, and also the landing gear, which transmits particularly forceful stresses to the fuselage on every landing. In addition, he does not believe that the design changes would have given the NG series fuselage a significantly longer life. He raised doubts, when I talked to him, that the tests met Boeing's design requirements. (Boeing disagrees. It asserts that the testing "provides a realistic simulation of complete flights," and adds that "all these loads were represented"; Boeing did not specifically answer my questions about whether the wing box and landing gear were part of the airframe that was tested.)”





I doubt efficacy of this test to replicate fatigue stresses in pickle forks. Furthermore Boeing probably thought fatigue of forks unlikely because they are primarily in compression.

Main job of the forks is to transfer weight of fuselage to wing, putting forks in compression. Secondary tension stresses come from fuselage pressurisation ie fuselage lifting away from wing. So, tension in forks arises from pressurisation together with negative G, and is probably not well replicated in fatigue test spectrum.

With forks we may have dodged a bullet, much as we did with Section 41 cracking on 747. See https://www.flightglobal.com/FlightPDFArchive/1991/1991%20-%200317.PDF Just by luck widespread cracks were first found in wreckage of JAL 123 in 1985, though nothing to do with crash. Likewise we almost dodged a bullet with 737 lap joints; except for poor Clarabelle Lansing

All of this I had in mind when penning https://www.smh.com.au/national/self-regulation-failed-with-banks-but-with-aircraft-in-can-kill-20191103-p536wk.html Plus I was irked by CASA and Qantas disingenuous platitudes downplaying significance of pickle fork cracking.

[pilot9248]

Thanks ozaub for the informative post. At first, I thought it excessive to demand inspections be carried out on all 737NGs regardless of flight cycles. However, with the latest developments of cracking apparently having been found in other locations than the inspection areas of the AD-mandates MOM, in conjunction with findings on aircraft below 22k flight cycles, I am becoming increasingly doubtful of Boeing’s approach. The multitude of existing issues in the area of Body Station 663 on all 737 minor models up to and including the NG does not exactly dispel my doubts. It is a very busy area in terms of loads going through the airframe, so one would hope that Boeing are being sufficiently conservative in their analysis. I believe it is safe to say that if an NG should ever suffer an accident due to failure of the joints in question, that would spell the end for Boeing (and the FAA as we know it). Hopefully that will never happen, but as engineers we shouldn’t have to summon hope.

[Just a spotter]

According to The Irish Times, 5th Nov 2019, Ryanair has confirmed 3 aircraft are affected by the pickle fork cracks;

The grounded planes have the registration numbers EI-DCL, EI-DAL and EI-DCJ. All three are more than 15 years old.

https://www.irishtimes.com/business/transport-and-tourism/ryanair-grounds-three-planes-due-to-cracking-between-wing-and-fuselage-1.4074196

JAS

[ManaAdaSystem]

Would you happen to know what the regos are for those two SAS aircraft?

Sorry, took a while to get hold of my SAS man.
LN-RPK. 737-700 with winglets. Has been with SAS since new. 19 years old.
SE-RET. 737-700 without winglets. Joined SAS in 2012. Previously with Virgin Blue/Virgin Australia reg VH-VBM. 17 years old.

[ozaub]

Thanks Turbo. I’m not advocating inspection before further flight; just inspect all 737s, oldest first.
It’s ludicrous for CASA and Qantas to insist there’s no risk below an arbitrary threshold. We simply don’t know; because 737 is only significant airliner that's never been properly fatigue tested. Except with passengers on board.
According to AD it only costs $170 to check for cracks. Just do it, instead of prattling about safety being paramount.

[MickG0105]

MOM gave airlines four weeks advance notice of the AD. Ample time to inspect most aircraft and have a "we've done it already" PR coup when AD came out.
That's not correct. The MOM was issued on 30 Sep 2019 1731 US PACIFIC TIME / 01 Oct 2019 0031 GMT. The AD was issued October 3, 2019. That's at best four days, not four weeks.

[MickG0105]

Sorry, took a while to get hold of my SAS man.
LN-RPK. 737-700 with winglets. Has been with SAS since new. 19 years old.
SE-RET. 737-700 without winglets. Joined SAS in 2012. Previously with Virgin Blue/Virgin Australia reg VH-VBM. 17 years old.
Thank you for the follow up.

So it looks we're seeing the cracking problem manifest itself in some aircraft built before 2004 but right across the whole NG range (-600, -700, -800 and -900), and both with winglets and without.

This may simply be a coincidence but Boeing moved the Commercial Airplanes - Fabrication Division's complex machining from their Auburn, Washington plant to their Portland, Oregon facility between April 2003 and January 2005. Among the parts packages that were transitioned from Auburn to Portland were pickle forks. The move gets mentioned in a Boeing Frontiers article. It's probably unrelated but you never know.

[megan]

It is a very busy area in terms of loads going through the airframe, so one would hope that Boeing are being sufficiently conservative in their analysis90,000 cycles is being quoted as the expected design life of the forks, fell some ways short it seems if that were the case.

[pilot9248]

Mick, I believe ozaub is referring to the previous MOM that informed operators of the issue, not the MOM that contains the inspection instructions.
I also do not advocate inspecting all aircraft before further flight. Generally, if it involves fatigue cracking, inspecting “too early” may be counterproductive.
Having said that, fatigue analysis is notoriously error-prone. I have seen exercises conducted throughout the industry to benchmark predictive analysis with subsequent fatigue testing to validate the results, and some of the results are so far off (to the wrong side) it’s truly shocking. That’s one of the reasons why testing is required for certification, even though the OEMs have been trying to get away with less testing and more analysis and modelling. Stating the obvious, testing is expensive.

[DaveReidUK]

That's not correct. The MOM was issued on 30 Sep 2019 1731 US PACIFIC TIME / 01 Oct 2019 0031 GMT. The AD was issued October 3, 2019. That's at best four days, not four weeks.

The issue date of the AD was September 30. October 3 was the "effective from" date.

[Australopithecus]

This issue makes me wonder about inspection scheduling to airline convenience. Since the FAA allows no revenue flights with cracks in primary structures, how long before an avaricious lawyer sues an airline for operating aircraft that were not inspected just prior to their last revenue flight after which cracks were then discovered? I am told by engineers that my airline is avoiding inspections at airports it would be difficult and costly to do a repair for example.

That an airline has the option to inspect, knowing that some of their fleet have cracks, but does not for convenience smacks of reckless endangerment. (I am not going to qualify or attempt to quantify any risk. But I do note that cracks result in an immediate grounding, which is all a jury needs to hear.)

[MickG0105]

Mick, I believe ozaub is referring to the previous MOM that informed operators of the issue, not the MOM that contains the inspection instructions.
Unless I'm mistaken the previous MOM first notifying operators of the issue - MOM-MOM-19-0530-01B(R1) - was released on 27 September 2019.

The issue date of the AD was September 30. October 3 was the "effective from" date.
Yes, quite correct. My mistake.

So, the upshot of all that is that there was about three days between the first Boeing MOM and the AD, meaning that
MOM gave airlines four weeks advance notice of the AD. Ample time to inspect most aircraft and have a "we've done it already" PR coup when AD came out.​
is not correct.

[artee]

Apparently Lion Air found cracks on two 737s with less than 22,000 flights.
https://www.smh.com.au/business/companies/pickle-cracks-found-on-newer-737s-spark-fresh-calls-for-qantas-virgin-inspections-20191106-p53828.html

[ozaub]

Mick, Turbo et al, I stand corrected re dates

[MickG0105]

A comment from joe bloggs on BB's article in The Australian today -

​​​​​​... Boeing issued a second Multi Operator Message on the 5th Nov that has extended the inspection area on the rear spar pickle fork & fail safe strap. So its now 8 fasteners to be visually inspected instead of the one common to the S-18 stringer at Body Station 663.75 left & right hand sides. For aeros that have reached 30,000 flight cycles the three airlines have 60 days to carry out this inspection and the lower threshold has been dropped to around 21,600FC to be inspected within 1,000 FC. The repeat inspection remains at 3,500 FC. Will see if any more cracks will be found apart from the 3 Qantas planes. Grounded to date. The FAA are yet to mandate the MOM as a new airworthiness directive or will they ammend the one issued last month - its watch this space...

Has anyone seen the 5 November MOM?

[RickNRoll]

Interesting situation for the planes that haven't been inspected yet. All planes that have the cracks are grounded until they are repaired. Most planes have not been inspected yet. The inspection threshold keeps dropping. The repair process is going to take a long time, not because it is difficult but for logistics reasons. Ten percent of planes inspected at higher cycles times are found to have the cracks (what percentage for lower cycle planes?). In theory there are quit a few planes still flying that should be grounded, although in practice they are safe. This at time when new 737 deliveries are frozen.

[Dave Therhino]

The planes that were previously inspected will have to be inspected again for cracks at the additional locations.

[ManaAdaSystem]

Thank you for the follow up.

So it looks we're seeing the cracking problem manifest itself in some aircraft built before 2004 but right across the whole NG range (-600, -700, -800 and -900), and both with winglets and without.

This may simply be a coincidence but Boeing moved the Commercial Airplanes - Fabrication Division's complex machining from their Auburn, Washington plant to their Portland, Oregon facility between April 2003 and January 2005. Among the parts packages that were transitioned from Auburn to Portland were pickle forks. The move gets mentioned in a Boeing Frontiers article. It's probably unrelated but you never know.

SAS are now in a process of changing from Boeing to Airbus, but a number of their 737’s are from the same time period as the two with cracks.
Yet, only two of them had cracks.
Not sure how many cycles they have, but they fly a large network of shorter domestic and Scandinavian routes, so I expect them to have a rather high number. 30000 and above?
Why one aircraft but not the one produced at the same time, operating in the same environment/area and with the same amount cycles ?

This is what puzzles me.

[MickG0105]

SAS are now in a process of changing from Boeing to Airbus, but a number of their 737’s are from the same time period as the two with cracks.
Yet, only two of them had cracks.
Not sure how many cycles they have, but they fly a large network of shorter domestic and Scandinavian routes, so I expect them to have a rather high number. 30000 and above?
Why one aircraft but not the one produced at the same time, operating in the same environment/area and with the same amount cycles ?

This is what puzzles me.
Yes, it's decidedly difficult to see a discernible pattern. LN-RPK and SE-RET were built just over two years apart and both have cracks. You've then got LN-RNO and -RCN built just months either side of LN-RPK; no cracks.

Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.

[EDML]

Yes, it's decidedly difficult to see a discernible pattern. LN-RPK and SE-RET were built just over two years apart and both have cracks. You've then got LN-RNO and -RCN built just months either side of LN-RPK; no cracks.

Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.

Obviously it is not (yet) predictable, why these cracks develop. Also the number of cycles is not consistent.

Makes it a lot harder to set preventive inspection requirements.

[MurphyWasRight]

Yes, it's decidedly difficult to see a discernible pattern. LN-RPK and SE-RET were built just over two years apart and both have cracks. You've then got LN-RNO and -RCN built just months either side of LN-RPK; no cracks.

Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.
A few possibilities:
Inconsistent production practices, built on Friday or with a bigger hammer for alignment.:)

Number and magnitude of hard landings or other outside the norm conditions, reported or not.

Tolerance stackup issue where some combinations of within specification parts result in higher stresses.
Design should account for this but obviously something was missed somewhere.

Normal distribution of failures where some items will simply fail earlier than others, in some fields testing until a certain percentage of a batch of parts fail is used to quantify expected lifetime using statistical methods.

I suspect that Boeing is hard at work trying to determine the common factor(s), of course none of the above are mutually exclusive.

[GlobalNav]

A few possibilities:
Inconsistent production practices, built on Friday or with a bigger hammer for alignment.:)

Number and magnitude of hard landings or other outside the norm conditions, reported or not.

Tolerance stackup issue where some combinations of within specification parts result in higher stresses.
Design should account for this but obviously something was missed somewhere.

Normal distribution of failures where some items will simply fail earlier than others, in some fields testing until a certain percentage of a batch of parts fail is used to quantify expected lifetime using statistical methods.

I suspect that Boeing is hard at work trying to determine the common factor(s), of course none of the above are mutually exclusive.

At least Boeing will be hard at work to make it look that way.

[Lyneham Lad]

South Korea grounds 13 737NGs with 'pickle fork' cracks (https://www.flightglobal.com/news/articles/south-korea-grounds-13-737ngs-with-pickle-fork-cra-462155/)On Flight Global:_
South Korea is the latest country to ground a number of Boeing 737NG found to have structural cracks, following worldwide inspections of the popular narrowbody.
The Ministry of Land, Infrastructure and Transport (MOLIT) inspected 100 737NGs and found that 13 contain cracks. Nine of these were identified in a first round of inspection in October and had clocked more than 30,000 flight cycles.

Four more were discovered with cracks, after a second round of inspections was completed on 10 November. These aircraft recorded between 20,000 to 30,000 flight cycles, says MOLIT.

Upon discovering the cracks, MOLIT notified Boeing, which then sent a team to South Korea on 31 October to commence repair work. The affected aircraft will all be repaired by January 2020, and each aircraft takes about two weeks, the ministry adds.

MOLIT did not specify which airlines operated the affected 737NG aircraft. Cirium fleets data indicates that South Korean carriers operate 153 737NGs, the bulk of them 737-800s.

At the heart of the inspections is a piece of hardware known as a "pickle fork", which connects the wing to the aircraft fuselage. Cracking of the hardware could result in structural failure, which affects the structure integrity of the aircraft and results in loss of control.

Regulators had urged airlines operating 737NGs with more than 30,000 flight cycles to inspect their aircraft immediately for cracks. Aircraft that have logged 22,600-30,000 cycles should be inspected within the next 1,000 cycles.

South Korea’s MOLIT has also put in place further measures in response to the latest spate of cracking incidents.

For instance, carriers taking delivery of new 737NGs must check for cracks before registering the aircraft in South Korea.

737NGs that have passed inspections this time round will also be “thoroughly managed” by MOLIT’s aviation safety inspector and will be re-inspected within the next 3,500 flight cycles.

South Korea joins several other countries in the Asia-Pacific region to ground 737NGs over pickle fork cracks.

Australia’s Qantas has grounded three 737-800s found to have cracks, while Indonesia has also grounded three, two of them operated by Sriwijaya Air and the third by Garuda Indonesia.

Media reports also state that Lion Air found structural cracks on two 737NGs with less than 22,000 flight cycles. The Indonesian low-cost carrier has yet to respond to FlightGlobal’s request for comment.

[DaveReidUK]

New FAA AD 2019-22-10 specifying inspection of all 8 fasteners instead of the original two.

[Dave Therhino]

Here's a link to the Boeing MOM in the FAA public docket. It shows pictures of the new inspection area.

https://www.regulations.gov/document?D=FAA-2019-0866-0002

[ozaub]

A couple of intriguing details to note about compliance times in Table 1 of new MOM (5 Nov).
1. If aircraft is BBJ with "Lower Cabin Altitude" STC ie higher pressure differential, then threshold for initial inspection is halved and repeat intervals are cut by 25%. That's drastic and suggests pressure loads were not well modelled in test or analysis.
2. If I'm reading MOM correctly, initial inspection interval after fork replacement is only 3500 flight cycles. Inspections don't start again from scratch.

[Bend alot]

A couple of intriguing details to note about compliance times in Table 1 of new MOM (5 Nov).
1. If aircraft is BBJ with "Lower Cabin Altitude" STC ie higher pressure differential, then threshold for initial inspection is halved and repeat intervals are cut by 25%. That's drastic and suggests pressure loads were not well modelled in test or analysis.
2. If I'm reading MOM correctly, initial inspection interval after fork replacement is only 3500 flight cycles. Inspections don't start again from scratch.
Yes I noticed that!

Seems the problem is not the pickle fork itself, but the by product of another "error/assumption/calculation" that causes the pickle to crack - and that is a much bigger fix than a fork change/s over the life of the airframe.

[400guy2]

... repeat intervals are cut by 25%.

Actually, it is worse than that: repeat intervals are cut *to* 25%. From page 20:


(a) For airplanes which have incorporated Boeing Business Jet (BBJ) Lower Cabin Altitude
...
All repeat interval compliance
times specified in flight cycles must be reduced to one‐quarter of those specified in Table 1.

[Grebe]

Yes I noticed that!

Seems the problem is not the pickle fork itself, but the by product of another "error/assumption/calculation" that causes the pickle to crack - and that is a much bigger fix than a fork change/s over the life of the airframe.


I'll still bet that the problem is one of ' casual ' final assembly issues at Renton. The basic picklefork attach is at Spirit in Wichita, and the final assembly at Renton is when wings are attached and the fail safe strap/picklefork is attached with the ' last' half dozen fasteners on each side. IMHO the ' fix' to be incorporated for the fleet will be to remove and replace the" last half dozen" with cold expansion ( aka coldworking ) re reaming, and installation of light interference fit fasteners.

And on those still on line at Renton, I'll bet such work will be carefully checked and or sequence modified starting on line # xyz123

I doubt there will be any significant changes in the picklefork itself :cool:

[FlexibleResponse]

Presumably, the aircraft that have been grounded with cracked pickleforks are having their pickleforks replaced.

Once a number of cracked pickleforks have been removed, metallurgical examination of the damage and cracking should identify the failure mode(s) and thus ultimately allow a long term fix.

Is anyone privy to any of the investigations or examinations of the actual damaged pickleforks to date and if the failure mode(s) have yet been identified?

[BlankBox]

https://simpleflying.com/qantas-737-crack-repair/

3000 hours??? ....can build a plane in that time /s :E

[GordonR_Cape]

Thanks for the link! The mythical man-hour: The length of the job may depend on the size of the team, and how many technicians can squeeze into a confined wheel bay, or remove skin rivets on the outside of the fuselage. Will be interesting to follow the learning curve.

[mayam13]

I have a doubt that the pickle forks are stressed more on landing than in flight..If so the following needs to be answered:
1. How many cycles/landings are allowed before it cracks
2.What is the tolerance zone
3.Did the cracks occur earlier than it should?
4.Why has not FAA asked for a design review?

[lomapaseo]

We've seen this kind of structural problem before in the pylon pins in B747 freighters
When you start designing load redundancies in multiple load paths you may miss a handoff of what path is now carrying the load and by how much vs flight operations. The assumed response is to cover this by high margins of safety over normal conditions.

Sometimes (rarely) some hidden condition pops up and creates abnormal stress conditions which now over many flights could result in early fatigue. Again even with fatigue cracking the concept still is failsafe as long as nothing else starts to fail.. The airworthiness action in this case addresses increased inspection early enough (inspections etal.) to maintain sufficient margins of flight safety.. Yes it is a pain-in-the a$$ for the operators but it doesn't always require an FAA design review or complete replacement of the design.

[Pugilistic Animus]

Aihkio

They're now using CATIA with great success...the 777 was designed almost entirely by computer

[SLF3]

How did this play out?

[DaveReidUK]

I think they got themselves out of the pickle in the end.

[rigpiggy]

As someone who cut their teeth on Boeing Structures with the B 727 -223 and also a few of years on B-757-200 it is very sad to see what has become of Boeing Airplanes. I also have about 6 years experience on B 737 - 800 Structures. To borrow an expression from a TV Commercial the B 737-800 " Is Not Your Daddy's Boeing". In my OPINION it is an example of Lean MFG Gone Wild. I can tell you that Boeing Airplanes do develop cracks and corrosion in Primary Structure Members over time. What is concerning about this story and my experience with the
B 737 -800 is how early into the service life these Structure members are cracking.The B 727's were much older when we started seeing these kinds of problems. A lot of the problems with the B 737 -800 APPEARS to be Design issues to save weight as compared with the B 727 that was built like a tank. I do not recall issues with the B 727 Pickle Forks but the Wing Rib Chords were cracking on the 727 much later in the life cycle of the Airframe.
Example B 737-800 - In an apparent effort to save weight the Skin and Stingers above the cabin floor line were made in a manner that Boeing had to go back and add Bonded Doublers to the Skin and Stringer Hats. These were added to stop vibrations and buzzing of the Airframe and are called Acoustic Dampeners . We experienced a lot of cracking on the Main Frames where the Lower brackets of the Overhead Bins attach to the Frame. We discovered these on numerous B 737-800 frames in their 1st Heavy C Checks which were relatively low time aircraft. The repair includes stop drilling / routing out the crack - cold working the holes - and installing a doubler. We did not see this on the B 727 and B 757 Generation aircraft this early into the life cycle. All these repairs just add weight back to the aircraft that Boeing seemed to be avoiding in the original design and MFG. .
The first generation B-737 were built like the B 707 and B 727 Airframes : Heavy Duty. Even with the Stringer 10 Skin Lap Corrosion issues they were much better built airframes in my opinion. The Aloha B 737 Proved this in Hawaii.Pictures are available on Goggle Images because this forum will not let me upload pictures.
remember engineering nowadays isn't about making it strong. More about"just strong enough"