Boeing’s 737 in another pickle, Part 2

By Bjorn Fehrm

October 8, 2019, ©. Leeham News: The FAA Air Worthiness Directive (AD) for high time Boeing 737 NGs regarding cracked rear pickle forks was issued Thursday last week.

Boeing had a call with 737 NG operators today giving the latest information regarding the cracks. Of the 500 first inspected aircraft, 5% had cracks. These aircraft are now grounded. Boeing is setting up repair lines with the US line starting work on the first NG this week. The expected repair time is three weeks for the first aircraft.

Figure 1. The rear pickle forks carrying the main loads from the wing to the fuselage crack near the rear wing spar. Source: Leeham Co.

It’s the rear spar pickle forks which crack

The cracks form in the outer chord of the rear pickle forks (Figure 1) and the behind lying safety straps, just where they pass from the rear spar of the center wingbox to the fuselage side of the aircraft, Figure 2.

Figure 2. Crack location on pickle forks and the behind lying safety straps. Right side fork shown Source: Leeham annotation of Boeing graphic.

This is also the area where the fuselage side has a cutout to let the main landing gear strut fold in with its attached wheel.

The combination of a wing center-wingbox pushing wing and landing gear forces into the fuselage in an area where the fuselage has a large cutout for the folded main landing gear makes this area complex regarding forces and how these work the parts during flights.

The stress spectrum in the area can also have been affected by the later fitting of winglets on the NG, not foreseen in the original design of this area.

Winglets change the pressure distribution of the wing to a distribution spreading further outboard, by it increasing the wing root bending moment.

Repair line started, parts production critical, temporary repair investigated

The results of the first week of inspections are 5% of the inspected aircraft have cracks with the lowest flight cycle aircraft with cracks at 23,600 flights.

Boeing has commanded full speed ahead with the production of replacement pickle forks and a repair line is set up at Victorville by its AOG (Aircraft On Ground) team. The first aircraft starts the replacement of its rear pickle forks Friday. It’s expected to take two to three weeks for the first job, which also serves as the master for a Service Bulletin from Boeing how to carry out the repair. Further AOG repair lines will be set up in Europe and Asia.

There shall be 25 shipsets of pickle forks available in October which covers the aircraft which cracks found to date. The safety straps also need replacement but these are less complicated parts.

Boeing is now investigating if a temporary repair can be designed and approved so aircraft with cracks can continue to fly until they can be scheduled for repair. It’s also checking if the demounted forks can be repaired to ease the demand for new forks as the inspections find further aircraft needing repair.

The wider inspection will give statistical data so the spread in cycles for cracks can be better understood. It could be the cycles shall be counted from when Winglets were fitted to the aircraft. This is one of the areas which are now needing further work and data to understand better.

82 Comments on “Boeing’s 737 in another pickle, Part 2

  1. One would think that the effects on structural components/joints due to increased wing root bending moment would have been considered in full before approving/installing winglets. I wonder if cracks have appeared on any 737 NGs without winglets?

    • One would think that, but then again…

      Sounds like it’s a design fault then, with potential for a fleet wide impact.

      I wonder what the repair is. If it’s dropping in an exact same part, then what that’s saying is that these aircraft are going to need another repair by 50,000 cycles. That’s not great. And, if it is the same part, it alse means they’ve not reassessed the loadings at all, they’re be simply running with the old design that is known to be inadequate. That in turn raises the question, what other parts of the airframe are being stressed beyond the original design intent?

      Apparently the AD sees this as an interim repair, not a final solution. So that fits.

      But what this means is that 5% of aircraft >23,000 cycles are going into a lengthy repair, and then probably sometime in the nearish future 95% of >23,000 cycle aircraft (the ones not already broken) will have to have a comparable repair, possibly more if a re-analysis of the structural loads finother oversights. Then the original 5% will have to have the same repair sometime before ~50,000 cycles.

      What this means is that Southwest’s entire fleet (average age 20,000 cycles apparently) of NGs is on the cusp of requiring major work. I bet they’re looking forward to that.

      I also wonder what the other regulators around the world are thinking about all this.

      It’s interesting to contemplate the options for a proper permanent repair. Suppose they design a stronger, stiffer pickle-fork. Great, that won’t break, but then it’s simply shunting the excess load elsewhere in the airframe. The consequences of this could ripple out as modifications across that whole section of airframe. Ouch. If they deisgn a more flexible one, well then that’s simply not taking the loads as it was before and other parts bend more as a result.

      It’s a bit like identifying that part of a jelly is failing under too much stress, and changing that part’s stiffness whilst expecting the rest of the jelly to simply accomodate the new loadings.

      • Seems to me there may be an interim repair for those not ydet cracked- mainly be properly reworking the fasteners- holes in that area via remove, coldwork, ream, install first oversize interference fit, and closely check nearby clevis portion for clearance.

        And incorporate the same coldwork technique on ALL yet to be assembled.

        For details of how coldwork – fatigue improvement works ( a Boeing development over 40 years ago ) look up Fatigue technology in southcenter WA

        • If it is incorrect manufacturing, requiring rework as you describe, then that’s got to mean that the entire fleet has to be looked at. And they’d have to go back across the entire production run to see what else wasn’t manufactured correctly too.

          There’s already a lot of manufacturing quality problems across Boeing (Charleston 787, KC-46, Apache), and this would add 737-NG to the list. If one concludes that Boeing haven’t been making anything properly for decades, then one might reasonably ask whether Boeing’s manufacturing certification is worth the paper it’s written on, and whether any Boeing can genuinely be considered to have been built to design?

          EASA have said that they see the recent problems with the MAX to be as much about how Boeing operate as a company as about the poor design of MCAS. This pickle fork problem is, ultimately, more of the same. At some point EASA are going to conclude that there’s too many problems with how Boeing runs itself to be able to trust anything they’ve built. Perhaps this is what pushes EASA beyond what they’ll accept and ground the lot.

          AFAIK there’s nothing in the rule book about how a regulator is supposed to react to the wholesale collapse of quality control in a senior manufacturer; I suspect they’re winging it, but they’ll be setting a precedent one way or other.

      • I wonder if SW will continue to hold to their ideal of putting all their eggs in one basket. Interesting that a CEO would hold to that tenant. But the pride of the Boardroom…

        • Who knows, but there’s a lot going on to suggest that they may have picked the wrong basket for their eggs.

          For any Boeing-only airline, these recent events have surely got to have forced them to at least reconsider.

          The same’s likely true of Airbus-only outfits, but so far there’s nothing to suggest that adding Boeing to their mixes would be a good idea.

          It was quite widely reported that Southwest were taking a good look at the A220 family, in Europe, going for a ride, chatting to Airbus and the operators.

          • According to latest whether forecast hell is still far from freezing so Southwest will not buy anything apart cheap Boeing. They are eyeing for A220 because they are eyeing for more discounts from B. Cheap planes are basis. I don’t know to whom they are praying but a lot, but not buying anything which is not B. Same for Ryanair. Michael O’Leary gives a lot of thanks to some airtravel god for Thomas Cook bankruptcy so they can buy A320 for Laudamotion and save parcialy next summer season.

          • Got to be a lot of bad blood and finger point between BA and LUV. We’re talking of billions of dollars of lost revenues ((and wages) see SW Pilots Assoc. lawsuit against Boeing.) Moxy, Jet Blue, Delta A220-300s (the former two with probably 150 seats per plane.) If not A220s, SW maybe thinkin’ EM195-E2s. They don’t use a feeder airline, so that might play into it.

          • Pablo, well, Southwest might still be thinking along Boeing-only lines, but it does kinda depend on Boeing having something viable to sell.

            Once upon a time the thought of Boeing going bust was laughable. Yet here we are, certain that it’s less laughable than it used to be; we just don’t know what the odds actually are.

            In the worst case there’d be a rush for Airbuses, and any airline wise enough to have already placed a commitment (even non-binding) with Airbus first will be less impacted.

            Also this year’s events may well have provoked the more thoughtful airlines into calculating whether or not cheap Boeings are actually worth it. Southwest have lost a pile of cash this summer, possibly more than the price difference between MAX and NEO.

            They won’t want that being a regular occurrence, so perhaps they’re looking at Boeing to see if the company really does mend its ways, and switching to Airbus if they don’t like what they see. In the long run Southwest have shareholders to please too, and a Boeing-only strategy that’s found wanting might upset the shareholders. It would be a very startling decision for Southwest to switch, but it would look like a positive long term decision, good for the management’s kudos…

          • Till the hell get frozen, it means till Boeing get bust.

            For now Southwest already ordered 3 MAX simulators – B will pay for them as promised 1 mln per plane if training required, will receive from B compensation and more because if Southwest will switch – everybody will switch. So they are investing still in Boeing – because they can live without it, and B will invest every way in Southwest – because they can’t live without it also.

            Boeing, boldly as always, with FAA soon will make return MAX to the sky in US claiming it is safe, but envy EASA (because of tariffs slapped by Trump) and others complies (especially who robs technology from US companies and also slapped by tariffs / embargo) – will not allow this wunderbar aircraft of course because of political reasons. [my very gray case scenario that unfortunately closes to be future narration of Boeing or FAA or Trump to explain to America why nobody trusts MAX]

          • If you factor in the three week repair time to replace the Pickle Forks (PF) in the hundreds of LUV 737NGs, that’s going to wear down SW management even more. SW has got to be thing – was the PF a short cut on that series of 737s, too? Inspecting the PF is just a short time out of service. Replacing them? Like I noted, I’ve read three weeks. I think the takeaway on what has transpired with Boeing over this last year, will be SW at some point in the near future will have a more diverse fleet. If Boeing got on the stick and tried to launch a NSA, it would be 8-10 years before it carried passengers. Also, there is just no way they are not going to try and fill $275 billion worth of MAX orders.

        • I suggest that depends on its broader strategy – does it want the economy of scale of something like a 787 (or someday a MM) on busy runs, or operating cost savings of something like the A220 on runs not filling 737s or expand into smaller markets?

          WestJet did the latter by adding large turboprops. Not that WestJet is smart any more.

          One size gives scheduling flexibility.

      • It might indeed be a design flaw, either in the Pickle Fork itself or the mating parts.

        But it could also be that the Pickle Fork was manufactured incorrectly (incorrect alloy, machining, heat treatment, surface treatment, etc).

        Or the Pickle Fork could be installed incorrectly at Boeing (improperly shimmed, incorrectly drilled holes, et).

        The ultimate fix will depend on the core problem. If the Pickle Forks were improperly installed or were a bad batch of parts then replacing them with new and good parts of the same design should be a permanent fix. Otherwise it’s only a temporary repair, and if the lowest flight cycles to cracking is 23,600 then shouldn’t the first inspection be around 10,000 to 15,000 cycles?

        • Yep. You can figure on factor of 2 on that 23,600.

          Unless they can demonstrate via analysis crack growth predictions similar to what is evident, and that the lifecycles of the currently cracked aircraft parts are of some standard deviation fit around their predictions. Then it might go down to 1.5

          But will they have load data? Maybe. They might be able to hack something off MAX.

        • What if the web the pickle fork is flexing and that bent the tip of the pickle for. Just like bending a piece of tinplate back and forth.
          Is there cracking in the web too ?
          Any way it’s going to cost big $

      • Keep in mind this is economic panic time, get the airplane back into revenue service, that’s worth cost downstream.

        I’d hope that Boeing can do something to the design/installation , that may take several months, depending in part on how much full-scale testing is required. (They need to have a better understanding of stresses in the area, repairing the high-cycle airplanes may give them some insight into fitup stresses (forcing parts into place can create unwanted stress on them or mating parts).) More engineering and testing resources consumed.

        As for finding during cargo conversion, I guess the airplane was opened up to reinforce floor beams if they are in the modification to freighter (package express use can cope with lower floor loading limit than general cargo use, but a lower loading limit limits future economics). People have accepted limits, IIRC ABX operated DC-9s with standard pax doors, not investing in large cargo door, probably using narrow containers. OTOH PW’s 727-100Cs and 737-200Cs going into the High Arctic had heavy freight sometimes (food and heavy parts/tools – I saw a 5000 pound chunk of SS pipe that was full of sensors to measure tilt of the well hole to help keep it straight, probably very sophisticated up there near the north magnetic pole).

    • I’d check to see if the affected planes have been flown by Ryanair pilots, it’s always a crash landing with those guys 👍

    • Great question Albert! You would think that Bjorn would have thought of asking that very question before drawing conclusions. It would be refreshing, for a change, to see a reporter actually do their homework and report using facts versus relying on rampant speculation (eg “fake news”). There are exactly 161 737-NG aircraft remaining in-service (of about 6,500 produced) eligible to receive the Blended Winglet STC. Thus 97.5% of the world fleet of 737-NG’s operate with Blended Winglets installed. With facts in hand, I find it very interesting that, of the first 7 aircraft discovered with the subject cracking (3 -800’s, 3 -700’s, and 2 -900’s), 3 of them do NOT have Blended Winglets installed and still operate with their original wingtips. Also, of the first 25 aircraft discovered with the subject cracking (11 -800’s, 12 -700’s, and 2 -900’s), 8 of them do NOT have Blended Winglets installed (5 -700’s, 1 -800, and 2 -900’s). Look at those facts and then tell me that Blended Winglet installation has anything to do with this issue…? In addition, the component of the very small structural load increases due to Blended Winglets at side-of-body, are likely in a direction that has little or no influence on the pickle fork. It is quite disturbing that we live in a time where “reporters” can write whatever they want (speculate) and then wait for the facts to emerge.

      • @Patrick

        May I ask where did you find such precise data of subtypes affected by cracking?

      • A winglet related hypothesis is being raised Patrick, Bjorn is not drawing a conclusion. But thanks for the insider data, that is up to the extent, one can actually rely on it without a second source.

  2. Given how the cracks were discovered by chance during a freighter conversion inspection, what would have been the consequence of this part cracking all the way to failure in flight?

    • I can only presume that the ones being converted to freighters had been flying around with them cracked all the way through. Not sure how they’d have cracked whilst sitting on the ground.

      If so, then there’s less structural redundancy and possible overload of other parts of the structure. I don’t know whether or not it’d be on the edge of break-up. We know they didn’t crash, but that might simply have been luck (no turbulance, smooth landing, low payloads etc).

      It’s possible to determine the cracking history from a detailed examination – i.e. was it fast and all at once, or did the crack grow slowly with time. I suspect the latter – that’d fit the AD’s differing timescales for >30,000 (to be inspected within the week) and >20,000 cycle aircraft (to be inspected within the year).

      • Curious to know what the failure of this single piece of structure would have led to.

        It reminds me of the A380 wing rib feet cracks and how they were discovered by chance after the QF32 mishap. IIRC those would have caused the wing skin to come off causing structural damage but it would have taken hundreds or thousands of these pieces to fail before that happened.

        Here we have 2 large pieces on each side which if even one gives way, could probably result in a catastrophic structural failure, unless there are other parts to take off some of the loads. But would we even know that.

        • QF32 exposed it prematurely. IMU the regular check interval would still have been fully sufficient to expose the cracks long before they had grown to be dangerous.

          No NG has shown cascading failure from pickle fork cracking yet.
          Q: how much more cycles needed to force some structural failure caused by further growing of observed cracks?

  3. Any idea how Southwests 700s are affected?
    I’m guessing that the combination of max and pickle fork will peak at around 1000 airliners missing from the global fleet. It’s amazing how the industry manages this.

    • Maybe. 700s don’t weigh as much – it’s possible that they won’t be as badly affected. There are a lot of possible variables at play here that can’t be understood without a lot more data. Speculation is fine, so long as we don’t confuse it for reality.

      • 2 are SW aircraft, as of yesterday. Does SW have any 800s over 10 yo? I don’t think so, they’re probably 700s. Lighter, well looked after and damages from aircraft which seem to have been delivered years apart, judging by cycles. It will be interesting hear the cause. Assembly QA comes to mind.

  4. Any idea how Southwests 700s are affected?
    I’m guessing that the combination of max and pickle fork will peak at around 1000 airliners missing from the global fleet. It’s amazing how the industry manages this.
    By the way, well done Bjorn, this is the best and up to date easily available information at the moment.

  5. This is really bad news (I mean the 5% rate) but at least it is now identified and repair seem mastered.

    Any idea if it is part batch related or a real design flaw?

    • It could be a production issue, possibly in combination of decent quality control.

  6. How many NGs have over 20000 cycles? No indication of why? Redesigned part? Any non winglet equipped aircraft affected?

  7. Bjorn, if there are to be 25 shipsets of pickle forks available in October, does this mean that BA understand what is causing the cracks ?

    It seems to me, due to immediate availability, that these new pickle forks are potentially standard items, and not re-designed, and thus would indicate that BA don’t believe there is a need to change the design of the pickle forks or reinforce them.

    Cracks in an item at 23,600 flights, but designed to last 90,000 cycles is significant isn’t it ?

    Did BA in fact change the pickle forks from Classic to NG, and from NG to MAX? I can’t believe that they wouldn’t have taken the new wing loading / wing pressure distribution / winglets into account.

    The wider inspection data will be very interesting.

    I’m presuming that BA will pick up the cost for the repairs in order to protect the resale value of all other NG aircraft.

  8. Detailed design consideration of the NG blended winglets
    I had thought that the benefit was just reduced fuel consumption ( from cruise drag) for longer flights but there were other improvements.
    “Low-speed testing showed a significant reduction in takeoff and landing drag and a significant benefit in payload capability for certain operations.

    Maybe the discussion about high wing loads during speed brake usage could be relevant to the pickle fork cracking causes.

  9. Do you know if they are going to continue with just visual bore scope inspections, or use some other inspection methods (ultrasonic etc), to try and gather some information on the source of the cracks?
    Will they be requiring a retrofit every X number of cycles until they better understand the cause of the cracking?

  10. Repairs are going to involve messing with an awful lot of complicated systems and it’s going to be done in a hurry. I don’t like the sound of that. Maybe I missunderand how these things work.

  11. Well we have all those MAX planes not being built right now!

    More curious , in the static test, do they swing the gear?

    Clearly they down’t test with wiglets if they did not exist at the time.

    You can add another mark on Boeing for not replacing the 737 before the NG.

  12. Considering that the engine is on the wing, I don’t understand how the winglets would affect the wing/fuselage bending moment. In fact the winglets may reduce the bending moment because they could balance the drag of the fuselage.

    Speaking as a complete layman here obviously.

    • From Boeing detailed analysis of the structural changes, they didnt seem to consider the wing-fuselage join at all and were only concerned with the outer wing structure and flutter loadings

    • Forget about the engine; it’s a smaller load than the weight of the fuselage and doesn’t nullify the shear and bending from the portion of wing outboard of it. Taking the same lift force and pushing it further out increases bending moment at the root. The drag is irrelevant; not only is small, but it’s acting about the strongest axis of the wing.

      • Talking of wing twist movement will the greater upward twist of MAX fans play havoc with rear pickle fork yet again? Does anyone know if pickle forks are identical throughout 737 production?

        • henry bond. It seems like it’s only the 737-NG’s affected according to Boeing
          The 737 NG, or Next Generation, was introduced in 1997 and is the third generation version of the best-selling Boeing airplane. The 737 MAX, which was grounded in March after two fatal crashes in five months, is not affected by this issue, Boeing said


          • This issue is horrific… Boeing says no pickle forks on the Max? If that is true what was the substantive replacement attachment system? Why no mention of that major change at the time it should have been being tested and approved by FAA AS PART OF NEW CERTIFICATION? Oh that’s right, everything Boeing did with the Max was intended to be shortcuts as to timing and cost…. and to avoid anyone suggesting full review was indicated prior to certification. One thing is clear to me reading this thread, to wit, the pickle forks, straps and whatever else in the fuselage and attachment system should be required to be replaced often and repeatedly… I don’t trust anything Boeing or the FAA say at this point… there will be hell to pay when a wing rips off an NG… AND, ON THAT POINT, HOW MANY OLDER 737s, cargo or otherwise, have pickle forks damaged that we found out about only by accident?

    • Moment is load times distance. Since the load of the 900ER is greater than the 800, is greater than the 700, what models are cracking at what hours?

      • A breakdown would be good.

        It’s not as bad as it could have been! So perhaps a very dim light is shining for Boeing. It could have been crazy.

        • A breakdown is what we would have got if the problem hadn’t been discovered!

        • Phillip-the dim light is coming from the dim bulbs in the chicago corner offices powered by MDC juice- been that way for a few decades. Would be interesting to check age of planes with cracks versus mfg date versus MDC buyout

          • Erudite comments… Boeing used to be run by legit safety first engineers. “MDC juice” who changed all that have prima facie been all about shortcuts, winks and doing everything as fast and inexpensively as possible.What else has been done we have yet to learn about on the NG fleet of some 7k planes and the now DOA Max? It may already be too late to “fix it”… and the “new” ceo has been in the C Suite for over a decade..l not the answer!

  13. In the mean time anyone for a pickle and cheese on rye swilled down with a strong mug of Yorkshire tea.

  14. I wonder where I could find FAA report after NG’s inspection?

    As many I would like to know how many aircrafts are affected in Southwest or Ryanair? Which subtype is affected most? Is there a pattern?

    5% of 500 is many, but this 5 /500 numbers sounds too round for me, like a B’s press release ultimately, they are not exact data.

    This case, with others known, talks much about quality decline in B, no matter if it’s design / manufacture / assembly.

  15. Fox reports LUV found 2 planes with broken forks and has grounded them. Exact type and age not listed.
    Not clear how many were inspected to yield this result.

  16. Not easy to calculate complex areas. Obviously the cycle tests they made gave wrong results, especially if they changed something later. If they had done tension optic tests they might have found areas where cracks might happen. Are they working with tension optics?

  17. I’m not sure that the winglets would have a big effect to be honest.

    Its a bearing load on the lower fastener row that is causing the problem – which is not from moment but from direct vertical load from wing to fuselage.

    The upper fastener row is in bearing/bypass load (which would be higher load), but look like bigger fasteners so maybe net stresses are down to the point they aren’t as susceptible to crack growth. Possibly an easy fix for Boeing there (enlarge the lower fasteners) if they have the edge distances to work with.

    The moment will only really be visible to the wing spars and wingbox – but that joint to the fuselage doesn’t really care. A bigger wing bending moment might see more loading of the fillets around that pocket, but not the fasteners.

    The joggle to the left of the faster group on the image also looks a bit of a challenging area – wouldn’t be surprised if any fix includes a padup or similar in that region.

  18. Quote:

    “The pickle fork – the component attaching the wing structure to the main aircraft fuselage – is manufactured to last 90,000 flight cycles. But issues have now been found on aircraft which have done far less mileage.”

    Cycles but……

  19. At least the Max will be back in service. American just announced they’ll resume flying it January 16.

  20. Seems the rate of occurrence is now agreed at 5%, but beaucoup questions remain:

    1) what subtypes or groups are affected? Is there a subset of 6800 frames to be targeted? Or is it a mere passage of time issue? Why is the impact on Gol so high? Is LUV next?
    2) will simple replacement suffice? Or will redesign be needed in time?
    3) will this be a “ rolling” problem? I.e. will newer frames become affected as they age?
    4)are second replacements likely to be required at 50000 cycles and the again at 75000 cycles?
    5) three weeks for repairs for each frame sounds very low. Availability of parts and staff may stretch the outcome considerably.

    What an unholy mess.

    • I wold not call it an unholy mess, I would call it interesting and beyond fortunate the was not an in air breakup that lead to it.

      The details are going to be most fascinating as Mr. Spock would say.

      And maybe, just maybe this will lead to the major purge at Boeing that is so desperately needed.

      Are there other ticking time bombs?

      • The Classic series had the fuselage fatigue failure problem in those two Southwest 737-300s. The maintenance fix costs meant the rapid retirement of that series from the fleet – at mid age- and a buyup of as many used 737-700s as they could find. Now its a fatigue problem for the NG series pickle forks and they dont know the true cause yet – Im thinking it was connected to the critical wing load case with the speed brakes extended during descent before the final approach. However wing loadings are complex and the added winglets changed things as well.
        Back before the Max was announced the plan Boeing had talked about was a NSA coming into service about now, instead we have all these issues with the 737 and no new replacement in sight.

        • You start your new (airframe) project when the established one has its best time and offers lots of profits.
          But Boeing’s accounting shew that profit years back already. It been “used”: share buy back and other nonessentials.

        • Load cycle knowledge is essential, that was missed for the horizontal stabilizer of the 707-320C – cycles caused by turbulance from wing with full flaps was not recognized. The accident prompted Boeing to work on filtering service reports to spot patterns, many cases of loose fasteners in the horizontal stab had been reported and easily fixed but no one connected the dots.

          As for the crown skin failures in SWA service, remember the Aloha fliptop, IIRC thick paint and poor inspection conditions contributed. Flight crew handled the unprecedented situation well, had to communicate with hand signals due noise level. Wish I’d made extra effort to go to Tacoma when by-then Captain Mimi [Johnson?] spoke there.

          I’m curious as to why the NG was different body-wing join then the MAX different again (though by the time of its design the general shortcomings of the NG design would be known, especially the need or a life limit, which airlines don’t like – had enough of that with the Viscount and other designs, older designs may not have had two wing spars).


      Covers the maintenance cycles of the 737 as modifies for the NG
      “One of the advantages of the 737 NG over the 737 Classic is that Boeing allow for intermediate base C checks before the D check to reduce the time, work and expense required at the D check. Operators that have a maintenance programme with a P48 check can have a base check interval of eight phases (hence the name P8), where one phase = 500FH, and so have six base checks in the base maintenance cycle. These will be P8, P16, P24, P32, P40 and P48 checks, often referred to as C1, C2, C3, C4, C5 and C6 checks. The P8 check will have an interval of 4,000FH and up to 18 months. The maintenance cycle will therefore have an interval of 24,000FH and up to nine years.

      A typical P48 check or (D check) can take 20-30,000 man-hours, 2 months to complete and cost several million dollars”

      Boeing is going to very much shift the pickle fork replacements into the heavy maintenance cycle or at least one of the C checks.

      • Apropos:
        is the “superior, cheaper vs Airbus” way Boeing has set up for cyclic maintenance just another domain were margins were shaved down to microns?

        If yes my guess is that we will see further fall out from that “over-optimizing” process in the future. In another domain it is called blowback. 🙂

    • If ships with 35k cycles go into freighter conversion …
      ( How many cycles do freighter on average do? 1.. 2? )
      .. few frames will reach 50k cycles.

      • NMAing the 767 with the NEO treatment and some Frankentanker accessories ..
        will create a 767MAX. Kind of Toast from the get go.

        In the initial pair of offerings ( 767 vs A330 ) the A330 flew circles around the 767 performance wise tided over in select domains: tanker via politics, freight via price.

        GenX 2b should perform ?slightly? worse than the Trent 1000 TEN ( basic layout and tech improvements.) ok, the engine should be PIPable but the layout thing should stand.

        • Yes, a 767-400Fneo is maybe some cargo Airlines wish with Uncle SAM backing, not much of a competition for the A330neo as a pax jet, the 767-400neoER with its old systems and limited cargo hold for LD3’s. Still the 767 has only been reengined once (JT9D-7R4 to PW4060, CF6-80A to CF6-80C2) so one more time is pretty logical but way too late. If UPS and FedEx orders 200ea with updated cockpit, systems, carbon wings, landing gears like Boeing does on the 777X it could trot along profitable for another 15 years as a freighter.

    • Interesting concept, the 767 belly is limited by not fitting LD3 containers, a good airplane, -400 has raked wingtips IIRC and newer flight deck instrumentation. Good flight deck windows (Boeing copied Douglas concept as on DC-10 and C-17, low drag low noise).

    • Iacocca turned Chrysler around by leveraging the K-Car FWD sedan design into larger cars and the product concept he brought from Ford – a garageable minivan, which sold very well. Similar to Airbus and Boeing Chrysler then perpetuated that compromised design under reflowed skin in the early 90s – akin to what people accuse Boeing of doing with the 737. (The Caravan has compromised suspension to facilitate low overall height (it is a smooth road vehicle) and large turning radious due to larger tires to handle increased weight of extended body version and more options.

      Later Alan Mulally from Boeing turned Ford around. In part by motivating employees, in part by being more sensible about models IIRC. (A small example of cheapness in models was Toyota selling more pickups when gasoline prices zoomed upward again, because it offered a small V-8 engine option whereas Ford only offered a large V-8).

  21. As best I can understand the illustration of crack location you provide, I note it appears to be the end row of two and the fasteners are smaller than in the other row.

    Stress on rows varies, DC-10 designers tapered fuselage splices to equalize stress at fasteners. (This is more complicated geometry.)

    Discontinuities in structure are likely places for fatigue. Way back, the Found Brothers’ floatplane cracked at the wing-fuselage joint where a heavy fitting was attached to a sheet web.

  22. With the news today that cracks have been found in other locations, maybe it is appropriate to add another comment.

    I don’t know if this has been widely discussed already, but there are two possible reasons for cracking. The first is if the forces were known, but the components for one reason or another were not able to withstand the stress. The second is if the forces are not what was expected, so the part(s) weren’t designed for the actual forces.

    As far as the second possibility is concerned, it is possible that the additional stress might have been caused in the production process. For instance if the pickle forks, or an adjacent part, had to be forced into alignment. That would fit the idea that additional cracks have been found is adjacent components, or in other areas of the pickle forks.

  23. I am curious if there have been updates on this issue. It would be great to get some updated statistics on the planes affected. And also if there is any better understanding of a root cause. lastly, I am somewhat surprised that international airlines have been more open about number of planes affected while it seems that US carriers (Delta, United, American) have not been as transparent.

  24. Certainly, problems like these rarely have one root cause, but typically come about due to a myriad of issues…

    Certainly Robert Wilkinson’s comment is spot on…

    The NG was the airplane that deviated from the Boeing historical use of structural die forgings for the Picklefork and, instead, with the advent of high-speed aluminum machining, went to stretcher-leveled plate (and related large cross-section open die forgings). These latter two products took a hit on mechanical properties, and were prone to porosity in the center. Just maybe…

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