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

[Scuffers]

Originally Posted by Smythe

Watch the winglets bounce around the next time you fly, then watch wings without them....cyclical loading with the extra mass at the end of the moment arm.

Source: leeham
https://leehamnews.com/2019/10/08/bo...pickle-part-2/

so you're suggesting it's fatigue from the wings wagging and the winglets change the resonant frequency?

I can see that, right up to the point that surely the airframe is designed to cope with this? and adding mass to the wing will lower it's resonant frequency?

I don't subscribe to this, I would suggest it' way more likely that a change in manufacturing has caused this.

[Aihkio]

Then, who does ?

Hmmm..... Who might that be? In charge of a Boeing airplane production ......

[Fly Aiprt]

Originally Posted by Aihkio

Hmmm..... Who might that be? In charge of a Boeing airplane production ......

;-)
Of course I know...
The point is, despite all this turmoil the FAA doesn't seem willing to regain an overseer position vis a vis Boeing.

[maxxer]

https://investors.boeing.com/investo...s/default.aspx

Doesnt look good if the only horse they have is the 787

[Fonsini]

I recall watching a documentary about Boeing whistleblowers who worked in QA reporting issues with sub-standard outsourced 737NG components a while back.

I wonder if those chickens have come home to roost....

Oh Boeing, how the mighty have fallen.

[rickyricks]

Originally Posted by Fonsini

I recall watching a documentary about Boeing whistleblowers who worked in QA reporting issues with sub-standard outsourced 737NG components a while back.

I wonder if those chickens have come home to roost....

Oh Boeing, how the mighty have fallen.

I believe that would be the Ducommun scandal I referred to earlier? One other thing. I read somewhere that pickle forks are made from AA7075-T3 - does anyone know this to be true? Most 7xxx alloys are not used in tension, which seems to be the role of a pickle fork, and certainly not in a naturally aged temper (i.e. age hardened at room temperature) as such tempers are unstable in 7xxx alloys. I would have thought a 2xxx damage tolerant alloy more appropriate for pickle forks.

[Grebe]

Originally Posted by rickyricks

I believe that would be the Ducommun scandal I referred to earlier? One other thing. I read somewhere that pickle forks are made from AA7075-T3 - does anyone know this to be true? Most 7xxx alloys are not used in tension, which seems to be the role of a pickle fork, and certainly not in a naturally aged temper (i.e. age hardened at room temperature) as such tempers are unstable in 7xxx alloys. I would have thought a 2xxx damage tolerant alloy more appropriate for pickle forks.

You might be correct re diff in alloy and temper. May well be the diff between costs and production techniques between the alloys- all "solved" by using the cheapest.


But the above being said- This SLF non structural engineer suggests that the landing loads and skipped production techniques re hole prep ( drill, coldwork, ream, interference fit being the baseline ) when skipped ( saves time ) and/or improperly checked tooling size ( coldwork mandrel ) is the most likely issue. ( see Fatigue technology ) disclaimer I have NO ties to Fatigue technology- almost opposite for ancient reasons not pertinent here

Add the file to fit- pound to suit- and here we are.

[B727223Fan]

Grebe : and/or improperly checked tooling size ( coldwork mandrel ) is the most likely issue. ( see Fatigue technology )

If the cracks are originating at the fastener holes I do agree with your statement.

Cold Working of holes is very important to relieve stresses as well as proper fastener hole size and proper installation of Hi-Loks.
Hi-Loks are usually installed interference or close tolerance fit.

https://www.aerofastener.com/products/hi-lok-pins

Shot Peening and Flap Peening are also important post assembly maintenance processes on various structural members and parts that also relieves stresses.

Note: I posted this info for folks who have never cold worked holes or installed Hi-Loks.

[ST Dog]

Originally Posted by rickyricks

If the pickle forks were installed without the correct drilling procedures, for example, then one ought to expect some early cracking to appear.

My suspicion is incorrect cold working one or both fasteners. Could have been incorrectly specified too.

[Grebe]

Originally Posted by B727223Fan

Grebe : and/or improperly checked tooling size ( coldwork mandrel ) is the most likely issue. ( see Fatigue technology )

If the cracks are originating at the fastener holes I do agree with your statement.

Cold Working of holes is very important to relieve stresses as well as proper fastener hole size and proper installation of Hi-Loks.
Hi-Loks are usually installed interference or close tolerance fit.

https://www.aerofastener.com/products/hi-lok-pins

Shot Peening and Flap Peening are also important post assembly maintenance processes on various structural members and parts that also relieves stresses.

Note: I posted this info for folks who have never cold worked holes or installed Hi-Loks.

Thanks - also applies to lock-bolts which use a swaged collar- and at one time early on in 737 program ' taper-loks" may have been used but I'm not sure.

As to cracks and holes - somewhere in this thread are some photos which show exactly that- and early on I had postulated just that kind of problem.

MY guess then and now - is probalby a production change tg ensure coldworking of that area, and a repair of non cracked holes via coldwork and first oversize- awkward to be sure- but cheaper in long run

[daved123]

[QUOTE=B727223Fan;
Note: I posted this info for folks who have never cold worked holes or installed Hi-Loks.[/QUOTE]
That would be me then - which prompted a google and found a good explanation at ENG-TIPS
Thanks for the lead
DaveD

[GogogoStopSTOP]

Dave could you post a link to this Boeing inspection data and possible images of the cracks, thanks.

[Grebe]

PICS OF ACTUAL CRACKS

Originally Posted by Dave Therhino

The Boeing MOM referenced in the AD is available to the public in the public rule making docket, which can be viewed at:

https://www.regulations.gov/document...2019-0711-0002

The pdf contains great pics of actual cracks I've repeated it here since many recent posters have probably missed it

[Cheltman]

I read that when it was changed to a forging there was a material spec change. Does anyone know what grade of aluminium it now is?

[Grebe]

Originally Posted by Cheltman

I read that when it was changed to a forging there was a material spec change. Does anyone know what grade of aluminium it now is?

AFIK it was always a forging - do you have any credible source that it was other ?

[Loose rivets]

Mention on PPRuNe about them being milled from the solid. They talked of waste but I assume the dross was collected. Whether it can be recast or used for something else I don't know.

A mill can make short work of a lot of metal, but it's possible that high speed milling might not be normalizable - or whatever has to happen to it.

[nonsense]

Originally Posted by Scuffers

OK, as an engineer, I am struggling to understand how winglets would make any difference to the loading on this fork?

My thinking is this, the plane's body mass has to be supported by the wings, makes no odds if the wings are bigger/smaller or more/less efficient, they still have to hold up the same mass.

Remember first year solid mechanics?
Bending moment is the product of force by distance.
An aircraft supported at the wingtips would exert a much higher bending moment at the wing roots than one supported beneath the engines, close to the fuselage.
The winglets alter the distribution of the wing outwards by making the wing tips more efficient, thus increasing the bending moment at the fuselage.

Originally Posted by Fly Aiprt

If the wing and fuselage structure were infinitely rigid, indeed there would be not problem.

Regardless of the stiffness of the structure, increasing the lift at the wingtips moves the effective location of the total lift in the wing outwards, increasing the bending moment at the fuselage.

[Smythe]

I can see that, right up to the point that surely the airframe is designed to cope with this? and adding mass to the wing will lower it's resonant frequency?

The airframe was not designed to handle the load from the winglets...the winglets came later...

I find it interesting that only pax ac have found cracks so far...

The issue was found by Boeing Shanghai on the new C2F line July 2019).....What about all of the previous C2F conversions done by others..., why was nothing found there?

Are freighters (and C2F) not included in the group the FAA wants tested?

[Pilot DAR]

I'm thinking about it differently, and, purely speculating, as 737's are outside the scope of my work. As the cracks appear at the last aft fastener holes of a four fastener cluster, I'm not thinking that there is a lot of motion or load between those four fasteners, they're going to act as a unit, even if the spar is bending a little in that area. The presence of a crack indicates a load exceeding the local capacity right there, it's not the whole cluster, just that one last line of two fasteners. So what load separates the area of structure of that cluster of four fasteners (and those above) from those below? Not so much wing bending, but yaw torsion between the wing and the fuselage. The force which would have to be resisted if lots of rudder were being applied. I've always wondered about the applicability of Va in the yaw axis, and this defect continues my wonder about that. As those pickle fork fittings would appear to be the sole structure which resist a wing to fuselage yaw load transfer, perhaps they're just not quite up to it. The presence of winglets could aggravate a yaw reaction a little (winglets would not reduce yaw reaction anyway!).

But, of course, that's all speculation on my part. I'm just thinking about aging aircraft and primary structure cracks, as I'm dealing with them on other aircraft these days. Obviously a load in direction or magnitude, or both, is exceeding what the engineers accounted for, and as we consider repairs, we must consider what load is being carried, which was originally inadequately considered.

[Grebe]

Originally Posted by Pilot DAR

I'm thinking about it differently, and, purely speculating, as 737's are outside the scope of my work. As the cracks appear at the last aft fastener holes of a four fastener cluster, I'm not thinking that there is a lot of motion or load between those four fasteners, they're going to act as a unit, even if the spar is bending a little in that area. The presence of a crack indicates a load exceeding the local capacity right there, it's not the whole cluster, just that one last line of two fasteners. So what load separates the area of structure of that cluster of four fasteners (and those above) from those below? Not so much wing bending, but yaw torsion between the wing and the fuselage. The force which would have to be resisted if lots of rudder were being applied. I've always wondered about the applicability of Va in the yaw axis, and this defect continues my wonder about that. As those pickle fork fittings would appear to be the sole structure which resist a wing to fuselage yaw load transfer, perhaps they're just not quite up to it. The presence of winglets could aggravate a yaw reaction a little (winglets would not reduce yaw reaction anyway!).

But, of course, that's all speculation on my part. I'm just thinking about aging aircraft and primary structure cracks, as I'm dealing with them on other aircraft these days. Obviously a load in direction or magnitude, or both, is exceeding what the engineers accounted for, and as we consider repairs, we must consider what load is being carried, which was originally inadequately considered.

FWIW I note some sort of clevis fitting in the pics near the cracked area- which is on the rear spar to wing box area. Could the clevis area be part of the landing gear arrangement and during landing with a heavy load ( short haul/cycle ) and/or hard landings wind up pushing partly sideways thru the landing gear ' side struts' or some sort of connection ???.

BE interesting to find the flight time vs cycles for the carriers that have found the highest incidence of cracks or other high-over stressed indications.