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

[B727223Fan]

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


The outer wing was modified with different structure parts before the winglets were installed on the post factory retro-fit NG aircraft.


"For example, a BBJ retrofit, accomplished according to an FAA supplemental-type certificate, involves the following tasks. (This listing does not constitute a complete work instruction package.)"

• Removal and replacement of the outboard upper and lower skin panels .
• Removal and replacement of rib 25, which is third from the outermost rib ).
• Installation of stiffeners across rib 25.
• Cutting of the closure rib (rib 27) and trimming of the two spars
• Installation of the new center section of rib 27 and the new winglet attach fitting
• Installation of the spar attach fittings .
• Installation of the aft-position light.
• Installation of the winglet ."

"For airplanes in production, the wings are strengthened throughout the wingbox to accommodate the winglet loads with full use of the speed brakes to the in-flight detent position. The in-production modification meets the same design criteria as those for the retrofit. However, during production, structural strengthening is accomplished by increasing the gage of spars, stringers, ribs, and panels. Rib 27 incorporates bolt hole patterns that allow attachment of either a winglet or a standard wingtip. The winglet is installed in final assembly."

http://www.boeing.com/commercial/aer...let_story.html

[M609]

SAS has found cracks on two aircraft. One that is allready leaving their fleet. The other one will have the pickle fork replaced.

[DaveReidUK]

Originally Posted by Smythe

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

The AD applies to all N-registered 737NGs. It doesn't differentiate between passenger and cargo aircraft.

[HarryMann]

Originally Posted by meleagertoo

Despite several thousand hours on the NG I have never heard of pickle forks and the OP's link doesn't work.

Anyone care to explain?

i believe we in the UK call them what they are... fishplates 😊

[Fly Aiprt]

[Smythe]

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

The outer wing was modified with different structure parts before the winglets were installed on the post factory retro-fit NG aircraft.

This was strengthening for the wingtip and replacement of the spar with one with boltholes for the winglet...the only stiffened element was the spar third in from the wingtip...

The rest of the wing and the pickle fork connector between the winbox and fuselage remained unchanged.

The AD applies to all N-registered 737NGs. It doesn't differentiate between passenger and cargo aircraft.

Thanks...it seems odd the cracks (so far) have only been found on the pax version....

[ManaAdaSystem]

Originally Posted by M609

SAS has found cracks on two aircraft. One that is allready leaving their fleet. The other one will have the pickle fork replaced.

One 700 with winglets and one without.
Any idea of the number of cycles?
NG’s have been with SAS for a very long time. From the late 90’s?

[GordonR_Cape]

Originally Posted by Smythe

Thanks...it seems odd the cracks (so far) have only been found on the pax version....

Probably just numbers in service, as a fraction of the NG fleet. I don't have all the figures, but the majority of cargo operators seem to be using the B737 Classic (-300 to -500). See: https://en.wikipedia.org/wiki/List_o..._737_operators

[B727223Fan]

The production models had even more structure modifications.

"However, during production, structural strengthening is accomplished by increasing the GAGE of spars, stringers, ribs, and panels.".


"For airplanes in production, the wings are strengthened Throughout the wingbox to accommodate the winglet loads with full use of the speed brakes to the in-flight detent position. The in-production modification meets the same design criteria as those for the retrofit. However, during production, structural strengthening is accomplished by increasing the gage of spars, stringers, ribs, and panels. Rib 27 incorporates bolt hole patterns that allow attachment of either a winglet or a standard wingtip. The winglet is installed in final assembly."


I don't think anyone here has suggested that changes were actually made to the fail safe strap / pickle fork at the wing center section on the winglet models..
In fact my posted link states that inboard wing changes were minimal or unnecessary. Maybe Boeing should re-think this position.
It is apparent that the early Generation NG aircraft were designed / produced / sold without winglets in mind.



"Structural changes.
After completing the studies of the toe angle and speed-brake angle, structural material for the mid- to outboard wingbox was still required. (Because the inboard wing had sufficient strength margins, structural changes to that area were minimal or unnecessary.) To minimize the adverse effects of the wing structural modifications on flutter, wing torsional stiffness was maximized in relation to bending stiffness."

After reading the link info [#221] it appears to me that Boeing strengthened the mid and outer wing but did not make any structure modifications to the inner wing or wing center section.
[made the outer wing stronger which had the effect of transmitting more wing loading into the wing center section that was not modified]

[Winemaker]

After reading the link info [#221] it appears to me that Boeing strengthened the mid and outer wing but did not make any structure modifications to the inner wing or wing center section.
[made the outer wing stronger which had the effect of transmitting more wing loading into the wing center section that was not modified]

I may be stupid, but I don't see how the wing load is greater with the winglet. The load is aircraft weight and g loading; that hasn't changed. I can see stiffening the wing tip as the winglet is increasing the local loading, but the final load carried by the wing box is constant. The wing load profile will change, but not the total load.

[Grebe]

Originally Posted by Winemaker

I may be stupid, but I don't see how the wing load is greater with the winglet. The load is aircraft weight and g loading; that hasn't changed. I can see stiffening the wing tip as the winglet is increasing the local loading, but the final load carried by the wing box is constant. The wing load profile will change, but not the total load.

Uhhh look at the wing- with and without a winglet. Compare to lever arm pivoted about one side of the wingbox. Without a winglet, assume a ( hypothetical for ease of calculation ) load of say 1000 lbs 20 feet from pivot poInt= 20000 foot lbs acting on pivot point at edge of wingbox

Now with a winglet, assume a load ( due to aerodynamics in both cases ) of say 1000 lbs at 22 feet from pivot poInt = 22000 foot lbs .
This is because in simplified terms, the winglet changes the aero load ( lift) distribution a bit further outboard, thus increasing the ' lever' arm.
I have deliberately not counted extra weight of winglet, or tried to model actual aero lift numbers, etc. Simplified numbers for illustration only- not an aero engineer, etc.

For a detailed description- go to Aeropartners site.

"For airplanes in production, the wings are strengthened throughout the wingbox to accommodate the winglet loads with full use of the speed brakes to the in-flight detent position. The in-production modification meets the same design criteria as those for the retrofit. However, during production, structural strengthening is accomplished by increasing the gage of spars, stringers, ribs, and panels. Rib 27 incorporates bolt hole patterns that allow attachment of either a winglet or a standard wingtip. The winglet is installed in final assembly."

http://www.boeing.com/commercial/aer...let_story.html

Then point is its the LEVER arm change in lift distribution that adds to the torque-bending loads at the junction of wing and wingbox, NOT jusrt the minimal difference in weight and slightly longer wing.

[DaveReidUK]

Originally Posted by Winemaker

I may be stupid, but I don't see how the wing load is greater with the winglet. The load is aircraft weight and g loading; that hasn't changed. I can see stiffening the wing tip as the winglet is increasing the local loading, but the final load carried by the wing box is constant. The wing load profile will change, but not the total load.

There are two pairs of pickle forks on the 737, with the AD referring to cracks found in the rear pair (at STA 663.75).

It's possible that the presence/absence of winglets alters the lift distribution along the wing, and if that moves the C of P then it may well be that the proportion of the wing load carried respectively by the front/rear pickle forks changes, even if the total load remains constant.

[Fly Aiprt]

Originally Posted by Winemaker

The wing load profile will change, but not the total load.

So the bending and torsional moments will change.

This might help :
https://tamarackaero.com/insights/20...se-on-winglets

[KelvinD]

Are these "pickle forks" or fishplates related to the items that were reportedly poorly manufactured years ago?
https://www.pprune.org/archive/index.php/t-239918.html
There is an Al Jazeera you tube video, produced in 2010 on this same topic. As far as I could gather, the US Dept of Justice stepped in and halted law suits etc relating to the issues raised by a couple of whistle blowers. In the video, reference is made to 3 737NG accidents which all had one thing in common: the fuselage in each case broke into 3 parts upon impact in pretty much the same location. I was a bit baffled by the terminology used in the documentary, they refer to "failsafe cords" (or chords?)

You Tube

At around 11mins in to the film, you will see evidence from Boeing that one of the most common defects with the parts as supplied from the manufacturer was "hole mislocated".
(unrelated to the safety issue, I was baffled by the process here: The parts are manufactured in California, shipped by train to Wichita where they get joined up with other parts to become fuselage sections. Which are then shipped by train to Seattle, presumably going via California??)

[Winemaker]

Ah, thanks. I hadn't looked at the wing as a torque generator on the center structure, but it obviously is, and moving the load outboard would increase the torque because of the changed moment arm and, consequently, increase the load carried through the pickle fork(s). I hope I've got this right now!

[Grebe]

Originally Posted by KelvinD

Are these "pickle forks" or fishplates related to the items that were reportedly poorly manufactured years ago?
https://www.pprune.org/archive/index.php/t-239918.html
There is an Al Jazeera you tube video, produced in 2010 on this same topic. As far as I could gather, the US Dept of Justice stepped in and halted law suits etc relating to the issues raised by a couple of whistle blowers. In the video, reference is made to 3 737NG accidents which all had one thing in common: the fuselage in each case broke into 3 parts upon impact in pretty much the same location. I was a bit baffled by the terminology used in the documentary, they refer to "failsafe cords" (or chords?)
https://www.youtube.com/watch?v=IaWdEtANi-0
At around 11mins in to the film, you will see evidence from Boeing that one of the most common defects with the parts as supplied from the manufacturer was "hole mislocated".
(unrelated to the safety issue, I was baffled by the process here: The parts are manufactured in California, shipped by train to Wichita where they get joined up with other parts to become fuselage sections. Which are then shipped by train to Seattle, presumably going via California??)

NO there is no relationship between those parts and the picklefork issue- and trains do NOT go thru calif to get to seattle ( normally )

[Grebe]

Originally Posted by Winemaker

Ah, thanks. I hadn't looked at the wing as a torque generator on the center structure, but it obviously is, and moving the load outboard would increase the torque because of the changed moment arm and, consequently, increase the load carried through the pickle fork(s). I hope I've got this right now!

YEP ! of course in the real world, there are a multitude of other factors to be considered making the actual values at different flight or certification regimes difficult to know and maybe even measure. But the basics are still the same.

[david340r]

If I understand correctly moving centres of lift outwards also moves them backwards requiring h stab to, on average, trim slightly more nose up than without, thereby slightly increasing any tensile loads that may be imposed on the rear forks?.

[Grebe]

Originally Posted by david340r

If I understand correctly moving centres of lift outwards also moves them backwards requiring h stab to, on average, trim slightly more nose up than without, thereby slightly increasing any tensile loads that may be imposed on the rear forks?.

That is way out of my wheelhouse- need both an aero and structural engine- ear and a senior level birdman..

[Aihkio]

Moving lift towards the tips increases AND wings with normal airfoil sections have AND tendency anyway. To counter AND moment one needs a downward force from horizontal tail. As far as I can see this all adds up to a compressive load on the fork.