Go Back  PPRuNe Forums > Flight Deck Forums > Rumours & News
Reload this Page >

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

Rumours & News Reporting Points that may affect our jobs or lives as professional pilots. Also, items that may be of interest to professional pilots.

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

Old 14th Oct 2019, 21:22
  #241 (permalink)  
 
Join Date: Aug 2000
Posts: 1,331
So if winglets are the cause, how do you guys explain the cracked -700 without winglets?
ManaAdaSystem is offline  
Old 14th Oct 2019, 21:31
  #242 (permalink)  
 
Join Date: Mar 2019
Location: French Alps
Posts: 271
Originally Posted by ManaAdaSystem View Post
So if winglets are the cause, how do you guys explain the cracked -700 without winglets?
We don't know much at the moment, but they might be a contributing factor.
Fly Aiprt is offline  
Old 14th Oct 2019, 22:21
  #243 (permalink)  
 
Join Date: Sep 2019
Location: USA
Posts: 3
Originally Posted by ManaAdaSystem View Post
So if winglets are the cause, how do you guys explain the cracked -700 without winglets?
Most of the discussion on this thread about winglets is the additional stresses that they impose on a wing. I don't think anyone has stated here that winglets are the sole possible reason for the cracking of the pickle fork There are other possible reasons why the pickle fork could crack including High CYCLES vs Hours.Folks on this thread have brought up many possibilities for the cracking of the pickle fork including improper manufacturing or assembly processes.
B727223Fan is offline  
Old 14th Oct 2019, 22:37
  #244 (permalink)  
 
Join Date: Nov 2006
Location: Auckland, New Zealand
Posts: 423
Originally Posted by B727223Fan View Post
There are other possible reasons why the pickle fork could crack including High CYCLES vs Hours.Folks on this thread have brought up many possibilities for the cracking of the pickle fork including improper manufacturing or assembly processes.
Any chance that repeated overweight or hard landings could have been the cause
Chris2303 is online now  
Old 15th Oct 2019, 03:13
  #245 (permalink)  
 
Join Date: Mar 2019
Location: On the Ground
Posts: 109
737s cannot jettison fuel.
Takwis is offline  
Old 16th Oct 2019, 00:41
  #246 (permalink)  
 
Join Date: Oct 2019
Location: usa
Posts: 5
737 ng picklefork cracks actual cause

The actual cause of the pickle fork cracks is because of the the holes being over drilled by around 6 thousands. The holes are grossly oversize and the bolts that run through the part and the fail safe strap are not supporting the part. The bolts are supposed to be snug fit so when a plane lands the bolts and part share equal stress and not have a stress issue. The bolts are not touching the sides of the holes hardly at all therefore any gap between the bolt and the part contributes to all stress being put on the fork part and fail safe strap, which has holes where the bolts are.
lc3m4n23 is offline  
Old 16th Oct 2019, 15:17
  #247 (permalink)  
 
Join Date: May 2011
Location: NEW YORK
Posts: 561
Originally Posted by lc3m4n23 View Post
The actual cause of the pickle fork cracks is because of the the holes being over drilled by around 6 thousands. The holes are grossly oversize and the bolts that run through the part and the fail safe strap are not supporting the part. The bolts are supposed to be snug fit so when a plane lands the bolts and part share equal stress and not have a stress issue. The bolts are not touching the sides of the holes hardly at all therefore any gap between the bolt and the part contributes to all stress being put on the fork part and fail safe strap, which has holes where the bolts are.
Wow, if that is true, there is a serious manufacturing deficiency. Is there any source reference that you can share?
etudiant is offline  
Old 16th Oct 2019, 15:34
  #248 (permalink)  
 
Join Date: Sep 2019
Location: leftcoast
Posts: 34
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.


Originally Posted by Grebe View Post
It will be interesting to see or find out just where the cracks are. Obviously at this time, its just a guess. However, as a GUESS and based on experience dealing with fastener issues in large parts on 707 and 767 ( 707 that had been in service for a long time ) and tooling for 767-here is my SWAG based on the relatively low key descriptions.

A) The cracks are probably around or spreading from Fastener holes, probably those drilled " by hand" during the LEAN manufacturing process which are less than about 3/8 in diameter.
B) As such there is of course an argument that the crack would simply progress to the next hole in the pattern ( since drilling a small hole at the ' end ' of such crack is considered to be a ' crack stopper ' - which is true for a lot of ' sheet metal ' issues.

C) again , just a guess, but for 40 plus years, thre has been available and used a three to four step process to prevent such cracks, which can be done for all sizes of hole, even large holes during fabrication while still in large tooling- drill plates , etc.

D) in general- the steps are 1) drill a hole slightly smaller than final size 2) insert a thin sleeve into hole 3) insert a special mandrel such that when pulled back thru the hole it expands the sleeve and hole. 4) Ream the hole which will usually be slightly out of round to final size.

On assembly, insert bolt as a tight fit.
E) in some cases and sizes, the same process can be used without a sleeve but with a expanding mandrel.

F) This leaves a major prestress around the hole and provides a significant improvement in fatigue life.

The process was patented by Boeing in the late 1960, and a local firm called Fatigue technology was founded- developed from the previous firm called Industrial Wire and metal forming as I recall. And major first use ( from memory ) was on AWACS.
The process- tooling has been the subject of several related patents, and is still used by virtually all aircraft manufacturers
It can be used to prevent or stop cracks from further progress.

Again MY SWAG is that to save time some $$$ - or due to a temporary lack of sleeves or just plain skipping the sequence ( hard to detect when inspection is only on final hole size )

So depending on location and accessibility, the fix would be to remove bolt, expand hole, ream hole, insert new oversize bolt and voila, a terminating fix.

Just have to wait and see- If someone has access to documentation as to real issue and location, would be interesting to see how close I came
Grebe is offline  
Old 16th Oct 2019, 16:12
  #249 (permalink)  
 
Join Date: Apr 2008
Location: on the ground
Posts: 236
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.
nonsense is offline  
Old 16th Oct 2019, 16:23
  #250 (permalink)  
 
Join Date: Mar 2019
Location: Vantaa, Finland
Posts: 0
There is a chance of calculating a bolt joint where - as Nonsense said- bolts do not carry shear. With shear carrying bolts and variable hole diameters and varying location tolerances not a chance. In spite of the calculation capacity existing experimental data is still used, that database is huge.
Aihkio is offline  
Old 16th Oct 2019, 16:30
  #251 (permalink)  
 
Join Date: Mar 2019
Location: French Alps
Posts: 271
Originally Posted by nonsense View Post
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.
Fly Aiprt is offline  
Old 16th Oct 2019, 16:35
  #252 (permalink)  
 
Join Date: Mar 2019
Location: Vantaa, Finland
Posts: 0
A slip can usually be seen. Usually the problem is incorrect tightening moment or procedure depending on the fastener type.
Aihkio is offline  
Old 16th Oct 2019, 17:04
  #253 (permalink)  
 
Join Date: Oct 2019
Location: usa
Posts: 5
I am the source lol

Originally Posted by etudiant View Post
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.
lc3m4n23 is offline  
Old 16th Oct 2019, 17:29
  #254 (permalink)  
Moderator
 
Join Date: Aug 2006
Location: Ontario, Canada
Age: 58
Posts: 4,269
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.
Pilot DAR is offline  
Old 16th Oct 2019, 18:23
  #255 (permalink)  
 
Join Date: Mar 2019
Location: Vantaa, Finland
Posts: 0
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.
Aihkio is offline  
Old 16th Oct 2019, 18:25
  #256 (permalink)  
 
Join Date: Mar 2005
Location: Poplar Grove, IL, USA
Age: 53
Posts: 839
Originally Posted by nonsense View Post
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.
IFMU is offline  
Old 16th Oct 2019, 19:24
  #257 (permalink)  
 
Join Date: Sep 2019
Location: leftcoast
Posts: 34
Originally Posted by IFMU View Post
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
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 ..

Last edited by Grebe; 16th Oct 2019 at 19:54. Reason: added a bit of background
Grebe is offline  
Old 16th Oct 2019, 19:55
  #258 (permalink)  
 
Join Date: Oct 2019
Location: usa
Posts: 5
Originally Posted by Grebe View Post
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.
lc3m4n23 is offline  
Old 16th Oct 2019, 20:00
  #259 (permalink)  
 
Join Date: Mar 2019
Location: Vantaa, Finland
Posts: 0
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.
Aihkio is offline  
Old 16th Oct 2019, 20:05
  #260 (permalink)  
 
Join Date: Aug 2019
Location: Rocket City
Posts: 4
Originally Posted by Grebe View Post
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.

Originally Posted by lc3m4n23 View Post
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.

ST Dog is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.