|
So if winglets are the cause, how do you guys explain the cracked -700 without winglets?
|
Originally Posted by ManaAdaSystem
(Post 10594384)
So if winglets are the cause, how do you guys explain the cracked -700 without winglets?
|
Originally Posted by ManaAdaSystem
(Post 10594384)
So if winglets are the cause, how do you guys explain the cracked -700 without winglets?
|
Originally Posted by B727223Fan
(Post 10594429)
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.
|
737s cannot jettison fuel. |
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.
|
Originally Posted by lc3m4n23
(Post 10595329)
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.
|
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
(Post 10581584)
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 |
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.
|
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.
|
Originally Posted by nonsense
(Post 10595796)
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.
Maybe the reason why those cracks are taken so seriously : the joints have slipped. |
A slip can usually be seen. Usually the problem is incorrect tightening moment or procedure depending on the fastener type.
|
I am the source lol
Originally Posted by etudiant
(Post 10595752)
Wow, if that is true, there is a serious manufacturing deficiency. Is there any source reference that you can share?
|
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. 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. |
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. |
Originally Posted by nonsense
(Post 10595796)
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.
|
Originally Posted by IFMU
(Post 10595905)
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.
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 .. |
Originally Posted by Grebe
(Post 10595768)
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. |
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. |
Originally Posted by Grebe
(Post 10595952)
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.
Originally Posted by lc3m4n23
(Post 10595329)
The actual cause of the pickle fork cracks is because of the the holes being over drilled by around 6 thousands.
|
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 |
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. 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. |
Originally Posted by ST Dog
(Post 10595995)
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. 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. |
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? |
Originally Posted by Grebe
(Post 10596185)
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.
I've been involved with similar stack ups and they were always clamped up then final work was done on the assembly. |
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. |
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. |
Originally Posted by Webby737
(Post 10598456)
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. 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 . . |
9 out of 42 at Korean Air and others airlines in South Korea 737NG's grounded!
https://simpleflying.com/south-korea...9-boeing-737s/ |
Originally Posted by Grebe
(Post 10598537)
MDC has been more than a " cancer" its been a plague. And apparently the worst is yet to come . .
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... |
Originally Posted by jmmoric
(Post 10603089)
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... 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. |
Originally Posted by etudiant
(Post 10603197)
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. |
Any more news on picklefork issue from workers or BA AOG crew ?
|
Anybody got the total number at this point? With the 9 Korean NGs and the Ryanair, we gotta be pushing 50 |
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 ?
|
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? |
Just curious how many airframes have ‘failed’ so far ? Thanks David |
Originally Posted by FlexibleResponse
(Post 10605915)
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).
|
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/comp...30-p535xo.html |
Guardian says of 810 jets inspected so far 5% have failed
|
| All times are GMT. The time now is 12:36. |
Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.