737NGs have cracked 'pickle forks' after finding several in the jets.
A comment from joe bloggs on BB's article in The Australian today -
Has anyone seen the 5 November MOM?
... Boeing issued a second Multi Operator Message on the 5th Nov that has extended the inspection area on the rear spar pickle fork & fail safe strap. So its now 8 fasteners to be visually inspected instead of the one common to the S-18 stringer at Body Station 663.75 left & right hand sides. For aeros that have reached 30,000 flight cycles the three airlines have 60 days to carry out this inspection and the lower threshold has been dropped to around 21,600FC to be inspected within 1,000 FC. The repeat inspection remains at 3,500 FC. Will see if any more cracks will be found apart from the 3 Qantas planes. Grounded to date. The FAA are yet to mandate the MOM as a new airworthiness directive or will they ammend the one issued last month - its watch this space...
Interesting situation for the planes that haven't been inspected yet. All planes that have the cracks are grounded until they are repaired. Most planes have not been inspected yet. The inspection threshold keeps dropping. The repair process is going to take a long time, not because it is difficult but for logistics reasons. Ten percent of planes inspected at higher cycles times are found to have the cracks (what percentage for lower cycle planes?). In theory there are quit a few planes still flying that should be grounded, although in practice they are safe. This at time when new 737 deliveries are frozen.
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Thank you for the follow up.
So it looks we're seeing the cracking problem manifest itself in some aircraft built before 2004 but right across the whole NG range (-600, -700, -800 and -900), and both with winglets and without.
This may simply be a coincidence but Boeing moved the Commercial Airplanes - Fabrication Division's complex machining from their Auburn, Washington plant to their Portland, Oregon facility between April 2003 and January 2005. Among the parts packages that were transitioned from Auburn to Portland were pickle forks. The move gets mentioned in a Boeing Frontiers article. It's probably unrelated but you never know.
So it looks we're seeing the cracking problem manifest itself in some aircraft built before 2004 but right across the whole NG range (-600, -700, -800 and -900), and both with winglets and without.
This may simply be a coincidence but Boeing moved the Commercial Airplanes - Fabrication Division's complex machining from their Auburn, Washington plant to their Portland, Oregon facility between April 2003 and January 2005. Among the parts packages that were transitioned from Auburn to Portland were pickle forks. The move gets mentioned in a Boeing Frontiers article. It's probably unrelated but you never know.
Yet, only two of them had cracks.
Not sure how many cycles they have, but they fly a large network of shorter domestic and Scandinavian routes, so I expect them to have a rather high number. 30000 and above?
Why one aircraft but not the one produced at the same time, operating in the same environment/area and with the same amount cycles ?
This is what puzzles me.
SAS are now in a process of changing from Boeing to Airbus, but a number of their 737’s are from the same time period as the two with cracks.
Yet, only two of them had cracks.
Not sure how many cycles they have, but they fly a large network of shorter domestic and Scandinavian routes, so I expect them to have a rather high number. 30000 and above?
Why one aircraft but not the one produced at the same time, operating in the same environment/area and with the same amount cycles ?
This is what puzzles me.
Yet, only two of them had cracks.
Not sure how many cycles they have, but they fly a large network of shorter domestic and Scandinavian routes, so I expect them to have a rather high number. 30000 and above?
Why one aircraft but not the one produced at the same time, operating in the same environment/area and with the same amount cycles ?
This is what puzzles me.
Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.
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Yes, it's decidedly difficult to see a discernible pattern. LN-RPK and SE-RET were built just over two years apart and both have cracks. You've then got LN-RNO and -RCN built just months either side of LN-RPK; no cracks.
Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.
Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.
Makes it a lot harder to set preventive inspection requirements.
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Yes, it's decidedly difficult to see a discernible pattern. LN-RPK and SE-RET were built just over two years apart and both have cracks. You've then got LN-RNO and -RCN built just months either side of LN-RPK; no cracks.
Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.
Similar anomalies bob up in other fleets. Qantas's oldest B738, VH-VXA, cracks; -VXB, which came off the production line within a day and that actually has about 200 more cycles than -VXA, no cracks.
Inconsistent production practices, built on Friday or with a bigger hammer for alignment.
Number and magnitude of hard landings or other outside the norm conditions, reported or not.
Tolerance stackup issue where some combinations of within specification parts result in higher stresses.
Design should account for this but obviously something was missed somewhere.
Normal distribution of failures where some items will simply fail earlier than others, in some fields testing until a certain percentage of a batch of parts fail is used to quantify expected lifetime using statistical methods.
I suspect that Boeing is hard at work trying to determine the common factor(s), of course none of the above are mutually exclusive.
A few possibilities:
Inconsistent production practices, built on Friday or with a bigger hammer for alignment.
Number and magnitude of hard landings or other outside the norm conditions, reported or not.
Tolerance stackup issue where some combinations of within specification parts result in higher stresses.
Design should account for this but obviously something was missed somewhere.
Normal distribution of failures where some items will simply fail earlier than others, in some fields testing until a certain percentage of a batch of parts fail is used to quantify expected lifetime using statistical methods.
I suspect that Boeing is hard at work trying to determine the common factor(s), of course none of the above are mutually exclusive.
Inconsistent production practices, built on Friday or with a bigger hammer for alignment.
Number and magnitude of hard landings or other outside the norm conditions, reported or not.
Tolerance stackup issue where some combinations of within specification parts result in higher stresses.
Design should account for this but obviously something was missed somewhere.
Normal distribution of failures where some items will simply fail earlier than others, in some fields testing until a certain percentage of a batch of parts fail is used to quantify expected lifetime using statistical methods.
I suspect that Boeing is hard at work trying to determine the common factor(s), of course none of the above are mutually exclusive.
South Korea grounds 13 737NGs with 'pickle fork' cracksOn Flight Global:_
South Korea is the latest country to ground a number of Boeing 737NG found to have structural cracks, following worldwide inspections of the popular narrowbody.
The Ministry of Land, Infrastructure and Transport (MOLIT) inspected 100 737NGs and found that 13 contain cracks. Nine of these were identified in a first round of inspection in October and had clocked more than 30,000 flight cycles.
Four more were discovered with cracks, after a second round of inspections was completed on 10 November. These aircraft recorded between 20,000 to 30,000 flight cycles, says MOLIT.
Upon discovering the cracks, MOLIT notified Boeing, which then sent a team to South Korea on 31 October to commence repair work. The affected aircraft will all be repaired by January 2020, and each aircraft takes about two weeks, the ministry adds.
MOLIT did not specify which airlines operated the affected 737NG aircraft. Cirium fleets data indicates that South Korean carriers operate 153 737NGs, the bulk of them 737-800s.
At the heart of the inspections is a piece of hardware known as a "pickle fork", which connects the wing to the aircraft fuselage. Cracking of the hardware could result in structural failure, which affects the structure integrity of the aircraft and results in loss of control.
Regulators had urged airlines operating 737NGs with more than 30,000 flight cycles to inspect their aircraft immediately for cracks. Aircraft that have logged 22,600-30,000 cycles should be inspected within the next 1,000 cycles.
South Korea’s MOLIT has also put in place further measures in response to the latest spate of cracking incidents.
For instance, carriers taking delivery of new 737NGs must check for cracks before registering the aircraft in South Korea.
737NGs that have passed inspections this time round will also be “thoroughly managed” by MOLIT’s aviation safety inspector and will be re-inspected within the next 3,500 flight cycles.
South Korea joins several other countries in the Asia-Pacific region to ground 737NGs over pickle fork cracks.
Australia’s Qantas has grounded three 737-800s found to have cracks, while Indonesia has also grounded three, two of them operated by Sriwijaya Air and the third by Garuda Indonesia.
Media reports also state that Lion Air found structural cracks on two 737NGs with less than 22,000 flight cycles. The Indonesian low-cost carrier has yet to respond to FlightGlobal’s request for comment.
The Ministry of Land, Infrastructure and Transport (MOLIT) inspected 100 737NGs and found that 13 contain cracks. Nine of these were identified in a first round of inspection in October and had clocked more than 30,000 flight cycles.
Four more were discovered with cracks, after a second round of inspections was completed on 10 November. These aircraft recorded between 20,000 to 30,000 flight cycles, says MOLIT.
Upon discovering the cracks, MOLIT notified Boeing, which then sent a team to South Korea on 31 October to commence repair work. The affected aircraft will all be repaired by January 2020, and each aircraft takes about two weeks, the ministry adds.
MOLIT did not specify which airlines operated the affected 737NG aircraft. Cirium fleets data indicates that South Korean carriers operate 153 737NGs, the bulk of them 737-800s.
At the heart of the inspections is a piece of hardware known as a "pickle fork", which connects the wing to the aircraft fuselage. Cracking of the hardware could result in structural failure, which affects the structure integrity of the aircraft and results in loss of control.
Regulators had urged airlines operating 737NGs with more than 30,000 flight cycles to inspect their aircraft immediately for cracks. Aircraft that have logged 22,600-30,000 cycles should be inspected within the next 1,000 cycles.
South Korea’s MOLIT has also put in place further measures in response to the latest spate of cracking incidents.
For instance, carriers taking delivery of new 737NGs must check for cracks before registering the aircraft in South Korea.
737NGs that have passed inspections this time round will also be “thoroughly managed” by MOLIT’s aviation safety inspector and will be re-inspected within the next 3,500 flight cycles.
South Korea joins several other countries in the Asia-Pacific region to ground 737NGs over pickle fork cracks.
Australia’s Qantas has grounded three 737-800s found to have cracks, while Indonesia has also grounded three, two of them operated by Sriwijaya Air and the third by Garuda Indonesia.
Media reports also state that Lion Air found structural cracks on two 737NGs with less than 22,000 flight cycles. The Indonesian low-cost carrier has yet to respond to FlightGlobal’s request for comment.
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Here's a link to the Boeing MOM in the FAA public docket. It shows pictures of the new inspection area.
https://www.regulations.gov/document...2019-0866-0002
https://www.regulations.gov/document...2019-0866-0002
A couple of intriguing details to note about compliance times in Table 1 of new MOM (5 Nov).
1. If aircraft is BBJ with "Lower Cabin Altitude" STC ie higher pressure differential, then threshold for initial inspection is halved and repeat intervals are cut by 25%. That's drastic and suggests pressure loads were not well modelled in test or analysis.
2. If I'm reading MOM correctly, initial inspection interval after fork replacement is only 3500 flight cycles. Inspections don't start again from scratch.
1. If aircraft is BBJ with "Lower Cabin Altitude" STC ie higher pressure differential, then threshold for initial inspection is halved and repeat intervals are cut by 25%. That's drastic and suggests pressure loads were not well modelled in test or analysis.
2. If I'm reading MOM correctly, initial inspection interval after fork replacement is only 3500 flight cycles. Inspections don't start again from scratch.
A couple of intriguing details to note about compliance times in Table 1 of new MOM (5 Nov).
1. If aircraft is BBJ with "Lower Cabin Altitude" STC ie higher pressure differential, then threshold for initial inspection is halved and repeat intervals are cut by 25%. That's drastic and suggests pressure loads were not well modelled in test or analysis.
2. If I'm reading MOM correctly, initial inspection interval after fork replacement is only 3500 flight cycles. Inspections don't start again from scratch.
1. If aircraft is BBJ with "Lower Cabin Altitude" STC ie higher pressure differential, then threshold for initial inspection is halved and repeat intervals are cut by 25%. That's drastic and suggests pressure loads were not well modelled in test or analysis.
2. If I'm reading MOM correctly, initial inspection interval after fork replacement is only 3500 flight cycles. Inspections don't start again from scratch.
Seems the problem is not the pickle fork itself, but the by product of another "error/assumption/calculation" that causes the pickle to crack - and that is a much bigger fix than a fork change/s over the life of the airframe.
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Actually, it is worse than that: repeat intervals are cut *to* 25%. From page 20:
(a) For airplanes which have incorporated Boeing Business Jet (BBJ) Lower Cabin Altitude
...
All repeat interval compliance
times specified in flight cycles must be reduced to one‐quarter of those specified in Table 1.
...
All repeat interval compliance
times specified in flight cycles must be reduced to one‐quarter of those specified in Table 1.
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I'll still bet that the problem is one of ' casual ' final assembly issues at Renton. The basic picklefork attach is at Spirit in Wichita, and the final assembly at Renton is when wings are attached and the fail safe strap/picklefork is attached with the ' last' half dozen fasteners on each side. IMHO the ' fix' to be incorporated for the fleet will be to remove and replace the" last half dozen" with cold expansion ( aka coldworking ) re reaming, and installation of light interference fit fasteners.
And on those still on line at Renton, I'll bet such work will be carefully checked and or sequence modified starting on line # xyz123
I doubt there will be any significant changes in the picklefork itself
Presumably, the aircraft that have been grounded with cracked pickleforks are having their pickleforks replaced.
Once a number of cracked pickleforks have been removed, metallurgical examination of the damage and cracking should identify the failure mode(s) and thus ultimately allow a long term fix.
Is anyone privy to any of the investigations or examinations of the actual damaged pickleforks to date and if the failure mode(s) have yet been identified?
Once a number of cracked pickleforks have been removed, metallurgical examination of the damage and cracking should identify the failure mode(s) and thus ultimately allow a long term fix.
Is anyone privy to any of the investigations or examinations of the actual damaged pickleforks to date and if the failure mode(s) have yet been identified?
https://simpleflying.com/qantas-737-crack-repair/
3000 hours??? ....can build a plane in that time /s
3000 hours??? ....can build a plane in that time /s
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Thanks for the link! The mythical man-hour: The length of the job may depend on the size of the team, and how many technicians can squeeze into a confined wheel bay, or remove skin rivets on the outside of the fuselage. Will be interesting to follow the learning curve.
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I have a doubt that the pickle forks are stressed more on landing than in flight..If so the following needs to be answered:
1. How many cycles/landings are allowed before it cracks
2.What is the tolerance zone
3.Did the cracks occur earlier than it should?
4.Why has not FAA asked for a design review?
1. How many cycles/landings are allowed before it cracks
2.What is the tolerance zone
3.Did the cracks occur earlier than it should?
4.Why has not FAA asked for a design review?
We've seen this kind of structural problem before in the pylon pins in B747 freighters
When you start designing load redundancies in multiple load paths you may miss a handoff of what path is now carrying the load and by how much vs flight operations. The assumed response is to cover this by high margins of safety over normal conditions.
Sometimes (rarely) some hidden condition pops up and creates abnormal stress conditions which now over many flights could result in early fatigue. Again even with fatigue cracking the concept still is failsafe as long as nothing else starts to fail.. The airworthiness action in this case addresses increased inspection early enough (inspections etal.) to maintain sufficient margins of flight safety.. Yes it is a pain-in-the a$$ for the operators but it doesn't always require an FAA design review or complete replacement of the design.
When you start designing load redundancies in multiple load paths you may miss a handoff of what path is now carrying the load and by how much vs flight operations. The assumed response is to cover this by high margins of safety over normal conditions.
Sometimes (rarely) some hidden condition pops up and creates abnormal stress conditions which now over many flights could result in early fatigue. Again even with fatigue cracking the concept still is failsafe as long as nothing else starts to fail.. The airworthiness action in this case addresses increased inspection early enough (inspections etal.) to maintain sufficient margins of flight safety.. Yes it is a pain-in-the a$$ for the operators but it doesn't always require an FAA design review or complete replacement of the design.