Boeing's latest delay for the 787 Dreamliner was blamed on a 57-day strike by machinists, but the program's schedule already faces additional pressure by new disclosures about improperly installed fasteners.According to sources across the program, the number of fasteners needing replacement range from about 2,500 to 5,000 per aircraft or shipset. Boeing publicly estimates that less than 3% of fasteners installed to date will have to be removed and reinstalled.
Boeing underscores that, "the issue is with installation of the fasteners, not the fasteners themselves."
Significant engineering and machinist resources across the program are being devoted to solving this problem inside Boeing and structural partner facilities as quickly as possible.
The challenge to the programme schedule centres on getting fasteners removed and reinstalled, as well as the potential damage to the composite material that could occur.
"The risk involved is that some of the fastener holes will need to be oversized. This is a common practice on in-production repairs," said one veteran engineer.
"Fastened structure is designed to allow for future reworkability, primarily for in-service repairs."
Boeing faced time consuming repairs on Dreamliner One following the July 2007 rollout when temporary fasteners caused damage after being removed to make way for permanent ones.
Boeing is re-training all 787 machinists in its Everett facility on new fastener installation procedures. Compounding the problem, sources say, is the slow pace of workers returning following the conclusion of the IAM strike. Machinists have until November 10 to return to work, according to the strike resolution. Only machinists who have completed the re-training are permitted to work on the aircraft again.
Those familiar with the fastener situation tell FlightBlogger that the problem originated in two separate types of fastener installation on the four flight test and two ground test aircraft, as well as the more than a dozen shipsets currently at supplier partners.
The first problem stems from the holes drilled to affix titanium and carbon fibre together. When holes are drilled into titanium, a burr is often left on the edge of the entry side of the hole. Because of the extraordinary strength of titanium, when a fastener is installed in the hole, the head will sit on the burr rather than flush against the surface.
With the head of the fastener resting on the burr, the loads will be distributed on that one spot rather than evenly across the surface. In addition, in the event of high side-to-side shear loads, in a worst-case scenario, the high-strength titanium burr could cut the fastener undermining structural integrity.
Titanium is used in key structural areas of the aircraft such as the joined sections in the fuselage and horizontal stabilizer.
Sources say the fastener problem was first discovered on the engine pylons on the static test airframe. The pylons have been removed from all aircraft in Everett and returned to Spirit AeroSystems in Wichita, Kansas for repair.
All major structure partners, with the exception of the wings supplied by Mitsubishi, are impacted by this problem, including Vought, Global Aeronautica and Alenia.
Fastener Anatomy
The re-training of Boeing staff covers preparation of the holes after they are drilled prior to fastener installation. Typically, after a hole is drilled the edges of the hole are slightly sloped to remove any possible debris or irregularities and creating a symmetrical sloping surface at edge of the hole.
There are generally four methods of preparation, all essentially identical with increasing size. In size order from smallest to largest a hole can have a deburr, fillet relief, chamfer or countersink to accommodate a fastener.
According to several sources in Everett, WA and Charleston, SC, in many cases, holes were prepared using one of these four methods, but the specification for installation was often unclear and size of the required slope was insufficient.
The second fastener problem finds its roots in the fastener shortage that challenged Boeing early in the program. Fasteners of varying lengths were installed improperly creating the possibility of structural instability.
The fastener tolerances are known as 'protrusion maximum' and 'base minimum.' Protrusion maximum addresses the amount of allowable thread on the bolt to emerge out of the nut to avoid any threading, or bottoming out, of the shank or non-threaded area of a fastener inside the hole.
In some situations where the fastener is too large, too much force can damage the fastener during installation by threading the shank rather than sitting flush against the surface undermining the entire fastener system.
Using longer fasteners is perfectly acceptable if washers are used.
The drawback for using larger fasteners, another veteran engineer says, is inevitable weight gain that comes with using larger parts than originally intended.
The base minimum keeps from having any threads in the hole of the fastened material.
If the shank of a fastener is too short then there is a chance the threaded part of the bolt is inside the hole and not enough thread is running through the nut. In this case, the result would be a compression of the two surfaces rather than using the whole fastener for stability.
Ideally a nut will sit snugly at the end of the bolt's thread allowing the entire shank to be entirely inside the fastened material creating a structurally sound connection.
The remedy: remove the incorrectly sized fastener and replace it with the proper fastener.
Though the timeline to solve the problem is unclear, Boeing will likely be hard pressed to release guidance for first flight and delivery until this situation is well in hand and production staff are back at work.
The re-training of Boeing staff covers preparation of the holes after they are drilled prior to fastener installation. Typically, after a hole is drilled the edges of the hole are slightly sloped to remove any possible debris or irregularities and creating a symmetrical sloping surface at edge of the hole.
There are generally four methods of preparation, all essentially identical with increasing size. In size order from smallest to largest a hole can have a deburr, fillet relief, chamfer or countersink to accommodate a fastener.
According to several sources in Everett, WA and Charleston, SC, in many cases, holes were prepared using one of these four methods, but the specification for installation was often unclear and size of the required slope was insufficient.The second fastener problem finds its roots in the fastener shortage that challenged Boeing early in the program. Fasteners of varying lengths were installed improperly creating the possibility of structural instability.
The fastener tolerances are known as 'protrusion maximum' and 'base minimum.' Protrusion maximum addresses the amount of allowable thread on the bolt to emerge out of the nut to avoid any threading, or bottoming out, of the shank or non-threaded area of a fastener inside the hole.
In some situations where the fastener is too large, too much force can damage the fastener during installation by threading the shank rather than sitting flush against the surface undermining the entire fastener system.
Using longer fasteners is perfectly acceptable if washers are used.
The drawback for using larger fasteners, another veteran engineer says, is inevitable weight gain that comes with using larger parts than originally intended.
The base minimum keeps from having any threads in the hole of the fastened material.
If the shank of a fastener is too short then there is a chance the threaded part of the bolt is inside the hole and not enough thread is running through the nut. In this case, the result would be a compression of the two surfaces rather than using the whole fastener for stability.
Ideally a nut will sit snugly at the end of the bolt's thread allowing the entire shank to be entirely inside the fastened material creating a structurally sound connection.
The remedy: remove the incorrectly sized fastener and replace it with the proper fastener.
Though the timeline to solve the problem is unclear, Boeing will likely be hard pressed to release guidance for first flight and delivery until this situation is well in hand and production staff are back at work.
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