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New Fastener Problem Plagues Boeing 737s
Workers failed to install nut plates with anti-corrosion coatings
Doug Smock, Contributing Editor -- Design News, November 25, 2008
Fastener problems continue to affect Boeing.
Deliveries of 737s are now being delayed as Boeing replaces nut plates that had been installed on the aircraft since August 2007. Specifications require the nut plates to have an anti-corrosive cadmium coating. For reasons that aren't clear, coated and uncoated nut plates were intermingled in bins, making it difficult for assemblers to pull the right parts. The problem was discovered last August at a Spirit AeroSystems factory in Wichita, KS. The assembly plant provides fuselage and wing components for Boeing aircraft.
The nut plates are used to fasten bundles of wires and other parts to the inside of fuselages. There are thousands in every aircraft. According to one source, fewer than 30 percent of the nut plates installed since August, 2007 are defective.
"We're replacing them as we find them," Boeing spokeswoman Vicki Ray told the Associated Press. "Also to be addressed is the in-service fleet, and we're still working on a plan for that." She says the defective fasteners are not an immediate safety risk. Boeing plans to inspect 394 of the 737s already in service.
Cadmium coatings are applied to ferrous and nonferrous metals to provide resistance to corrosion. Like zinc, cadmium also provides sacrificial protection to a substrate such as steel by being preferentially corroded when the coating is damaged and small areas of the substrate are exposed. Electroplating accounts for more than 90 percent of all cadmium used in coatings, and is normally specified in thicknesses between 5 and 25 mm.
The Boeing Materials Group did pioneering work on the use of cadmium coatings in aircraft all the way back to the 1960s. Significant work has been conducted on replacing the toxic cadmium in recent years, but the recent problem at Boeing was strictly a supply chain error.
As previously reported in Design News, fastener shortages were a major problem in the early production ramp-up of the Dreamliner 787 program. At first, there was just a shortage of fasteners. Later, it was reported by Boeing that there also were installation problems. Some of the fasteners in the Dreamliners were not flush, and there was a gap between the structure and the head of the fastener, according to Boeing spokeswoman Mary Hansen. Another problem: The fastener's pin was at times the incorrect length.
"We're going to remove and replace every one and work with our partners to get this fixed as quickly and effectively as possible," Hansen said.
The Federal Aviation Administration issued a new directive in August about the need for more inspections of a fastener problem affecting cargo doors on 747s.
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The problems to Boeing 787 should be looked at the general view of design of the airplane.
It should be good if anyone looked to the histoty of the old Doulglas DC-10. These histories have some point in common.
Marcio V. Oliveira - 2009-29-1 06:25:31 EST -
Isn't Cadmium one of the heavy metals that were replaced by zinc? The thickness has to be wrong, 5 to 25 mm seems way too much.
Thomas M Anderson - 2009-5-1 16:36:07 EST -
Sounds to me like the design engineers need to get out into the shop more! Or at least talk to the shop management. They will be amazed at what happens to their designs, when reality strikes!
Ben Bullen - 2008-30-12 08:45:30 EST -
Boeing has been in the business long enough to not have any of these fastener problems. Shortages yes, but not head placement problems or fastener length problems. Head placement is controlled by installation tooling, fastener length discrepancies come about because call out does not take into account material thickness tolerances, and protective finish thicknesses. The CAD drivers invariably stack parts to nominal sizes and neglect to include finish coatings and as a consequence the fastener ends up being shorter than required. The shop is aware of this and uses a grip length gauge to determine the real size of fastener required and draws that from the store for installation. Over time there is a shortage of fasteners of a given size (the real life situation) and a surplus of the drawing callout size.
John Mitchell - 2008-2-12 12:12:51 EST -
"Electroplating accounts for more than 90 percent of all cadmium used in coatings, and is normally specified in thicknesses between 5 and 25 mm."
Surely that is nopt the correct unit of measure: what SHOULD it have said?
Ronald G. Darner - 2008-2-12 12:03:42 EST
