Airline Safety: Attention Fastens on Nuts and Bolts
In-flight accidents lead FAA investigators to inspect Boeing production line.
Prompted by several recent accidents, federal aviation investigators are tightening their focus on the nuts, bolts, and fasteners that hold crucial sections of modern airplanes together. They are also reportedly poised to fine the Boeing Co., the world's largest manufacturer of airplanes, for problems relating to the installation of fasteners along its production line.Skip to next paragraph
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Individually, nuts and bolts are usually not that critical. But there are cases in which a single sheared bolt brought down a plane. And if several are missing, or weakened by age or corrosion, "the results can obviously be catastrophic," says an aviation expert.
The federal focus first shifted to nuts and bolts last May, when a 20-foot section of wing ripped off as Delta pilots were landing a 767 in Texas. The cause: four failed bolts.
Then in December, a 737 owned by SilkAir plunged from the sky in Indonesia, killing all 104 people aboard. Dredges are still dragging the muddy waters of the Musi River in Sumatra for scraps of the almost-new plane, but the discovery that 26 fasteners were missing from the tail section set off alarm bells.
The FAA called for an immediate inspection of all Boeing 737s built since 1995. As a "precautionary measure," the agency also sent inspectors to the Boeing plant in Renton, Wash., and Wichita, Kan., where the plane's tail section was built. The SilkAir 737 had rolled off the production line only 11 months ago - a time when Boeing was ramped up to meet record production levels.
"Missing a row of fasteners is the kind of error ... that can be critical enough to cause an aircraft accident," says David Marx, an aviation-safety consultant in Tacoma, Wash.
Last week, Boeing sent a letter to its customers stating the missing fasteners were not the cause of the SilkAir crash. But that has not allayed concerns about the airplane's condition. Missing fasteners have caused a fatal accident before.
In 1991, the National Transportation Safety Board (NTSB) concluded that 47 missing fasteners along the horizontal stabilizer of a Continental Express Embraer 120 - not a Boeing plane - caused it to break up in flight, then plummet to the ground. All 14 people aboard were killed.
Over the past 20 years, the NTSB found that failure of various nuts, bolts, or fasteners caused more than a dozen accidents in the US alone. Over the same period, the FAA has issued hundreds of calls to airlines to inspect these parts.
While modern airplanes are built with plenty of backup for key mechanical features like engines, hydraulic systems, and electrical systems, that's not true for nuts and bolts.
"The structure of the aircraft is the one place we don't have a lot of redundancy, so if we miss a row of fasteners, it becomes pretty critical," says Mr. Marx.
Most accidents in the US involving nuts and bolts resulted in only a few minor injuries, like last spring's Delta wing-flap incident. After the 20-foot section ripped off, the pilot managed to bring the jet down safely and no one was hurt.
But the incident still raised a red flag. The FAA immediately ordered all owners of 767 model planes that had flown at least 25,000 hours to inspect their wings for similar bolt problems. Twenty-five percent had loose or missing bolts.
"The number is disturbing on the face of it, but without knowing the details, the specification, where those bolts were, it's difficult to judge," says James Mar, professor emeritus of aeronautics and astronautics at the Massachusetts Institute of Technology.
Neither the airlines nor the FAA wanted to disclose the specifics of the wing-flap problem. After the Monitor filed a Freedom of Information Act request, the FAA released records of four airlines - United, US Airways, American, and a cargo carrier. Delta and TWA refused access to their documents.
The records show a wide range of problems in those planes with loose or missing bolts on their wings. On one 767, owned by United, 16 of 24 bolts inspected either failed a torque test or needed to be replaced. Inspection of a US Airways plane found two loose bolts, and "no factory torque seal was in place at the time of inspection." American simply replaced all the suspect bolts on eight of the nine planes it inspected.