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.
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.
Two of the 26 United planes found to have loose bolts are mentioned in a whistle-blower lawsuit first reported by the Monitor in October. In that case, a former Boeing production-line employee, who worked for the company from 1986 to 1993, alleges that the manufacturing giant cut corners on quality to beat production records. The worker, Timothy Kerr, documented dozens of instances in which he alleges holes were misdrilled, bolts were improperly torqued, and inspectors routinely approved such work without checking it.
During the recent inspection of 767 wings, a United plane cited in the Kerr case was found to have two loose bolts on the right wing. In his sealed affidavit, Mr. Kerr writes that while that plane was being produced, an inspector approved 100 holes drilled for bolts in the tail section "without checking any of them."
"The inspector just stamped the paperwork off as OK," the affidavit states. "So they don't know if any of those holes are good..... This is typical.... it's not just one or two rare instances where Boeing let garbage go, it's typical."
United refused to comment on that specific allegation, but a spokeswoman insisted that all of the company's airplanes are airworthy. "If we do see a problem, particularly if we see something coming off the line, we'd go back and work with the manufacturer on it," says Mary Jo Holland, United spokeswoman.
Boeing is prohibited from commenting on the allegations because the case remains under seal, but it stands by its products. "We have a lot of faith in our manufacturing processes, including inspection, and have no doubts about the quality of the airplanes that leave the factory," says Boeing's Russell Young.
Aviation experts say problems with nuts and bolts can originate along the production line or during maintenance work. In the case of the Continental Express Embraer 120 that crashed in 1991, workers from one shift failed to tell the oncoming shift the fasteners had been removed and needed to be replaced.
"They do a tremendous job of being sure that no single mechanical failure can bring down an airplane, but when it comes to the human error, especially in maintenance, they haven't made very good strides," says Wayne Glover, editor of GroundEffects, an aviation maintenance newsletter based in Redmond, Wash.
A recent FAA study found that 1 of every 250 times a plane leaves the gate, there is some maintenance error. But the vast majority have no impact on safety.
In the SilkAir 737 crash, investigators are examining the plane's maintenance records. But because the plane was only 11 months old, they're also focusing on Boeing's production line and other 737s built since 1995.
Seventeen of the 194 planes inspected after the December crash were found to have cracked putty around bolts, or loose or sheared bolts. A new 737 owned by Continental was also found to have four missing fasteners along the horizontal stabilizer on the tail.
The FAA says none of these cases imperiled flight safety. But it's still investigating how some of the fasteners came to be missing.
"That's the thing that's very, very baffling," says Ronald Wojnar, manager of the FAA Transport Airplane Directorate. "How can planes leave the factory with hardware missing, notwithstanding the normal quality checks and operations that occur?"
Last month, the FAA spent more than a week at the Boeing plant in Renton on an emergency inspection. According to a confidential Boeing document leaked to the Seattle Post-Intelligencer, FAA inspectors found machinists installing fasteners of the wrong length, using improperly certified or calibrated tools, and failing to follow assembly instructions.
Both Boeing and the FAA refused to comment on the details.
"If they found anything that was a safety issue, we would have taken some action on it," says FAA spokesman Tom Dorr.