BY SEAN BRODERICK
Valuable lessons learned in areas like fuel systems and wiring will mark the TWA 800 investigation as a groundbreaker in aircraft non-structural systems research, leading to revamped maintenance and design practices.
Last month's National Transportation Safety Board (NTSB) meeting to go over the probe's final draft report produced little news. Four recommendations (see related story, p. 24), 25 findings, and a probable-cause statement that's been evident since the early days of the investigation were the meeting's nuts and bolts. Far more revealing was evidence cited by NTSB staff that helped the board make its conclusions.
Not long after the July 17, 1996, break-up of the TWA Boeing 747-100 just off the coast of Long Island, N.Y., investigators were convinced that a center fuel tank explosion touched off the disaster. Examination of the wreckage turned up damaged fuel system components, including fuel quantity indication system (FQIS) wiring, probes, and terminal blocks, that helped the board craft its theory. Poor-condition wiring, circuit breakers, and even some evidence of improper maintenance were also pulled from the ocean floor, leading to a wider concern beyond fuel tank safety.
Seeking to put the conditions found on TWA 800 into perspective, the board decided to look at aircraft that were about the same age as the TWA jumbo, which had 93,303 hrs. and 16,869 cycles when it went down. The board looked at several other old widebodies, including two former TWA 747s and an Air France jumbo -- all recently retired. Investigators found things similar to what they discovered on TWA 800: high-voltage and low-voltage wires in the same bundles, wire insulation chafing due to sagging against nearby objects, open-ended splices in so-called "wet areas" of the aircraft that could lead to short circuits, and unsafe concentrations of splices that could stress wires.
"We concluded the condition of wiring in TWA 800 was not atypical with aircraft of its age, and it had been maintained in accordance with accepted industry practices," said Robert Swaim of NTSB's aviation engineering division.
Even though TWA 800 was one of the older 747s still flying, the board's findings suggested that age was only part of the reason why the plane's guts were in such poor condition. A study of bundles pulled from several of the guinea-pig aging widebodies by Raytheon concluded that wiring tends to age at different rates based on its environment. In other words, wires exposed to more harsh conditions will degrade more quickly than wires that are less prone to varying levels of moisture and extreme temperature fluctuations.
The board also made several discoveries about accepted industry practices related to wiring that gave it concern. For instance, manufacturers are not required to make distinctions between safety-critical wiring (such as wiring leading into a fuel tank) and other wiring (such as for in-flight entertainment systems). Wiring for systems with drastically varying importance to a plane's airworthiness are routinely be placed side-by-side, with only a 1/4-in. separation of bundles required. Also, because wiring is generally considered an on-condition item, inspection guidelines usually call for no more than a "general visual inspection," with no requirement to check wires for, say, brittleness that might not be obvious to the naked eye.
Even more disturbing was NTSB's conclusion that standard wiring practices manuals offer little help to mechanics. Many airline workers don't consult the manual because its sheer bulk makes it inconvenient to haul away from a reference desk. Those that do rely on the standard-practices manual get "extreme levels of detail" on some tasks like installation, but basic information like how to safely clean substances like lavatory fluid from wire bundles is nowhere to be found.
The number of potential wire hazards unrelated to aircraft age convinced the board to rip open a wide range of jets, including some right off the assembly line that hadn't been handed over to customers. In 1998, investigators did checks on 25 aircraft and included their results in the TWA 800 docket. What they found convinced them more than ever that, in Swaim's words, "Insufficient attention has been paid to wiring conditions until recently."
Problems found during the 25 spot-checks covered a range of concerns, from so-called accepted industry practices leading to potential hazards to downright sloppiness. Each aircraft, regardless of age, harbored at least several foreign objects that posed threats to wire bundles. In-service jets had everything from washers and tools that may have been left behind during the last maintenance check to dinner knives that apparently made their way down below by slipping through gaps in the cabin walls. Not even brand-new aircraft were immune to the foreign-object threat: one Boeing 747-400 had a 1.25-in. metallic drill shaving left over from production line work penetrating a wire coating. Similar shavings were found on aircraft that had structural repairs and modifications, the board said.
Inattentive maintenance explained some of these oversights. The board is concerned, however, about how non-intrusive industry practices and the lack of regulatory persistence contributed to the problems it found. General visual inspections that cover electrical system components like wire bundles and circuit breakers are one example. Mechanics often are told not to disturb wire bundles looking for hidden problems, because they might do more damage by moving wires than is caused by normal wear-and-tear. Unfortunately, potentially damaging objects like the ones found in the 25 airplane checks done two years ago could work themselves into and behind wire bundles, leaving them hidden from a mechanic's sight.
NTSB wants FAA to tackle these problems in several ways. In addition to improved training for mechanics on how to spot and deal with electrical system problems, the board wants new certification standards and technological leaps. Possible advances include using fewer wires or working in new technologies like fiber optics.
The safety board also wants FAA to follow up on past crackdowns. Following a 1991 in-flight fire incident on a Delta plane, the board told FAA that special attention needs to be given to lint accumulation on electrical system parts. FAA took action, but the 1998 inspections found dangerous amounts of build-up on breakers and other key parts, leading the board to conclude that FAA wasn't following through.
Ensuring that FAA's other actions since TWA 800 -- including some 40 airworthiness directives on fuel tank systems alone -- don't suffer the same neglect will be the key to making the four-year, $35 million investigation pay off.
"The real challenge for all of us," said NTSB board member John Goglia, "is to not let this work go away, to not forget the lessons learned. We must continue with the diligence to make sure every single item [NTSB cited] is brought to a satisfactory conclusion."
Sean Broderick is Commercial, Maintenance, and Safety Editor for AviationNow.com.