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On day four of its hearings, the National Transportation Safety Board focused on the danger of aging aircrafts.

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TOM BEARDEN: The focus of today's hearing was aging aircraft. Did the fact the TWA 747 was a quarter of a century old contribute to the accident that killed 230 people?

JIM HALL, Chairman, NTSB: We do not know what caused the TWA 800 tragedy. One of the factors that has been widely reported and one is considered is the age of the aircraft.

TOM BEARDEN: The aviation industry's wake-up call about aging aircraft came in 1988, when much of the forward cabin of an Aloha Airlines Boeing 737 ripped away in flight because the aluminum fuselage had cracked under the stress of thousands of takeoffs and landings. Everyone had thought the regular maintenance and inspection programs required for all aircraft would have detected cracking long before anything like this could have happened. A host of new procedures specifically designed to deal with aging aircraft were subsequently put in place. A Boeing engineer testified the program is working.

ROBERT VANNOY, Boeing Engineer: The detailed examination that was conducted of the accident airplane, the 25-year-old accident airplane, and the lack of any significant corrosion or cracking does provide additional confidence the programs are working. I've been in a position over the last 15 years to review all the incoming data from the fleet, and it's become very evident to me that since these programs were established about 10 years ago, you know, the number of aging airplanes have exceeded our design objectives, have exceeded--you know, have gone way up--but the serious reports that we've been receiving have gone way down.

TOM BEARDEN: The NTSB wants to know if new procedures are necessary to inspect the wiring in older aircraft and whether today's inspection procedures will detect latent failures in those systems.

JOHN HALL: As you found system problems they have been addressed through recommendations or service bulletins.

ROBERT VANNOY: That's right.

JOHN HALL: Do you know how many of those there have been? And have there been any in regard to the electrical system of the 747?

ROBERT VANNOY: I checked through a lot of databases and results of the fleet surveys. I did not find anything relating to electrical systems.

JOHN HALL: Were you looking for those problems, or were you just looking at structural problems and as you maybe system--a system problem came to your attention, that was addressed, or were you looking for both?

ROBERT VANNOY: We were looking for both. We had systems specialists on our team, and they basically looked over the whole airplane, you know, everything from cable runs to door systems, and the cockpit, anything they could look at.

TOM BEARDEN: An Air Force expert showed examples of how wiring could chafe and fray on older military aircraft, which on rare occasions have shorted out, producing an arc, and even more rarely a fire.

GEORGE SLENSKI, Engineer, Wright Laboratory: This is typical wiring in a fighter aircraft, if you could zoom in on that. As you can see, there's quite a bit of wiring moving back and forth in there in that aircraft. These bundles are almost like tree trunks in the aircraft, and one of the problems of inspection, every time you disturb that bundle, you can induce more damage. A lot of times the wiring problems we're seeing are chafe-related due to handling during maintenance or maybe even during initial installation. And we try to make sure the people working on the aircraft, the maintenance troops, are aware of wiring problems, how the wire fails. They can look for these types of damage sites during normal maintenance of avionics. But typically, again, you're not going to get in there and disturb the wires. When you may see it, though, is when you're removing other avionics for either modification or repair, you need to go ahead and look at the wiring at the same time to see if there are any problem areas. So we ask people to do an overall inspection when they're in the area of the aircraft.

TOM BEARDEN: The board has been pursuing the idea that a spark from damaged wiring inside the center wing fuel tank of TWA 800 might have been the ignition source that caused the tank to explode.

ALEX TAYLOR, Boeing Engineer: We have 147 747 wires which are--airplanes--which are wired with the poly-x wire. We have no record of any incident of arc tracking taking place on any of the wires on any of these airplanes.

JOHN HALL: Mr. Craycraft.

KEN CRAYCRAFT, TWA Engineer: May I add to that that TWA has had no experience whatsoever of arc tracking of this wire on a 747.

TOM BEARDEN: FAA supervisory maintenance inspector Bill Crow said that might do more harm than good.

BILL CROW, FAA Maintenance Inspector: In listening to the testimony today I couldn't help but sit back there in the observers' area and recall the many times that I've been in fuel tanks and knowing what that critical environment is. Another critical environment is the aircraft wiring. And the point that I'd like to make regarding that, calling those critical environments, is I'd like to give you an analogy, if I could, and the analogy would be one that runs to someone that is a surgeon, if you will. If a person was to go in for major surgery at some time and he had a specific symptom, then that surgeon would go in there and he would do--he and his team would go in and do the work that needed to be done. And while they were in that general area, he would look around in there and see what else he could find. But I don't believe that a surgeon would go in without symptomatic cause or purpose and intent and perform surgery in an area that was critical to the well-being of that patient. And for the same purpose as our mechanics that are represented by the IAM and other people that are maintaining these airplanes, we like to go into these areas that are considered to be critical areas, critical environments on a need basis. If we open those areas, if we open those fuel tanks, if we get into systems maintenance where there's no symptomatic indication of a problem, sometimes, as was brought out in the expert testimony before, you cause more damage than you may have during the life cycle of that particular airplane. Before we look at this TWA 800 accident and the people that have lost loved ones and all of the things that come around this in the flight standards right now. We really have no probable cause, as you have no probable cause. We have ideas. But it's very difficult to stand firmly in that position and not waiver until such time we do have a finding that we can really work with. And I think it's important to note that when the flight standards does have that information, they will take the swift and immediate action to make sure that that is remedied.

TOM BEARDEN: FAA representatives said systems checks, which would include wiring, were being developed and would be added to the aging aircraft program next year. But they went on to say wiring is not ignored under existing procedures.

BILL CROW: It's not as if as we speak there is not programs in place that are not necessarily required by the FAR but that are encouraged by the FAA to identify systemic problems that will show degradation of the systems. There are a lot of devices; there are a lot of processes, a lot of programs in place that support the continued airworthiness of these airplanes that are designed by engineers and built by manufacturers, purchased by our air carriers, flown by our pilots, and maintained by the maintainers. So I want the general public to know that once an airplane leaves the drawing board and once an airplane leaves the manufacturer, once that airplane is in service, that airplane is maintained on a daily basis.

JIM HALL: If the FAA and the NTSB find problems within a particular aviation accident investigation, I don't think you are saying that the FAA is going to wait until there is a probable cause to act on those problems.

BILL CROW: No, sir. You're exactly right. We would not wait, and we would cooperate very professionally and effectively with all of the other entities that are working. But one of the most difficult things to do in this world is to identify a corrective action for a discovery that has yet to be discovered.

TOM BEARDEN: Late this afternoon, the board heard testimony on ways to reduce the flammability of fuel tanks, including a military system the airline industry has already said is not viable for commercial service. The board will pursue the subject again tomorrow on the final day of the hearings.

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