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On day two of its hearings, the National Transportation Safety Board focused on what might have caused the fuel tank to explode.

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TOM BEARDEN: Today's testimony focused on what might have caused the center wing fuel tank of the TWA 747 to explode. The NTSB was particularly interested in the fuel probes--rod-like devices that measure the level of fuel inside the tank. The probes use a weak electrical current to make that measurement. There's been speculation that a failure in the wiring system could have introduced a lot more electricity into the system, causing a spark that might have ignited the fuel-air mixture inside the tank.

LOU TAYLOR, Honeywell Engineer: The normal operating voltage for tank units in the center wing tank we operate these at five volts. One of the things we did also to try and put this in perspective, you can hang onto the wiring, you don't feel a thing. You have no idea if this is on or off if you're holding the wires. So this is an extremely low energy system.

BOB SWAIM, NTSB Investigator: I'd like to jump ahead a little bit and ask you if there's ever been any fuel tank ignitions through the fuel quantity indication system of any airplane that you know of.

LOU TAYLOR: No, none that we're aware of.

BOB SWAIM: None that you're aware of?

LOU TAYLOR: None that we're aware of.

TOM BEARDEN: Even so, the Boeing Company, builder of the 747, has launched a program to inspect the center wing tanks and the entire fleet, nearly a thousand aircraft. But NTSB Chairman Jim Hall didn't seem impressed.

JIM HALL, Chairman, NTSB: I appreciate you gentlemen, and I appreciate the industry and the things you represent, the Boeing Company, and--but, you know, it's 16 months since this accident occurred, and to be sitting here and saying we're going to do something that takes 16 months and add two and a half years, and it's just a recommendation, I get criticized for being frustrated, but, to me, that's frustrating. How many classic 747's are there? We know that this accident occurred with a classic 747, is that correct? I know that's correct. So do we know how many classic 747's there are?

JERRY HULM: I believe there's approximately 750.

JIM HALL: Of the 970?

JERRY HULM: Correct.

JIM HALL: So when you're talking about the classics, you're talking about the majority of the fleet?

JERRY HULM: Correct.

JIM HALL: Okay. Let me try to understand one other thing now. If the manufacturer of this Boeing 747 puts out a service bulletin, how does the FAA deal with that in terms of an AD? How long does that take? Because obviously what we're being told is a service bulletin is a recommendation, not a requirement, and for flying in this country we look to the FAA to the regulations for safety, so what is the process? Are you waiting until a recommendation, the service bulletin comes out to consider it?

DANIEL CHENEY, FAA: As you're aware there's issues that are unfolding on this investigation and have been unfolding late summer and this fall.

JIM HALL: What has not changed is that we had a 747 explode. We agreed early on that the center fuel tank was the cause--was what brought the aircraft down. That hadn't changed or evolved, and what has not changed is we don't know what the ignition source was. So what I'm trying to do is find out what has Boeing done. That's why I'm asking you all. This is a public setting, a public record. What has Boeing and the FAA done, since we don't know, to try to take prudent steps so there's--that fault tree is in place to be looking at every possibility that could have caused this accident so that when I'm asked a question, Mr. Chairman, you know, should the people be flying the 747, I can answer as I try to do, you know, yes, the industry and the FAA, the government regulators, we don't know what caused this accident, but we're doing everything that you do, or I would do in those situations to prudently protect the American public. So if you gentlemen could tell us what you're doing, that's what I'd like to know.

JERRY HULM: Well, I can address some of that. You know, what we've done since the accident, you know, the service bulletin is just one action to that. We've done, you know, an extensive review of all of the components that are involved in the system and from top to bottom, you know, over the last year and a half, we haven't been idle. It's been a very extensive review. It's involved a lot of people within Boeing, within Honeywell, within different parts of the industry, and looking at all these parts trying to determine this accident, what could have been the cause, we do have the fault tree. We've gone through that, and the NTSB has looked at that. We've got the inspection program not only for the center tank but we address the fuel boost pump issue with the wiring and the conduits, and we've done a complete inspection of all U.S.-registered aircraft for that conduit, making sure that the sleeving that's protecting that wiring is intact.

JIM HALL: And how many service bulletins have come out of all that work?

JERRY HULM: We've got the scavenge pump service bulletin; we have the service bulletin for the conduit inspections; we've got the center tank inspection service bulletin.

TOM BEARDEN: Boeing's service bulletin is only a recommendation to its customers and doesn't have the force of law. Chairman Hall wanted to know if the Federal Aviation Administration would make the recommendation mandatory by issuing an air-worthiness directive to the industry.

DANIEL CHENEY: We do plan to take mandatory action on the center tank inspection when all of the issues are included in that. We're very concerned about multiple entries to this tank. We want--we want to enter it one time and do the right things, one time, fix the things that we believe should be fixed, and fix them right. And the current schedule of having that bulletin is--

JIM HALL: And you don't think that Boeing's service bulletin does that?

DANIEL CHENEY: Not yet. There are things that are being added. And that's the revision. There are components within that tank that need to be looked very carefully at, and the current bulletin simply doesn't do that.

JIM HALL: If we had a situation where the FAA wanted to order an inspection of all the classic fleet, how long do you think that would take for the ones that operate in this country, Mr. Cheney?

DANIEL CHENEY: We have to consider the entire impact.

TOM BEARDEN: This afternoon the board saw a videotape of two explosive tests conducted on a one quarter scale model of a center wing tank. One of them resulted in a spectacular fireball as the internal structure of the tank was ejected in much the same way investigators think that TWA 747's tank failed.

JOSEPH SHEPHERD, California Institute of Technology: We have found that combustion occurs in a complex fashion within the center wing tank. But, in all cases, the pressure within the tank increases quickly once the plane has propagated through the bay in which ignition has occurred. It appears that the damage observed in the crushed wreckage could have been produced by ignition in any of the bays. Our testing has been designed to examine specific features of the explosion that might be produced by various ignition locations. And that testing is still in progress.

TOM BEARDEN: Tomorrow, the NTSB will examine the possible sources of ignition inside fuel tanks and discuss how the age of the aircraft affects all of the factors under scrutiny.

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