Remarks of Jim Hall, Chairman
National Transportation Safety Board
before the FAA/SAE Conference on Fuel Flammability
Washington, D.C., October 7, 1997


I want to thank the Federal Aviation Administration and SAE-Aerospace for inviting me to speak to this very important conference.

On July 17, 1996, TWA flight 800 exploded over the Atlantic Ocean shortly after taking off from Kennedy Airport. The loss of the 230 persons aboard that flight shocked our nation; indeed, it shocked the world. It also triggered the largest transportation accident investigation in American – and possibly world -- history, led by the National Transportation Safety Board.

The American people have spent tens of millions of dollars so far in this investigation, and while we have made significant progress, we cannot say at this time how long it will be before we complete the investigation.

The fact that we have not as yet issued the probable cause of this accident has masked a very important fact about the investigation, and that is that we know what happened to the aircraft.

What has brought us here is the event that started the breakup sequence: there was a fuel/air explosion in the almost-empty center wing tank. What ignited the explosive fuel/air vapors in that tank is not yet known.

A big issue is whether the precipitating event was a mechanical malfunction or an act of sabotage. That is why we have had two parallel investigations, ours and the FBI’s. Our finding so far is that there is no evidence of a bomb or a missile impact in the wreckage, but of course the investigation continues.

Looking at mechanical causes, we know that there are numerous potential sources of ignition in a variety of fuel tanks despite the FAA certification process to engineer-out sources of ignition.

When we began this undertaking, we were surprised at how little was known about the explosive characteristics of Jet A fuel, what it takes to ignite the vapors associated with fuels, the temperature and vibrational characteristics associated with airplane fuel tanks, and the vapor concentrations in the tanks.

That is why we have been conducting tests in laboratories all around the country and even overseas, which I’d like to summarize for you:

Laboratory Studies of Jet A Fuel: An extensive series of studies of Jet A fuel has been carried out at the California Institute of Technology to determine the flame speed and pressure characteriscs of Jet A vapors, as well as minimum ignition energy as a function of pressure and altitude. You will be hearing about the results of some of this work from Dr. Joe Shepherd today.

Flight Tests on Boeing 747/100 Aircraft: Nine test flights were conducted in a Boeing 747/100 aircraft to measure temperature, pressure, vibration and vapor concentrations in the fuel system. These test flights were completed on July 19, 1997. The objective of these tests was to determine the environmental conditions in the center wing tank at the time of the explosion on TWA 800, and under flight conditions designed to minimize fuel flammability, consistent with the Board’s recommendations to the FAA.

Chemical Characterization of Jet A Vapor: Laboratory tests to determine the chemical composition of Jet A fuel vapor as a function of temperature and ratio of ullage to liquid volume have been carried out at the University of Nevada, Reno.

Bruntingthorpe B-747/100 Explosion Tests: A series of tests was conducted in England to examine the sound spectral characteristics of explosions in a 747/100 aircraft, and also to examine the collateral damage created when small high explosive charges were placed on the exterior surface of the center wing tank. This test program also evaluated the effects of a fuel/air explosion in the center wing tank.

Quarter Scale Explosion Testing: A quarter scale model of the 747 center wing tank, having multiple compartments, is being constructed to carry out explosion testing. This is being conducted for the Safety Board by CalTech and Applied Research Associates at a site near Denver, Colorado.

Computer Modeling: Computer modeling studies are being carried out simultaneously with the quarter scale experimental work by Sandia National Laboratories in Albuquerque and the Christian Michelson Research Institute in Norway to determine if we can adequately model on a computer the explosion dynamics that occur in the fuel tank and ultimately determine the location of the ignition.

The NTSB agrees that the task of engineering out ignition sources is an important and desirable one, but we also recognize that it is a task we might never accomplish. That is why the Board issued its recommendations last December.

It was the explosive nature of the vapors in the fuel tank that allowed the blast – whatever its origin – to bring down flight 800. We believe that mechanisms exist that could, even in the short term, reduce the probability of a recurrence.

Our four recommendations to the FAA, which have been added to the Board’s Most Wanted List, urged both short-term and long-term actions to reduce the potential for a fuel/air vapor explosion in the center tanks of Boeing 747s, as well as in fuel tanks of other aircraft. We suggested possible means to reduce the explosive potential of the fuel vapor, such as adding cold fuel to the center tank before takeoff, providing insulation or other methods to reduce the transfer of heat from the air conditioning units beneath the center tank, or inerting the tank by replacing the explosive vapor with a harmless gas.

We are not saying that these changes will prevent every accident in the future. The Safety Board agrees with the FAA and the industry that the policy of eliminating sources of ignition should continue. As I’ve stated, the problem is that there is no way we can assure ourselves that all ignition sources will ever be eliminated; TWA 800 shows that they haven’t been all eliminated yet.

If there is a source of tension between the NTSB and the FAA on this issue, it is that the request for comments wasn’t issued until April, with comments not due until August. The Board expected a more timely response.

It is no secret that the industry has not embraced our recommendations. But I’m happy to say that since those recommendations went out, industry has turned its attention to the problem. I read with great interest the response of ATA, AEA, AAPA, AECMA and AIA – filed jointly – to FAA’s call for comments on proposals. While it disagrees with our recommendations, importantly, it contains two pages of "industry initiatives."

For example, industry plans to undertake a survey of aircraft or major fuel tank inspection programs to verify the integrity of wiring and grounding straps; the conditions of fuel pumps, fuel lines and fittings; and the electrical bonding on all equipment. This program will include not just Boeing 747s, but also Airbuses and aircraft of other manufacturers.

Folks are looking seriously at heat sources under center tanks. There is movement toward making Jet A fuel even less explosive. All to the good, if pursued with vigor. And, of course, both the FAA and the SAE are to be commended for organizing this conference.

It is healthy that the industry engage in a thorough examination of the practicality and benefits of our safety recommendations. That is what is going on now with the center fuel tank recommendations. What I don’t want to see, though, is a reflexive action by industry that leads to our recommendations being rejected out of hand.

It was asserted shortly after the crash of TWA flight 800 that, even if the center wing tank exploded, it could not have brought down the plane. In addition, we were told that center fuel tanks of 747s are virtually never in an explosive state. I think most of us are convinced otherwise by now.

The American people can be proud of the total effort expended by its government and the aviation industry in pursuit of the cause of the TWA tragedy. This week’s conference is an example of how those two segments of our society are working together to eliminate the flammability of vapors in fuel tanks and therefore try to ensure that another flight 800 doesn’t happen.

The Safety Board will continue to follow every avenue that might lead us to what ignited the center wing tank of flight 800. As you probably know, we’ll be holding a public hearing in Baltimore the first week of December. But, regardless of what we find to be the ignition source of that blast, we must do all we can to render these fuel tanks non-explosive. I hope this gathering provides big steps in that direction.

Thank you for allowing me take part, and good luck on achieving the goals of this conference.

Jim Hall's Speeches