February 18, 2004
Captain Ray Lahr (ret.)
18254 Coastline Drive
Malibu, CA 90265
Letters to the Editor
Los Angeles Times
202 W. 1st Street
Los Angeles, CA
The FAA plans to mandate a fuel tank system using nonflammable nitrogen gas. It is a good idea in theory, but it just isn't practical. Early in the jet age, we were using a volatile fuel, and we needed protection against fuel tank explosions. Parker Hannifin developed a prototype system and demonstrated it at its facility near LAX. It was a workable system, but the disadvantages were immediately obvious.
Any nitrogen system is going to require another plumbing system on the aircraft to get the nitrogen from its source to the fuel tanks. In addition to the weight and space penalty, installing such a system would be a retrofitting nightmare.
Then where do you get the nitrogen? You can store it in a large pressurized container, but that is another weight and space penalty with its own safety hazard if the container ruptures. And believe me, it takes a lot of nitrogen as I will explain in a moment.
The other option is to generate the nitrogen as needed. Air is about 80% nitrogen. We just need a device on the aircraft to separate the oxygen out of the air before we send the nitrogen to the vapor space in the fuel tanks. That sounds good until you consider how much and how fast the nitrogen is needed.
As you know, the density of air decreases as you climb. The rule of thumb is that it decreases by half each 18,000 feet (the fuel tanks are vented to outside atmosphere.) In other words, at 36,000 feet, the density is about one fourth of the density at sea level. At the end of a flight, the tanks are relatively empty, so as you descend, you would need to generate enough nitrogen to fill the tanks at sea level pressure. That is a daunting requirement, and it would require substantial equipment and substantial fuel to operate the equipment (oxygen/nitrogen separation is not energy free).
The answer then is the same as the answer now. Use a fuel that is less volatile. Such a fuel was developed, and there has never been a fuel tank explosion using this new fuel while it is in the liquid state. That includes TWA800. Jet A fuel will burn when it is atomized. That is the way it is burned in the engines. It is sprayed into the combustion chamber. But a fuel tank with liquid fuel in the bottom of the tank will not explode due to a spark under the conditions of TWA800. I refer you to the research of Commander William S. Donaldson at www.twa800.com .
The center wing tank of TWA800 did ultimately explode, but it was a secondary explosion. The liquid fuel first had to be violently agitated. The initiating explosion was military ordinance. How that ordinance was delivered and by whom has not been precisely determined, but that is the problem the FAA and other government agencies should be addressing instead of focusing on an unsubstantiated spark of unknown origin.
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