META: ENGINEER:airline fuel tanks..WAS: Galileo Rules!

Michael Lorrey (retroman@together.net)
Thu, 18 Feb 1999 22:08:22 -0500

Spike Jones wrote:

> the fuel pump on my truck is inside the tank, submerged in gasoline,
> unless the tank is nearly empty. i would think the vapor pressure
> of octane on a cool day would be lower than that of kerosene on
> a warm day, yet there is never enough oxygen in a car's gas tank
> to cause an explosion, even assuming a faulty fuel pump. that
> kapton has problems is acknowledged, but why would a spark
> inside a fuel tank cause an explosion?
>
> what is your experience? i fly on these boeings about 40 times
> a year. i would hate to hafta worry that a tank could explode
> at any time. {8-[ spike

There are several ways:

  1. fuel leaks. aircraft fuel tanks can be notoriously leaky, especially planes on international flights. the fuels used in different countries tend to have some important variances to them, and ground crews in many other countries are not english literate and can easily mix up a request for JP-4 for JP-5, 4 for 3, etc. if they are not properly trained. I do know that JP-5, for example has no trouble at all leaking through the standard seals used on tanks made to hold JP-4. When I worked on F-15's in the USAF we deployed to the Navy's Miramar NAS to fly as agressors against the Navy Top Gun students. The F-14's use JP-5, while the F-15 uses JP-4, but there was no JP-4 on hand. What did we do? We used JP-5. Within the first 24 hours all of our fuels specialists were working non-stop to plug the leaks that were developing in the seals. The Navy boys helped out immensely. Apparently one specific brand of chewing gum works as an excellent temporary sealant for JP-5 tanks....I don't recall the brand other than it was a bubblegum.

Given this potential for fuel leaks (also likely to increase when under increased structural loads from climbing, I can imagine that a sufficient leak could have developed through the seals, around screws, etc. and the leaked fuel could have ignited on a neighboring wire harness, especially if it leaked on a cannon-plug at a bulkhead interface connection.

b) arc-throughs. The power supply cables for the forward distribution busses run by the fuel tanks as I recall, but any wire with sufficient current that had chafed against the fuel tank wall could possibly have chafed through, arced into the fuel tank wall, and arc-welded a hole in the tank, providing a point of ignition.

Keep in mind that this particular tank was nearly empty. Nearly its entire volume was filled with fuel vapor, and and air that had filled the space when it had been vented as it emptied. Such a situation is far more potentially explosive than a full tank of gas.

I don't think your statement that there is never any oxygen in a tank is true. The reason why tanks don't blow up so often is that the pumps and sensors are built so well that they almost never expose bare conductors to the inside of the tank. One interesting feature that some planes have on their fuel systems is that they have current sensors on both the hot and ground legs of the circuit. This allows the system to measure if the insulation is breaking down and permitting too much current to go to ground outside of the ground leg of the circuit. I do not know if that model of 747 have such a monitoring system or not.

The reason why I think we will never have a conclusion to this case is because it happened over water. If it had happened over land, we would have known long ago what happened to the plane, but there is so much of the plane (especially in the area of that fuel tank) that is still missing, that I am afraid we will never know. 

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