Fighting Fire with Foam

David Evans’ "Safety in Avionics" column in the June 2001 issue (page 78), titled "Time to Stop Fuel Tank Explosions" reminded me of an article I read many years ago. It was about wing-tip fuel tanks on fighter aircraft and filling the tanks with foam, which took up about 5 percent of the volume. The foam prevented the fuel from sloshing around in the tank and it also prevented explosions when a bullet hit the tank.

The article had a photo of a fuel tank just after a bullet was fired through it. There was a small flame at the bullet hole, but the tank did not explode.

Would foam be useful in the fuel tanks of commercial aircraft?

Donald Weinstein
Fair Lawn, N.J.


Yes, there are various types of foam that would virtually eliminate the hazard posed by flammable vapors in the fuel tanks of transport-category airplanes. I dare say the foam you referred to is reticulated foam. There is another type, expanded aluminum mesh, which does the same job. A government-industry task force in 1998 rejected both of the substances as too costly.

A second government task force is looking more closely into various schemes employing nitrogen-enriched air or liquid nitrogen. The weight savings from these approaches are considerable, in comparison to foam. (Another problem with reticulated foam, but not with aluminum mesh, is that is has to be treated as a hazardous material when removed for any in-tank maintenance.)

The nitrogen gas-based approach shows some promise. One technique, using liquid nitrogen stored in an insulated bottle, was tested successfully by the Federal Aviation Administration on a DC-9 nearly 30 years ago.–David Evans.

A New Diskette Standard

Your "Avionics Systems Design" in May (page 49), titled "The Pitfalls of Long-Term Storage," caught my attention with regard to the 5.25-inch diskettes. Living in Argentina, I have found that it is very hard to obtain that kind of media used as a standard in the automatic or in the less computerized testers. My question is this: Do you know if there is any work being done to change this standard to one like the usual 3.5-inch diskettes?

Roberto Klemm
Aerolineas Argentinas SA


Most machines that have 5.25-inch drives will take a 3.5-inch drive, possibly with an adapter plate (the bios, or basic input/output system, usually supports this). Also, a dual-drive machine with an old and new drive can serve as a quick way of converting media.

The 5.25-inch media (1.2 Mbytes) was essentially discontinued years ago, and the 3.5-inch drives (1.44 Mbytes) became "standard" because they fit both desktop and laptop computers. Nothing else has emerged as a substitute for small local storage, although many persons now use a CD recordable (CDR) or CD rewriteable (CDRW) 5.25-inch optical drive for local storage, but it offers much greater size (650 Mbytes).–Walter Shawlee 2.

Just One Antenna

As a former GTE Airfone technician and now a United Airlines technician, I found your Product Focus on antennas (Aug. ’01, page 50) to be very informative. There is only one small error that I wish to point out regarding the air phone antenna.

On aircraft such as the MD-80 or B737, the Airfone system uses one ARAD (airborne radio) unit that is connected to one antenna. That one antenna helps the ARAD simultaneously transmit and receive four phone calls. The other antenna (there are only two, not three) is there, should a second ARAD be installed to permit eight simultaneous phone calls. All narrowbody aircraft that I have worked on employ only one ARAD.

One resounding truth is that baggage load vehicles break a lot of antennae. An excellent article.

Fernando Foje
United Airlines

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