Re: 757 flight recorder located

From:         Jon Ward <>
Organization: The End at Infinity
Date:         29 Feb 96 01:24:23 
References:   1 2
Followups:    1
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  "John Bay" writes:
> Would it be possible to make a flight recorder that would transmit all its
> stored data upon request, thereby eliminating the necessity to physically
> recover the box?

I may be wrong, it's a while since I did my electromagnetics course...

The problem is sea water. Sea water contains sodium chloride (and various
other things), and this makes it an electrolyte - it can conduct electricty.
Because it conducts, this makes it harder for electromagnetic radiation
such as radar, radio transmissions and gamma rays to penetrate (there's
a load of maths to do with skin depths, conductivity etc. which I will omit
for my sanity as well as yours). Now sea water may not be as good as a
conductor as copper, but it's a darn sight better than air. Once your
flight recorder is in a couple of hundred of feet of water, you might as
well put it in a lead-lined box, and drop it down a coal-mine.

There is also the question of weight - adding a transmitter to a black box
(and batteries) which can survive a crash or explosion means extra pounds.
Every flight you take when it doesn't crash costs you money. (Insert
religious figures and terrain features here).

Now here's a question for those people who's EM theory is a bit more up to
scratch - can you count a sea or ocean as a Faraday cage?

Jon Ward
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