A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why aren't you supposed to use a radio receiver on an airliner?

October 30, 1987

Dear Cecil:

On every commercial airliner, the little "card in the seat pocket in of you" warns you not to use AM or FM receivers because they might interfere with navigation equipment. How so? I could understand not operating a transmitter, since any aircraft has a large number of radio receivers, but why would a simple portable radio be a problem?

Cecil replies:

You probably have the idea that radio receivers are passive, vegetable-like devices incapable of making trouble for anybody, and at one time, I suppose, you would have been right. But technology has made great strides. Today a receiver can wreak every bit as much environmental havoc in its way as a transmitter — and not just in the form of boom boxes on the subway, either.

Most modern receivers use something called a "local oscillator," which is sort of an internal transmitter. The oscillator generates signal A, which is mixed with the somewhat raw incoming signal B to produce nice, easy-to-work-with signal C. There's usually some sort of shielding around the oscillator, but it's not always effective and sometimes errant signals leak out to make life difficult for other radio equipment nearby. If the other equipment happens to be an aircraft navigation device, somebody could wind up digging furrows with a $25 million plow. So do your bit for air safety and bring a tape player instead.

Interesting digression department: Radios aren't the only gizmos spewing electrical graffiti into the ozone--so do lots of other things, such as electric typewriters (remember them?). Typewriter static isn't a threat to airplanes, but it did get the Pentagon to thinking: jeez, what if the bad guys figure out how to pick up that stuff and translate it? So the National Security Agency set up a program called TEMPEST to test "information processing systems" to see if they give off "compromising emanations," the better to spy-proof government offices. If you want to do the same for your office, Uncle Sam can send you its official list of TEMPEST-tested products. Who knows, it could be the status symbol of the new millennium.

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