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If every passenger on an aircraft had an RF-emitting device turned on, there is no telling what secondary RF could be generated and at what frequency and power. Old radios operated using Beat Frequency Oscillation (BFO) which combined and overlapped waves to create new ones, similar to harmony in music. Such combined oscillations could create 'new' frequencies that might interfere with navigational systems, though it's unlikely. Aircraft IFR equipment are highly shielded and far enough away from hand-held device frequencies to be theoretically safe, but with a new device coming out every day, it is impossible to verify each and every one of them for safety. So just make a single, simple rule that lasts only for 20 minutes or so and enforce it. I was on a Continental flight recently that was held on the tarmac for a minute while the stewardess came back and announced that a cell phone was on that must be turned off. Sure enough, some teen-ager pulled one out of her purse and flipped the switch. Perhaps there are now devices within some commercial aircraft for detection of spurious RF frequencies?
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