Nobody ever gets this right.

GSM cellphones (from AT&T, T-Mobile and most non-US carriers) cause potentially harmful interference. CDMA cellphones (from Verizon and Sprint) do not.

This is due to the different ways that GSM and CDMA allow multiple phones to operate on the same band at the same time. CDMA does it by slightly varying the frequency of a continuous transmission. GSM does it by having the phone turn its transmitter on, fire off a short burst of information, then turn the transmitter off again. Unfortunately, and as far as I know unintentionally, this results in GSM phones sometimes turning their transmitters on and off at a frequency within the human audible range.

The problem is, any device containing an amplifier and a length of wire - like a pilot's headset - is potentially a radio receiver. All these radio receivers pick up every transmitted signal. Devices that were intended to be radios contain circuits to amplify the desired frequency and reject all the undesired ones. But a pair of headphones assumes that the only signal on the wire is the one that came from the jack. With CDMA, this is not true - there is added signal at a frequency range thousands of times beyond the headphones' ability to reproduce, and also beyond human hearing even if they could. So nobody cares. But with GSM, there is sometimes added signal that the headphones *can* reproduce, and the human *can* hear.

You can easily test this at home. Put a GSM cellphone next to your turned-up computer speakers and make a call to it. You hear "brrrrrp-brp-brp-brp" through the speakers. Now do the same thing with a CDMA phone. You don't hear anything.

A pilot in congested airspace could be seriously impaired in his ability to communicate with ATC if his headset keeps picking up GSM interference. In the home test with the computer speakers, you will probably notice that the effect goes away once the phone is about 4-6 feet away from the speakers, and you aren't normally within 4-6 feet of the pilot, so even GSM phones aren't normally a problem. But there's wiring in all kinds of places on an airliner. If a passenger with a GSM cellphone happens to be sitting next to the radio antenna wire, it could geniunely cause a problem.

In reality, a large airliner probably has several powered-on cellphones on every flight, just because people forget to turn them off. But "several" is different from "hundreds" and hopefully the phones stay away from the weak spots. On rare occasions a powered-on cellphone coincides with a weak spot and you get interference. This is why you occasionally see the flight attendants run through the cabin searching for a powered-on cellphone. Presumably the pico-cell installers check the cabin shielding and improve it where necessary.

So I think it would be perfectly safe to allow CDMA cellphones but not GSM. But this is probably logistically impossible. If you tell passengers that CDMA is okay, they won't know what you're talking about. If you tell them Verizon and Sprint are OK, they will assume the choice of carriers is just some business shenanigans, and will happily turn on their AT&T iPhone. Flight attendants won't be able to keep up with the changing technology and tell which device is which.

Which means we're stuck with the rules as they are, at least until some new GSM standard eventually fixes this problem (which they seem in no hurry to do).

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