Fliers Must Turn Off Devices, but It’s Not Clear Why

Put your cellphone near your FM radio. Listen in amazement as the digital chatter between your phone and some remote tower causes interference in your radio reception and affects your listening pleasure. Now imagine that same thing happening to the digital signals in the airplane's electronics while attempting to operate the craft. Catastrophe, chaos, etc. ad nauseum.

Realistic? Doubtful for many reasons. But that's the fear the bureaucrats, pilots, and passengers possess.

Personally, I don't mind the switch-off since it keeps obnoxious people from yakking on their phones (now, if only the airlines would offer infant-free flights...).

EMC is black magic and airplane safety is another and thus nobody is willing to say that using random RF devices onboard of plane is safe - for one reason it's almost impossible to test. By the way owner's manual for my new car contains paragraph that explicitly disallows usage of any RF transmitter inside the car that does not have external antenna (and I would assume that most car manufacturers include similar paragraph in their manuals and also that everybody ignores them).

Cellular networks and cell phones are not designed for relative speeds and RF propagation modes of high flying commercial aircraft. Most access methods of digital wireless networks depend on precise timing where propagation delays are significant and thus need to compensate for relative movement of phone and base station and there are pretty low limits of how fast movement can be compensated (it's almost practical to drive car faster than speed limit of GSM1800). Also higher levels of cellular protocol stacks are not exactly prepared for situation when phone sees large number of accessible cells with quite strong signal and this set changes very quickly.

This sums it up pretty well: "The regulatory agencies and aviation industry take the position that any increased risk is unacceptable if it is avoidable."

There's some technical channel sharing problems where phones have access to more towers than designed on land. The FCC can't predict what would happen if phones were allowed to stay on in regards to cell roaming. They also don't know the effects on the avionics.

It looks like they're developing 'picocells' to have an on-board tower, but there are struggles with the different cellular bands. Though, there are some European carriers that have been successful with just letting people keep their phones on.

That said - I think if your device doesn't have a radio on it, it should be allowed to stay on. Which probably won't fly given the 'if it's avoidable' rationale.