andrewwm

The thing is, electronic interference is an additive, not binary, situation. So the more devices you have going, the more powerful the interference gets. Most devices these days are fairly well behaving and add minimal, if any, electrical disturbances.

But some off-brand bought from China that looks like a real iPhone may not be so well behaved and may be emitting electronic interference like it's going out of style. The point is that the FA can't tell just by looking whether a device is going to be well behaved or not. And, moreover, if everybody (150 people) are all using devices simultaneously as the plane takes off, you can image the difference in interference (because interference is additive) versus the one guy sneaking in a game of Sudoku on his iPad despite being told to turn it off.

My understanding is that most (and certainly all essential) flight instruments/components are shielded from electromagnetic interference. But until plane manufacturers can guarantee that all devices that might be useful during flight are shielded, then all devices will have to be off during takeoff.

Maybe it's only a 1 in a 10,000 chance that any device will be enough to moderately interfere with the radio or directional finding. And maybe it's a 1 in 10,000 that this interference will cause a crash (tower calls in "change altitude collision immanent" plane: "what's that? all i heard is static, some yahoo is using his knockoff phone from China") - but until the FAA can nail down both of those probabilities given that potentially everyone on the plane might be using devices at once and some of them might not be well-behaved and that those joint probabilities be in the realm that the agency feels is unlikely to ever cause a real-world crash, devices need to be off.