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You Don’t Need to Use Airplane Mode on Airplanes
(gizmodo.com)
This is a most excellent place for technology news and articles.
No they don't. GSM is all over the place with frequency, but it never goes down to 118-137 MHz which is where air traffic sits. It's just that mobile phones will increase transmission power to reach cell towers and that can produce that annoying disturbance. Phone won't mess up instruments, just annoy people trying to talk and get your ass safely from the ground.