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During take offs the flight attendant makes sure that mobile phones are switched off (or maybe turned in airplane mode), why is that necessary? (I suspect that there is something to do with physics that's why it was posted here.)

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    $\begingroup$ It might have more to do with electrical engineering or electronics. $\endgroup$ – probably_someone May 10 '19 at 13:13
  • $\begingroup$ One reason why they made that rule was that the phone companies asked them to make it. The physics part of the reason was simple: A plane flying five miles above the average terrain had line-of-sight to hundreds of cell towers back in the when they made the rule--maybe tens of thousands today. If your phone actually was able to communicate with hundreds of towers, that could confuse and overload their network. I don't know whether a modern cell phone actually can emit a signal that can penetrate the airplane's skin. The technology today is different from back when they made the rule. $\endgroup$ – Solomon Slow May 10 '19 at 13:40
  • $\begingroup$ Because 1) a poorly designed mobile might interfere with poorly designed avionics, and 2) the cell networks below you aren’t designed for a bunch of phones with line of sight to lots of towers while moving at 1000 km/hr. Physics per se has nothing to do with it. $\endgroup$ – Jon Custer May 10 '19 at 13:40
  • $\begingroup$ There's an Aviation SE site, where this question has already been asked and answered. There's probably interesting physics issues to be picked up from there, but I'm not particularly convinced that the bare naive question as asked here really is about physics. $\endgroup$ – Emilio Pisanty May 10 '19 at 17:52
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    $\begingroup$ I'm voting to close this question as off-topic because it belongs on Aviation. $\endgroup$ – Emilio Pisanty May 10 '19 at 17:59
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As others have pointed out, one reason is that the cell towers couldn't really deal with the signals.

The other, and this is more theoretical in nature, is that the working frequencies of the transmitters in the phone, and the CPU and other chips, might give off signals that interfere with the navigational receivers. This isn't entirely crazy, we had a deskside-PDP-11 machine that you could tell was running from the next room because you could hear it on the radio.

That said, the frequencies in question are very widely separated for exactly that reason, and it seems there are no proven examples of this ever happened in spite of millions of hours of phones being left on by mistake.

Update: if you're curious why even having a receiver is an issue, read over the wiki's article on superheterodyne and the ASV radar.

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  • $\begingroup$ Now could that be the reason for those "missing" airplanes ? $\endgroup$ – user207455 May 10 '19 at 14:23
  • $\begingroup$ No, although some claims to that effect have been made. In every example, this was found to not have actually occurred. $\endgroup$ – Maury Markowitz May 10 '19 at 14:31
  • $\begingroup$ So, you say they worked that out for the ones not found yet... good going... $\endgroup$ – user207455 May 10 '19 at 14:43
  • $\begingroup$ No, I said they found zero examples of interference with instruments, ever. It is difficult to imagine that this problem only occurs on certain flights, and always causes the aircraft to disappear. $\endgroup$ – Maury Markowitz May 10 '19 at 14:53
  • $\begingroup$ This is why physics is not the venue for this. "There are no proven examples of this ever happened" is largely irrelevant - it's not how aviation works. Safety is paramount, and the bar is "has been proved to be safe", not "has not been proved to be unsafe". (There are no proven examples of an MCAS bringing down a 737 Max 8 - would you feel comfortable flying in one?) Let's not have a discussion about aviation safety with information provided by armchair aviators - there are professionals for this, and this is not their site. $\endgroup$ – Emilio Pisanty May 10 '19 at 17:58

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