What Happens When You Don't Put Your Phone on Airplane Mode on an Airplane?
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Nikita Schmidt, private pilot:
Your phone will probably annoy a few pilots and air traffic controllers. But, most likely, not badly enough for them to take action against you, if that’s what you want to know.
You may have heard that unpleasant noise from an audio system that occasionally happens when a mobile phone is nearby. A phone’s radio emissions can be very strong, up to 8W; they cause this noise due to parasitic demodulation. I actually heard such noise on the radio while flying. It is not safety critical, but is annoying for sure.
Of course, there is plenty of attenuation between phones in the cabin and the pilots’ radio. However, if say, 50 people on board are inconsiderate enough who can’t be bothered to switch their cell radio off, there will be 50 phones constantly looking for cell towers at maximum power. That is a lot of radio pollution.
When in-flight cellular service is provided, there is a cell station right beside those phones. They communicate at very low power without causing any disturbance. The Wi-Fi signal is much weaker (100mW) than GSM at its peak, and I never heard of it causing any problems.
It is common courtesy. By switching your phone to airplane mode you show your appreciation to the people doing their jobs to get you where you want to be.
Have you ever heard what it sounds like when you leave your phone on a Hi-Fi speaker while you are receive an incoming call, especially older phones?
Now, in this clip it's not very loud, but it can be. It makes bzzzt-bzzzt-bzzzt-bzzt-bzzt. Now imagine that noise in the pilot's headphones while he is receiving a critical bit of information from ground control. Imagine that noise, constantly, from the phones of over 100 passengers! Or worse yet, that noise interfering with some navigation signal that was sent so as to avoid a collision with another plane. Unlikely, but possible. Ask yourself: Do you really want to annoy the pilot charged with getting you safely to your destination?
To compound matters, the weaker the signal your cell phone picks up from the tower, the more it amplifies its signal to try and get a response (and the more battery it uses). Planes with on-board cell coverage, allow your phone to communicate using very low power, or Wi-Fi. When you put your phone in airplane mode, the GSM/3G Radio inside your phone is completely disabled and you can still use the phone for other functions. Here's another perspective:
Summarized by: "There simply isn't enough time to check every single device and its particular function."
Yes, it is annoying. But eventually you come to appreciate the few hours of "radio-silence" you're forced to observe. Until you discover an airliner with on-board Wi-Fi.
So why is Wi-Fi allowed? Because its emissions are around 100 times less powerful than that of some 3G radios. And that's not to say that newer planes aren't made to specifically be resilient against any potential harm that being showered by a pile of unexpected photons can bring.
Here is a list of in-flight emergencies that had actually been caused by electronic devices. If you read the narratives on that page, you will find reports of all the things above mentioned happening, as well as electronics getting crushed in aircraft seats and tables and catching fire — hardly things you want happening on your flight!