Because the FAA is not sure if your particular phone emits the same RF signals that the plane's systems use. Your phone probably won't interfere with the plane's navigation system, but the only way to be sure is to have the FAA certify every single device which is probably not going to happen given how often new models come out. The rules have been changed for quite a while now to allow you to use them in flight, that's why onboard WiFi is available now.

America is the only country has has these ridiculous rules, and there hasn't been a single accident blamed on someone's cell phone or laptop. Feel free to just pretend you're following the rule.

Your phone is capable of transmitting on frequencies that aircraft equipment might use. This theoretical interference is as far as it goes.

>America is the only country has has these ridiculous rules

It's still a rule in the EU until June 2023

During takeoff and landing is when the rules are applied and the practical reason is you need to pay attention to the flight attendants and overhead Announcememnts.

Because there have never been and never will be real studies to determine whether this is actually a dangerous thing or not. The fear is that your device emits a signal on a frequency that will interfere with flight systems and cause problems or a crash.

Starting in the early 2000s, they would make you turn off ALL electronic devices just in case and you weren't even allowed to read a personal organizer or perhaps an ebook reader. It took them 10 years or so until it was finally decided that your Bluetooth headphones weren't going to crash a plane, and now they all have WiFi on board anyway.

There may come a day when they stop caring, but for now, it's all restricted out of fear/paranoia rather than being known for sure to be a problem. As someone who has flown for decades, I have observed that many (MANY!) people have really never cared to follow these instructions and we have yet to hear of a crash being caused by electronic interference.

Well, don't forget that they also want things put away so that there is less being tossed around the cabin to injure passengers in an emergency.

Planes use special radars called altimeters pointed at the ground to determine how far the plane is from the ground.

The FAA, which makes up all the rules for airline flights in the USA, isn’t 100% sure that the cell phone frequencies we use to make calls don’t sort-of overlap (and therefore interfere) with the altimeter.

Technically, the FCC (who regulate radio frequencies among other things) has made efforts to ensure that these “lanes” of frequencies stay separate, but manufacturers of altimeters were lax with their standards and the FCC was lazy in enforcing that altimeters in airplanes stayed in their lane, and that’s why you have to put your cell phone in mobile mode.

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