For safety reasons, the Federal Flights Commission (FCC) restricts the use of phones and other smart devices during flights. Over the last few years, however, there has been public pressure to change FCC rulings, (via Washington Post) but aviation officials, for the most part, have erred on the side of caution and have continued to restrict their use while flying. While it’s never been completely proven that using your phone when up in the clouds can cause your plane to crash, it is always better to be safe than sorry.
A phone that is not in airplane mode can interfere with and potentially disrupt communication systems on your flight. How? For wireless devices to work, it emits and receives signals from the nearest cell towers. The further away it is from a transmission tower, the stronger its signal has to be. On a flight, your phone is more than 40,000 feet away from a tower, so its strength will increase to be able to reach the receivers below. These signals are able to disrupt the pilots’ navigational and landing systems on board—which is probably the last thing you want to do on a flight.