You'e on a flight. You're sitting comfortably in your seat with your seat belt on, ready to take off and then this happens - 

“Please set your portable electronic devices, including any mobile phones, to flight mode.”

The minute you're on a flight, you make sure that your cell phone is either switched off or on flight mode. Even if you forget, the flight's safety announcement will remind you of the same. If you're still not listening, the flight attendant will make sure that you do! Because, if not, it will mess up with plane's electrical systems and the plane might just blow up mid air. You get the drill, right?

Source: lyertalk

Well, no. That's not what's going to happen.

Most of us believe that it's just a procedure and not following it will do nothing, while some of us are sure that the plane will just crash and everybody will die. The truth is, both the assumptions are incorrect. Till date, there is no such evidence that proves that, if not switched to flight mode, signals from your cell phone can cause a plane's malfunction leading it to a crash. In short, no plane crash has ever happened because a passenger forgot to switch his cell phone to flight mode.

So why exactly are we asked to turn on the flight mode when we are on a flight?

Source: vipbags

Because the signals can jam the network on the ground.

There are multiple reasons for the safety concern. As you are travelling very fast, and are more than 10,000 feet in the air, the phone signals will try to connect to various towers and will send out stronger signals because the weaker the signals your cell phone picks up from the tower, the stronger the signals it sends out to try and get a response. Basically, your cell phone communicates with several cell towers and attempts to maintain a connection at all times and if the towers are far away, the phone will boost its signal so it can keep communicating with the towers. This has a potential to interfere with the cellular signals on the ground.

Source: thepointsguy

So the pilots don't get annoyed.

Another important reason why you need to switch your phone to flight mode is because you don't want to annoy the pilots. You may have heard the unpleasant noise from an audio system, that occasionally happens when a mobile phone is nearby, right? A phone’s radio emissions can be very strong and can go up to 8W which causes this noise due to parasitic demodulation. Now imagine that noise on the pilot's headset, while he is receiving a critical information from ground control. Imagine that noise, constantly, from the phones of over 100 passengers! Can you imagine how annoying it can get? You need to switch your phone to flight mode because it's common courtesy, and by doing so, you show your appreciation to the people doing their job to get you where you want to be.

Source: The Wall Street Journal

In a blog post for Airline Updates, a pilot wrote - 

"It is not safety critical, but is annoying for sure. If say 50 people on board are inconsiderate enough who can’t be bothered to switch their mobile phone off, there will be 50 phones constantly looking for cell towers at maximum power. That is a lot of radio pollution."

Now, do you really want to annoy the pilot who is about to get you safely to your destination? I don't think so. So just switch off that cell phone the next time you're on a flight. We can all do without it for a few minutes.