I once left my phone turned on (accidental oversight) during a short training flight. After landing, I saw the battery was almost completely drained. In flight, cell phones “see” a LOT more cellphone towers than they normally do while stationary (or even driving) on the ground. It sends a “handshake” signal to EACH tower it sees (this is primarily to determine if roaming fees would apply should you make a call).

Think about that. Now imagine if every passenger and pilot left their phones on… thousands of people airborne over any major city, completely overwhelming the cellular system with “here I am” signals just by having their phones powered up.

Source: https://www.quora.com/Do-pilots-keep-their-phone-on

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