FYI, use airplane mode

[jackal24]

I do a lot of long flights. I've found that if I leave the cellular radio on during a flight, I will go from 100% to dead on a 6 hour flight. Put the watch in airplane mode and no problems

 

[Tim1954]

Apart from the fact that you're not meant to have your phone on during flight as it may interfere with navigation instruments and it's against aviation regulations....
In any case, cell towers are aimed low, they have a low angle of radiation for terrestrial users. Your device is constantly searching and trying to log into networks with marginal signal, thus depleting the battery. Airplane mode disables all radios and you can use the device to play games or music etc without any problem.
Some airlines have apps that work on wifi, so you can access in-flight entertainment on your device.
You get told at every flight to turn your devices OFF. . Please do so in future

 

[Jaycemiskel]

Lol this is kinda common sense, plus like Tim said, you're not supposed to have cellular radios on anyway.

 

[jackal24]

True, but it is easy to forget to do to your watch

 

[PookiePrancer]

Here's what you do if you want to keep the connection to the phone to control the music app: airplane mode on the phone, Bluetooth on by itself. You never have to mess with your watch, and since the watch is connected via Bluetooth...No crazy battery drain. Same for the phone, since the only thing it'll be connected to is the watch.