So I recently did 2 long haul flights (~11 hours each), and before the first flight, I turned it off as usual, with around 30% battery, and when I arrived, the battery was dead, and wouldn't turn on.

After the 2nd flight, I turned off the radio first, then the device, and had the same result starting with a 50% battery.

Any ideas? I'm running 4.7.0.75.

I do several flights a week and have zero battery drain when the unit is off.

Could it be you're just letting it go to sleep and not turning it off? When sleeping it's probably constantly searching for a network and that is a major battery drain. I fly all the time and mine never loses battery when off.

That seems plausible

Can you explain the differences. Does holding the power button not turn it completely off?

I selected the "Power Off" option on the menu, and waited for it to power down so that pressing a button (other than holding down the power button of course) would not bring the screen back up. I would presume this would be "Off".

I would bet it hit the time you have it set to turn on automaticlly. try turning off next work connections too when you are on a flight.
my guess.

I had this happen recently too. I had about 30% left, turned off the radio but did not do a full power off so I could use it on the plane. My flight was only about two hours, I never used it, but when I got to the other end it was dead as a brick? Since then I now turn the radio off first and then do a full power off, defeats the purpose of using it "offline" on the plane though. Also, I don't have any auto turn on or off settings, so that won't be it?

several times i had a cam on the flightdeck with me on long flights and when i try turning the cam on it has no batt left . the same goes for the phone batt i think but not sure yet . when ever i have a flight to some parts in mexico where u r over g level this hapens offten .