Recently, when traveling with my iPad Pro and Apple Pencil, the typical announcement to put devices into airplane mode was made. Dutifully, I followed the crew instructions. Shortly after takeoff, I wanted to sketch out an idea.
I pulled out my Apple Pencil, fired up my up the iPad Pro and opened Apple’s Notes app. After tapping on the sketch icon with my finger, I touched the Pencil to the screen and nothing.
“Maybe my pencil was not charged,” I thought. I swiped down from the top center of my screen to reveal the widgets and check the charge status. Nothing.
The fact that both my iPad battery level and pencil level did not appear was momentarily confusing.
I removed my Pencil cap, placed the lightning end into the lightening port of the iPad Pro and a notification appeared alerting me that Bluetooth was off. Did I want to turn it on? I tapped “Turn On.”
Then, my pencil charge status appeared.
I now realize that airplane mode turns off Bluetooth too. Without Bluetooth communication – no working Pencil.
Remember, simply turning Bluetooth back on while in airplane mode does not result in the Pencil working. One must actually turn on Bluetooth and insert the Pencil into the iPad to re-establish the link between the two devices. You can swipe up from the bottom of your iPad and tap on the Bluetooth icon to turn it back on or Settings > Bluetooth > On. There is a support article by Apple that mentions the fact that you need to pair your pencil again after turning on airplane mode.
Basically, Airplane mode makes your iPad forget your pencil and you have to reacquaint the two together by turning on Bluetooth and re-inserting your pencil. Hopefully, this solution solves a problem you may have experienced.
iPad Notebook 
This hasn’t happened to me yet, but it was only a matter of time. I have not seen this issue written up anywhere else. Thank you for solving what surely would have been a really annoying mystery to me, and many others I’m sure.
This is actually a great way to save the battery life of your Apple pencil! Every time you move your Apple pencil, it wakes up and drain the battery a little bit. If you carry it everywhere with you in a bag but don’t use it much, you still drain the battery! Once it’s unpaired it just stays off until you plug it again in the iPad! And then you are good to use it right after pairing it in because it’s not depleted!
thanks great info