The Mac Observer (see linked article below) has a great piece on how you are able to use Bluetooth headphones like Apple’s AirPods on an airplane. Most people think that once you turn on Airplane mode you can’t use Bluetooth but that isn’t the case.
One thing this article doesn’t mention is a new feature in iOS 11 that leaves Bluetooth on when you are currently using Bluetooth headphones and enter into Airplane mode. It used to be that when you did that your headphones would stop working until you manually turned back on Bluetooth after enabling Airplane mode. But with iOS 11 it puts you into Airplane mode without severing the Bluetooth connection.