How do Apple Watch and iPhone communicate with one another in airplane mode?

Question

My iPhone and WiFi-only Apple watch are sync'd in such a manner than when putting the watch in airplane mode, the phone also enters airplane mode. My understanding is that airplane mode disables all radio activity: WiFi, cellular, and Bluetooth.

When entering airplane mode on the watch, presumably the watch tells the iPhone to enter airplane mode as well. So now both devices are in airplane mode.

But I'm curious about what happens when airplane mode is turned off. Let's say I turn off airplane mode on the watch. WiFi and Bluetooth are now restored on the watch. Fine. It then reaches out to the iPhone to tell it to turn off its airplane mode. But theoretically the iPhone shouldn't be able to respond, should it? All of it's radios are off after all. Yet somehow it works.

How is the watch able to give instructions to the iPhone when the iPhone's radios are off?

Answer 1

By default Bluetooth is left on and that’s how the devices communicate.

• https://support.apple.com/en-us/HT204234

When you turn on Airplane Mode, it turns off all radios except for Bluetooth. If you turn off Bluetooth while you're in Airplane Mode, your device will remember the next time you turn on Airplane Mode.

The primary risk to aviation safety is ground based interference of radar altimeters, but many regulations on cellular network power and transmissions exist still out of an abundance of caution or other concerns like presumably more attentive passengers during higher risk phases of flight like takeoffs and landings.

Answer 2

When you enable Airplane Mode, Bluetooth and Wi-Fi can still be enabled. This is because Bluetooth and Wi-Fi are typically approved at all times during flights.

You can turn off Wi-Fi or Bluetooth and it will fully turn it off (unlike when Airplane Mode is off, where turning it off Bluetooth retains Watch connectivity) with a diagonal line through the icon.

After turning on Airplane Mode, review the Bluetooth and Wi-Fi settings to see the current status. Airplane Mode will remember the choices you made, so if you turn off Bluetooth and Wi-Fi it will turn them off again in the future.

https://support.apple.com/en-us/HT204234