SMS works over the cellular network and uses the cellular carriers' messaging infrastructure. Your device cannot "fall back to SMS" during the flight, because (presumably) your device is in airplane mode with only WiFi enabled. You cannot use SMS over the internet. You can use Messages by Google (which uses RCS), iMessage and several other messaging services over the internet. The messaging-only feature on AC provides you with a very filtered connection to the internet, with the filtering designed to make sure only text messages via specific internet-based messaging services will actually go through. This same filtering is also making sure that you cannot use your phone's WiFi calling service offered by your cellular carrier.