During a server maintenance, you can stay on the flight tracking page in offline mode and continue your flight even if the server is not accessible.
At the end of the flight, a dialog will indicate that you need to wait for the server to be up and reconnect to OnAir in order to resume and register the flight.
Use the “Retry” Button or restart OnAir to do so.
WARNING: If you shut down OnAir, the flight is not tracked anymore and when you resume the flight, the aircraft will be positioned where it was when you shut down OnAir.