If you cannot Hotspot or tether your phone to your iPad, try this…
I fly with two different Non-WiFi Android tablets. Since I know where I am going, I open the Fly App at home, where I have WiFi, and open the Maps and scroll around in order to lock them into the Queue, Now, you do need to know where you are going, the Maps that are Queued are not that large.
Here is a test, Pick an area away from your present location. Open the fly App, open the Maps and scroll over to that location. Pick an imaginary Home Point and zoom in, then slowly zoom out.
Now shut the app down, turn off the WiFi, and turn off the iPad. Now turn the iPad back on (WiFi should still be Off…), open the App, open maps and scroll around and check how good the resolution is as you scroll away from that Home Point. That will give yo an idea of how good the map will appear…