Force quit might be needed when using Pix4Dcapture:
Force quit Pix4Dcapture and restart the app when changing the drone that is selected for flying.
1. Double-click the Home button of the iOS device. This brings up the app switcher.
2. Scroll the apps to find the app to force quit.
3. Tap the corresponding app window and swipe it up off the screen (upwards or downwards).
4. Go back to the Home screen of the iOS device and select Pix4Dcapture.