Make sure that the date/time on your device is set correctly. If the date or time on the device is changed this can cause issues within the game. If the date/time on your device is correct and you are still having this issue you can follow these steps to correct it.
-Change the date on your device to far enough in the future for all your planes to land.
-Open the Pocket Planes app and confirm all of your planes have landed. (Do not start any new flights).
-Set the date back to the correct date and open the game.