Pocket Planes uses iCloud sync between devices. Make sure you have iCloud enabled on both devices, that you have the Documents and Data set to ON in the iCloud settings as well as iCloud sync enabled in the Pocket Planes settings menu.
Alternatively, you can backup your entire device with iTunes or iCloud and restore your game onto the new device.
Note, Pocket Planes only uses Game Center for achievements and iCloud for syncing between devices and does not maintain a backup of your game once deleted from all devices. If you've deleted the app from all of your devices you will need to rely on a recent iTunes or iCloud device backup for restoring your game.