Does MotionX-GPS HD work when my iPad 3G in Airplane Mode?

No unfortunately not. When in airplane mode, the GPS chipset is disabled by the iPad. Third party iPad applications cannot override or alter the airplane mode functionality to enable the GPS chipset.

If you want to ensure network data is not used while using MotionX GPS HD, remove your SIM card. The A-GPS (Assisted GPS) chipset in the 3G iPad is designed to use cellular or WiFi data to speed up the acquisition of your GPS signal, so you may need to wait 15 minutes or longer to acquire a signal if your SIM has been removed.

You may also go to your iPad’s Settings to turn off 3G, Cellular Data, and Data Roaming. On your device go to Settings > General > Network for these options.