Use a State -> Display State (off) context in your profile.
Then, in your task, you can either:
Use the action Net -> Airplane Radios [ Bluetooth: Off Cell: Off NFC: Off Wifi: Off Wimax: Off ] # this will set what radios are DISABLED when airplane mode is turned on.
Plus use Net -> Airplane Mode [ Set: On ]
Or, you can:
Use the action Net -> WiFi [ Set: Off ]
Plus use the action Net -> Mobile Data [ Set: Off ]
Both methods have positives and negatives. Turning the cell radio off will also affect calls. Turning mobile data off doesn't work on all devices, and Google apps may turn it back on.