from Reg Article May 07 2009: US air traffic faces 'serious harm' from cyber attackers
The solution is obvious, create a network of VPN nodes with multiple redundant routes, that utilize end-to-end encryption and authentication and connect your 'computers' to that. Now don't tell how/why it can't be done, tell me how it can be !
"when hackers took control of Federal Aviation Administration computers in Alaska. By exploiting the administration's interconnected networks"
"Two separate attacks in 2006 hit the FAA's remote maintenance monitoring system and its air traffic control systems. The latter forced the FAA to shut down a portion of ATC systems in Alaska"
"The report went on to fault the FAA for employing woefully inadequate IDS, or intrusion detection systems. .. none of the IDS sensors monitor mission critical ATC operation systems"