On 9/11, the terrorists turned off the transponders on three of the four hijacked aircraft. With its transponder off, it is possible, though more difficult, to track an aircraft by its primary radar returns. But unlike transponder data, primary radar returns do not show the aircraft’s identity and altitude. Controllers at centers rely so heavily on transponder signals that they usually do not display primary radar returns on their radar scopes. But they can change the configuration of their scopes so they can see primary radar returns. They did this on 9/11 when the transponder signals for three of the aircraft disappeared.94
http://www.9-11commission.gov/report/911Report_Ch1.htm
Interesting. I’d just assumed that the radar return would be easily able to distinguish a lear from a 777.
In light of the tinfoil theories the plane might be reused I find that tidbit disturbing. I’d thought it would be difficult for such a large plane to get close enough to a big metro w/o being busted by radar returns.
If the flight controllers aren’t using the returns in that way could a 777 could ‘lie’ as a learjet and get close to a major metro area?