Just to be accurate;
There are places in this country that "turning off the transponder" will make you invisible. Flight 93 flew through one of those areas. The US is a big country and no, radar coverage is not seamless. As far as the "truthers" claims go, Total BS.
Just to really be accurate the primary radar return was always present on radar, it's only that the specific sector (controller) working the aircraft at Indianapolis could not see it.
Rather than getting into technicalities about it, consider it a data processing issue, not a radar issue at all. At the time the FAA was starting to implement ATC Center Sectors with no primary capability at the radar display. Since a transponder was/is REQUIRED in Class A airspace (above 18,000' MSL) and if an aircraft loses it's transponder it MUST exit Class A airspace and since that Sector only worked high altitude traffic (on Jet Routes above 18,000' MSL) it made sense.
Remember Indianapolis Center did not know about the events in NY yet, so they were concerned about an aircraft loss, not one that had turned around and headed the other way. By the time they realized the REAL SITUATION AA77 was several hundred miles East of it's last known position.
After the fact, AA77's entire track from Dulles to just short of the Pentagon can be plotted via radar returns.
