The Riddle of the Transponders

What was the value-added benefit for the 9/11 hijackers in turning off their transponder signals?

The planes remained visible to radar; the transponders merely ID'd the flights. And yet the transponders of all four flights were switched off. What was gained?

I think the answer is found in the proliferation of wargames on September 11, particularly the exercise called "Vigilant Guardian": the live-fly simulation of hijackings in the US Northeast staged by the Joint Chiefs and NORAD the very morning of the attacks. (Health advisory to coincidentalists: chew carefully before digesting.)

At one time on 9/11, as many as 22 aircraft appeared to be hijacked. Suddenly, the virtue, now verging on necessity, of switching off the transponders becomes evident. With loss of transponder signals the planes became bogies, and discriminating real from simulated hijackings became next to impossible.

This confusion compounded the paralysis already introduced to the system by drawing most of the Eastern seaboard's combat-ready interceptors into Northern Canada for the wargame "Northern Vigilence," and changing the standing orders for a shootdown in June 2001 by removing the discretion of field commanders and placing it solely in the hands of the Secretary of Defense.

For more regarding the multiple wargames of 9/11, see chapter 19 (a .pdf is available online here) of Michael Ruppert's soon-to-be-released Crossing the Rubicon.