Humint Events Online: About Those Transponders

Wednesday, November 24, 2004

About Those Transponders

One big mystery from 9/11 is why the hijackers turned off the airplane transponders. While it is possible they thought this rendered the plane invisible, if they knew enough to turn off the transponders, then they should have known that the planes could still be detected by normal radar. Moreover, since they were fairly sophisticated hijackers (in theory), they should have known that turning off the transponders actually alerts air traffic control that there is a problem with the plane.

So why turn off the transponders?

One reason is that it may have allowed a plane switch. Thus if a hijacked plane landed at, for instance, a military airbase at the same time as another plane was taking off, there would likely be only one plane visible by radar. The hijacked plane could land, be switched with another plane, and then the hijacked plane could take off later and be thought of as a different plane.

Yet, in light of the wargames that were running on 9/11-- turning off the transponders takes on a new meaning. That is, transponders are another name for IFF beacons, where IFF means Identify Friend or Foe. Thus, by turning off the transponder, a plane becomes an enemy by definition since it is not showing an IFF signal.

Thus the hijacked planes became "bogies", in essence. And given that the Northern Guardian wargame dealt with Russian fighters, it is intriguing to speculate that turning off transponders on the hijacked planes somehow merged into the exercise.

Note-- one of the real oddities of 9/11 for me (and others), is that none of the pilots on the four planes ever signaled there was a hijacking to air traffic control. This can be done easily by punching in 7500, which is the code for a hijacking, on the transponder. I just have a hard time believing that every pilot was caught completely by surprise by the hijackers-- and that none of the pilots heard the struggles outside the cockpit, and none of the pilots ever had time to either alert air traffic control or to type in the hijacking code on the transponder.

Second Note--Another oddity is that flight 175 apparently CHANGED its transponder twice before shutting it off. I don't think anyone understands what that was all about. Was it possibly a wounded pilot trying to type in the hijacking code-- or the hijacker trying to be tricky?


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