The Mode S transponders aboard Boeing 767 and 757 aircraft, such as those used on 9/11 as "flying bombs," deliver aircraft identification and altitude and can supplement FAA’s radar by "providing ATC and traffic alert collision avoidance system (TCAS)-equipped aircraft the ability to determine position and heading information," according to DoT. "A lesson from 9/11 is the importance of ensuring continuous transponder communication with ATC, regardless of a hijacking.

"Without the transponder switch in a fully active position, ATC can track an aircraft only by primary radar, which does not indicate aircraft identity and altitude," states DoT. "The loss of this information causes other aircraft to lose awareness of the flight in progress."

Another issue for the task force to consider has been power management. Besides disabling the transponder by switching the unit to "standby" on the control panel, hijackers could simply disconnect the appropriate circuit breaker. To prevent this would require changing the circuit breaker switch to ensure that the power supply was switched from the bus that hijackers could control to a secure power source.

At 8:21, American 11 turned off its transponder, immediately degrading the information available about the aircraft. The controller told his supervisor that he thought something was seriously wrong with the plane, although neither suspected a hijacking. The supervisor instructed the controller to follow standard procedures for handling a "no radio" aircraft.107

Minutes later, United 175 turned southwest without clearance from air traffic control. At 8:47, seconds after the impact of American 11, United 175's transponder code changed, and then changed again. These changes were not noticed for several minutes, however, because the same New York Center controller was assigned to both American 11 and United 175.The controller knew American 11 was hijacked; he was focused on searching for it after the aircraft disappeared at 8:46.123

The failure to find a primary radar return for American 77 led us to investigate this issue further. Radar reconstructions performed after 9/11 reveal that FAA radar equipment tracked the flight from the moment its transponder was turned off at 8:56. But for 8 minutes and 13 seconds, between 8:56 and 9:05, this primary radar information on American 77 was not displayed to controllers at Indianapolis Center.142 The reasons are technical, arising from the way the software processed radar information, as well as from poor primary radar coverage where American 77 was flying.

From 9:34 to 10:08, a Command Center facility manager provided frequent updates to Acting Deputy Administrator Monte Belger and other executives at FAA headquarters as United 93 headed toward Washington, D.C. At 9:41, Cleveland Center lost United 93's transponder signal. The controller located it on primary radar, matched its position with visual sightings from other aircraft, and tracked the flight as it turned east, then south.164

Originally posted by Reheat
Transponders on missiles?

Guidance was to be a simple radio control MCLOS system for use against daytime targets, but night-time use was considerably more complex because neither the target nor the missile would be easily visible. For this role a new system known as Rhineland was under development. Rhineland used a transponder in the missile for locating it in flight (as read by a radio direction finder (RDF) on the ground) and a radar unit for tracking the target. A simple mechanical computer guided the missile into the tracking radar beam as soon as possible after launch, using the transponder and RDF to locate it, at which point the operator could see both "blips" on a single display, and guide the missile onto the target as during the day.

A second development was underway that used only a single cross-shaped radar beam that was rotated while pointing at the target. Like the Rhineland system the missile was first directed into the beam via the transponder, and from there would keep itself centred in the beam. It did this by listening to the radar signal, if it was off course it would hear pulses instead of a steady signal, and automatically place itself back in the middle of the beam.

Also, not ONE pilot pushed the call button and radioed a distress call..imagine that!! A cockpit being rushed by a couple of small men with nothing but suposed box cutters..and not ONE thumb could push the radio button...funny huh?

At 09:28:17, a crew member in the cockpit began shouting, "Mayday! Mayday! Mayday!" over the radio amidst sounds of violence. A Cleveland Air Traffic Controller replied "somebody call Cleveland?", but received no reply.

Thirty-five seconds after the first Mayday call, the crew made another transmission. Someone in the cockpit shouted, "Mayday! Get out of here! Get out of here!"