When the first plane struck the World Trade Center, people were grief-stricken by thoughts of the tragic accident. When the second plane hit, those visions became an appalling, terrorizing nightmare. But by the time the third plane hit the Pentagon, nearly an hour after the first hit, hard questions were being asked: how could this have happened, why hadn’t air traffic controllers and the military done something? Nearly two hours after Flight 11 struck the first Twin Tower, the fourth plane came down near Pittsburgh, and the questions got even harder. Could the President have ordered the U.S. military to shoot down a commercial airliner carrying civilians? Was there sufficient time to grasp the situation and give that order?
Even now, as recovery and rescue operations continue, and we assess the terrible cost–human, economic, physical, and emotional–the questions linger and the clamor for answers grows more insistent. What should air traffic controllers have known and when? What should they have done? Where were the fighter jets?
To answer these questions, we start with another, more basic one: how do planes, pilots, and air traffic controllers communicate? Basically via radios and transponders. Radio communication between pilots and the ground is constrained by the sheer number of planes a controller handles at any given time: pilots and controllers use as few words as possible to request clearances for altitude changes, or to report other planes in the vicinity. Controllers therefore rely heavily on the transponders onboard airliners to provide a steady stream of data.
To double-check our own understanding of the system, IEEE Spectrum asked an air traffic control expert, who requested anonymity, to review how the transponder system works.
How transponders work
Airlines file flight plans for all their aircraft. A plan may be modified to accommodate weather systems, for example, but when a pilot radios an airport tower for clearance to take off, the procedure is always the same. The pilot is asked by the controller to squawk an assigned code on the transponder that ties the aircraft to the flight plan filed for it. The pilot manually dials the code into the transponder.
The secondary radar system used in air traffic control interrogates the transponders aboard aircraft much as electronic toll collection systems like E-ZPass interrogate the transponder in a car. But the aircraft transponder sends back more data than a vehicle ID–it sends what controllers call the data block: the aircraft’s call sign (airline code and flight number), altitude, airspeed, and squawk code. The data block shows up on the controller’s display screen.
In an emergency, predefined codes that alert controllers to certain situations can be dialed in to the transponder: 7700 indicates a general emergency, while other codes mean electronics failures, hijacking, or lost communications. Switching to one of these codes transforms the data block on the controller’s display into a flashing block meant to attract attention.
Losing transponder contact with an airliner would prompt a controller to ask the pilot to reset the device. When all communication with an aircraft breaks off, procedures are in place that allow controllers to alert the military to a developing situation and to track the plane using primary radar. Primary radar is tied into the perimeter air defense radar for joint use by air traffic controllers and the military. It paints the skin of the aircraft to give controllers an idea of where the plane is, but does not give airspeed or altitude or any other data. It also is a line-of-sight system, so terrain can prevent the system from giving even limited positional data about the plane.
So what happened?
Mark Clayton of the Christian Science Monitor was able to piece together the air traffic controller’s story of American Airlines Flight 11, the first plane to strike the World Trade Center. Here’s a summary of his story [see his complete article at http://www.csmonitor.com/2001/0913/p1s2-usju.html].
Takeoff was routine for the flight. The transponder’s squawk code was given by the controller, dialed in by the pilot, and worked properly. About 15 minutes after takeoff, the controller requested that Flight 11 climb from 29 000 feet to 31 000 feet. Nothing happened. Repeated requests to climb went unanswered. Changing to an emergency radio frequency also produced no results, nor did it prompt any kind of reply from the plane. Even this was not unduly alarming to controllers, who sometimes lose radio contact with aircraft. Within seconds, however, the data stream from the transponder stopped. Controllers hoped it was just an electrical problem, until the plane, visible via radar, turned south.
At this point the pilot on Flight 11 pushed a button on his yoke (the plane’s steering wheel) that enables him to fly and speak with controllers without taking his hands from the plane’s controls. He pushed it intermittently during the brief flight, letting controllers hear the "heavily accented voice" of at least one hijacker. At this point controllers knew they faced a hijacking situation.
Do we shoot it down?
Procedures dictate that controllers alert the U.S. military when a hijacking is known to be under way. The typical response is for the Air Force to scramble intercept jets. Two F-15s were dispatched from Otis Air Force Base, in Falmouth, Mass., but Flight 11’s radar signal was lost at almost the same moment, somewhere over Manhattan.
Similarly, according to published reports, fighter planes were scrambled to intercept the plane that eventually hit the Pentagon. They, too, were just not in time to intercept the hijacked plane before it reached a target.
Had they been in time, though, would the commanders have had the moral fortitude to order fighter jets to shoot down a commercial airliner carrying civilians? President Bush, on the recommendation of Vice President Cheney, apparently decided to do just that, after both Twin Towers were struck. But it was too late. For reasons that will no doubt be thoroughly explored in the coming months, data from the flight control system could not be assimilated fast enough to get fighters where they needed to be–airborne and ready to complete missions not dreamt of in the worst of nightmares.