``Squawking'' its way into US airspace, the American Airlines DC-10 appears as a small, glowing line at the upper right corner of George Molberger's radar screen - about 100 miles out over the Atlantic. Traveling more than six miles high at about 480 miles an hour, Flight 51 easily catches Mr. Molberger's attentive eye as he gauges its position relative to a dozen other jets traveling through this high-altitude chunk of airspace.
Here at Boston Center, one of 20 Federal Aviation Administration Air Route Traffic Control centers around the United States, controllers govern 163,000 square miles of airspace. Like a relay runner grabbing a baton, Boston Center takes over guidance of planes that have just left the space handled by airport controllers.
Jets like Flight 51 must highlight themselves using an on-board transponder that ``squawks'' a plane's location louder than radar echo alone. That's helpful, because at this altitude big jets that are six miles apart can converge in 20 seconds.
Molberger radios Flight 51 to make a 20-degree course change that will take it a few miles out of its way, causing it to fly behind a slower DC-9.
``On any given night, a sector can go real smooth,'' says Daniel Bunce, a five-year ``full performance'' controller at Chicago Center in Aurora, Ill. ``If you've got a 10-year veteran working, no thunderstorms to worry about, flow restrictions are working, and the other center fed you the correct number of airplanes with the proper mileage in-trailing [separating] each flight - it all goes smoothly.''
But in bad weather, or with traffic approaching 20 to 25 planes in a sector, keeping jets separated can be challenging. An overworked or inexperienced controller can complicate matters.
``If you're trying to move planes as fast as you can talk ... then in a real troublesome situation you have to make the exact right call at the right time, one after another,'' says Mr. Bunce. ``Experienced veteran controllers might be able to pull that off, but a lot of times the newer controllers - and I myself am one of them - can't do it.''