From an AWST article:
The FAA's 325 en route centers, Tracons and towers, and the air traffic
controllers who staff them, "did a superb job" getting all aircraft out of the sky
in little more than 2 hr., Schuessler said. But it was all in a day's work,
however extraordinary the day, according to John Carr, National Air Traffic
Controllers Assn. (NATCA) president.
At any given time before Sept. 11 there would be about 5,000 aircraft
airborne, Carr commented, and when FAA ordered all aircraft down, there
were 4,546. The number had been reduced because of the ground stop 19 min.
earlier. Airplanes were landing at their destinations during this period, but
none were taking off.
Thus controllers weren't handling more aircraft than they
usually do; they were simply bringing all of them down, Carr noted. They
landed them about twice as fast as they would have done normally, but with
no takeoffs the numbers weren't unusual. Controllers used minimum spacing
and maximum efficiency, Carr said, but "we didn't cheat." They broke no
rules. With no commingling of departures and arrivals, and no need for en
route separation of climbers and descenders, the job was simpler in many
respects. "Everybody was a descender," Carr said. "This was not unlike a
very, very heavy arrival rush for every airport in the country."
Considering that controllers hadn't ever tested or trained for clearing the sky,
the process was surprisingly smooth, Carr said. The easiest aircraft to deal
with were commercial flights that were near their destinations, and general
aviation (GA) aircraft--low, slow flyers, perhaps in the vicinity of the airports
they were headed into or out of. For the GA
pilots, it was a question of
sending an order to land, finding the nearest available and suitable airport,
directing them to that airport and bringing them down.
The hardest part often turned out to be overcoming disbelief among pilots,
even a few airline pilots, that everyone was going to land. Some GA
flying VFR didn't know what had happened.
Another problem was commercial aircraft that were too big for the nearest
available airports. It was up to flight crews to determine whether they could
land at airports to which they were being directed, and in some cases airline
operations people told pilots where the carrier wanted them to land. These
factors produced what Carr termed "give and take between flight crews and
controllers" about where to land. "There were more than a few instances
where controllers had to talk pilots into understanding what they were being
directed to do," he said.
Aircraft headed for the U.S. from other
countries were not admitted after the
order to land. Halifax, Nova Scotia, was
one of several Canadian airports filled to
capacity by arrivals from Europe. Two
days later the aircraft were permitted to
leave for the U.S., starting with this
Coordination between en
route centers, Tracons and
airport towers was normal.
Whatever facility was
controlling an aircraft at 9:45
continued to control it until a
pilots and controllers "is no
different during an
emergency than it is during a
normal operation," Carr said.
"Air traffic control is a very
tightly woven net of
responsibility. It is not unlike
a relay race, with airplanes as
batons." Aircraft were passed
in sequence from centers to
Tracons, to towers, to ground
controllers, to gates. "It
wasn't hard," Carr said. "It's what we do. We work airplanes . . . We didn't
find it to be even the least bit extraordinary. We found it to be challenging,
extremely important, deserving of due caution and care . . . But I don't think
you could find a single controller who would tell you they did something
extraordinary that day. They did their job."
"That's so stupid! If they're so secret, why are they out where everyone can see them?" - my kid