March 2, 1996
The very idea of going around in circles in a hold at 150 knots
is pretty ridiculous, particularly when it goes on so long that
you end up diverting to an alternate. Makes you wish you'd taken
the bus. Nonetheless, the feds really do want to minimize airborne
holding and they use something called Flow Control to do it.
Flow Control was originally implemented as a temporary measure
to regulate traffic during the 1981 PATCO strike. It has survived
to become a permanent part of the system. The program is managed
by the ATCSCC (Air Traffic Control System Command Center), formerly
located in FAA HQ in Washington, and recently relocated to hightech
quarters in Herndon, Virginia.
The program is complex and while it covers stuff that's not always
important to GA pilots, you can still be affected by Flow Control.
What they're trying to do, essentially, is to balance the arrival
supply with the arrival demand.
Because any airport has the potential of receiving more instrument
arrivals than it can handle (as happens during special events
such as the Indy 500 or the Flying Farmer's Convention), even
small airports are candidates for the dubious benefit of traffic
management. Here's a thumbnail synopsis of how it works.
Step one is to determine the arrival rate, which is the number
of airplanes per hour that the airport can handle. The arrival
rate is determined by on-airport ATC, since it's influenced by
almost anything: runway availability, wind, weather, staffing,
the number of departures expected sharing runways with arrivals,
even the phase of the moon (no kidding-a bright moon can make
it easier for pilots to see the airport and other airplanes, allowing
more efficient visuals).
Every airport is different, but under ideal conditions, O'Hare's
arrival rate will be as high as 105 airplanes per hour. Under
worst case conditions (say, one slippery runway available for
all operations), it might be as low as 18.
ATCSCC takes a look at the anticipated demand for the period in
question and compares it to the arrival rate. They actually work
in 15 minute increments, but for our discussion, we'll use an
hour. "Demand" consists of all airplanes that expect
to land at the airport in the hour under consideration, including
those airborne and those on the ground awaiting departure.
If the demand is less than the supply, no problem. Airplanes en
route keep coming, departures go on schedule, and the Flow Control
guys relax and drink coffee. But if 100 airplanes are expected
to arrive during the hour, and the arrival rate is 80, 20 airplanes
will enter the terminal airspace with no place to go, except around
in circles at some holding fix. This is not considered to be a
The solution is to lower the demand for the hour by 20 airplanes.
This could be accomplished with en route holding, but because
airborne holding wastes fuel and increases controller workload,
this isn't a preferred method. Instead, as soon as the ATCSCC
sees excess demand developing, they'll start issuing EDCTs (Expect
Departure Clearance Times). In effect, they artificially lower
demand by ground-holding 20 airplanes that haven't left yet. These
are sometimes called "gate holds," since the airplane
doesn't push back from the gate on schedule, but waits until it's
close to the ATC-dictated departure time. Airports without enough
gate space may park the airplanes in a run-up pad or other holding
The problem with this scenario is that the airplanes hit with
EDCTs of, say, one hour from the original proposed departure time,
are now going to be arriving in the next hour. If the next hour
is as busy as the last, the airport will have 20 more arrivals
to add to the 20 excess arrivals it already had, for a total of
40 airplanes it can't handle. So, the same scenario repeats, and
while it might seem logical for the original 20 airplanes to receive
priority this time around, it doesn't always work that way. Due
to the pecking order imposed, some (or all) of the original 20
that took the brunt of the blow last hour may get hit again.
So, what is the pecking order? At the top of the heap are airplanes
that are already airborne, since en route holding is a last resort
and rarely used. Airplanes inbound from Alaska and Hawaii usually
get a free pass, too. Their long en route times make it difficult
to judge how busy things will be when they arrive. Airplanes departing
foreign points for arrival in the U.S. are exempt for similar
reasons, which is just as well. Can you imagine an independent-minded
French controller heeding the flow-control demands of some FAA
Which leaves the domestic flights from within the conterminous
United States. And among those domestic flights, the shorter the
flight, the better the chances it will be hit with some sort of
ground delay. After all, if the airport is saturated, airplanes
that are closest to the destination present the most immediate
problem. Airplanes that are close by, but not yet airborne, are
the easiest to hold back. Unfortunately, they're often the last
to be released when things improve. That's because their very
proximity makes them useful pawns in the traffic-management game.
The slightest change in any single factor in the equation can
affect the entire plan. Let's say that out of those 80 airplanes
that the ATCSCC thought would arrive during a particular hour,
10 are arrivals from New York airports. But due to a stuck mic
on one of New York tracon's frequencies, all of the airplanes
take an unexpected 30-minute delay.
Now there are 10 extra arrival slots, but airplanes that are holding
on the ground at distant locations can't possibly get to the arrival
airport in time to fill the gaps. Suddenly, airplanes being held
on the ground at relatively close-in airports become a hot commodity.
Nearby Centers and terminals start scrambling to find airplanes
to fill the vacant slots.
This being the case, the closer the departure airport is to the
arrival airport, the more erratic the accuracy of the ground-delay
times. If you're given an EDCT delay from an airport that's a
short distance from the impacted arrival airport, keep your handheld
tuned to clearance delivery while you're cooling your heels. You
may be called to serve on short notice.
So, is the point of Flow Control to avoid airborne-holding altogether?
Surprisingly, the answer is no. In fact, airborne holds of 5 to
15 minutes are considered to be ideal, in that they keep pressure
on the airport, assuring that the final-approach courses are always
full, with no gaps caused by a lack of arrivals to fill them.
For Traffic Management folks, success is measured only by a constant
flow of airplanes on the final approach courses, at minimum spacing,
with airborne-holding being kept to an acceptable level. The fuming
of pilots on the tarmac at sunny locations hundreds of miles away
is of little consequence. Lucky for them, flow controllers only
have to look at the big picture, not the annoyance on the faces
of passengers who thought they had no time to spare, so they went