Posted by: Korry | December 20, 2011

Techie Tuesday: ATC Delays

The next week and a half is one of the busiest travel periods in the year (Thanksgiving earns the top spot). So aside from being ready to use the travel tips I wrote about here and here, there’s something else you should be prepared for: Delays. And while some delays are easy to understand, especially when the delay deals with thunderstorms and snowstorms that are right outside the airport terminal window, other delays are more difficult to comprehend. Chief among these difficult delays is the Air Traffic Control delay, formally known as the ground delay program. For this week’s Techie Tuesday, I hope to shed a little light on why ATC delays exist so that if you’re subjected to one you will understand what’s really going on.

ATC delays can basically be summed up as preventing 10 pounds from being shoved into a 5 pound bag. More appropriately, they’re designed to prevent 10 airplanes from being squeezed into airspace and runways designed for 5.

Lets take a typical busy (probably a hub) airport and assume it has two runways. One is most likely for landing and one is most likely for takeoff. On a good day (clear skies, low winds, etc.) the airport can land, let’s say, 30 airplanes per hour max. This is called the Airport Arrival Rate and basically represents what happens as one plane lands and not but a few seconds after it taxis clear of the runway the next plane is touching down. One after the next…again and again…30 planes per hour, and it looks like this:

Airport Flow - Normal Day

The problem is that life is rarely if ever perfect. Some weather starts to enter the airport area and controllers have to add additional space between arriving planes…or the pilots and air traffic controllers don’t fly or control flights as perfectly efficient as is needed to achieve the maximum arrival rate into the airport. Now, instead of landing 30 planes per hour, the airport can only handle 25…or 20…or in extreme situations 5 or 10 arrivals per hour. In other words, the airport has a “flow” issue and it looks like this:

Airport Flow - Bad Weather / Reduced Flow

Once this starts to happen, ATC has only two options available: put flights that are already airborn into holding patterns (where planes fly oval patterns to in essence pull over to the side of the road) or hold flights that have yet to take off on the ground. The second option is most ideal as it is easier on the controllers’ workloads and more cost-effective for the airline. This is called a “Ground Delay Program” and flights affected by them are given Expected Departure Clearance Times, or EDCT’s (pronounced “E-dicts”).

The Air Traffic Control System Command Center in Herndon, Virginia runs the show. They determine how to space the flights so that the actual airport arrival rate does not exceed an airport’s capabilities. It’s pretty complicated stuff, but they basically look at the expected arrival times of all the flights scheduled to a particular airport and then put them in order. Sometimes your flight’s EDCT may be just a few minutes past your original departure time, and other times–such as when you’re flying to an airport being hit with a nasty winter storm–you may have EDCTs that stretch into several hours. (By the way, if you’re on an international flight other than Canada, you’re most likely not subject to U.S. flow control).

The bottom line is that ATC delays are not just random events. There actually is a method to their madness. Without the central ATC Command Center managing flow control (which is relatively new), you would likely be experiencing more in flight holding or even diverts…which I’m guessing isn’t really something you’ve put on your holiday wish list this year.

Do you have a technical question about airplanes or the airlines? Email me at and ask away!

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