June 17, 2014

Why You Are Delayed: How Air Traffic Control Delay Programs Work

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Written by: Phil Derner Jr.
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If you follow NYCAviation on Twitter, you are well aware of our delay update reporting. But what are things like a “ground stop,” and who’s behind these delays?

Flight delays happen for many reasons. Airlines can impose delays on themselves (which, needless to say, they don’t like doing any more than you), for mechanical or operational reasons, or for a multitude of possible reasons. But when weather hits, most massive large scale delays are put into place by the decisions of air traffic control (ATC).

Before you start getting angry at ATC for your vacation starting two hours late or missing a connecting flight, there is a very specific reason for its decision – your safety. When enroute or local airport weather slows or prevents flights from operating, aircraft need somewhere else to go and airspace can become very congested. The best option in this case is to delay flights from departing so as to space out that traffic into a controlled, safe flow into your destination airport.

Imagine driving in your car and there’s an accident up ahead that closes two of four lanes of highway. Now all of those cars have to squeeze into those two lanes, slowing them down or bringing them to a total standstill. It is the same way with aircraft that following specific routes (airways) in the sky, but the only difference is that an aircraft cannot stop in mid-air; that standstill is not an option.

With that, there are various traffic management initiatives (TMIs) that ATC can implement in order to space out that traffic.

Ground Delay Program

Ground Delay Program (GDP) is the name for what are essentially arrival delays into a given airport. In a GDP, ATC will assign each individual flight going into the constrained airport to a specific departure time. These can be minor and even unseen or realized by passengers, or drag on for many hours depending on where you are coming from and what/if any of the constraints that caused the GDP are impacting your particular path of flight.

Ground Stops

Ground Stops are ATC’s way of saying “okay, everyone stop and give us a second!” A ground stop will prevent flights from chosen regions from departing until a determined time. That time is for an update only, though ATC does give a heads up on whether there is a low/medium/high chance of the ground stop extending. Often times, a ground stop will lead to a GDP as a way to recover from it.

Airspace Flow Program

Airspace Flow Programs (AFPs) are something that becomes fun for dispatchers because they involve making operational decisions based on routings. An AFP is similar to a GDP, but for a region of airspace as opposed to a specific airport. This is usually implemented because of something like a thunderstorms, and ATC literally draws a line in the sand which is usually a specific boundary of airspace that a control center manages (such as the line where flights would enter Cleveland Center from the west, for example). This line brings with it a proposed delay amount on a flight by flight basis, if the flight intends to cross that line. An airline can avoid the delay altogether by simply flying around the line. The airline has to make a choice on which one is more viable. Trying to fly around the line to avoid a 35 minute delay, but adding 40 minutes to enroute time might not be worth it. Also, can the aircraft even carry that much fuel? Many variables come into play. (Note: these decisions are made at the airline’s headquarters level, not at the airport; don’t waste your time yelling at a gate agent or other employees.)

Miles in trail

On average, the separation between aircraft in busy airspace is kept at five miles behind one another. On days with weather or extra congestion, ATC may assign miles in trail, or, increased separation. This separation may be assigned to a specific fix or separating departures from a particular airfield, with 20 or 30 miles separation not being uncommon.


As mentioned earlier, of course, an airplane does not have the option to pull over to the side of the road. However, the closest to that would be a holding pattern. A holding pattern is a point in the sky (a waypoint or over a navigational aid) that ATC tells a plane to hold at. Upon reaching that point, the flight will fly in a racetrack pattern (4 minutes per lap) until ATC releases them to either continue, or maybe to hold at another point thereafter.

Holding can take place for several reasons. Common ones include a thunderstorm over the airfield that prevents flights from landing. Thunderstorms tend to be fast moving so hopefully arrivals can be told to do a few turns in holding and then come in once the storms have cleared the field. That is, if the volume of traffic and the flights have enough fuel to hold long enough. Otherwise, you’ll start seeing flights divert to other airports.

Note: Pilots can request to be placed into holding by their own request. There may be a mechanical issue or a checklist that they must address before continuing on, and ATC usually has no problem accommodating.

As annoying as those delays are, they are intended to keep the airspace manageable by air traffic controllers, all in the interest of safety.

What conditions cause airspace volume delays? How can that be forecast? What does the implementation of these initiatives mean to pilots and dispatchers from their side of the operation? We’ll address that later. Stay tuned!

Phil Derner founded NYCAviation in 2003. A lifetime aviation enthusiast that grew up across the water from La Guardia Airport, Phil has a background in online advertising and airline experience as a Loadmaster, Operations Controller and Flight Dispatcher. You can reach him by email or follow him on Twitter.

About the Author

Phil Derner Jr.
Phil Derner founded NYCAviation in 2003. A lifetime aviation enthusiast that grew up across the water from La Guardia Airport, Phil has aviation experience as a Loadmaster, Operations Controller and Flight Dispatcher. He owns and operates NYCAviation and performs duties as an aviation expert through writing, consulting, public speaking and media appearances. You can reach him by email or follow him on Twitter.



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