Flow control

Ground delay program

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An air traffic control ground delay program or FAA Flow Control is a traffic flow initiative that is instituted by the United States Federal Aviation Administration (FAA) in the National Airspace System.

This program is usually instituted when the following events occur at an airport:

  • inclement weather (i.e., reduced visibility, thunderstorms, snow),
  • a large volume of aircraft going to an airport or en route to another airport in the same line of flight,
  • an aircraft incident, closed runways,
  • a condition that requires increased spacing between aircraft, such as theInstrument Landing System (ILS) approaches vs. visual flight rules (VFR) approaches.

The main factor for determining if a ground delay program is needed is a number called Airport Arrival Rate (AAR). This number is set by the controlling air traffic facility. When the AAR is reduced by the supporting ATC facility, the Air Traffic Control System Command Center (ATCSCC), in Warrenton, Virginia(Vint Hill Farms Station) institutes a Ground Delay Program.

Ground delay programs (GDP) can affect various sections of the United States‘ airspace, as well as airports in Canada. That is because Nav Canadainherited the agreement between the FAA and Transport Canada that the Ground Delay Program would be implemented for departures from Canadian airports. GDPs are always assigned a “scope” and to a specific “center” or tier. For instance, if Atlanta was affected by a ground delay program, air traffic control could institute delays for just ZTL (Atlanta Center) or each center touching ZTL. It is possible that the delay may affect centers on the second Tier; that is, each center touching the first tier. Sometimes the scope of the ground delay programs is set by mileage; that is, all departures that are less than 1,500 miles from the affected center can be included.

These programs usually last for several hours and average delay minutes can vary as conditions change at the said ATC controlled area.

Each aircraft en route to a particular area is assigned an Expect Departure Clearance Time (EDCT). There are various ways that the FAA computes these delays. To streamline traffic demand the FAA utilizes a computer system called Enhanced Traffic Management System (ETMS)[1] which is used by Traffic Management Personnel to predict, on national and local scales, traffic surges, gaps, and volume based on current and anticipated airborne aircraft. Traffic Management Personnel evaluate the projected flow of traffic into airports and sectors, then implement the least restrictive action necessary to ensure that traffic demand does not exceed system capacity. This is also called a “wheels up time” and is when the FAA expects to give takeoff clearance. Aircraft have 5 minutes on either side of their EDCT to depart or they will be assigned a new EDCT and expect further delays.

When conditions improve, or when demand decreases, the ATCSCC begins running compressions. This is when the ATC facility can accept more traffic or users; that is, airlines cancel flight plans, thus causing openings. This causes other EDCT times to change and decrease delays.

It is to the flight planner’s advantage to file a plan as early as possible to get the earliest EDCT time possible. Once the AAR has been met, planes that filed later in the day will be pushed into the next time block while those that filed early will be assigned an EDCT closer to the time they filed (first come, first served).


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.



Understanding Flow Control


No one needs to tell you that air traffic congestion can delay issuance of your IFR clearance or cause you to suffer the dreaded Hold for IFR Release call with an indefinite departure time while the Hobbs goes tick, tick, tick.

NextGen, the FAA’s wide ranging overhaul of the Air Traffic Control System, is supposed to eliminate many delays by making the most efficient use of airspace and airports. In part, this is an acknowledgement that simply building more airports is not the answer due to cost, space and environmental issues to name a few, even though airports are responsible for about a third of flight delays.

Safety is the invisible elephant constraining any proposal to increase capacity. There is only so much airspace and so many airports. How then to make best use of what we have?

Flow Control

The answer is flow control. By regulating the rate at which aircraft enter congested resources such as airport airspace to a level no greater than the resource can accept, bottlenecks can be mitigated if not avoided altogether.

A related term, metering, regulates the time of arriving traffic into a terminal area so as not to exceed a pre-determined terminal acceptance rate.

IFR Slots are one way ATC exercises flow control. Back in 1968, traffic saturation at five critical airports (LaGuardia, Kennedy, Newark, Dulles and O’Hare) forced the FAA to institute the high density rule that limited the number of IFR departures and arrivals during certain hours of the day. The resulting time slots were allocated to air carriers for IFR landings and takeoffs during a particular 30 or 60-minute period. That rigid process has since become more flexible, but it has also become more conservative relative to the evolution of streamlined ATC technology. Ironically, the slot system at times becomes a constriction point working against the very efficiency it was designed to create.

Ground Delay Program

Bad weather forces runway arrival rates down, due in part to time-consuming SIDs and STARs. In 1999, Ground Delay Program (GDP) software introduced a form of flow control in which it reduced airline arrival slots for aircraft headed toward weathered-in airports. As its name implies, aircraft are held on the ground at their origin, resulting in less expense and greater safety than in-flight holds.

Today’s Enhanced GDP reassigns slots made vacant by cancellations or delays and fills them with operating flights.

Lasso that LAHSO

One easy way to increase runway capacity is via LAHSO—Land and Hold Short Operations, something we can do very well in GA.

At airports with operating towers and intersecting runways or taxiways, the tower may ask, “Can you land and hold short of runway 31, 3600 feet available.” They can also specify a hold-short point on a runway.

It’s up to you to accept or reject it as you wish, but student pilots or those unfamiliar with LAHSO should not participate. A LAHSO clearance does not preclude your right to reject the landing and go around, but once you accept the clearance you are obligated to comply with it like any other clearance. If you subsequently need to reject the landing, you are expected to safely separate yourself from other aircraft and promptly notify the controller.

Airports that conduct LAHSO are listed on page O-1 of the FAA U.S. Terminal Procedures Publication as well as in the A/FD. If LAHSO are in effect, that fact will be stated on the ATIS or AWOS.

In the future LAHSO may be extended to include wet runways where mixed commercial and GA operations are in effect and might include multiple hold-short points on a runway.

Surface Management

Surface Management Guidance and Control System (SMGCS) consists of a computer-generated low-visibility taxi plan for airports able to launch or land when the RVR is less than 1200 feet—for both air crews and airport vehicles. SMGCS (pronounced SMIGS) designates taxi routes to and from SMGCS run- ways and displays them on a special SMGCS Low Visibility Taxi Route monitor in the tower.

A SMGCS chart makes it possible for pilots and drivers to find their way during low visibility and keep aircraft and vehicles where they are supposed to be. Jeppesen publishes Low Visibility Taxi Route Charts for cockpit use.

Strategically, SMGCS is the ground component that aligns with stream-lined arrival and departure management and the en route components of free flight. Taken together, they are supposed to form a system that minimizes delays and enhances safety during each phase of flight.

Save Time With TEC

TEC stands for Tower En Route Control, and is referred to as tower en route or tower-to-tower. This is a misnomer because you don’t talk to towers en route, you talk with TRA-CONs. Filing a TECroute means that you stay in approach control airspace all the way and never enter the en route system controlled by ARTCC. It is designed to service non- turbojet aircraft going to and coming from metropolitan centers at altitudes below 10,000 feet and for flights of two hours or less. Beyond that length, extensive coordination between facilities may create undue delays.

TEC flights may experience the same departure, destination and en route delays as any other ATC-controlled aircraft would. In this case you may prefer an alternate destination airport with no delays.

There are no special pilot or equipment requirements because TEC uses the Victor airways. TEC routes are published in the A/FD. Simply specify TEC in the Remarks section of your IFR flight plan. You have the flexibility of filing to a satellite airport near the major primary airport via the same routing.

Not all approach control facilities may operate up to the maximum TEC altitude of 10,000 feet. This may be a concern if you want to get above any weather.

Historically, TEC became important back in 1981 when the controllers went on strike. This is because TEC was designed to be an overflow resource in the low-altitude system. It worked pretty well.

Preferred IFR Routes

Preferred routes are published in the A/FD for both low and high altitude stratum. They are also available as an option in most flight planning soft- ware. Preferred IFR routes have the advantage of minimizing in-flight route changes, are designed to use airways in an efficient, orderly way and help systematize air traffic flow. The more you use preferred routes, the better for all participants in minimizing departure, en route and arrival delays.

File By The STARs

One of my favorite ways of filing is to find a STAR that takes me to my desired airport and then flying it almost from departure. By doing this, you minimize delay by following a standard, published route that weaves you through sometimes complex airspace.

For instance, when I go north, I file the Bairn Three Arrival into the Orlando, FL (KSFB) area from Palm Beach (PBI), which is just five miles north of my home airport. Usually the controllers will fly me west of PBI and join the PBI 330 radial to the northwest. I can fly into any of the five airports in the Orlando area, which provides more flexibility in flight planning.

FAR 91.103 (a)

This provision of the FAR requires the pilot to be aware of any known traffic delays of which the PIC has been advised by ATC. You can take this one step further and ask for published NOTAM delays during your flight briefing. Although the readout can be somewhat cryptic, you can get the essence by reading it slowly. Note that it begins with the acronym ATCSCC – the ATC Systems Command Center in Warrenton, VA. ATCSCC regulates air traffic when weather, equipment, runway closures, or other conditions place stress on the National Airspace System.

ATCSCC’s Enhanced Traffic Management System (ETMS) predicts traffic surges, gaps, and volume based on current and anticipated airborne aircraft. Personnel evaluate the projected traffic flow into airports and sectors, and then take the least restrictive action needed to assure that system capacity is not exceeded.

Within ETMS, the Monitor Alert analyzes traffic demand for all airports, sectors, and airborne reporting fixes in the continental U.S. It automatically displays an alert when demand is predicted to exceed capacity in a specific area. Again, personnel examine the circumstances and then provide routes and spacing to promote traffic flow.

Some things we can control and others not. The FAA is doing its best to minimize system delays. We can help ourselves, however, by making use of what they give us and by being intelligently creative in our IFR filings. As always, being smart on the ground will make your flight easier, safer and faster.

This article originally appreared in the June 2013 issue of IFR Refresher.