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Traffic Flow Management

Introduction

As pilots, we all understand, at some level, what air traffic control (ATC) is. Most of us immediately envision a controller looking out over an airport from a control tower or huddled over a radar scope.

There is, however, an entirely different side to the control of air traffic that is largely invisible to many pilots – traffic flow management (TFM).

TFM is largely unknown to many pilots, including most of us in the general aviation (GA) world. The main reason for this is that TFM generally does not impact VFR operations, with a few limited exceptions. Low-altitude IFR pilots may also not encounter TFM on a regular basis, unless they are operating into or out of high-density airports or along portions of the east coast.

As a general rule, pilots flying IFR in the flight levels, in and around busy airspace, are the ones most likely to encounter elements of TFM at some point.

So, what is TFM? How does it impact general aviation pilots? Why is it important to know about? Most importantly, what steps can pilots take to minimize the delays they encounter? These are all questions that we aim to answer in this resource.

What is TFM?

TFM is a function of air traffic control that seeks to balance air traffic demand with capacity – making sure there are not too many airplanes in one place at the same time. It is accomplished by using various traffic management initiatives (TMIs) like ground stops, ground delay programs, and reroutes, to keep traffic at manageable levels. The process of TFM is overseen by traffic management units (TMUs) at Air Route Traffic Control Centers (ARTCCs) and by FAA’s Air Traffic Control System Command Center (ATCSCC).

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FAA defines TFM as “the craft of managing the flow of air traffic in the National Airspace System (NAS), based on capacity and demand.” But, in simple terms, it is how ATC ensures there are not too many aircraft in a particular place at the same time.

ATC, in the best-known sense, is all about the safe separation of air traffic and is executed by front line controllers, as seen in control towers, ARTCCs, and Terminal Radar Approach Control facilities (TRACONs). This is, as noted above, the most visible aspect of ATC and the one that pilots are most familiar with.

TFM, on the other hand, is accomplished by air traffic managers who are, in many cases, nowhere near a radar scope. To be clear, most of those involved in the TFM process are trained controllers and are located in ARTCCs and TRACONs. However, they are usually working in what are called traffic management units (TMUs), which communicate with their counterparts in other facilities and, ultimately, report to the ATCSCC.

The ATCSCC is located in Warrenton, VA and is the facility that, among other things, coordinates the overall flow of air traffic across the NAS. Their job, and that of the TMUs, is to keep traffic flowing as efficiently and safely as possible, while minimizing delays.

Ironically, while one of the chief goals of the ATCSCC is to reduce delays, decisions to implement delay programs, metering, reroutes, and other initiatives, either come from, or are coordinated, with that facility. At times, it is necessary to introduce delays into the system in order to help avoid or alleviate larger systemic delays.

As the above definition of TFM states, the goal is to balance available capacity (airspace, slots, runways, ramp space, etc) with demand (the number of aircraft trying to utilize that capacity). When demand exceeds capacity, we get congestion and delays.

Why does knowing about TFM matter?

Understanding TFM allows pilots to see the big picture of what is going on around them in the NAS. This can help them to anticipate or avoid delays.

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We all know that, sometimes, it is helpful to step back and look at the big picture. This is, in essence, what TFM does – it looks at the 60,000-foot view of the NAS. Pilots sometimes tend to see things through a bit of tunnel vision – they focus on moving from point A to point B without considering what is happening more than a few miles off their route.

For example, the Command Center often gets inquiries from pilots who are frustrated that they are not being allowed to depart due to convective weather. The complaint is usually something along the lines of “there aren’t any storms around me – just clear skies.”

In reality, there may be weather over the horizon from the pilot’s location that is pushing other traffic into his or her route of flight. Or, there may be thunderstorms impacting the arrival fixes for their destination – not the airport itself – slowing the overall flow of traffic to that airport. These are things that a pilot might not see with too narrow a scope.

Learning to look at the NAS with a bit wider view can pay dividends when trying to anticipate or avoid delays.

What does TFM look like?

Management of air traffic using TFM is segmented into two main domains of flight – terminal and enroute. For each of these domains, there is a specific set of tools, called traffic management initiatives (TMIs) that air traffic managers use to keep things running efficiently. Real-time information about what TMIs are being used can be found online at https://nasstatus.faa.gov/.

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One way to break down the various aspects of TFM is by looking at the regimes of flight – terminal (departure and arrival) and enroute. The terminal environment is where most low-altitude IFR, as well as some VFR, aircraft encounter impacts.

Typically, what needs to happen is that ATC needs to slow down/spread out the flow of traffic in order to alleviate bottlenecks. In order to accomplish this, they use a series of what are called traffic management initiatives (TMIs). These are the tools in the toolbox that are available to ATC. Public information about what TMIs are in use is available at https://nasstatus.faa.gov/.

It helps to note that there are some semantics when differentiating between departure and arrival delays. For example, the issue is often too many arrivals at the destination airport and most TMIs are targeted at managing those arrivals. But in order to do that, flights need to be slowed down before departing from their origination airport. So, while the cause of the delay is the destination, the delays are experienced as departure delays.

The TMIs themselves also require a bit of explanation – and that is what the next portion of this resource will look at.

Scope

Since not all aircraft are captured in every TMI that is issued, a scope must be defined that will determine whether a flight will be assigned a delay or reroute. There are several different ways that scope can be defined, based on the direction and distance the aircraft is coming from.

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While TMIs can happen anywhere in the NAS, they usually don’t involve all air traffic (though sometimes they do). For example, there might only be a need to slow flights down within 500 miles of an airport or flights coming from one or two specific ARTCCs.

(By the way, it’s helpful to be familiar with the abbreviations for the 20 ARTCCs in the contiguous US, shown in the sidebar.)

As a result, most TMIs include some form of scope or, put another way, the parameters that define which aircraft are going to be captured by whatever TMI is being used.

There will almost always be a time-frame provided, but beyond that, scope can be defined in a few different ways - by distance, by facility (usually ARTCCs), or by “tier.”

A distance-based scope is pretty straightforward. For example, a ground stop for an airport might only capture aircraft departing from within 1000 miles.

A “center-based” scope might look something like ZJX+ZMA+ZTL, meaning that aircraft are only being captured if departing from within Jacksonville, Miami, or Atlanta centers.

A tier-based scope requires a bit more explanation. 1st Tier refers to centers immediately adjacent to the facility in question, while 2nd Tier adds the centers adjacent to the 1st Tiers. For example, a 1st Tier ground stop for ATL (which is located in ZTL) would affect aircraft coming from ZID, ZDC, ZJX, ZHU, and ZME. A 2nd Tier stop for ATL would affect aircraft coming from those 1st Tier centers, plus ZOB, ZNY, ZBW, ZMA, ZFW, ZAB, ZKC, and ZAU. (FAA has an online Tier Info tool that can help you visualize this.)

Terminal TMIs

To manage traffic in the terminal (departure and arrival) domain of flight, there is a set of TMIs that traffic managers can use to slow traffic down to a particular airport – ground stops and ground delay programs (GDPs). Both of these TMIs manifest themselves as delays when departing from the origination airport. Ground stops are the most restrictive of the two, but are usually shorter-lived and do not result in expect departure clearance times (EDCTs). GDPs typically last longer and DO result in EDCTs being issues to affected aircraft.

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When there are issues (often called constraints) at an airport, we will generally see terminal TMIs being used. Again, the point of these is to slow down (meter) traffic to or from an airport. This can be due to weather (the usual culprit), excess volume, runway construction, staffing, or a few other reasons.

The TMI chosen to deal with the situation will depend on a variety of factors, including the duration of the constraint. Whichever TMI is used, they only generate delays for arrivals to the airport. While there can be accompanying departure delays (if, for example, there are thunderstorms near the field), terminal TMIs only impact the arrival side.

Ground Stops

Arguably, the most restrictive form of TMI is the ground stop – so much so that, if there are overlapping TMIs in use, the ground stop supersedes all others. A stop is generally needed when there are too many aircraft heading to a particular airport at one time.

Basically, a ground stop at an airport means that all arriving aircraft, within the scope of the stop, will be held at their origination airports and not permitted to depart until the stop is lifted. While a ground stop obviously can’t capture airborne aircraft, flights that are already enroute to the stopped airport might encounter airborne holding, speed restrictions, or, in the worst case, might need to divert to another airport.

Note that, on many occasions, a stop is implemented to address a future issue or is not a stop for all aircraft – meaning that you might observe aircraft continuing to land at the stopped airport. Traffic managers try to impact the fewest number of aircraft possible to address the constraint, so ATC will usually try to limit the scope.

If you are captured in a ground stop, you will be issued an update time – the time at which the stop is expected to be cancelled or re-evaluated (and possibly extended). Unfortunately, many times this update time is communicated as an expect departure clearance time (EDCT). This is technically not accurate, since you can’t necessarily expect to depart at the update time – it’s just that – an update.

Since the issue is at your destination airport, there is very little that you can do to extricate yourself from the stop. You can switch to a different airport or just wait it out. That said, trying to evade the stop by filing to a different airport and then switching back once airborne is STRONGLY discouraged – ATC actively watches for pilots doing this and it usually is not going to end well.

Generally, ground stops are designed to only run for a relatively short time – usually an hour or two. Anything longer than that and there is usually pressure to turn the stop into a ground delay program (GDP), which we’ll talk about next.

One other note on ground stops. Some controllers will call almost anything a “stop” when it is often not quite accurate. Other TMIs, such as miles-in-trail (MIT) for example, also involve holding you on the ground at your origination airport – the effect is the same, but the cause is different. If you are curious, feel free to ask ATC questions (if they are not too busy, of course) or browse to https://nasstatus.faa.gov/ for more information.

Ground Delay Programs (GDPs)

The other popular tool that ATC will use for terminal constraints is the GDP. While a ground stop could be described as an “on/off switch,” a GDP is more of a dimmer switch – it is designed to meter traffic to an airport over a period of hours.

Initially, traffic managers will need to set an arrival rate for the airport – the number of arrivals that they can accept each hour, based on conditions. The rate can fluctuate as conditions change – for example, an arrival rate can drop, as thunderstorms move through the terminal area, then jump back up once the weather has moved away.

Once the rate has been established, automation is utilized to figure out how many (and which) flights need to be delayed to ensure that too many aircraft don’t show up in any one time “bucket”. To make that happen, the system issues EDCTs to aircraft captured in the scope of the program, calculated to put each aircraft into an available slot. This is the reason EDCTs exist – to serve as a reservation of sorts.

GDPs are generally designed to run for several hours and can be adjusted as needed. Those adjustments are called “revisions.” A GDP might be revised up if conditions improve or downward if conditions worsen, meaning your EDCT might get better or worse. Sometimes, ATC will even implement a ground stop to address a sudden overage, followed by a downward revision. This usually means you are in for a long day.

As with ground stops, your options for relief from a GDP are limited to choosing a different airport or waiting it out. Remember, since the constraint is at the destination airport, trying a different route will not help.

Enroute TMIs

For flights in the enroute domain, the primary TMIs used are reroutes and airspace flow programs (AFPs). Structured reroutes are used to move traffic from one route of flight to another and are often comprised of numerous routes between point A and point B. AFPs operate similarly to GDPs, but instead of controlling flights to a particular airport, they control flights through a particular piece of airspace (meaning they are not airport-specific).

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The second group of TMIs are those that impact traffic in the enroute environment. While there are several different types, the two most common are reroutes and Airspace Flow Programs. The key thing to note is that, generally speaking, enroute TMIs are implemented to control the flow of traffic through a piece of airspace rather than to or from specific airports.

Reroutes

When ATC wants to move traffic from where it currently is onto another specific route, that is, by definition, a reroute – and there are several different types.

Reroutes are sometimes required (RQD), but are other times only recommended (RMD) or even simply put out as “FYI”, to be used if the pilot wishes.

If you are flying a relatively short flight, below the flight levels, you are less likely to encounter a formal reroute. For higher-flying traffic, operating over greater distances, reroutes take on more significance to the NAS and are more formalized.

Most formalized reroutes are pulled from, or adapted from, the National Playbook. This is a publicly accessible, regularly updated document that contains numerous pre-coordinated routes designed to be put into place very quickly or with minimal coordination. The playbook is very much analogous to a football playbook, where a coach reads what the other team is doing and calls a play. In this case, traffic managers look at the constraints and implement an appropriate route in response.

Reroutes currently in place are published on FAA’s National Airspace System Status web page, with each “reroute” actually composed of a SET of reroutes, dependent on the city pair. Most routes are structured with an origination segment and a destination segment - matching up the "ORIG” and “DEST” segments results in a complete route.

Using the screenshot as an example, this required reroute would assign a flight from IAD to ROC the route: JERES J211 LEONI WEVEL ROC.

A list of reroutes currently in place can be found by visiting FAA’s Current Reroutes web page.

Airspace Flow Programs (AFPs)

When ATC needs to slow traffic through a specific piece of airspace over a period of several hours, they will typically consider using an AFP. The idea is to ensure that no ATC sectors get overloaded with more traffic than they can handle. Keep in mind that an AFP might be run to deal with a constraint hundreds of miles from where you are – remember, step back and look at the big picture.

An AFP is basically just a line in space that is drawn in response to a constraint – usually weather or excess volume through a piece of airspace. Any aircraft crossing that line are subject to delays – that is, issued EDCTs. With AFPs, the exact city pair is less important than where the AFP is located. In other words, if there is an AFP in place across northern Florida and you are flying from Orlando to Atlanta, it doesn’t matter if your destination is PDK or FTY – you are still crossing the AFP and will get a delay.

AFPs are most commonly used to manage traffic around severe weather, but are also used to manage excess volume – for example, it is common to see them in place during the holidays for traffic between the northeast and FL. They usually run for several hours and can run independently or in conjunction with other TMIs like ground stops or GDPs for specific airports. Vertically, the floor is usually set at between 12,000 feet and FL 200, while the ceiling is usually set at FL600. This means that you can many times fly under an AFP (and avoid an EDCT), but usually not over it.

This screenshot shows an example of a set of AFPs being run to control traffic southbound into Florida. Any flights crossing either JX1 or JX3 southbound would be issued EDCTs. Graphic courtesy of FAA.

For those captured in an AFP, there are typically a few more options than with ground stops or GDPs. Many times, there are “route-out options” available – that is, routes that will take traffic around the lateral boundaries of the AFP. They will be documented in the Command Center advisory describing the AFP. As mentioned above, flying under the AFP may be an option as well. Note, however, that due to coordination issues, you can’t take off, climb to FL250, then duck under the AFP at FL150, and then climb back up to FL250 or higher – once you’re low, you’ll need to stay low.

AFPs tend to be dynamic in nature, meaning that they often get revised multiple times. This means, if you are issued an EDCT, expect that it could change multiple times before departure.

As noted earlier, information about active AFPs can be found on FAA’s National Airspace System Status web page – a graphic depiction is generally provided. Also note that AFPs are given 6-character names, always beginning with “FCA” (which stands for “flow constrained area”).

Other Enroute TMIs

There are several other types of TMIs that pilots might encounter, but many of these manifest themselves similarly. For example, Time Based Flow Management (TBFM) and Miles-in-Trail (MIT)/Minutes-in-Trail (MINIT) are all designed to meter traffic once airborne. While they work differently and accomplish slightly different things, the effect is the same to most pilots – you end up with a wait on the ground before departure and may encounter speed restrictions enroute.

What can you do?

While avoiding delays in not always possible, there are some options that can be tried. Flexibility is key, as many delays can be minimized or avoided entirely by simply scheduling the flight during non-peak days/hours. In addition, GA operators many times have the luxury of switching to a different destination airport – something scheduled carriers cannot do. Delays stemming from enroute TMIs can sometimes be avoided by flying around or under the constraint. Finally, valuable tactical assistance is available from flight plan service providers for a cost.

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Understanding what TFM is beneficial and can be very helpful. However, most pilots will be left wondering what they can do when getting caught in a delay.

In many cases, the unfortunate reality is … well, not a lot. Often, the airspace is simply saturated and there are no reasonable alternatives to just waiting it out. However, there are things that you can try.

First, while more preventative than reactive, you can also do some advance planning and schedule your flight to avoid peak hours. If flying during a busy holiday season, consider adjusting your departure to go a day earlier or later than most people do.

Conditions permitting, you can opt to fly VFR instead of IFR – but, it is important to note that, while VFR traffic is not captured in TMIs, it still must be able to fit into the flow – this is not always possible and ATC could decline to accommodate you.

If caught in a ground stop or GDP, the most obvious option is to switch to a different destination airport that is not encountering delays. (However, as noted earlier, you need to be willing to actually go to that airport. We do NOT recommend changing your destination to get out of a delay and then trying to switch back to the original destination once airborne. This can disrupt other flights who are waiting in line and will tend to aggravate ATC.)

For enroute delays, the main option is to alter your route of flight to avoid congested areas. This is where it is important to understand how AFPs work and where the AFP boundaries are located. It is sometimes possible to fly around AFPs and usually possible to fly under them.

Finally, there is help available from many flight plan service providers like ARINC, Honeywell Flight Sentinel, Jeppesen, and a few others, as well as from NBAA Air Traffic Services, which offers a subscription service. These services are all tied into FAA’s Command Center, can provide tactical assistance, and, many times, can provide delay reduction. There is a significant cost involved, but these services can pay dividends for those who need to get from point A to point B as quickly as possible.

Conclusion

TFM is a complicated thing, with many moving parts, lots of additional acronyms to learn, and with resulting impacts that are often difficult to understand. However, for those operating in busy airspace, and particularly in the flight levels, having at least a basic understanding of TFM can help you to understand what is going on around you and can, in many cases, help you to avoid delays.

For those interested in learning more, here are several additional resources: