Delays are an inevitable part of being a pilot, and there's a lot that goes on behind the scenes when you're given an ATC delay. Here's what you should know about how it works, and the terminology you should use with air traffic controllers...

Who's Calling The Shots?

Air traffic over the United States is managed by Air Route Traffic Control Centers (ARTCCs), select Terminal Radar Approach Control Facilities (TRACONs), and the Air Traffic Control System Command Center (ATCSCC). Traffic Management is a separate unit of the FAA staffed by air traffic controllers who analyze the demand on the nationwide air traffic system. They make decisions based on their experience, coupled with software analysis to implement traffic management initiatives which are relayed to individual En Route and Terminal controllers/facilities.

Managing the flow of traffic isn't limited to the FAA, however. Air traffic controllers, air traffic managers, pilots, and airline dispatchers all work together to meter the flow of traffic in our skies. Here's what the normal communication process looks like when delays are issued throughout the ATC system:

With So Much Sky, What's The Problem?

Separation standards for aircraft create a challenge for ATC, especially at the busiest airports in the nation. Miles-in-trail and minutes-in-trail are two types of separation standards that require ATC to meter the flow of traffic in the sky.

Miles-in-trail describes the number of miles required between aircraft departing an airport, over a fix, at an altitude, through a sector, or on a specific route. Enough space must be maintained between aircraft to keep safe separation, with occasional larger gaps to fit in "merging" traffic along aerial routes or weather deviations.

Minutes-in-trail is how much time is needed between aircraft operating along the same route. Time-based separation is used for aircraft transitioning to/from non-radar airspace, usually over offshore or oceanic routes.

Busy airports have a figure called an "Airport Arrival Rate" (AAR), which is the number of aircraft that an airport can accept within one hour. The AAR is a variable number which takes into consideration runways in use, weather conditions, and NAVAID limitations.

In the example below, New York's LaGuardia Airport has an AAR of 38 aircraft per hour during normal operations, as indicated by the white line on the chart below. As you'll notice, there are peaks above this line at different times of the day. If the airport won't be able to handle the heavy flow of traffic, a delay program might be issued to meter the flow of traffic, which we'll discuss below.

Ground Delay Programs (GDP)

A Ground Delay Program is a traffic management procedure where aircraft are delayed at their departure airport in order to manage demand and capacity at their arrival airport (FAA). Flights are assigned departure times, which in turn regulates their arrival time at the impacted airport. Low ceilings, thunderstorms, wind, snow, or equipment outages are all reasons why a GDP might become necessary.

The FAA uses software called Flight Schedule Monitor (FSM) in an attempt to make the demand equal capacity for particular airports. All aircraft filed to a GDP airport are assigned an Expect Departure Clearance Time (EDCT) by FSM. We'll cover EDCTs in detail below. Let's say the winds at LaGuardia pick up and the Airport Arrival Rate is reduced. A GDP is issued from 1600UTC with an AAR of 34 until 2000UTC, which then increases to 36 for the remained of the GDP.

Airspace Flow Programs (AFP)

In 2006, the FAA released Airspace Flow Programs. "An AFP is a traffic management process that identifies constraints in the en-route system, develops a real-time list of flights that are on filed routes into the constrained area, and distributes EDCTs to meter the demand through the area. Think of flow programs as the FAA's way to meter traffic through airspace full of route-interrupting hazards like thunderstorms.

To avoid AFP delays, filing new routing around an AFP area with ATC could remove an EDCT time.

Expect Departure Clearance Times (EDCTs)

EDCTs are given by ATC during both GDPs and AFPs. You probably won't know what reason you're being issued an EDCT, because it's entirely out of your control. EDCTs are essentially a window of time that ATC has given your flight for departure. Most of the time, you can depart within +/- 5 minutes of your assigned EDCT. Click here to see if your flight has an EDCT from a national GDP.

In some cases, a "call for release" program might be in place for your flight. This is not something you'll know as the pilot. For instance, you might have an EDCT of 1500z, but the controller still must "call for release" to get a "wheels-up time" so you can fit into the overhead flow of air traffic.

Here's what one TRACON controller we spoke to wants pilots to understand:

If you miss an EDCT, you'll have to get a new EDCT which could be a subsequent delay as long, or longer, than your original delay. Airlines can request new EDCTs for your flight directly from ATC. At airports with a control tower, the controller has a process for requesting a new EDCT for your flight. At non-towered airports, you'll have to contact flight service or a nearby ATC facility directly for a new EDCT time.

EDCTs are somewhat flexible. When conditions causing a GDP or AFP are diminished, you might receive a new, sooner EDCT. ATC or dispatchers will notify crews of updated EDCT. Airlines can also swap EDCTs for multiple flights to meet their schedule needs. If the airline needs a particular flight to arrive close to on-time, they can swap an earlier EDCT from another flight.

Ground Stops (GS)

According to the FAA, "GSs are implemented when air traffic control is unable to safely accommodate additional aircraft in the system." An airport can be ground-stopped for select departure airports, select air traffic tiers, or simply for all arrivals. Usually, GSs don't apply to aircraft departing from international destinations across the globe; so a lot can change during a 14-hour flight from across the globe. Here are a few reasons why GSs are used:

• Severely reduced capacity situations: weather below approach minimums, severe weather reducing usable routes, major equipment outages, or catastrophic events.
• Precluding extended periods of airborne holding.
• Precluding sectors from reaching saturation levels.
• Precluding airports from reaching gridlock.

Is Your Flight Delayed?

You can find a list of current GDPs, AFPs, and GSs on the FAA's ATCSCC Advisories page or the FAA OIS page. If your flight is a part of a national GDP (not "call for release"), you can click here to see your EDCT. If you fly for an airline, your dispatcher and company EFB might be able to provide you with updated EDCTs more specifically than those found on public FAA web pages.

Putting It All Together

On the FAA OIS page (image below), there's both a national Ground Delay Program in place for Chicago's O'Hare Airport and a Ground Stop. For inbound aircraft, the GS takes precedence over any flow delay. If your flight originates in the GS tier, you won't be departing until the GS is lifted.

Low ceilings have reduced KORD's AAR to 76 aircraft per hour, increasing to 96 and finally 106 later in the day. Inbound delays are averaging 50 minutes, with a maximum delay of 450 minutes. Outbound delays are much shorter, averaging 15 minutes. It's a lot easier to depart aircraft into low ceilings, then it is to manage the flow of dozens of airplanes arriving in low ceilings.

What Should You Tell Passengers?

"I don't understand why we're delayed. There's no weather here, or between us and Chicago." It's something you've probably heard from passengers questioning your flight is delayed.

An easy analogy you can use with passengers is how on-ramps to busy highways have stoplights to meter the flow of traffic. ATC has to control the amount of traffic entering the sky to prevent gridlock. And when congestion earlier in the day builds up, it can take hours for the airspace to restore to full capacity.

If you want to know more, read through this FAA Traffic Flow Management presentation. Do you have more questions? Ask in the comments below!