When severe weather or runway closures affect flight schedules, both airlines and airports have to work together, along with the Federal Aviation Administration's Air Traffic Control, to safely manage the traffic in the skies and gate congestion on the ground.
Think of airways as the highways of the sky. With so many flights arriving and departing in all directions, weather and other circumstances can slow down or prevent flights from operating, causing the airspace to become congested as flights are rerouted. Much the way traffic on a freeway backs up during a lane closure as cars merge into fewer lanes, the same is true with aircraft in the skies above. Unlike cars that can sit at a standstill during a traffic jam, aircraft can't stop mid-air.
When an airport is impacted by weather, it reduces the number of flights it can land and take off in a given hour. For instance, during peak times at Atlanta's Hartsfield-Jackson International Airport, 120 flights can arrive each hour. Dense fog, thunderstorms or other inclement weather will reduce that arrival rate—when that happens, the FAA will need to put in place initiatives to manage the airspace.
To help ease traffic congestion and space out aircraft along the airways to prevent in-air holding, the FAA will implement various Traffic Management Initiatives such as a Ground Delay Program or Ground Stop.
When Ground Delay Programs or Ground Stops are issued, Delta's Strategic Planning Team in the airline's Operations and Customer Center, coordinate with the FAA and Air Traffic Control group to discuss airport constraints. Once those are determined, Delta's Strategic Planning Team will manage and prioritize the arrival times for both Delta's mainline and connection carrier flights and pass that information on to customers.
Ground Delay Programs
Ground Delay Programs are put in place to control air traffic volume to airports where the number of flights scheduled to arrive at an airport exceeds the number of arrival slots available during a certain period of time. They are put in place to ensure the arrival demand at an airport is kept at a manageable level, to prevent aircraft from extensive holding or diverting to alternate airports.
Ground delays are typically applied when there are thunderstorms, low visibility or low cloud ceilings. Under a Ground Delay Program, Air Traffic Control will delay flights that are destined for a "constrained" airport and assign each flight a Controlled Departure Time—also known as a "wheels up time". Flights that have been issued a Controlled Departure Time are not permitted to depart until their assigned time. These departure times are calculated in such a way as to measure the rate that traffic arrives at the affected airport; ensuring that demand is equal to acceptance arrival rate.
During a Ground Delay Program, Delta's Strategic Planning Team will use software programs, mandated by the FAA, to look at all of the Controlled Departure Times and strategize ways to mitigate those delays. For example, if Flight 202 is already delayed for a mechanical issue and won't be able to depart at its assigned departure time, Strategic Planning will alter its delay time and assign Flight 305 the departure time instead in an attempt to move the flight out earlier and reduce the delay. Additionally, the Strategic Planning Team will swap Controlled Departure Times between flights to minimize the operational impact for the greatest number of customers.
It is important to note that Ground Delay Programs affect arrivals to a constrained airport, not departures. Also, each Ground Delay Program only affects that particular airport.
A Ground Stop is a procedure requiring aircraft that meet specific criteria to remain on the ground at the origin airport. Ground stops are considered the most restrictive form of traffic management because they hold all aircraft at their departure airports until conditions at the destination airport allow for their arrival.
Ground Stops are applied when Air Traffic Control is unable to safely accommodate additional aircraft in the system. They are typically used in severe weather, equipment outages, and other major disruptions to prevent extended periods of airborne holding or to prevent airports from reaching gridlock.
Like Ground Delay Programs, Ground Stops are airport-specific and only affect arrivals to a specific airport—not departures. Also, Ground Stops generate only one Controlled Departure Time and continue until its "update time," after which it is extended or canceled. Occasionally, Ground Stops transition into Ground Delay Programs if the constraint continues long enough.
The intent behind Traffic Management Initiatives, like Ground Delay Programs and Ground Stops, are to balance the demand with capacity – both at airports and in the airspace. Most importantly, these initiatives are implemented to provide safety within the national airspace system. Though these delays may cause a significant impact to Delta's operation, the Strategic Planning Team works closely with various teams in the Operations and Customer Center, such as Crew Tracking, Fleet System Operations Managers and Aircraft Routing to ensure a smooth recovery within a 24-hour time period.