I've heard several terms referring to air traffic control procedures that restrict movement for a certain period of time: ground stop, ground delay, and gate hold (plus others I'm sure I'm forgetting at the moment). How exactly does each of these procedures work? And do they affect arrivals, departures, or both?
@Pondlife has good quotes and I'll supply some of the direct knowledge he requested.
Your destination airport cannot accommodate arrivals -- at all. In this case, they will disallow all planes destined for that airport to take off from wherever they are.
Some examples of what might cause ground stops are:
- Weather impacting standard arrival routes, probably more than one. Aircraft are already in holding patterns and they are running low on altitudes/fixes to put us, and/or those of us in holds are getting close to diverting for fuel.
- Airport or runway closure. If you can't land, you probably shouldn't take off.
- All gates are occupied. If planes cannot depart, there may be nowhere to park arrivals.
You'll note that many times you'll see ground stops described as "Tier 1", "Tier 2". In short, the T1 ARTCCs are those that directly border with the ARTCC of the destination. T2 ARTCCs are those bordering the T1 ARTCCs and so on. Sometimes only T1 departure airports are affected by a ground stop, and sometimes it is everyone.
Ground stops are issued with update times, usually (in my experience) 45-60 minutes later. They are subject to cancellation at any time, or extension with updated in another 60 minutes.
These are like the Ground Stops but the airport knows it can accommodate you, just not exactly when you planned to be there. The reason is most often weather at the destination airport. Runway closures, accidents, construction, plowing/de-icing can all cause these delays also.
In good weather, airport A may land 90 aircraft per hour. The helpful airlines have scheduled 110 airplanes to land in one particular hour (no slots at this airport!). Low clouds and high winds have moved in and now airport A can only land 40 airplanes per hour. Now of the 110 airplanes, 70 of them will be delayed. The airlines and ATC will work out which 70 planes those are, and which 40 planes will be allowed in that hour, and start issuing ground delay times. Of course, these delays spill over into the next hour and cascade.
When a ground delay program is in effect, you will get a "Wheels up time" from ATC, which is when you have to be airborne. Not sooner, and not much later (3-5 minutes usually). Getting into your spot in the arrival flow depends on you taking off at this time. In these scenarios ATC can figure out how/when to get you to your destination, if this isn't the case, that is usually when the Ground Stop program gets started.
Gate Hold Procedure
There is too much happening out on the taxiways, and no room for you. Stay at the gate!
Reasons for gate holds:
- De-icing is in effect and not enough room for everyone to line up at once.
- The line to takeoff is too long
- Your departure fixes are closed/saturated (This one happens a lot in the NYC Tracon)
In these cases, ATC will issue you a time to start and pushback via clearance delivery (at big airports) or ground control (smaller airports). Sometimes ramp control can be helpful and help coordinate things... usually not. When your time is up, you push and taxi and contact ground control. Not any different than normal except you are given a specific time to do it.
The FAA Air Traffic Organization has issued a booklet titled "Traffic Flow Management in the National Airspace System" which was specifically written for pilots and s flight operations personnel. They cover Traffic Flow Management (TFM) in great detail (68 pages) but here is a small portion covering the most commonly used traffic management techniques. Each section also includes an extensive "How does it work" sub-section which you can look at if you want more details. I would highly recommend that any pilot read the entire booklet (while it is 63 pages, it is booklet format so it goes quickly)!
A GREAT website that the FAA has allows you to see a list of, and the current status of all of the following programs in one place: http://www.fly.faa.gov/ois
Ground delay programs (GDP)
A Ground Delay Program (GDP) is a traffic management procedure where aircraft are delayed at their departure airport in order to manage demand and capacity at their arrival airport. Flights are assigned departure times, which in turn regulates their arrival time at the impacted airport.
GDP will normally be implemented at airports where capacity has been reduced because of weather—such as low ceilings, thunderstorms or wind—or when demand exceeds capacity for a sustained period.
GDPs are implemented to ensure the arrival demand at an airport is kept at a manageable level to preclude extensive holding and to prevent aircraft from having to divert to other airports. They are also used in support of Severe Weather Avoidance Plan (SWAP). (See section on SWAP.)
Airspace flow programs (AFP)
Airspace Flow Programs (AFP) were introduced in Summer 2006 and marked a significant new step in en route traffic management. The principal goal for the initial deployment was to provide enhanced en route traffic management during severe weather events. An AFP is a traffic management process that identifies constraints in the en route system, develops a real-time list of flights that are filed into the constrained area, and distributes EDCTs to meter the demand through the area.
Ground stops (GS)
A ground stop (GS) is a procedure requiring aircraft that meet specific criteria to remain on the ground. The GS may be airport specific, related to a geographical area, or equipment related.
Severe weather avoidance plan (SWAP)
SWAP is a formalized program that is developed for areas susceptible to disruption in air traffic flows caused by thunderstorms. Each air traffic facility may develop its own strategy for managing the severe weather event. Their plan then becomes part of the overall daily operations plan.
Special traffic management program (STMP)
A STMP is a long-range strategic initiative that is implemented when a location requires special handling to accommodate above-normal traffic demand, e.g., Masters golf tournament, NBAA Convention, etc.
Gate hold is not a traffic flow management technique, but is instead used by airports to keep you from starting engines and taxing just sit in line burning fuel when you can't takeoff due to a delay program. It makes everybody's life better if you don't leave in the first place!
At least for the US, the specific terms you mentioned are defined in the Pilot/Controller Glossary:
GROUND STOP (GS)- The GS is a process that requires aircraft that meet a specific criteria to remain on the ground. The criteria may be airport specific, airspace specific, or equipment specific; for example, all departures to San Francisco, or all departures entering Yorktown sector, or all Category I and II aircraft going to Charlotte. GSs normally occur with little or no warning.
GROUND DELAY PROGRAM (GDP)- A traffic management process administered by the ATCSCC; when aircraft are held on the ground. The purpose of the program is to support the TM mission and limit airborne holding. It is a flexible program and may be implemented in various forms depending upon the needs of the AT system. Ground delay programs provide for equitable assignment of delays to all system users.
GATE HOLD PROCEDURES- Procedures at selected airports to hold aircraft at the gate or other ground location whenever departure delays exceed or are anticipated to exceed 15 minutes. The sequence for departure will be maintained in accordance with initial call-up unless modified by flow control restrictions. Pilots should monitor the ground control/clearance delivery frequency for engine start/taxi advisories or new proposed start/taxi time if the delay changes.
Wikipedia has some more information:
But I suspect that a really detailed explanation is probably only in the ATC manuals and procedures (and is probably far too long and complex to post here). I'm neither a commercial pilot nor a controller, though, so hopefully someone with more direct knowledge can comment.