What happens when there is too much traffic?
Center traffic management units (TMUs) set Airport Arrival Rates (AAR) and Airport Departure Rates (ADR) in conjunction with the ATC Command Center. These rates are based on airspace and airport design (how many runways) and controller availability/capability. When the demand exceeds capacity, traffic management initiatives meter the flow of air traffic.
How does TBFM work?
According to the FAA, TBFM provides time based metering applications to merge, space, and sequence flights to improve NAS efficiency. In other words, when we decide how many airplanes can fit in the airspace, TBFM provides a way for traffic managers to ensure that the flow of traffic remains at or below the maximum rate. SimTraffic implements a set of Traffic Management Tools to do this that are similar in function to the ones used by FAA TMUs.
What is a Departure Clearance Time?
One way that TBFM handles high traffic demand is to schedule a 'slot' for each aircraft and to hold the aircraft on the ground at their departure field until a controlled takeoff time. This is called a Ground Delay Program (GDP) and it works by issuing Expect Departure Clearance Times (EDCTs). If you are aligned with a virtual airline, they may be able to request an EDCT up to 48 hours before your scheduled departure time. If you are unaligned, ATC will request an EDCT for you when you request IFR clearance.