Air Traffic Flow Management

Air traffic flow management (ATFM) matches the number of flights to the number that air traffic control can safely handle. A controller can work only so many aircraft in a sector in an hour, and a runway can accept only so many arrivals; weather, staffing, equipment outages and special events all reduce those figures. When the flight plans filed for a sector or an aerodrome would exceed its declared capacity, flow management delays or reroutes some of them before they depart.

The central idea is simple: a predictable delay is best taken on the ground. An aircraft held at the gate with its engines off burns almost nothing and endangers no one, while the same delay flown in a holding stack costs fuel, crowds the airspace and loads controllers and crews. Flow management is a planning service that works upstream of ATC; it does not separate aircraft. Europe runs it through the EUROCONTROL Network Manager, the United States through the FAA's Air Traffic Control System Command Center.

Why flow management exists

Flow management depends on knowing the demand in advance. Flight plans, and above all the repetitive flight plans that airlines file in advance for their scheduled services, let the flow management unit predict the traffic load in each sector and at each aerodrome. Where the predicted load exceeds capacity, operators are told of likely delays and flights are given later departure times or different routes, so that delays are absorbed on the ground rather than in the air.

ICAO's PANS-ATM adds an obligation on ATC at the departure end: operators, or their nominated representatives, are to be told of substantial delays and in any case whenever the delay at the departure aerodrome is expected to exceed 30 minutes. The operator can then decide whether to board, refuel or cancel.

EUROCONTROL Network Manager (CFMU)

EUROCONTROL was set up by an international convention on co-operation for the safety of air navigation, signed in Brussels in December 1960. Flow management for the European network was centralised in its Central Flow Management Unit (CFMU); the same function is now performed by the Network Manager Operations Centre (NMOC), and many texts, including current AIP entries, still refer to CFMU rules.

The Network Manager sees every IFR flight plan in its area, because the plans pass through its Integrated Initial Flight Plan Processing System (IFPS). Air navigation service providers declare the capacity of their sectors and aerodromes. When demand for one of them will exceed that capacity, a flow restriction called a regulation is applied, and the flights that cross it become regulated flights: each is given a take-off time that spaces the traffic entering the restricted volume.

The system works only if the flight plans are accurate. In the IFPS zone a delay (DLA) or change (CHG) message is required once the estimated off-block time (EOBT) will be exceeded by more than 15 minutes. A DLA can put the EOBT back but never bring it forward. The Network Manager's flight activation monitoring watches for flights not reported airborne after their expected take-off time and, if they stay on the ground, suspends their flight plans until the operator sends a DLA. A flight can also be suspended for other reasons, such as the closure of its destination.

ATC slots

The take-off time given to a regulated flight is its calculated take-off time (CTOT), allocated by the NMOC and commonly called an ATC slot. The operator receives it and passes it to the crew. The flight must take off within the slot tolerance window, from 5 minutes before to 10 minutes after the CTOT, and the departure aerodrome's ATC shares with the operator the task of keeping it there; a flight that will miss it needs a revised slot. At Lyon, when the automatic departure sequence is not in use, a regulated flight's off-block time is its CTOT minus the standard local taxi time, the calculated off-block time (COBT).

An ATC slot must not be confused with an airport slot. At a coordinated airport, the slot coordinator allocates each scheduled operation a time to use the airport, which appears as the scheduled off-block time (SOBT); in France the coordinator is the association COHOR. The flight plan's EOBT must be consistent with that airport slot, and a plan without one can be suspended at the coordinator's request. An airport slot is a scheduling right; an ATC slot is a take-off time allocated on the day because of capacity.

At airports using airport collaborative decision making (A-CDM), the airport, airlines, handlers and ATC share one departure sequence, which is also sent to the NMOC so that slots can be allocated on better data. The French AIP entries for Lyon and Nice define the main times:

The TSAT already takes the NMOC slot into account, and the crew must receive it with the same priority as a CTOT. At Lyon a flight that has not received its departure clearance by TSAT plus 2 minutes, or left its stand by TSAT plus 5 minutes, drops out of the sequence until a new TOBT is sent.

FAA Command Center

In the United States, the Air Traffic Control System Command Center (ATCSCC) manages the national flow. It issues the traffic management initiatives: ground stops, ground delay programs with their EDCTs, reroutes, and during convective weather the playbook routes and coded departure routes of a severe weather avoidance plan.

Some capacity limits are handled by reservation instead. At slot controlled airports, unscheduled operations need an advance reservation, and the AIM refers to 14 CFR Part 93 Subpart K, High Density Traffic Airports, for the details. The FAA's Airport Reservation Office takes reservations through the e-CVRS system from 72 hours before the operation; flights with declared emergencies do not need one. Special traffic management programs cover events and temporary losses of capacity, such as EAA AirVenture Oshkosh, a Super Bowl or a major runway closure, and are announced by domestic notice.

Ground stops and ground delay programs

A ground stop is one of the most restrictive initiatives, and it overrides all others: aircraft that meet stated criteria, usually those bound for an airport or area that cannot accept them, are kept on the ground at their departure airports until it is lifted. It is the tool for a sudden problem, such as a line of thunderstorms over the destination or an unplanned runway closure.

A ground delay program (GDP) is the planned version. When an airport's arrival capacity is expected to fall, for example with low ceilings or a runway out of use, the program meters the traffic bound for it by assigning each flight a departure time that spaces the arrivals to the reduced rate. The delay is taken at the departure airport, where it costs little, instead of in holding at the destination.

Expect departure clearance times

The time assigned under such a program is the expect departure clearance time (EDCT). The AIM defines it as the runway release time assigned to an aircraft included in a traffic management program, and expects the aircraft to depart no earlier than 5 minutes before and no later than 5 minutes after it. It reaches the crew through the flight plan system and the airline, often by ACARS.

The crew's task is the same in both systems: plan pushback and taxi so that the aircraft reaches the runway inside the window, without calling for push so early that the aircraft sits at the holding point with engines running. Long taxi holds still burn fuel, so the fuel on board is watched against the plan. Other FAA departure restrictions serve the same purpose locally: a hold for release in the IFR clearance stops the pilot departing under it until ATC issues a release, and a release time gives the earliest time the aircraft may depart (see ATC clearances).

Severe weather avoidance plans

Convective weather is the main reason for large-scale reroutes. A severe weather avoidance plan (SWAP) is put in place when thunderstorms block the routes into or out of a busy area. The Command Center publishes playbook routes and coded departure routes that keep traffic clear of the weather, dispatchers refile flight plans on them, and crews receive the new route before departure or en route by ACARS or CPDLC. The FAA's en route CPDLC service includes airborne reroutes, and its message set includes a traffic flow management reroute. Ground stops often follow a convective SIGMET over a major airport.

Preferred and coded departure routes

Preferred IFR routes are published in the Chart Supplement to organise the flow between major terminals. Filing them gives ATC the route it expects and reduces the chance of a reroute before departure.

A coded departure route (CDR) is a pre-coordinated alternative route identified by an eight-character code. During weather reroutes the Command Center can move flights onto CDRs, and dispatchers refile the flight plans on them, so a route agreed in advance replaces one improvised on the day.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.