What is an ATC Sector?
Air Traffic Control (ATC) providers (units) often sub-divide the volume of air that they control. This shares the workload between different control teams. The sub-divisions are called Sectors. A small unit may only need a single sector - larger units may need many sectors.
Who Decides How to Divide Up The Air?
Governments agree about which country will look after the different parts of the air through international agreements.
The air is sub-divided within national or agreed boundaries. A volume of air should be small enough to allow ATC to safely control the number of aircraft passing through it.
ATC providers usually have the responsibility to decide how to sub-divide the volume of air that they control. Individual ATC work teams may also have the ability to decide on how the air gets divided. They may open, close, split, and combine volumes of air depending on the workload.
It's important to remember throughout this discussion that ATC Sectors do not physically exist: they are the result of agreements. These only exist as lines on maps and ATC displays.
Sector Boundaries
Sectors have horizontal, vertical, and time boundaries.
For example, a simple sector at an airfield may extend out to 10 miles from the airfield and from the ground to a height of 10,000 feet. It is may be open from 06:00 to 22:00.
Most Sectors are in a complicated environment: they may be next to, above, or below other sectors. The boundary between sectors may be horizontal, vertical, or both. Different ATC units may control adjacent sectors.
In some countries a national (government) provider may control all the air except for the volumes around airfields where different providers for each airfield control local sectors.
Different units may need to apply different separation distances between aircraft depending on the staff, equipment, or applicable rules. It is not unusual for aircraft to be considered separated in one sector and yet require a larger separation before entering another sector. This is because of different minimum separation rules in the two sectors.
How Aircraft Transit Between Sectors
Aircraft may generally only enter a sector with the permission of those that control that sector. ATC working concepts require that controllers must know about all the aircraft in their sector and know when aircraft enter or leave. This is to ensure safe separation between aircraft.
Permission to enter a sector may be on an individual aircraft basis or by written agreement. Those in charge of adjacent sectors agree how aircraft may transit between the sectors.
Agreements usually require aircraft to transit under certain conditions.
- at specified vertical levels (at a single level or in bands of levels)
- on a specified published air route(s)
- at a particular speed(s)
- separated from other aircraft by specified distances
- coordinated first (prior warning)
- at a stated rate (number of aircraft per hour or a time between each aircraft)
- at a time or between times
The conditions are often stated explicitly in written form related to particular sectors but may be stated generally as part of international or national ATC rules. Controllers in adjacent sectors ensure that aircraft transit between sectors according to the agreed conditions. ATC monitor the progress of the aircraft and issue corrective instructions to the crews if necessary.
Transit Solutions
I discuss some specific sector transit situations below to show how the transit theory applies in practice. I've used some descriptive terms to show how the methods differ. Any of the methods may be the best for any particular ATC environment depending on the circumstances.
A 'Hard' Solution For Accepting Aircraft Into A Sector
Aircraft must enter according to written conditions (see above) but each also needs permission from the accepting controller. This allows the accepting sector to limit the rate that aircraft can enter on an aircraft by aircraft basis. If an accepting controller doesn't give permission for an aircraft to enter a sector then it must not enter. This is why I've described this solution as 'hard': not because it is difficult.
The acceptance method varies.
- a telephone or radio call
- one controller physically pointing out the aircraft to another controller on the RADAR display if they sit next to each other
- using electronic methods of highlighting aircraft on RADAR displays where the displays communicate electronically
Until the receiving controller accepts an aircraft the releasing controller must not allow the aircraft to enter the next sector. The releasing controller must offer the aircraft for transit in sufficient time for the receiver to notice and accept before the aircraft enters the next sector. Controllers usually set up their RADAR displays to mainly see their own sector but also see a short distance further so that see what's coming next.
Advantages
If for any reason a receiving sector is too busy, or has equipment failures, then no more aircraft enter the sector. This automatically limits the workload. This solution is simple and therefore easy to understand.
Disadvantages
Controllers need the time to offer and accept aircraft and will only have a certain time to do these tasks. This creates workload. Aircraft travel at high speed and usually follow expected paths. The releasing controller must offer aircraft in sufficient time for receiving controller to notice and accept. The offering controller must be able to stop it entering the next sector if it isn't accepted.
If the next sector does not accept an aircraft then the workload of offering controller increases. Airspace and procedure designers should consider this factor when designing the sector working arrangements.
The next sector may sometimes not accept an aircraft because they didn't notice that it was being offered. Controllers can be distracted or focused on a particular situation during the critical hand-over period.
The workloads from offering and accepting aircraft are part of the basic sector workloads because they are unavoidable and constant tasks.
The airspace design should closely match the user (airlines, airports) requirements. This is in terms of possible descent and climb profiles, speeds, and tracks. A less than ideal airspace structure will increase workload and decrease efficiency for both controllers and aircraft crews.
A 'Soft' Solution For Accepting Aircraft Into A Sector
Aircraft enter the next sector according to written conditions. This may involve prior warning but the aircraft don't require individual permission. If the receiving controller needs to stop an aircraft coming in to the airspace then she will tell the releasing controller.