Air traffic control
What ATC is for
Three objectives, met by one service in three parts.
Tower, approach and area
One unit for each part, and each with its own word on the radio.
Prevents collisions on and around the aerodrome: in the circuit, on the manoeuvring area, on take-off and landing, and between aircraft and vehicles.
- Positions
- Aerodrome controller: the runway, and the air around the aerodrome.Ground: the manoeuvring area, except the runways.Clearance delivery: start-up and ATC clearances for departing IFR flights.
- Works by
- Sight, first of all. Radar and surface movement radar where the tower has them.
Arrivals and departures at one or more aerodromes below the TMA. Turns and descents build the arrivals into an orderly flow, and departures are kept clear of each other and of the arrivals.
- Sits
- In the tower, where traffic is light. Otherwise in a terminal control centre, or in the same centre as area control.
- Works by
- Mostly radar. Procedural is possible.
Controlled flights en route. The airspace is split into sectors, each on its own frequency, and aircraft are handed from one sector to the next.
- Often two controllers
- Executive: the radio, clearances and instructions.Planning: coordination, and the traffic still to come.
- Works by
- Mostly surveillance. Procedural in some parts of the world.
The words on the radio are in LAW 003.
Who is given it
Air traffic control service goes to four groups of flights.
Clearances and instructions
How ATC does it: clearances, instructions, and coordination with the units around it.
For controlled flights only. Issued solely to expedite and separate traffic, based on the traffic known at the time.
Never permission to break a rule, and it relieves the pilot-in-command of nothing. A pilot-in-command who finds one unsuitable may ask for an amended one.
Requires the pilot to take a specific action: climb, descend, turn, reduce speed, proceed to a point.
The call sign, then the instruction. Several can go in one message: descend and turn, for example.
Departure and arrival clearances are the exception to the order. The clearance limit is a significant point, an aerodrome or a controlled airspace boundary, and it is the destination aerodrome once the units ahead have been coordinated.
ATC coordinates with another unit whenever an aircraft might otherwise conflict with that unit’s traffic, and before transferring control of an aircraft to it.
Holding
Waiting in the air for the next clearance.
A predetermined manoeuvre that keeps an aircraft within a specified airspace while it awaits further clearance.
- Congestion and approach delays
- Thunderstorms
- Low visibility
- Snow clearing
- The end of a missed approach
- Checklists, on the pilot’s request
Procedural or surveillance
Two ways of building the picture. The job is the same; the minima and the tools are not.
Based on flight plan data and pilot position reports, kept up to date by reports, estimates and visual observation.
Its tool is the flight strip: LAW 009.
Based on surveillance data, shown as an image on a surveillance display.
The display shows aircraft positions and movement accurately.
Procedural control is used where there is no surveillance cover, such as oceanic airspace, in TMAs and CTRs quiet enough not to need a surveillance system, and as the backup when every surveillance system has failed.
Separation
The ways ATC keeps aircraft apart, and the numbers worth knowing.
Safety nets watch over it: STCA for separation, MSAW for terrain and APW for airspace. They are in LAW 009.