The position
An approach controller owns a volume of airspace roughly 30 to 40 nautical miles around one airport, from the ground up to a defined ceiling. Every arriving aircraft is transferred into that airspace by an area controller, and every departure is transferred out of it.
The controller does not fly aircraft. They issue instructions by radio; the flight crew reads each instruction back and complies. Three positions divide the work:
- Approach
- Sequences arrivals onto the instrument approach and separates traffic in the terminal area. This is the position Talk to Tower simulates.
- Tower
- Owns the runway. Issues landing and take-off clearance once approach has delivered an aircraft to final.
- Ground
- Moves aircraft between the runway and the gates on taxiways.
Separation standards
Two aircraft under radar control must be kept apart by at least one of the following. Either alone is sufficient; both being breached at once is a loss of separation.
- Laterally — 3 nautical miles.
- Vertically — 1,000 feet.
Flight crews cannot see other traffic reliably, particularly in cloud or at night. The controller holds the only complete picture of the airspace, which is why separation is a controller responsibility rather than a pilot one.
The instrument landing system
The ILS is a pair of radio beams transmitted from the runway that an aircraft can follow to the touchdown zone without the crew seeing the runway.
- Localizer (LOC)
- A vertical beam aligned with the runway centreline. It tells the aircraft whether it is left or right of centre, and by how much.
- Glideslope (GS)
- A beam inclined upward from the runway, normally at 3°. It tells the aircraft whether it is above or below the correct descent path.
- Established
- The state after an aircraft has captured both beams. Its autopilot then tracks them down to the runway, and approach control transfers it to Tower.
An aircraft can only capture the beams from a limited region of sky. At VABB runway 27 that region is approximately 10 to 14 nautical miles from the threshold, within about 30° of the runway heading of 270°, and below the glidepath. An aircraft arriving above the glidepath has nothing to capture, and one crossing the centreline at a steeper angle passes through the beam without capturing it.
Positioning aircraft into that region is the controller's core task, and it is why radar vectors exist at all.
Instruction set
Approach control works through a small, fixed vocabulary. Each instruction assigns one value, and the crew reads it back before complying.
- Heading
Speedbird 214, turn right heading 240 — an absolute magnetic heading between 001 and 360, with the direction of turn stated.- Altitude
Speedbird 214, descend and maintain 2600 — an altitude in feet above mean sea level.- Speed
Speedbird 214, reduce speed 180 knots — indicated airspeed. Speed control is how spacing between arrivals is built.- Approach clearance
Speedbird 214, cleared ILS approach runway 27 — permission to capture the beams. It is not a guarantee of capture; the geometry must already be correct.- Handoff
Speedbird 214, contact Mumbai Tower 118.100 — transfer of communication once the aircraft is established.
Sequenced together, those five instructions take an arrival from the sector boundary to the runway: turn it onto an intercept heading, descend it below the glidepath, slow it for spacing, clear the approach, confirm capture, transfer to Tower.
Reading numbers
Digits are spoken individually so they survive a noisy radio: 240 is “two four zero”, not “two hundred forty”. Three digits are altered to avoid confusion between similar sounds — 3 is “tree”, 5 is “fife”, and 9 is “niner”.
Callsigns use an airline's radio telephony designator rather than its trading name. British Airways flight 214 is “Speedbird two one four”; the radar label reads BAW214.
In the simulator
Talk to Tower implements the above at Mumbai with published runway geometry and ILS procedure data. Clearances can be spoken or typed, crews read them back, and aircraft comply according to their own performance. It is a game and a training aid, not certified software, and is not for operational use.
