How Air Traffic Control Hands a Flight From Tower to Tower
A flight passes through tower, terminal radar and en route controllers before landing. Here is who controls what, and how each handoff works.
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On a typical airline trip, your plane is looked after by several controller teams in turn: the departure tower, a terminal radar room, an en route centre, then another terminal room and the destination tower. Each team owns a block of airspace, and the plane is passed from one to the next by radio and computer. This article explains the roles and the handoffs, using the United States system as the example.
Three kinds of facility
A 1997 report from the US National Academies describing air traffic control in the United States says the Federal Aviation Administration (FAA) runs 20 en route centres, almost 200 terminal radar approach controls (TRACONs) and hundreds of airport towers. Wikipedia's page on the National Airspace System gives the same three-way split and says the whole system has over 600 facilities. Those numbers have changed since 1997, and some sources now say 21 centres, so treat the counts as approximate.
The tower
Tower controllers sit in the glass cab at the airport and see the aircraft directly. According to the National Academies report, they manage pushback, taxiing, take-off and landing. The work is often split between a ground controller, who handles taxiing, and a local controller, who handles the runways. At small airports one person may do both.
Towers use their own eyes, voice radio, flight strips (paper or electronic notes on each flight) and, at busy airports, surface radar. A tower controller authorises take-off; once the plane is airborne, tower control ends and the plane goes to the next facility.
The terminal radar room
TRACON controllers work in a radar room, often without windows, and manage the airspace around one or more airports. Departure controllers take aircraft from the tower and guide them upward; approach controllers take arrivals from the en route centre, line them up and hand them to the tower for landing. Wikipedia notes that some airports have no TRACON, in which case flights go directly between the tower and the centre, and some short low flights stay within TRACON airspace the whole time.
The National Academies report says controllers must keep aircraft apart in three dimensions. For flights under instrument rules (IFR), the separation standard it gives is 1,000 feet vertically or 3, 4 or 5 miles sideways, depending on aircraft size, because larger aircraft leave stronger wake turbulence.
The en route centre
Centres guide aircraft across the country at high altitude. Wikipedia says the US has 20 contiguous areas of airspace above 18,000 feet, each managed by one centre. The National Academies report adds that aircraft fly along defined routes at assigned flight levels, with eastbound traffic generally at odd levels and westbound at even levels, and that required separation is larger than near airports: 5 miles, plus 1,000 or 2,000 feet vertically depending on altitude.
A busy sector is often staffed by two people. One, on the radar side, watches separation and talks to pilots. The other, on the data side, handles coordination and flight-plan information. In quiet periods one controller may do both jobs.
How radar and flight data work together
Two kinds of radar are involved. Primary radar sees anything that reflects radio waves. Secondary radar asks a transponder in the aircraft to reply, which supplies identity and altitude. The report describes the computer system joining both sources, then displaying a data block next to each target with the call sign, aircraft type, altitude and ground speed. The report describes equipment of its time, and newer systems have since replaced much of it, but the basic idea of combining radar returns with flight-plan data remains.
The handoff
Wikipedia describes the usual order: a departure goes from the tower to a TRACON and then to a centre, and an arrival goes from the centre to the destination TRACON and then to the tower. The National Academies report describes how this works in practice:
- Tower to TRACON: a mix of voice coordination, flight-strip data passed by computer, and the pilot changing radio frequency.
- Between TRACON and centre, or between sectors: usually an automated handoff, where the receiving controller accepts with a keypress when ready.
- If automation fails: controllers fall back on voice, and the pilot is told the new frequency.
The pilot hears "contact" followed by a new controller and frequency, and from that point the new team is responsible.
Over oceans
Radar does not cover the open ocean. The report says oceanic controllers rely on position and intent reports from pilots, and use larger separation standards to compensate. Other countries organise their facilities differently, though the idea of passing aircraft between tower, terminal and en route controllers is widely shared.
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