Swanwick: where it is all controlled from
Most of the airliners crossing your screen are talking to a single building in Hampshire. The Swanwick centre, operated by NATS, opened in 2002 and took over the en-route control work previously done at West Drayton.
It houses two distinct operations that people often conflate:
- London Area Control — en-route traffic at higher levels across England and Wales. This is the sector handling an aircraft cruising from Amsterdam to Dublin that never comes near a UK airport.
- London Terminal Control — the busy, low-level, high-workload airspace around the London airports, sequencing arrivals and departures.
Military air traffic control operates from the same site. Scottish airspace is handled separately from the NATS centre at Prestwick.
Swanwick controls a vast area, but it is not a radar station in the sense of a dish on a hill. It is a control centre: radar feeds arrive from sites all over the country and are presented to controllers there.
The London TMA
The London Terminal Control Area is the block of controlled airspace covering the South East. It exists to keep the arrival and departure streams of several major airports — Heathrow, Gatwick, Stansted, Luton and London City — from interfering with each other in a very small piece of sky.
Much of the TMA is Class A airspace, the most restrictive class in routine UK use. Everything inside flies under Instrument Flight Rules with an ATC clearance and is separated by the controller. There is no casual transiting through it.
This is why the traffic pattern looks so orderly: aircraft in the TMA are not choosing their own routes. They are being fitted into a sequence.
Airspace classes, briefly
| Class | In practice |
|---|---|
| A | IFR only, clearance required, all traffic separated. The core of the London TMA and the upper airway structure. |
| C | IFR and VFR both permitted with clearance. In the UK, principally above FL195. |
| D | Controlled but more accommodating — used around many regional airports. VFR traffic gets clearance and traffic information. |
| E | Controlled for IFR; VFR may enter without clearance. Used sparingly in the UK. |
| G | Uncontrolled. Pilots are responsible for their own separation, though radar services are available on request. |
Class G — often called the "open FIR" — is where most light aviation lives, and it covers a great deal of southern England below the controlled airspace. Those slow-moving aircraft pottering about beneath the airways on the map are usually in Class G, squawking 7000 and going wherever they like.
The holding stacks
When arrivals outpace the runway, aircraft are held in a stack: a racetrack-shaped orbit, with aircraft layered vertically and taken from the bottom as space appears at the airport. Watch one on the map long enough and you will see the whole column step down a level at a time.
Heathrow has four, and they are the classic sight on any tracker covering the South East:
- BOVINGDON (BNN) — north-west of London
- LAMBOURNE (LAM) — north-east, over Essex
- BIGGIN (BIG) — south-east, over Kent
- OCKHAM (OCK) — south-west, over Surrey
Gatwick's arrivals hold principally at TIMBA to the south-east and WILLO to the south-west. If you see a neat stack of aircraft turning circles over the Channel coast, that is almost certainly TIMBA.
A runway is the bottleneck. It can accept an aircraft roughly every 90 seconds at best, and that rate does not flex. Holding converts an unpredictable arrival flow into a queue the airport can actually absorb — inefficient, but the alternative is turning aircraft away.
Which way the airports are running
Aircraft take off and land into wind, so the prevailing south-westerlies mean UK airports operate on westerly runways most of the time. When the wind swings round, the whole flow reverses — and the map changes character completely.
Heathrow has two parallel runways and, on westerly operations, normally uses one for arrivals and one for departures, swapping them at a set time in the afternoon so the communities under each approach get a predictable half-day of relief.
Gatwick is the busiest single-runway airport in the world by movements. Its second strip is too close to the main runway to be used simultaneously and normally serves as a taxiway, coming into use when the main runway is unavailable.
If you are tracking an arrival and want to know which end it will land on, look at the direction of the other traffic already established on approach. They will all be doing the same thing.
Airfields in the southern England picture
| Airport | ICAO | IATA | Character of traffic |
|---|---|---|---|
| Heathrow | EGLL | LHR | Long-haul and short-haul, near-continuous streams |
| Gatwick | EGKK | LGW | Leisure and short-haul, strong seasonal peaks |
| Southampton | EGHI | SOU | Regional turboprops and short-haul jets |
| Bournemouth | EGHH | BOH | Leisure, cargo, training and maintenance |
| Farnborough | EGLF | FAB | Business aviation almost exclusively |
| Biggin Hill | EGKB | BQH | Business aviation and general aviation |
The four-letter ICAO codes are the ones that matter for tracking. EG prefixes every airfield
in the UK, so a code starting EG is domestic — EI is Ireland, LF
France, ED Germany.
Putting it together
Once the structure is familiar, the map becomes readable at a glance. High-level traffic in straight lines is the en-route airway system. Descending traffic converging on a point is a stack feed. Neat parallel lines at low level, all pointing the same way, are an approach sequence. And the aircraft meandering slowly at 2,000 ft with a 7000 squawk is somebody enjoying a Sunday afternoon in Class G, entirely legitimately, going nowhere in particular.