Gary Noakes finds out what goes on behind the scenes at some of the world’s busiest air traffic control centres.
If you’re looking at this page cursing the announcement that take-off has been delayed again, read on – it may not make you feel better, but it will hopefully give you a clearer idea of what’s actually happening.
Firstly, if you are sitting at Heathrow or any other of the world’s busier airports at peak time, don’t blame air traffic control for your woes. Even on the finest of days with no bad weather to take into account, getting off the ground at airports like Heathrow or JFK can take an hour between the aircraft pushing back and the wheels leaving the runway. This is simply the result of not having enough runway space at peak times.
As British Airways will confirm, building a new terminal can’t help it shift Heathrow’s 68 million passengers a year without delays if there are only two runways.
When you finally get permission for take-off, you are under control of the airport tower for the first 10-15 miles of your flight, as you will be on your return, when approach controllers will guide you in.
These people are responsible for giving instructions to pilots and putting aircraft into the correct sequence, for example giving priority to aircraft low on fuel due to stronger than expected headwinds. Approach controllers will also link inbound aircraft with the airport’s Instrument Landing System, a ground-based radar that, at major airports, allows landings in zero visibility.
After clearing the airport’s airspace, you become a dot on the radar at one of three control centres operated by National Air Traffic Services (NATS) until you are handed on to another country’s air traffic control (ATC). In hushed rooms at NATS’ Swanwick centre in Hampshire or at Manchester and Prestwick airports, you are one of the 220 million passengers a year overseen by the organisation, which controls all aircraft overflying the UK and the eastern Atlantic.
In the south-east, Swanwick oversees flights until they hand over to the control towers of the London airports, because as well as being the confusingly-named London Area Control Centre, covering England and Wales, it also houses the London Terminal Control Centre, which handles everything departing and approaching south-east England. NATS has a busy job, particularly around London, as each year it handles 2.5 million flights from 15 airports.
To ensure safety, NATS can only let into its airspace the number of aircraft its controllers can handle given the conditions. So if you’re sitting on the ground and the pilot has said something about air traffic control delays, it means that weather conditions like high winds are playing havoc with take-offs and landings, particularly at airports with only a single runway, and that this is messing up the flow pattern in the sky; or that there is an inordinate amount of traffic, such as on a Saturday in August when holiday flights compete with the regular scheduled traffic.
At busy times, it is Eurocontrol in Brussels that can keep your aircraft on the ground. Eurocontrol’s Central Flow Management Unit oversees airspace across Europe and spots potential bottlenecks and overloads before they happen, re-allocating slots and rerouting planes to keep the system flowing.
Weather has a big part to play. A thunderstorm over a major airport can cause massive delays, as can fog. Fog slows down the flow of traffic, but the difficulty is not so much in the air. “Controllers are looking at aircraft on a screen. They can see even if it’s foggy or dark,” said a NATS spokesman.
The problem is on the runway, as landing aircraft cannot see the one in front of them. “In these conditions we have to reduce the flow rate, although in the sky aircraft are still separated by the same distance.”
This means that, instead of aircraft following each other in to land as normal, with a few seconds’ safety margin, they must wait for their final approach until the control tower is satisfied the preceding aircraft has cleared the runway. It is this that slows the system down.
The instrument landing system (ILS) is a vital component in keeping airports open during poor weather like fog. When the British Airways Boeing 777 crash-landed at Heathrow earlier this year, it came within feet of destroying the runway’s ILS beacons, which could have caused weeks of disruption instead of just days.
Any weather delay or problem on the ground means inbound aircraft have to queue in the sky, known as stacking. Stacking is where aircraft are put into spiral patterns over designated parts of the UK – flights going into Heathrow from continental Europe, for example, are stacked over Clacton. A gradual spiral descent is made until the aircraft is cleared to join the normal approach route.
Most frequent travellers know flying into a busy airport at peak times will mean 10-20 minutes in a stack. Airlines know this too, and build extra time into their schedules, so a flight that is 20 minutes late shows as on time. Surprisingly, NATS is unable to say what percentage of flights are stacked.
While in a stack, you quite often get a glimpse of the aircraft above and below yours, which can be unnerving, particularly when you realise you’re close enough to identify the airline with ease. Even frequent fliers sometimes get confused, because judging the relative height of the passing plane is extremely difficult, especially if the higher aircraft is larger than the one you are on.
Don’t panic, though – it is all tightly controlled by air traffic control, and if another aircraft seems very close, it is probably at the standard minimum vertical separation of 1,000 feet from yours. Similarly, at airports like Los Angeles, which have pairs of runways assigned for landings, it can be alarming when an aircraft parallel to yours appears to be heading for the same runway, but this is all quite normal – it’s how airports like LAX maximise capacity.
Even if a controller does make a mistake, there are plenty of back-up systems in place to ensure that it doesn’t result in a mid-air collision. Apart from the old-fashioned option of the pilot looking out of the window, there are three key safeguards. Firstly, all aircraft with more than 19 seats flying into European airspace must be fitted with a traffic collision avoidance system (TCAS), which warns of an impending collision and orders evasive action.
On the ground, there is Short Term Conflict Alert, a computer prediction that alerts potential dangers to controllers before they happen and SMF, the Separation Monitoring Function, which automatically alerts controllers to the loss of standard separation.
Mid-air collisions are extremely rare. The last involving a UK-crewed aircraft was in 2002 over Germany, when a DHL cargo plane en route from Bergamo to Brussels collided with a Russian passenger airliner flying from Moscow to Barcelona.
The accident was a culmination of many factors hampering a lone Swiss controller. In his confusion, he overruled the Russian aircraft’s TCAS system and told the airliner to descend instead of climb, putting it into the path of the cargo plane, which had correctly followed the TCAS instruction to lose height.
The UK Airprox Board investigates all near-misses in UK skies. Last year, there were 154 “airprox” incidents, 65 involving a commercial airliner. Five of these were classed as risk-bearing, but none carried an actual risk of collision.
Even without the electronic gizmos, the chances of a mid-air collision are very slim because of the rules on aircraft separation, which put planes at least five nautical miles away from each other – roughly six ordinary miles – or spaces them at the minimum vertical separation.
This, particularly in the south-east’s crowded airspace, is often a difficult juggling act without upsetting somebody – despite the economic crisis, the skies are increasingly clogged.
Over the last decade, Europe’s air traffic has grown by 50 per cent to more than ten million flights a year or 33,000 a day. Available airspace has grown by 80 per cent since 1990, but it is still not enough.
No regular traveller needs reminding of how the London airports have mushroomed over the last decade. As well as the no-frills airline boom, London City airport by itself has come from nothing to last year’s total of 2.9 million passengers. It now has permission to grow from 80,000 return flights a year to 120,000, which will see it handle 3.9 million people by 2010.
Stansted, which a decade ago was almost a white elephant, has received permission to increase its annual passenger numbers from its current total of just under 23 million to 35 million, using its single runway to handle 184,000 return flights a year.
This need for more capacity has also prompted the government to start redrawing airspace, creating new airways over the south-east. A current hotspot is between Potters Bar and Hatfield in Hertfordshire. Here, many flights from Heathrow, Luton, London City and RAF Northolt converge after departure, meaning they must ascend in a step pattern, rather than going straight up.
There is similar congestion approaching Stansted and Luton. Currently, there are two holding patterns serving both airports – which means you’re stuck if you are flying into Stansted and something blocks the runway at Luton. NATS’ proposal is for three stacks, one for Luton and two just for Stansted.
Stansted’s geographical position near the Essex coast gives it another advantage, in that it allows aircraft from continental Europe to adopt continuous descent approaches.
Put simply, continuous descent is a gentle nose-dive towards the runway from cruising altitude, rather than the step-down approach in congested airspace. Using this technique, aircraft stay higher for longer and burn less fuel, and at least give the illusion to the passenger of a speedy, direct approach, even if your Stansted-bound aircraft is told to slow down somewhere over Germany.
London City’s arrival and departure routes will also be tinkered with, giving them a smoother flow pattern.
A bigger project happening across Europe should also cut delays. The European Union has already dismantled barriers to airlines flying within its borders, but air traffic control is still run illogically according to national boundaries, which means aircraft are routed indirectly – a short journey from Brussels to Rome has to be overseen by nine different air traffic control centres.
The EU is attempting to rectify this with the creation of a Single European Sky, banishing national borders in the clouds. This is inspired by the system in the US – there, a country which is roughly the same size as the EU and has twice the air traffic, has half the number of air traffic controllers yet manages better punctuality rates. In 2003, when the European scheme was first mooted, America was 77 per cent punctuality against the EU’s 73 per cent.
Today, KLM estimates that e5 billion a year is wasted by airlines flying indirect routes over Europe and 16 million tonnes of CO2 needlessly emitted. Legislation is currently being debated in Brussels.
“This has got to be good for passengers,” said Simon Evans, chief executive of the watchdog Air Transport Users Council. “Apart from the safety aspect, it will manage growing demand and play its part in reducing delays.”
All this is probably little consolation if you still haven’t taken off, but there are signs that the situation is, finally, looking up…
