Page last updated at 09:33 GMT, Wednesday, 25 March 2009

Microwaves 'improve fog landings'

By Daniel Emery
Technology reporter, BBC News


Tom Symonds explains how the system works

Passengers flying into Heathrow in fog or poor visibility will be guided in using a new microwave-based system.

The existing Instrument Landing System (ILS) is susceptible to interference, meaning aircraft had to be spaced further apart on their final approach.

The new Microwave Landing System (MLS) is less prone to interference, meaning aircraft can now land at a faster rate.

Initially, the system will be used by British Airways' Airbus 320s, although other airlines are expected to follow.

On a clear day, about 44 planes an hour land at Heathrow.

BA Airbus A321
BA's Airbus will be the first aircraft equipped with MIS

However, if the visibility drops and aircraft have to use the ILS system to land on full autopilot, that figure falls to 24 aircraft an hour.

This is because the radio transmitter at the end of the runway needs good line of sight to the approaching aircraft, but because it is at the far end of the runway, planes have to land and taxi clear before a full signal is restored.

Not only does Heathrow's capacity fall significantly, but because long-haul international flights take priority, domestic and short haul passengers either find themselves circling London in a holding pattern, diverted to another airport, or find their flight has been cancelled altogether.

The new MLS allows an extra six aircraft an hour to land, meaning that while fog will still cause disruption, its effects will be less prominent.

Graphic showing MLS system
1: Azimuth guidance system helps pilots work out how far from centre of runway they are.
2: Elevation guidance tells pilots how far above ground they are, and helps aircraft descend on the correct path.
3: MLS covers a far wider area than that provided by ILS, and is much less prone to interference.
4: ILS transmits radio signals in a narrow beam, and can experience interference from other aircraft and terrain.
5: Up to 30 planes per hour can land at Heathrow using MLS. Only 20-24 planes per hour could do so using ILS.

In 2006, four days of heavy fog at Christmas stranded thousands of passengers and resulted in hundreds of cancelled flights to and from Heathrow.

It is thought the cumulative costs of the Christmas fog ran into the tens of millions.

Speaking to the BBC, British Airways' flight operations manager, Captain Tim Price, said that the financial argument in favour of MLS stacks up.

"If we had had this system in December 2006, then the system would have paid for itself within four days," he said.

'Great reputation'

Designed in the 1940s, the ILS system uses two radio signals - one transmitted at the far end of the runway and the other at the side on two separate frequencies - to guide the aircraft down on an approach making a horizontal angle of three degrees with the runway.

MLS, on the other hand, uses a single frequency in a band far removed from that of the ILS system to broadcast the azimuth and elevation (horizontal and vertical angle) data to the aircraft.

The National Air Traffic Service (NATS) says that the new MLS system will guide planes down along the same flight path, so as to not interfere with ILS landings.

BA aircraft in fog
Heathrow can be prone to fog, especially early in the morning

As such, it will not be implementing so-called curved approaches. Rather than the three degree approach in line with the runway, aircraft could - in theory - approach the airport from up to 40-degrees off the end of the runway, lining up with it a mile or so before touchdown.

Even without this feature in the short term, the space between aircraft will be reduced, resulting in more planes landing per hour.

For pilots, the display for the MLS and the ILS is identical, meaning that there is very little training to get air crew up to speed.

Professor Graham Braithwaite, director of the Safety and Accident Investigation Centre at Cranfield University, said that anything that reduced delays at Heathrow had to be welcome.

"This is a precision-approach tool and is something that the International Air Aviation Organisation endorses.

"The challenge for air traffic controllers (ATC), now that distance between planes is reduced, is ensuring you get a good mix of aircraft. The last thing you want is a Fokker 50 flying into the turbulence generated by a 747 flying ahead of it.

"Some aircraft are worse for feeling the effects [of turbulence] than others, but Heathrow ATC would know this better than anyone else and they have a great reputation."

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