French revisit MLS for Paris landing guidance

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French air navigation authority STNA is considering providing both Paris airports with microwave landing systems (MLS) after ordering installation of the equipment at Toulouse in order for Airbus to conduct MLS certification work.

Although MLS was once hailed as a future successor to instrument landing systems (ILS) the technology is seen as inferior to satellite-based navigation, with the result that the civil aviation industry abandoned MLS in favour of satellite-based guidance.

But the French move signals a possible revival of MLS as a tool for increasing capacity at Paris Charles de Gaulle and Paris Orly during Category III weather conditions. Precision landing equipment based on global navigation satellite systems (GNSS) are currently unable to provide the navigation accuracy required for Category III approaches.

STNA has ordered a single MLS system for Toulouse and has placed options on seven more: four for Charles de Gaulle, two for Orly and a single system for training purposes. It has contracted French air traffic control systems manufacturer Thales ATM – formerly Airsys ATM – to provide the MLS equipment.

“The main reason [for looking at MLS] is that lots of people realise that GNSS is not available for Category III and will not be for the next few years,” says a spokesman for STNA’s communications and navigation division. “We’ve not changed our mind [by resurrecting MLS] but are trying to solve a medium-term problem.”

ILS signals are sensitive to movement from aircraft and other vehicles and each ILS installation has a critical zone around it which must remain free from interference. But for MLS equipment this zone is much smaller. MLS installation would theoretically allow Paris air traffic controllers to keep aircraft more closely spaced during approach and landing without risking the MLS signal integrity.

“For ILS the sensitive and critical area which has to be protected is large,” says the spokesman. “With ILS we have to make sure this whole zone is clear. Arriving aircraft have a direct impact, and this is one of the reasons we have to keep them separated. But with MLS the biggest limitation for [closer] separation disappears.”

While the MLS installation in Toulouse will primarily enable Airbus to conduct airborne certification tests for British Airways aircraft – as part of the carrier’s individual drive to implement MLS for its London Heathrow operations – Air France is expecting to reach a decision on its own MLS strategy by the end of this year.

“If we decide to install MLS on the fleet it will only be to increase the landing capacity at Charles de Gaulle,” says an Air France spokesman. “Retrofit of this equipment is very costly. We need to have good information and know that the traffic increase in low-visibility conditions would be good enough to justify it – we can install MLS only if there is a significant increase in the number of low-visibility movements.

Air France would install MLS avionics on its entire short- and medium-range Airbus fleet. The carrier presently operates more than 100 aircraft from the Airbus A320 family, and the spokesman says that a full retrofit would probably take until around 2007 to complete.

He says that Air France would need an increase of around five or six movements per hour in low-visibility conditions at Charles de Gaulle in order to make the retrofit worthwhile, adding: “We’ve not yet received confirmation that the number of landings will be enough for the required return on investment. But we hope to obtain this data at the end of the year.”

French civil aviation authority DGAC, STNA, Air France and Paris airports operator Aeroports de Paris will continue discussions on whether to proceed with a full MLS implementation at Charles de Gaulle and Orly. The STNA spokesman says: “It is very difficult to say when a decision will be made. The decision really has to be taken quite rapidly but there is no real target date.”