Editor's Note

MLS: Back to the Future?

By Brian Evans | April 1, 2003
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To many experts in the United States, the microwave landing system (MLS) is past history–one of the many highly touted solutions to pressing aviation problems that, despite their promise, never reached operational maturity. Yet to experts in Europe, MLS only now is gaining the acceptance it deserves, and its future still lies ahead.

Which experts are correct? We won’t know for at least two or three years. By that time British Airways (BA), the UK National Air Traffic Service (NATS), and the British Airport Authority (BAA) should be able to determine whether their investments in MLS avionics (by BA) and airport ground installations (by NATS and BAA) are paying off.

The stakes are fairly high. Although none of the three organizations or Thales, which supplies both avionics and ground stations, is prepared to discuss costs, independent estimates put the initial NATS/BAA investment, for four advanced MLS precision landing systems to be installed at London’s Heathrow airport, at around $20 million. BA’s MLS avionics bill for receivers in its 60-plus new Airbus A320 variants has been estimated at between $20 million and $50 million. (The first airplane is scheduled to be delivered to BA in April 2004.)

BA opted for the Thales TLS-755 multimode receiver (MMR), which combines GPS, ILS and MLS sensors. The Airbus certification activity, using an MLS ground station at the company’s Toulouse airport, also will cover the manufacturer’s A318, A319 and A321 aircraft. Separately, Rockwell Collins has recently obtained a technical standard order (TSO) for its Model GLU-920 GPS/ILS/MLS multimode receiver. Pilots flying an MLS approach, using either unit, will find the guidance presentation identical to ILS. Since pilots already should be familiar with ILS, no transition difficulties are anticipated.

Why MLS in Britain?

Why are the British putting so much money into a landing system that the International Civil Aviation Organization (ICAO) adopted in the early 1980s for world use but subsequently dropped over 10 years later in favor of satellite landing guidance? Primarily, the British see MLS as a means to enhance airport capacity or, more specifically, to maintain normal capacity levels during very low-visibility conditions.

The UK has always prided itself as being the world’s leader in very low visibility operations, and Heathrow has provided "hands off" autoland quality landing guidance to appropriately equipped aircraft for more than 30 years, without a single accident. During this time, the UK’s NATS and its predecessors have operated the most demanding Category (Cat) III ILS guidance systems on its runways, allowing landings in near zero-visibility conditions. Last year, Heathrow replaced its Cat III installations with the latest technology ILS units from Norway’s Navia Aviation.

But even the latest technology cannot overcome multipath reflections of the ILS guidance beam, which is an inherent characteristic of the system. As a result, every Cat II/III ILS installation worldwide has a "sensitive area" around the runway it serves.

No aircraft or vehicles may enter this area without air traffic control (ATC) permission during low-visibility landing operations, since their presence may reflect the ILS signals and create false guidance to the landing aircraft. Taxiways to Cat II and III runways therefore bear signs showing "Cat II Hold" and "Cat III Hold" points well clear of the active runway.

Similarly, the ILS guidance signals received by an aircraft following another aircraft on final approach can reflect off the aircraft ahead, resulting in false guidance. This is usually not critical if the pilot of the following aircraft can see the runway ahead, but in low visibility, this can create a potentially hazardous condition.

A Defining Moment

One MD-80 crew experienced a startling demonstration of this effect at London’s Gatwick airport years ago. While following a Boeing 737 to land on Gatwick’s single runway on a bright sunny day, the MD-80 crew decided to make a practice "hands off" ILS autoland approach and touchdown. All went well until the MD-80 was approaching the runway threshold, just as the 737 turned off the runway at its far end. In turning, the 737 placed its vertical fin and rudder squarely in the beam of the ILS localizer antenna, located tens of feet beyond the runway’s stop end. The resulting beam deflection caused the MD-80’s autopilot to sense that the aircraft had rapidly drifted off the beam, and it promptly responded by putting the MD-80 into a steeply banked turn to recover, just feet above the ground. Only immediate action by the crew in overriding the autopilot prevented a tragic accident.

Because of this effect, the normal two- to three-mile separation between following aircraft is effectively tripled during UK Cat III ILS operations, resulting in a reduction of runway throughput to about 10 landings per hour, versus about 30 in normal visibility. It is here that MLS is expected to pay off handsomely, since the MLS scanning beam ignores reflected signals. It allows MLS-equipped aircraft to maintain normal operational spacing on their final approach and, therefore, airports to recover the capacity levels presently lost during low-visibility periods. "Heathrow airport is among the busiest in the world and MLS is expected to bring benefits to both our airline and airport customers," says Peter Wilde, NATS airport services director.

Cat III MLS installations are now under way at each of Heathrow’s four main runways. The Thales ground station contract also includes options for installations at Gatwick, Stansted, Manchester and all other major, and several medium-sized, UK airports, where they will be collocated with Cat II and III ILS equipment. All told, about 50 UK runways could offer MLS approach and landing guidance by 2010.

French Interest

In France the Service Technique de Navigation Aerienne (STNA)– equivalent to the Federal Aviation Administration (FAA) in the United States–is conducting cost/benefit analysis of MLS at the Paris Charles de Gaulle and Orly airports and evaluating its use at other sites. Other European nations are said to be considering MLS installations, with Italy reportedly in the lead.

FAA, however, is solidly committed to the eventual transition to GPS from ILS and has no future plans for MLS. The agency did install several Cat I systems at airports in the northwest United States and Alaska in the 1980s, but these are being gradually withdrawn.

Canada launched a major MLS development program in the late 1980s, but this was canceled, as well, following ICAO’s policy change in 1995. But MLS development activities continued in Europe after ICAO’s decision.

Interestingly, GPS and its European equivalent, Galileo, will, like MLS, be immune to the multipath reflections that bedevil ILS for low-visibility operations. This, of course, raises the question of why the UK has opted for MLS instead of GPS.

The answer is timing. The UK’s need to enhance its Cat III operations is imminent. But FAA expects only to achieve Cat I GPS certification by 2006. The most optimistic certification date for Cat III is around 2008/2009. However, some agency experts recently have expressed doubts whether GPS ever can meet Cat III technical standards.

Largest MLS User

Nevertheless, the FAA’s progress towards GPS Cat I and higher certification levels is certainly being followed closely by the world’s largest MLS user. With almost 40 MLS ground stations and some 1,000 aircraft at last count (and now possibly many more) equipped with MLS avionics, the U.S. Air Force has for many years been flying the microwave landing system in virtually every part of the world, from the far north to equatorial jungles. Transportable ground stations, for example, were used during Desert Storm and the subsequent Afghanistan campaign to guide USAF C-130 and C-17 transports–all equipped with MLS receivers–to unprepared landing sites.

Forecasting the future in avionics is always challenging. But one thing seems like a fairly good bet. The market for multimode ILS/GPS/MLS receivers looks strong for many years to come.

ICAO’s Decision on MLS

The microwave landing system (MLS) came into prominence in the early 1980s. The International Civil Aviation Organization (ICAO) recognized the increasing risks of commercial radio interference to the then-world standard precision approach aid–the 108-to-112-MHz ILS–coupled with the growing pressure on the 40 available ILS channels. It therefore decreed that all airports would convert to MLS by the mid-1990s.

At 5 GHz, MLS was essentially interference-free, and offered 200 separate channels. ILS also suffered from occasional guidance beam distortions, or multipaths, when its signals reflected off intervening objects, such as taxiing aircraft. MLS, on the other hand, was immune to multipath.

The United States had been a driving force behind ICAO’s adoption of MLS. But in 1995, the United States proposed that the transition from ILS to MLS be replaced by a gradual transition to the satellite-based GPS, which showed promise as a future precision landing aid. ICAO agreed to this change, though nations that wished to do so could proceed with MLS installations while retaining ILS.

Landing System Categories

An internationally adopted scale of categories describes landing guidance systems, whether they are ILS, MLS or GPS. Category, or Cat, I is the least demanding and Cats II and III are progressively more demanding of avionics, crew training and the quality of the ground equipment, among other things.

All categories have assigned decision heights at the runway threshold, below which the pilot may not descend unless the runway is in clear sight. (Otherwise, the pilot executes a missed approach.) Categories also have assigned approach visibility limits, called runway visual ranges (RVR), below which the pilot must not attempt a landing.

There are three categories, and one is broken down into three subcategories. They break down as follows:

  • Cat I instrument approaches and landings allow a decision height of 200 feet (60 meters) and an RVR of 1,800 feet (550 meters).

  • Cat II operations allow approaches and landings down to a decision height of 100 feet (30 meters) and an RVR of 1,200 feet (350 meters).

  • Cat IIIa allows a decision height down to 50 feet (15 meters) and RVR of 700 feet (200 meters).

  • Cat IIIb allows a decision height of 50 feet (15 meters) and RVR of 150 feet (50 meters).

  • Cat IIIc allows a zero-foot decision height with an RVR of less than 150 feet (50 meters).

Generally, Cat III operations are flown to touchdown under autopilot control. However, while Cat IIIc operations are well within the capabilities of current automatic flight control systems, relatively few airports have installed surface movement guidance systems (SMGS) that can accurately lead an aircraft to the terminal in almost zero visibility.

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