Alternatives to ILS gaining momentum

 - December 27, 2007, 10:49 AM

Is ILS, aviation’s trusted friend for the past half century, now seeing its last days? Probably not. Some observers believe it has many years of life ahead of it, yet newer technologies are slowly entering the scene, in such diverse settings as Norway’s fjords, Heathrow’s jam-packed runways, the icy wastes of Antarctica and at several major U.S. hubs. In these locations, the newcomers provide unique benefits that ILS either cannot provide, or provides at much higher cost.

In Norway, regional airline Wideroe, an SAS subsidiary, flies its Dash 8s into some pretty challenging locations, such as the coastal fjords along the country’s rugged coast. Weather can be bad, landing minimums have been high, and yet the many small communities depend on regular air service. Following the 1988 loss of a company Dash 7 with all on board after a beacon step-down approach into the small airstrip at Bronnoysund, 500 miles north of Oslo, Avinor, Norway’s air navigation service provider, sought a safer approach solution.

Signal reflections off the high ground surrounding the fjord prevented the use of ILS, but GPS could work there, provided local accuracy corrections were datalinked to approaching aircraft–the principle behind the FAA’s LAAS. Norway’s Park Air, a Northrop Grumman subsidiary, had many years ago developed a predecessor to LAAS, called SCAT-1 (for Special Category I), which–after lengthy evaluation and testing–was approved by Avinor and the European Aviation Safety Agency. The first installation was certified at Bronnoysund earlier this year. Regular service by Wideroe started in late November.

Over the next three years, Avinor plans to install 25 SCAT-1 systems across Norway, at remote airfields with difficult approaches. Initially, Wideroe’s Dash 8s, equipped with dual Universal Avionics GLS-1250 receivers, are limited to 400-foot decision heights (DHs) on Bronnoysund’s 3.8-degree glideslope, but that number is expected to be lowered as operators gain experience.

ILS Alternatives
At London Heathrow, microwave landing systems (MLS) are beginning to infringe on ILS’s turf. By the middle of this year, British Airways MLS-equipped Airbus A320s and A321s are expected to be flying Category IIIB autolands, with DHs of 50 feet or lower, to the airport’s parallel 09/27 runways, each of which have a Thales Category III MLS at either end. Here, British Airways will be stealing a march on its competitors. At Heathrow, aircraft are prohibited from taking off or landing until two minutes after a departing aircraft has flown past the ILS transmitter at the far end of the runway, due to the risk of lingering disturbances in the ILS localizer signals.

But by using MLS, British Airways Airbuses, equipped with Thales multimode ILS/MLS receivers, are immune to this problem and can use one-minute spacing, which is expected to yield at least six extra departures per hour. At Heathrow, which routinely operates at 98-percent capacity, this represents a strong operational benefit, which other airlines are reportedly eying enviously. Other U.S. and overseas carriers might follow suit, since Eurocontrol’s long-range plan is to supplement and eventually replace its Category III ILS with MLS.

Category III LAAS, once considered to be the replacement for Category III ILS, is now considered to be only a distant possibility, although Honeywell aims to have its Category I LAAS certified for private operation late this year. But Category III LAAS remains in developmental limbo and might, according to some industry observers, be shelved.

At the McMurdo, Antarctica research station Category I MLS is providing landing guidance to USAF C-130s on re-supply missions, as well as a locally based Basler turboprop DC-3 and several smaller twins that provide transportation for scientific expeditions on the ice cap. The U.S. National Science Foundation operates three MLS units at various locations, with another two or three installations planned.

Built by the Communication Division of the former Bendix, now part of Honeywell, 12 of the Category I systems were made in the late 1980s and installed at small airports in Alaska and northern Washington as operational evaluation precursors to the FAA’s plan to install MLS nationwide. But user acceptance was slow and, with the agency’s subsequent change of direction in 1995 in favor of NAS-wide GPS LAAS, they were gradually de-commissioned and warehoused.

Reportedly, their reinstatement in Antarctica came about because a large number of USAF C-130s, the type that provides the major re-supply link to the research station, are still equipped with MLS receivers, a legacy from the FAA’s earlier commitment to the system. By the time the FAA switched to LAAS, the USAF had purchased some 150 small, helicopter-transportable, tactical MLS stations and had equipped its tactical C-130 fleet with MLS receivers. The USAF has since deployed its small MLS units in Iraq and elsewhere, although those systems were found to be less successful when tested earlier at McMurdo.

But the commercial MLS serving McMurdo’s ice strip is also unusual, being on the seven-foot-thick frozen surface of a sea inlet that becomes a ship channel during the Antarctic summer. However, the strip is much closer to the base station than McMurdo’s main airstrip, located more than an hour’s drive away. As summer approaches, the sled-mounted MLS station is towed off the sea ice and stored until winter returns. When it is reinstalled on the ice for the following winter, an FAA Flight Inspection Challenger checks it before it can be used by the USAF and civil aircraft.

Competition from FAA Programs
Another threat to ILS’s longevity is the FAA’s WAAS. While widely regarded as simply a means of providing accuracy enhancement to GPS, developments in WAAS signal processing have sharpened its capabilities significantly, to the extent of supporting GPS 200-foot DH, Category I equivalent approach standards.

Before the middle of last year, GPS localizer precision with vertical guidance (LPV) approach procedures never went below 250-foot DHs. Now, 12 WAAS procedures have been published with DHs down to 200 feet, and the FAA’s Oklahoma City charting facility is reviewing the more than 950 earlier LPV procedures for their upgrade potential for lower limits. Eventually these could, and probably will, exceed the 1093 FAA ILS Category procedures.

The great benefit of WAAS is its lack of expensive ground equipment and therefore its nationwide availability that, theoretically, could provide precision guidance down to every relatively obstruction-free landing strip across the nation. It isn’t that simple, of course, because there are other safety considerations, such as lighting, runway markings and the contentious current requirement to have a taxiway parallel to the runway under instrument approach conditions. However, this rule is now under review, and is likely to be amended on a case-by-case basis, although it is expected to remain a requirement at non-tower airports where unmonitored back tracking down the runway is a necessity after landing.

Nevertheless, in the FAA’s unofficial view, WAAS represents a viable alternative to Category I ILS and could conceivably replace it over the years ahead, leaving ILS to handle the Category II and Category III needs of the major operators where there no longer appears to be any FAA interest in the GPS LAAS solution.

The transition from Category I ILS to WAAS certainly won’t happen overnight but, with the reported population of more than 30,000 WAAS-equipped general aviation aircraft today, combined with the near certainty that all future GPS receivers will include WAAS, coupled with the steady release of WAAS LPV procedures, the economic pressure on the FAA to gradually decommission ILS is certain to increase, just as it did for the NDBs the agency is decommissioning today.

So the clock is ticking for ILS. When will its time be up? Certainly not for many years, if only for the fact that, apart from MLS, there is no replacement in sight for the Category II and III requirement. This raises an interesting question: as the airlines in Europe move to an MLS environment and U.S. overseas operators fit multimode receivers to stay competitive, will the FAA dust off its original plan for NAS-wide MLS?