In February, London Heathrow became the latest European airport to opt for the microwave landing system (MLS) as its future precision approach landing aid. The UK’s National Air Traffic Services (NATS) ordered four Category III systems for Heathrow’s Runways 27R, 27L, 09R and 09L, all of which will be installed later this year. The NATS contract also included options for an additional 20 Cat III systems for installation at major hubs such as Gatwick, Prestwick, Manchester and Stansted and a further 20 Cat I systems for smaller regional airports.
Last year France ordered eight Cat III MLS installations for Paris Charles de Gaulle and other airports, following the lead of the Netherlands, which had earlier installed two Cat III systems at Amsterdam Schiphol Airport. And in Afghanistan, the U.S. Air Force is currently operating six Cat II mobile MLS stations at undisclosed locations around the country.
In the UK and France, the MLS stations will be colocated with the current Cat III ILS installations, which will continue to operate, and the MLS and ILS centerline and glideslope guidance will coincide throughout the approach. The different MLS and ILS transmitting frequencies preclude the risk of mutual signal interference.
One of the French MLS installations is at Toulouse, where it is used by Airbus for certification of the multimode receiver installation in some 87 A320 variants ordered by British Airways. The airline and NATS expect that MLS will substantially increase Heathrow’s current Cat III arrival rate of around 20 aircraft per hour. This low rate results from extended approach spacing, aimed at avoiding the risk of receiving misleading guidance signals caused by the preceding aircraft distorting the ILS localizer beam. MLS–and GPS LAAS– are immune to this effect, and MLS is expected to bring Cat III arrivals back up to Cat I levels of 44 aircraft per hour.
Amsterdam Schiphol’s MLS decision was also based on the risk of ILS signal distortion, following development plans to erect several large buildings on the extended centerlines of Runways 01 and 06, just outside the airport’s zoning boundary. Calculations showed that the buildings would cause severe bends and even splits in the existing Cat III ILS beams, causing them to be down-categorized to Cat I or less.
In Afghanistan, the USAF is employing six of its 37 mobile MLS systems to transmit landing guidance to C-130 and C-17 transports providing support to U.S. and allied forces in the country. Currently, around 500 C-130s are reported to be MLS equipped, and MLS avionics are standard on the C-17. Interestingly, this makes the USAF the world’s largest MLS operator, with more ground stations and airborne equipment than all other civil and military users combined. While testing continues with the military’s GPS-based joint precision approach and landing system (JPals), the USAF currently has no operational GPS landing guidance equipment, and enemy jamming of GPS remains a problem.
Will this MLS activity, in addition to current GPS approach developments, accelerate the demise of ILS? It seems most unlikely. Most airplanes today carry ILS and are certified from Cat I all the way up to Cat III. Recertification to add equivalent-category MLS or GPS avionics is expensive and, while possibly affordable by a corporate operator with one or two similar aircraft, can easily run into many millions of dollars for a large airline fleet.
To justify this expense, operators must satisfy themselves that low-visibility MLS or GPS approaches at ILS airports will, over time, be cost-beneficial, and that’s a challenging proposition. For example, while British Airways feels it has a strong business case for MLS in its short-haul European fleet, which is severely affected by daytime arrival delays, it has held off upgrading its long-haul aircraft, most of which arrive in the off-peak early morning hours. So the final word should probably go to the FAA official who recently said, “Forget the blue-sky talk–ILS is good for at least another 50 years.”