Air Safety — Coining Down without a Bump
A. When an aircraft starts to descend, passengers' stomachs sink, too. Over 40% of accidents happen during the relatively short time from the final approach to touch-down. Nevertheless, flying remains one of the safest ways to travel; even during approach and landing accidents are rare. For this, passengers must partly thank the Instrument Landing System (ILS), which was adopted by the International Civil Aviation Organization (ICAO) as a worldwide standard in 1949. Now a new standard is called for, to take advantage of advancing technology, but people cannot agree on which one.
B. Any new system has to be very good. The ILS enables aircraft to land even when weather conditions allow the pilot to see only 75 metres (250 feet) ahead. Indeed, the ILS signals are sometimes fed directly into the flight computer so that an aircraft cian land itself. The ILS signals come from two radio transmitters on the ground, each of which uses a different frequency.
C. The localizer transmitter is at the opposite end of the runway to the landing aircraft and sends out a beam centred along the runway: this shows the aircraft’s ILS receiver whether the aircraft is too far to left or right. The “glide-path” transmitter, which is close to the point where the aircraft will touch down, points a beam towards the incoming aircraft, angled up at three degrees: this tells the receiver whether the aircraft's descent is above or below the glide-path.
D. Although the ILS has proved highly reliable, it has three significant defects. The most serious is reflection: signals can bounce off buildings and confuse the system. Even an aircraft that is taxiing or parked can cause problems. Second, some of the ILS frequencies are close to those used by FM-radio broadcasters who have been given permission to increase the power of their transmitters, which will interfere with ILS. The third problem is the lack of frequencies for airports that are relatively close together.
E. So in 1978 the ICAO chose the Microwave Landing System (MLS) to replace the old ILS. The MLS also uses two radio signals from similarly sited transmitters to guide down aircraft, but there the similarity ends. The MLS uses narrow beams at a much higher frequency, sweeping back and forth across the path of incoming flights. A receiver on an aircraft registers when the beams pass and works out its angle to the runway from the timing of the signals. The new system is significantly less susceptible to interference than the ILS. And because its signals fan out wider, aircraft can make long curving approaches. This sometimes helps controllers manage traffic.
F. Recognizing the complexity of organizing an orderly changeover which would affect thousands of airports and tens of thousands of aircraft, the ICAO set January 1“ 1993 as a target for the MLS to take over. Airports around the world began to order and install MLS transmitters. However, in the early 1990s the smooth transition was thrown into chaos. America's Federal Aviation Administration (FAA) became increasingly convinced that the satellite Global Positioning System (GPS), which is owned and operated by America’s Department of Defence, was a better option.
G. This system allows anyone with a receiver to calculate his exact position from radio signals transmitted by a network of 24 satellites orbiting the globe. America’s armed forces and increasing numbers of civilian sailors, pilots and walkers use it to navigate with great accuracy. 'Die system works well and has the bonus that many aircraft already carry GPS receivers to help them find their way from one airport to another. Successful trials have already taken place and the FAA expects to authorize GPS landings in conditions where pilots can see at least 550 metres ahead by the end of the decade. This covers the many American airports which have generally good weather.
H. All this convinced the Americans to abandon MLS last year, upsetting European aviation authorities, which were concerned that in a war the American military might close down civilian access to the GPS. Earlier this year, a special meeting of ICAO considered the conflict and came up with a compromise which keeps airports happy but will require aircraft to carry extra equipment. Airports which want to stick with ILS can do so. Those which support MLS can introduce it in place of ILS and those which want to use the GPS (or its Russian equivalent) are to be given time to bring their technology up to ICAO standards.
I. For people who hoped for a world standard this looks like a mess. One answer may be the multi-mode receiver. These devices, which are now being developed by avionics firms in Europe and America, will give a consistent ILS-type display on the flight deck whether they are receiving signals from ILS, MLS or GPS. New approach systems might be accommodated by slipping more circuit boards into the receiver. Having lost the battle on the ground, the standardizers may yet triumph on the flight deck.
A reliable landing device is needed by all passenger airplanes. The device that has been in use since 1949 is called ILS , which is basically made up of two transmitters and a . It has been realized that some exist and pose safety hazards to landing aircraft.
Another landing device is the MLS, developed more recently. Compared with ILS, the MLS transmitters send out frequency signals, which are less likely to be interfered by such things as and FM radio signals. The latest development of science and technology has brought about ? GPS, which works on a completely different principle from its two predecessors. It relies on radio signals transmitted by satellites and trials on airplanes have proven successful.
With the above three different systems, are now having problems deciding which one to use as . One solution may be that airports can use anyone they like but aircraft will be equipped with a special device that responds to all of the three.