What goes up must come down again

There is a very old tale of a much battered and broken pilot being extricated from the wreckage of his aircraft. As the rescuer was heard to say, “If God had meant you to fly, son, He’d have given you wings!”.

A recent story in Dragnet outlined the operating principles of the Global Positioning System and hinted that eventually it might be refined to such a degree of accuracy that approaches and landings could be made to any runway in the world, eliminating the need for ground-based approach guidance. That day may be nearer than I thought. The July 1994 issue of Aviation Bulletin states that the Microwave Landing System – which was destined to replace the standard Instrument Landing System (ILS) by 1998 – will undergo no further development by the FAA and all efforts will now be concentrated on a GPS landing system. Many of you will know that Australia’s CSIRO played a large part in the development of MLS and it is sad to see this work possibly about to end without ever achieving its full potential. It illustrates how rapidly technology advances nowadays, and how quickly obsolescence sets in. Most Amateurs have at least a passing interest in aviation and avionics, and this story briefly outlines precision landing aids and why ILS is now outdated.

Common sense says you cannot manually land an aircraft unless you can see where you are going. Even if you pop out of the bottom of cloud exactly in line with the runway, it still takes a finite time to recognise the runway as such, assess any drift that may be present, and successfully land the aircraft. The faster the approach speed, the higher must be the cloud-base to give enough time to make this assessment. If the aircraft is slightly off the centre-line then more time will be needed to sort out the approach. It is generally accepted in the airline industry that a cloudbase of 200 feet (60 metres) and a visibility of not less than 2600 feet (800 metres) represents the lowest conditions under which a manual landing, i.e. one where the human pilot is flying the machine, is likely to be successful, assuming that you are more or less lined-up with the runway to start with. In worse conditions a fully automatic landing is the safest way to go. Much time, effort and money has been expended to achieve this happy state of affairs, but even today there are still only a limited number of airports in the world where fully automatic landings can be made.

Early aviators relied on the Mark 1 Eyeball and external visual cues to keep themselves orientated. Those foolish enough to stray into cloud and thereby lose visual reference found that the seat of their pants gave totally erroneous information, and only the lucky ones avoided an early meeting with their Maker. However, people like Sperry and Doolittle developed gyroscopic instruments to simulate the horizon and “blind flying” became a reality. Unlike the birds, who had more sense, man learned to fly safely at night and in cloud. In very short order, the developing science of radio was coupled with aviation and radio-navigation was born. Radio beacons on low and medium frequencies could guide an aircraft towards its destination but with limited precision, and the final approach and landing still relied on the Mk 1 Eyeball.  LF/MF non directional beacons (NDBs) are still widely used today as guidance to airfields, but their lack of precision and absence of glide-slope information restricts approaches to conditions of about 500 feet cloudbase and a couple of miles visibility, or better.

As VHF techniques improved, so the possibility of using the shorter wavelengths to generate precision radio guidance developed. By using carefully designed Yagi-Uda antenna systems and special modulation techniques, it was possible to define a beam stretching outwards from the runway centre-line. This, coupled with vertically radiating beacons located at specified distances from the runway threshold which had to be crossed at a given height, gave a basic instrument approach or “runway localiser” system. A high degree of piloting skill was necessary to remain on the centre-line and to estimate the required rate of descent to position the aircraft for its landing. Not long afterwards a similar UHF beam was added at right angles to, and along the path of, the localiser beam to give glideslope guidance and the ILS system was born.

ILS has been the standard means of precision landing guidance for many years now. The localiser uses one of 40 channels between 108.10 and 111.95 Mhz, with a paired glideslope between 329.15 and 335.00 Mhz. Selecting the localiser frequency automatically selects the paired glideslope frequency. The localiser extends out to a minimum of 18 NM (33 km) from the runway, and the beam extends 10 degrees either side of the runway centre-line. The glideslope is normally set at 3 degrees and intersects with the runway about 900 feet in from the threshold. The localiser antenna system is located at the far end of the landing runway, while the glideslope antenna is located just to one side of the runway at about 900 feet in from the threshold. (The glideslope antenna cannot be at the threshold because the beam must cross the threshold at a height, usually 50 feet, such as to give the aircraft a safe clearance at the end of the runway). Theoretically, if the pilot could “hang on” to the two beams right down to the ground, then a manual landing without outside visual reference is possible. In practice this is not feasible due to both human and system limitations, and 200 feet cloudbase with at least 800 metres visibility is considered to be the lowest safe transition from instrument flight to visual flight to complete the MANUAL landing. IF we can ensure both ground and in-flight systems’ integrity, then we can achieve a fully automatic landing. Let’s look briefly at some of the limitations, both human and system.

Beam width and aircraft response. The runway localiser beam extends 10 degrees either side of the centre-line, and the glideslope beam extends (nominally) 0.7 degrees above and below the desired approach slope. Clearly, the further you are from touchdown then the wider will be the distance between the beam edges. The pilot interprets his displacement from the beam centres by means of meters; early ILS meters were simply two D’Arsonoval meter movements at right angles and the pilot flew the aircraft so as to keep the two cross pointers centred on a small circle in the middle of the dial. Today the presentation is better, but the basic idea remains. When the aircraft is some miles from touchdown, a displacement of some hundreds of feet from the centre-line results in a small displacement on the meter and flying back to the centreline is a fairly slow and gentle process. Close in to the runway, the same angular displacement and the same instrument indication may equate to only a couple of feet displacement, and a slight twitch on the controls may mean the aircraft going outside the beams altogether. The interpretation of the ILS indicator coupled with the ever-narrowing beams imposes a limitation on the ability to “fly the beam” manually. Remember that the human pilot has to also interpret the aircraft’s performance by reference to other instruments such as the artificial horizon, altimeter, airspeed indicator, rate-of-descent indicator, and compass, as well as beam displacement. The human brain can only work just so fast. Manual landings in “zero-zero” are just not on!

System limitations. Some ILS beams are neither straight nor stable. Unless the ground over which the beams are located is both level and of constant conductivity, beam bending will occur due to reflections and/or changes in propagation velocity. False lobes occur either side of the main beams, necessitating the use of marker beacons (on 75 MHz) to confirm that the correct beams are being flown. Movement of other aircraft or vehicles in the vicinity of the antennas may result in the beams shifting erratically, and an aircraft ahead of another on the approach may also distort the beams.  Some of these limitations can be reduced by better antenna design, but not totally eliminated. A system used for fully automatic landings must be protected at all times to assure beam stability.

The auto-pilot can be coupled to the ILS system for automatic guidance, and this confers a number of advantages. Instead of one human brain controlling everything, you can have dedicated “brains” for each required function; one to control the aircraft’s glideslope, one to control the lateral displacement, others to control airspeed and so forth, and all continuously co-related by a “master brain” to fly the aircraft. Electronics are much better at detecting and correcting minute displacements from the desired centre-lines and suffer from neither fatigue nor being “up-tight” about the lousy conditions outside. Systems are triplicated or quadruplicated to provide extreme reliability and to “out-vote” any out-of-tolerance system. Hence the rule that when conditions are below 200’/800m, only automatic landings using approved and monitored ILS systems are permissible.

Noise minimisation is a constant concern at major airports. The standard ILS can offer only straight line guidance to the approaching aircraft, and also the system has a limited handling capacity. A system that could offer curved approaches in both lateral and vertical planes could offer clearance around particularly noise sensitive regions under the approach path, and would be a boon in mountainous regions where approach obstacles exist. The Microwave Landing System (MLS) offers just these features.

The Microwave Landing System operates on 200 channels between 5031 to 5091 Mhz (almost twice the frequency of the XYL’s microwave cooker) and uses beam scanning technology to provide proportional landing guidance to 40 degrees either side of the runway centre line and to 15 degrees in elevation. Coverage extends to 20 NM or 37 km from the runway. Unlike ILS with its fixed beams, MLS provides three-dimensional landing guidance within the scanned volume, permitting curved approaches with varying glideslope angles. This enables better noise-reduction approach profiles, and may permit more fuel-economical approaches as well. The much higher frequencies used lessen the siting problems associated with ILS, and the greater number of available channels reduces any risk of mutual interference in areas where many airports exist in close proximity. (There are some 36 airfields in the greater Los Angeles area, for example).  Additionally, several aircraft can use MLS simultaneously thus permitting an increased landing rate compared with ILS. MLS was approved for general use in 1987 and already some 760 MLS systems are installed in the USA. The original plan was to fully convert from ILS to MLS by 1998, but it now appears possible that further refinement of MLS to “zero¬zero” capability will not be undertaken because of the introduction of GPS. As always, there are problems to be overcome.

GPS Landing System. An earlier article outlined how GPS uses information from 24 (21 operational plus 3 orbiting on standby) accurately positioned satellites to give a three¬dimensional fix anywhere on or above the earth’s surface. Two levels of signal quality are transmitted, but at present only the coarse acquisition signal is available to civilian users. The military-use precision signal guarantees a position within 17.8 metres in plan position and 27.7 metres vertically. Techniques are available which are able to refine the “fix” to within one metre, and, as with both ILS and MLS, a precision radio altimeter can be used for absolute altitude information. The latest Aviation Bulletin infers that pressure will be applied to United States Air Force, owners of the GPS system, to make the accurate signals available for approved civilian users, thus permitting the system to be developed for highly accurate approaches to any airport but without the need for ground-based installations. Like MLS, GPS would allow curved approaches and variable angles of glideslope. A final decision on MLS or GPS has yet to be taken by the International Civil Aeronautical Organisation, but development of GPS will certainly continue. Quite apart from the possibility of precision approaches, GPS also has enormous potential for use in a world-wide air traffic control system; however one wonders about the security of such a system in the event of major hostilities.

ILS has served us long and well, but modern technology offers us more versatile systems. MLS will likely run in parallel with GPS for some years yet but the indications are that GPS – or its successor – will be the ultimate winner.

Clive – VK6CSW.

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