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Aviation History
1959
1959 - 0543.PDF
268 - ;....->. •--"-.-•-;:-. : PLYING AIDS . . . FLIGHT An over-the-shoulder view of a "hands-oft" automatic approach in a B.L.E.U. Vanity at k.A.E Bedford. The runway can be seen through the circular windscreen panel PRECISION FLIGHT Trends in Flight Control Systems ALL the new transport aircraft carry an autopilot and most of. them also have some form of integrated flight instrumenta-tion either working from the same references or designed to provide an independent monitoring facility for those references.In any future transport aircraft it is highly probable that the autopilot and flight controls will themselves be integrated. Coupling of the autopilot to the I.L.S. receivers is now virtuallystandard, so that automatic approaches, with or without automatic control of throttles, are a common procedure. In this way anairliner can be brought down to a height of about 200ft wherever the I.L.S. transmissions are not excessively affected by terrainconditions. The minimum approach height of 200ft is dictated by two principal factors. Firstly, the basic limitations of presentI.L.S. installations are such that their guidance in azimuth and, more particularly, in glide-path is not sufficiently accurate belowheights of this order. Secondly, if the pilot is to make a visual touch-down, he will require most of the time taken to descendthe last 200ft to recognize and align himself with the visual aids and to make the actual round-out and touch-down. One of the shortcomings of present lighting patterns is that theygive insufficient glide-path guidance. Conventional angle-of- approach indicator lights, using single lamps with red, orangeand green segments, have never proved satisfactory, although a much more powerful and elaborate example produced by theGeneral Electric Co. Ltd. was extensively tested at Blackbushe some time ago. Two further systems were evaluated by R.A.E.during last year. One was the Australian Cumming/Lane System consisting of two wing-bars of lights, one close to the ground andthe other elevated on poles, their relative heights being adjusted to define a given glide-path angle when viewed in line with eachother. The other was developed by R.A.E. and consisted of bars of light about 30ft wide, spaced at the fore and aft limits of thecorrect touch-down area. The bars were formed by powerful lamps, half of whose reflector area was covered by red lenses.The lamps and lenses were set within shallow boxes with both the red and white light rays emerging through slits cut in the box-ends. Each box was then tilted so that the pink intermediate light denned a three-degree glide-slope. Two parallel three-degreeslopes thus bracketed the correct approach path. If the aircraft was low, both pairs of bars would show red; if it approached high,both would be white. Ideally the nearer bar showed red and the further white. Extremely accurate guidance was given right downto touchdown and the bars also gave some bank reference. Visual range was considered to be at least two miles by dayand 8 to 10 miles by night. This system has been installed at Blackbushe, Aberdeen, Prestwick, Belfast and Liverpool. The limitations in accuracy in the present I.L.S. can be reducedconsiderably by redesigning the existing aerial systems in order to eliminate the distortions caused by reflection from terrain andbuildings. In the resulting trend to make the Iocalizer transmitter directional, the Pye I.L.S., which has been standardized for theR.A.F. and installed at many European airfields, can be considered the first stage. It uses a parabolic reflector which renders thebeam effectively uni-directional, but does not wholly concentrate transmitted energy in a narrow sector. Pye I.L.S. conforms toI.C.A.O. standards and installations are likely to be made at Prague, Moscow and Kiev. More recent developments both in Britain and America haveproduced localizer radiations limited to a 10 deg arc about the centreline. Parabolic reflectors, linear dipole arrays and slottedwaveguides have been used. Although the narrowing of the coverage is not necessarily desirable because the ability to makea curved instrument approach may be required in the future, it does avoid those reflections from ground features which causeirregularities. But the use of a narrow localizer might eliminate interference between neighbouring I.L.S. installations and makethe use of single universal I.L.S. frequency possible. Similar refinements can be applied to the glide-slope aerials and onedirectional installation in America consists of a pair of aerials mounted close to the ground alongside the runway. A remarkableimprovement has resulted in glide-path accuracy, even in areas where terrain conditions previously made provision of any glide-path at all virtually impossible. Localizer aerials which project some distance above groundcannot be mounted on the centreline close to the end of a runway and glide-path aerials similarly have to be positioned at least400ft to one side. The next stage in development has therefore been to mount the aerials flush with the ground, either buriedin the runway surface or laid out beside it. Such systems are effective, but would be very costly to install. Both localizer andglide-path radiation patterns would be confined to a given azimuth arc and they would not use the ground as part of the aerial system.An arrangement has been proposed by R.A.E. in which the glide- path aerial would consist of two flush, linear aerials buriedparallel to the runway, one of them 45ft long and the other 200ft. The localizer array, also flush, would be set midway along therunway, to cover both landing directions, in the form of three rows of nine strips, each 18ft long and spaced at 18ft intervalsdown the runway. Each row would cover 40ft of the width of the runway. With improved I.L.S. aerials of this type it can be foreseenthat the minimum height for accurate I.L.S. guidance might be reduced to as little as 50ft. Last month, Standard Telephones and Cables Ltd. issued apreliminary description of the STAN.7-8 I.L.S. system which is based on the mobile AN/MRN-7-8 produced by Federal Tele-phone and Radio Co. The British equipment is designed for permanent installation, but is functionally equivalent to MRN-7-8and complies with I.C.A.O. standard requirements. The localizer is denned by a linear dipole array 85ft wide and 12ft high, calledthe course aerial, which produces a directional localizer reference over an 18 deg arc. Behind it is another, smaller array called aclearance aerial which radiates a second localizer pattern over the whole 360 deg with a carrier frequency 9.5 kc/s different fromthat of the course array. The latter is unlikely to be disturbed by ground reflections and its secondary lobes will be masked by theradiation from the clearance aerial. The airborne receiver will easily receive both radiations, but will naturally suppress onewhere the course-aerial radiation predominates. The glide-path aerial consists of two wide-band dipoles mounted on a commonmast in conventional fashion. The whole equipment is self- monitoring and automatically changes over from main to standbytransmitters if distortion or fault occurs. Marker beacons are also provided. In bridging the gap between automatic approach and automatictouch-down a number of basic techniques have to be considered. Many of them have already been tried and sufficient landingsmade to allow a statistical assessment of landing probability. The first requirement is for an autopilot which can give the necessaryautomatic control. In case of autopilot runaway it is extremely unlikely that the pilot will be able to avert disaster by taking overmanually at any height below about 60ft. His intervention must therefore be excluded at this critical stage. With the single-channel autopilot now used at the R.A.E. Blind Landing Experi- mental Unit the probability of an incident during the landingmanoeuvre is about 4 in 100,000 landings. A built-in monitor
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