GB2127192A - Reflex visual approach slope indicator - Google Patents

Abstract

A sharp-transition visual approach slope indicator for mounting adjacent a runway to indicate to a pilot the correct angle of approach comprises a beam-forming lens (7), a red filter (11) in the focal plane of the lens and a lamp (1) and pair of reflectors (3,21) which illuminate the filter edge. The lens (7) appears red or white according to whether the angle of approach is too low or too high. The lamp reflector (3) has a central parabolic region and a peripheral region (29). These regions generate the central and peripheral parts of the light beam from the lens (7) and determine their relative intensities. Thus the reflex construction enables the polar distribution to be optimised. Furthermore the indicator is shortened and its mass reduced, thereby reducing the danger in the event of a collision. <IMAGE>

Description

SPECIFICATION Aircraft visual approach slope indicator This invention relates to aircraft visual approach slope indicagors for use on airfields.

Hitherto, such slope indicators of the sharp transition type, e.g. GEC unit types ZA737/2 and ZA757/2, employ two or three sub-units each comprising a focussing lamp/reflector unit, a filter at the focus and a beam forming lens which gives a beam width of about 1 5,.

This arrangement, while it works satisfactorily, has certain disadvantages. The unit must be rigid and since it is necessarily over a metre long and nearly a metre wide using the standard components the weight is very substantial. Such a mass represents a danger to an aircraft which may inadvertently run into it. It is cimportant that the whole unit be as frangible as is consistent with reasonsable durability and rigidity.

Unless a large reflector is used, conventional visual approach slope indicators can provide only a limited beam spread. Furthermore, conventional indicator designs can provide only a narrow choice of polar distributions.

An object of the present invention is to overcome or alleviate at least some of these disadvantages and to provide an economical and reliable visual approach slope indicator.

According to the present invention, an aircraft visual approach slope indicator comprises lamp means, beam forming means arranged to be illuminated by the lamp means by way of a focal plane of the beam forming means, back reflector means positioned in the light path between said lamp means and said beam forming means so that said light path is effectively folded, and colour filter means having a filter edge transition in said foacl plane, the arrangement being such that light from said lamp means is diffusely focussed through the region of said focal plane bordering said filter edge transition whereby the percieved colour of a beam emerging from the beam forming means undergoes a sharp transition as the angle of said approach passes through a particular angle.

The beam from the lamp means may be focussed directly onto said plane or it may be focussed by a separate reflector displaced from this plane. In the former case the colour filter means is preferably located on the reflector which serves to fold back the optical path and thereby enables the length of the indicator to be reduced.

Preferably the lamp means comprises a light source in combination with a focussing reflector. The lamp means may incorporate a further light source displaced from the focal point of the reflector but preferably approxi mately on its optical axis, thus producing a more strongly diverging beam from the beam focussing means whilst preserving the sharp colour transition. The further light source may be distributed light source.

The colour filter means may incorporate two or more parallel filter edge transitions separating filter means of different colours to give a graded indication of angle of approach to the aircraft.

An aircraft visual approach slope indicator in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic representation of a known indicator unit; Figures 2 and 3 are front and side elevations of this known unit; Figures 4 and 5 are diagrams of two embodiments in accordance with the invention; Figure 6 is a diagrammatic representation of a lamp arrangement for use in the embodiments of Figs. 4 and 5, and Figure 7 is a diagrammatic representation of another lamp arrangement for use in the embodiments of Figs. 4 and 5.

Corresponding reference numerals are used to indicate similar elements in the Figures.

Referring tothe known arrangement shown in Fig. 1, a light souce 1 is mounted in a reflector 3, light from the lamp being directed in a beam towards a red filter 11 and a beam forming lens 7. The filter has a horizontal edge intercepting the optical axis 1 3 in the focal plane 5 of the beam forming lens 7. The pilot, being very distant, receives a narrow parallel beam of light 9 which emanates from a focal point in the plane 5, the vertical position of this point depending inversely upon the pilot's position. When the pilot is on the optical axis the beam focus is on the filter edge transition. When the pilot is above the optical axis the beam focus is below the filter and the beam 9' is white. When the pilot is below the optical axis the beam focus is above the filter edge and the beam 9" is red.

Aircraft approach slope indicators have to be operable throughout a range of weather and temperature conditions. The lens 7 is therefore subject to condensation which does, of course, degrade the focussing ability. It is common practice therefore to incorporate a lens heater 1 5 to clear the condensation for rapid start-up from cold.

A further feature of the known indicator units is a heat sink 1 7 mounted against the lamp 1 to dissipate the substantial amount of heat generated in operation.

As shown in Fig. 2, there are commonly three complete lamp/filter/lens combinations in each unit to provide redundancy againt lamp failure. The lamp, filter and lens mount ings are accurately pre-aligned to provide re placeable, on site, by way of access hatches 19 indicated in Fig. 3.

The whole unit is mounted on three adjust able legs which are normally fixed in concrete.

In a precision approach path indicator (PAPI) layout a row of perhaps four such triple units are mounted on each side of the runway, the angles of the units (i.e., their optical axes) being incremented from each to the next by typically twenty minutes of arc so that the two adjacent the runway show red and the two outboard units show white when the aircraft is on the correct 3" (say) glide path.

The pilot will then perceive, on each side of the runway, a range of patterns from four red to four white according as he is low, slightly low, correct, slightly high or high.

Fig. 4 shows diagrammatically, one visual slope approach indicator in accordance with the invention. A lamp arrangement comprising a rellector 3 with a composite profile and a compact light source 1, mounted at the focal point of the reflector generates a beam 22 which has its main intensity concentrated in a parallel beam (indicated by dashed ray lines 24) but which also has components 26 and 27 which diverge onto a converging spherical or parabolic reflector 21. Reflector 21 focuses rays 24 onto the horizontal lower edge of a red filter 11 which is located at the focus of a plano-convex beam-forming lens 7.Since the central portion of lamp reflector 3 is essentially parabolic and the effective dimensions of light source 1 are typically only a few mm (i.e. the size of a lamp filament) the beam component 24' will be substantially parallel and lamp 1, 3 will appear very bright to a pilot approaching along the optical axis of lens 7. The outer region 29 of reflector 3 is strongly converging and shaped so as to direct rays 26 and 27 via reflector 21 onto the region 23 of the focal plane of beam-forming lens 7, which region is intersected by red filter 11. Light from lamp 1, 3 is thus diffusely focussed onto region 23. Beams 26' and 27' are formed from rays 26 and 27 respectively and are focussed at the edges of region 23.

Thus beam 26' (which is focussed within filter 11) is the lowest red beam and beam 27 is the lowest white beam. It will be appreciated that an infinite number of beams at intermediate angle will be generated at intermediate foci within region 23. It can be seen that the light emerging from lens 7 and viewed by the pilot of a distant approaching aircraft will appear red if viewed from below the optical axis of lens 7 and white if viewed from above this optical axis, since not only will the focus corresponding to the lowermost beam (area shown shaded) give a red beam, but all other foci in focal area 23 lying above the optical axis will as well. Although the red and white beams overlap spatially, the distributions of ray angles within them do not overlap, so that a distant pilot will see a sharp transition when his angle of view passes through zero with respect to the optical axis.It will be noted that the limits 26 and 27 of the beam are determined by the divergence of the diffuse beam 22 from lamp arrangement 1, 3. The intensity distribution of light from lens 7 is determined by the relative sizes of the central parabolic portion of reflector 3 and the converging outer portion 29 of reflector 3, and also the exact profile of portion 29. Portion 29 need not be clearly defined - for example reflector 3 may be elliptical in profile, the relative degree of deviation from parabolicity determining the intensity of the off-axis components of the light from lens 7.In practice the required profile for reflector 3 may be determined to a first approximation by drawing reflector 21, filter 11, light source 1 and lens 7, drawing parallel pairs of rays such as 24', 26' and 27' from lens 7, deriving the rays connecting reflector 21 and reflector 3 and constructing a profile for lens 3 which will reflect said derived rays onto source 1. A prototype for reflector 3 can then be constructed with suitable slots (to enable different parts of the reflector to be bent independently) and a prototype P.A.P.I. constructed. The required light distribution can then be obtained by bending the slotted portions of reflector 3.

The contribution of any particular reflector portion to the overall light distribution can be found by measuring the light distribution before and after blacking out that particular portion. In the arrangement shown the divergence of the beam is dependent on the profile of reflector 3, but in some situations a suitable diffuse beam 22 may be generated by a relatively large light light source (in comparison with the focal length) and a substantially spherical reflector 3, the light source being located at the focus of the reflector.

Being a reflex system the overall length of the unit is approximately half the length of the conventional Fig. 1 type and is consequently much lighter, cheaper and stiffer. Furthermore the heat from lamp 1 prevents condensation on the lens 7.

The transition sharpness is the same as in currently available with an arrangement such as in Fig. 1.

Since the back reflector 21 has no hole in the centre to accommodate the lamp a smaller lens 7 may be used (approximately half the diameter of thoe generally used) to obtain the same flashed area (candelas) as normal.

This small lens, in addition to being cheaper, has a far smaller mass (1 /6th to 1 /8th) for the same focal length and consequently heats up more quickly to disperse condensation which is a serious problem under certain climatic conditions - necessitating the use of special lens heaters such as that, 15, of Fig. 1. In Fig. 4a heat deflector 3" is shown which directs a stream of warm air over lens 7, thus rendering the use of heaters unnecessary, economising in power and simplifying circuitry.

The back reflector 21 may have a profile of comparable focal length to that of the lens 7.

Consequently the lamp position is not then critical as it is in conventional reflectors of short focal length.

The back reflector 21 being shallow, it may readily be pressed from sheet metal, thus effecting a significant cost saving over conventionally spun types. Throw-away maintenance of this component is now an economic proposition.

In another similar arrangement shown in Fig. 5, the filter 11 is coincident or integral with the back reflector 21, further simplifying the unit. Diffuse beam 22 is focussed directly onto the focal plane of lens 7. A colour filter 11 is mounted on a back reflector 21 in this plane. When used in this way the curvature of the reflector/filter combination may be utilised to compensate for the off-axis deterioration in transition normally experienced, caused by lens aberrations.

The curvature of the back reflector 21 is such as to ensure that trace rays from an observer off-axis up to the limit of the azimuth and vertical polar diagram are always directed on to the lamp reflector 3; thus the limits 26 and 27 of the beam from lens 7 are defined by the use of reflector 3 and the lens appears flashed over the whole polar range. This has not been possible hitherto.

By suitable choice of back reflector and lamp reflector profiles a wide range of polar light distribution to meet varying needs may be achieved.

Additional features of this optical arrangement are: (a) There is no exit pupil and therefore no danger to personnel from burning which normally requires the forward projecting hood shown in Figs. 2 and 3.

(b) Significant reductions in weight over existing types - very important to minimise damage to aircraft in the event of a collision.

(c) Reflectors and filters run cold - eliminating deterioration and cracking.

(d) Standard lamp holders can be used and the enclosure used as the lamp heat sink.

As the light source is effectively the size of the lamp reflector 3 almost any type of lamp arrangement may be used, e.g. battery operated automobile lamps, luminescent panels, discharge lamps, strobe lamps, domestic filament lamps or even bottled-gas lamps, without the necessity of altering the basic optics and still maintaining the basic transition performance.

Two or three light sources adjacently disposed may be employed to provide a further improvement in off-axis performance, and in addition, to offer standby facilities in the event of lamp failure thus enabling a reduction in the number of units currently necessary to provide this stand-by feature.

However a particularly advantageous lamp arrangements are shown in Figs. 6 and 7. The lamp arrangement shown may be substituted for the lamp arrangement 1, 3 in the visual approach slope indicator of Fig. 4. Referring to Fig. 6, the lamp arrangement shown comprises a light source 1 (preferably a filament lamp or other compact source) mounted at the focus of an accurately parabolic reflector 3. A further light source 28 (which may be somewhat larger than source 1) is located in the optical axis of reflector 3 in front of the focus and in use, the light sources produce a combined diffuse beam 22. The main intensity of the beam is concentrated in parallel rays 24 from compact light source 1.It can be seen from Fig. 4 that this prallel component of the beam will result in a parallel, high intensity beam 24' from lens 7 of the approach slope indicator with a correspondingly sharp redwhite transition. Source 28 produces a converging beam 26 which subsequently diverges from a real image 31, as indicated by the ray lines. It can be seen from Fig. 4 or Fig. 5 that these represent the outer limits 26', 27' of the beam from lens 7. The angle of divergence included by ray-lines 26', 27' (which is suitably approximately 24 ) can be easily controlled by varying the position of source 28. It will be appreciated that similar results can be achieved by locating source 28 within the focal length of reflector 3.Owing to the finite size of source 28, the parallel and diverging components of the beam blend and thereby ensure a smoothly varying intensity distribution of the beam. It can be seen from Fig. 4 that these represent the outer limits 26', 27' of the beam from lens 7. The angle of divergence (suitably approximately 24 ) can be easily controlled by varying the position of source 28. The diffuse nature of reflector 3 and finite sizes of light souces 1 and 28 cause the parallel and diverging beams to blend (so that an adequate beam will still be produced in the event of one lamp failing), and ensuring a smoothly varying intensity distribution in the beam from lens 7. However it will be appreciated that a distributed light source 28 and a compact source 1 may be used to give similar results.

Fig. 7 shows a composite reflector 3 consisting of two generally parabolic portions. A point light source 1 is mounted at the focus of each portion. The innner regions of reflector 3 are accurately parabolic and generate parallel beams 24. The outer regions are more strongly converging and generate rays 26, 27 which initially converge and then diverge from a diffuse focus (not shown). In use in the approach slope indicator of Fig. 4, the composite reflector 3 is positioned so as to direct divereging rays 26, 27 onto reflector 21, and these rays define the outer limits 26', 27' of the beam from lens 7. In use, the lamp arrangement of Fig. 7 is preferably disposed with the light sources 1 horizontally side by side. The solid rays illustrated in Fig. 7 will then define the azimuth angle of divergence of the beam from lens 7.

Although the arrangement of Fig. 6 gives a better overall light distribution, the arrangement of Fig. 7 gives a better light distribution than that of Fig. 6 in the even of failure of one of the light sources.

It will be appreciated that the arrangements of Figs. 6 and 7 may be modified for use in the approach slope indicator of Fig. 5 by adjusting the positions of light sources 1 and 28 to give an intense light beam focussed on the filter edge of filter 11 and a less concentrated beam falling on the immediately surrounding area.

Claims (18)

1. An aircraft visual approach slope indicator comprising lamp means, beam forming means arranged to be illuminated by the lamp means by way of a focal plane of the beam forming means, back reflector means positioned in the light path between said lamp means and said beam forming means so that said light path is effectively folded, and colour filter means having a filter edge transition in said focal plane, the arrangement being such that light from said lamp means is diffusely focussed through the region of said focal plane bordering said filter edge transition whereby the perceived colour of a beam emerging from the beam forming means undergoes a sharp transition as the angle of said approach passes through a particular angle.

2. An approach slope indicator according to Claim 1 wherein the lamp means is sufficiently close to the beam forming means to warm it in use and thus accelerate the evaporation of any condensation.

3. An approach slope indicator according to Claim 1 or Claim 2 wherein in use the light from the lamp means is focussed directly onto said plane and said optical path is folded at said plane.

4. An approach slope indicator according to Claim 3 in which said colour filter is mounted on the back reflector.

5. An approach slope indicator according to Claim 4 in which the back reflector is curved so as to compensate for aberrations in said beam forming means.

6. An approach slope indicator according to any preceding Claim in which the lamp means comprises a substantially point light source located approximately at a focus of a focussing reflector provided with a composite profile, the range of approach angles over which the indicator is visible being determined by the dimensions of said focussing reflector.

7. An approach slope indicator according to Claim 6 wherein the central portion of said focussing reflector is substantially parabolic and arranged to focus said light source onto said filter edge transition and the profile of the surrounding portion of said focussing reflector is such as to illuminate the region of said focal plane bordering said filter edge transition.

8. An approach slope indicator according to any of Claims 1 to 5 in which said lamp means incorporates first and second light sources displaced fro each other, said first light source being arranged to concentrate light on the area of said focal plane immediately surrounding said filter edge transition.

9. An approach slope indicator according to Claim 8 wherein said second light source is a distributed light source.

1 0. An approach slope indicator according to Claim 8 or Claim 9 wherein said lamp means comprises a reflector having an optical axis and said first and second light souces are located on said optical axis.

11. An approach slope indicator according to Claim 8 or Claim 9 wherein said first and second light sources are disposed side by side at the respective foci of a composite reflector having adjacent optical axes.

1 2. An approach slope indicator according to any of Claims 8 to 11 in which in normal operation the light from the first and second light sources overlaps sufficiently that, in the event of failure of either one of the light sources, a beam from the beam focussing means is visible over the normal range of approach angles.

1 3. An approach slope indicator according to any of Claims 1 to 5 wherein said lamp means comprises a distributed light source.

1 4. An approach slope indicator according to Claim 1 2 wherein said lamp means is a luminescent panel.

1 5. An approach slope indicator according to Claim 1 3 wherein said lamp means is a bottled-gas lamp.

16. An approach slope indicator substantially as described hereinabove with reference to Fig. 4 of the accompanying drawings.

1 7. An approach slope indicator substantially as described hereinabove with reference to Fig. 5 of the accompanying drawings.

18. An approach slope indicator as claimed in Claims 5 or 6 incorporating a dual source lamp arrangement substantially as described hereinabove with reference to Fig. 6 or Fig. 7 of the accompanying drawings.