These notes
are reproduced from the 1st edition of DCA Publication
No. 44 Pilots' Notes on the Visual Approach Slope Indicator System ("T"-System),
October 1963.
Note that this is not the current issue of this publication.
GENERAL
In D.C.A. Publication No. 43, the "Red-White" Visual Approach Slope Indicator System and its use were described. This system, being an I.C.A.O. standard, is being installed at certain Australian international airports. However, another Visual Approach Slope Indicator System, known as the "T"—VASIS, has been developed in Australia and will also be installed at several airports. The specific type and location of VASIS will be published in A.I.P./RAC-2 landing charts.
The "T"—VASIS system provides approach slope guidance by symbolic means as distinct from colour differentiations, colour being used only to provide a conspicuous warning signal when an approach is made which grossly undershoots the correct approach slope. The system is of variable intensity and will be displayed for use by both day and night. It is designed for use over ranges of at least four miles.
The symbolic nature of the system is such that it may be likened to an I.L.S. glide path indicator—the cross bar representing "on slope", and deviations from "on slope" appearing as one, two or three lights above or below the cross bar. The sensitivity is approximately the same as when flying within the three "dot" up or down positions on an I.L.S. glide path.
It should be noted that, as colour discrimination is not an element of the basic system and as the lights are "variable white", the inevitable yellowing of the lights when the intensity is reduced to avoid glare does not in any way affect the accuracy of the information presented to the pilot. The red signal appears as a gross under shoot warning only.
DESCRIPTION OF "T" - VASIS
The ideal layout of the "T"—VASIS is shown in Figure 1.
The system consists basically of two components—
(a) A cross bar of unidirectional lights arranged symmetrically at each side of the runway, at right angles to the runway centre line and situated at a point intersecting the aiming point. The aiming point, normally located 900 feet from the runway threshold, is selected to position the aircraft at the optimum height at the threshold.
(b) A longitudinal line of lights, which provides the direction of the "T", situated at each side of the runway, parallel with the centre line and bisecting the cross bars. The light sources are enclosed in boxes fitted with blades adjusted to give a sharp cut-off at either the top or the bottom of the light beam or to provide a red signal according to the location of the lights in the system.
Click here to see a photo of a T-VASIS installation.
The blades in the light boxes are so positioned that when on the nominal approach slope, 2°52’, the pilot of an approaching aircraft will see only the white cross bar of the "T". (Note—The angle of 2°52’ is internationally recognised as the optimum approach slope angle. However, this angle may be raised slightly having regard to the approach gradient available). In practice, small deviations from the approach slope cause the lights immediately above and below the cross bar to appear momentarily. It should be noted that the centre light of each cross bar is omitted to give greater prominence to the adjacent leg lights. Refer to Figure 2 for the pilot's view.
If the aircraft diverges slightly (approximately 0°01’, but see below under "Method of Use’’) above the nominal approach slope, the first light above the cross bar will appear, so that an inverted "T", signifying "fly-down" will be seen. Further deviations above the nominal approach slope will cause the second and third lights in the leg of the inverted ‘T’’ to appear. The second and third lights appear successively in steps of 0°07’ in elevation as the aircraft diverges above the nominal approach slope, so that when the aircraft is approximately 0°11’ above the correct approach slope an inverted "T" with three lights in the leg will be seen.
When an aircraft diverges slightly (approximately 0°01’, but see below under "Method of Use") below the nominal approach slope, the first light below the cross bar will appear so that an upright "T", signifying "fly-up" will be seen. Further divergence below the nominal approach slope causes the second and third lights in the leg of the upright "T’’ to appear. The second and third lights appear successively in steps of 0°05’ in elevation as the aircraft deviates below the nominal approach slope, so that when the aircraft is approximately 0°11’ below the nominal approach slope, an upright "T’’ with three lights in the leg will be seen.
Should the aircraft deviate below the angle of elevation at which an upright ‘‘T’’ with three lights in the leg of the "T’’ is first seen, the signal will not change until the angle of elevation has been decreased by approximately a further 0°30’ at which point the colour of the upright "T" will change from white to red as a warning of gross undershoot.
METHOD OF USE
Refer to Figure 2 below.
When an approach is made on the correct approach slope, only the cross bar will normally be visible. However, as the distance from threshold is reduced (at approximately 2 n.m.) a glimmer of light may he seen from both the first "fly-up" and "fly-down’’ lights.
As the aircraft deviates from the correct approach slope, lights will appear in the leg of the ‘‘T’’ to form either an upright ‘‘T’’ or an inverted ‘‘T’’. The "T’’ so formed constitutes a command signal to fly to regain the correct approach slope, i.e., upright "T"—fly-up; inverted "T"—fly-down.
As the "fly-up" and "fly-down" indications are given in steps, increasing deviation from the nominal approach slope brings more lights (to a maximum of three) into view in the leg of the "T". In effect, the length of the leg of the "T" becomes a measure of the correction required.
Read about the history of T-VASIS
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