Pilots Notes on the Precision Visual Glidepath
||These notes are reproduced from DCA Publication No. 37 Pilots Notes on the Precision Visual Glidepath (P.V.G.).|
The arrangements of lights as seen by the pilot on final approach is shown in Figure 1. The resemblance to the Artificial Horizon is self evident, and the system is used in the same sense as the Artificial Horizon.
Precision Visual Glidepath was developed jointly by the Human Engineering Group of the Aeronautical Research Laboratories, Department of Supply, and the Aviation Medicine and Flying Operations Divisions of the Department of Civil Aviation of Australia, with the co-operation of the Royal Australian Air Force, the major Australian domestic and overseas airlines and the Australian Air Pilots' Association.
Technical enquiries or comment may be addressed to: The Director-General, Department of Civil Aviation of Australia.
ARRANGEMENT OF LIGHTS
Figure 2 shows the arrangement of lights recommended for runways 150-200 feet in width. The dimensions specified provide an approach angle of 1 : 20 (2 degrees 52 minutes), an obstruction clearance plane of 1 : 30 from the threshold, and assume an aircraft wheel to eye height of 20 feet. The approach angle and obstruction clearance plane, however, may be varied to meet local requirements.
The essential elements of the P.V.G. are two bars of fixed lights arranged symmetrically on each side of the runway and at right angles to the runway axis. The inner bars comprise five white lights of a compatible intensity with the runway lights and set at runway level 900 feet up the runway from the threshold. The outer bars comprise three amber lights of the same intensity mounted on frangible plywood poles 25 feet above runway level and 500 feet nearer to the threshold. Alignment of the two bars defines an inclined plane at the approach angle. A low intensity white light is set in the middle of the runway in line with the white bar in order to provide an aiming point.
As an additional safeguard the obstruction clearance plane of the approach is defined by two high intensity red flashing warning lights which are so positioned and screened that they are not visible to the pilot on a normal approach but are seen immediately his wheels infringe the predetermined obstruction clearance plane. The warning light units are 'fail-safe' in that they contain multiple light sources, and they are controlled by actuators which leave the lights 'on' in the event of actuator failure.
More detailed information on setting up the P.V.G. is given in A.R.L. Human Engineering Note 3: "Installation Notes on the Precision Glidepath" by R. W. Cumming and J. C. Lane.
METHOD OF USE
of the Display:
In addition to providing glidepath information the bars form a valuable horizontal reference, which is not always naturally available but which is essential to derive the roll guidance necessary for accurate tracking in azimuth.
The sensitivity of the system is the apparent relative vertical movement of the bars for a given departure of the aircraft from the glidepath; this depends upon the longitudinal displacement of the two bars. By arranging the lights to form lines which do not quite meet, advantage is taken of the remarkable property of the human eye known as 'vernier acuity' which allows the detection of extremely small misalignments of lines. The lateral spacing of the lights in the P.V.G. has been designed to make the best use of this property so that adequate sensitivity is achieved without requiring large relative movement of the bars (which in turn would necessitate very high poles for the amber lights). Thus for a good approach the pilot should appreciate and act upon small misalignments, attempting to keep the two bars exactly in alignment rather than waiting for appreciable misalignments to appear. He will then be using his eyes in their most sensitive range.
In both accuracy and sensitivity the P.V.G. resembles the Artificial Horizon - it responds instantaneously and exactly, but with small movement of the display. Although mounted on the ground rather than in the cockpit, P.V.G. is in fact a precision flight instrument with a 'red-line' limit provided by the red warning lights.
On straight-in approaches in clear conditions the aid should be usable from a range of about 8 miles to the threshold, with the sensitivity progressively increasing as the runway is approached. When making an approach from a normal circuit the position of the aircraft in relation to the defined glidepath can be detected by reference to the relative position of the bars whilst still on base leg. This enables initial adjustments to be made prior to lining up on final approach.
THE FINAL LANDING PHASE
Required landing distance is normally a factored increase over stopping distance based on crossing the threshold at a predetermined speed relationship to Vso with a wheel clearance of 50 feet.
The bars of the P.V.G. have been positioned to bring the wheels of the aircraft over the threshold at about 25 feet. Thus, provided normal approach speeds are maintained, adherence to the defined glidepath right down to the point of flare-out helps to make effective use of the runway length available, while at the same time ensuring that touchdown does not occur short of the runway.
The aiming point light in the middle of the runway is intended to afford an elevation and alignment reference in what is normally the 'black hole' phase just prior to touchdown. This light is spring mounted to depress flush with the runway so that it cannot cause wheel shimmy or tyre damage.
P.V.G. is designed for approaches in Visual Meteorological Conditions, which constitute the majority of approaches made. It is not intended as a substitute for high intensity centre-line and cross-bar approach lighting which is for alignment guidance in reduced visibility conditions, when lights on the aerodrome cannot be seen until late in the approach. However the usefulness of the aid is unaffected by haze, ground fog or rain, provided the lights themselves are still visible, since its operation depends only upon seeing the relative positions of light sources.
SUMMARY OF PILOT NOTES
1. This approach aid is a precision flight instrument: fly it as one.
2. For a perfect approach keep the white and amber bars accurately in alignment until you cross the threshold. Do not allow large misalignments to build up.
3. Liken the display to the Artificial Horizon: The runway and white bars for the miniature 'aircraft'. The amber (outer) lights form the reference bar. Fly to the amber bar.
4. If you ever see the red warning lights: FLY UP.
FLY TO THE AMBER BAR
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