The regime in which the daylight visual range exceeds Allard's RVR

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Tuesday, 31 January 2006
The regime in which the daylight visual range exceeds Allard's RVR
A301 (Georgia World Congress Center)
J. P. Pichamuthu, Sir M. Visvesvaraya Institute of Technology, Bangalore, Karnataka, India

In flying operations, two visual ranges are of importance. One is the MOR, or meteorological optical range (RM), the other is Allard's runway visual range, or RA [Middleton, 1952]. The values of RM and RA are respectively, the maximum distances at which non-luminous entities (e.g., runway markings), and luminous objects (e.g., runway edge lights) are barely identifiable. The MOR is operationally significant only under daylight conditions, i.e., when the atmospheric brightness B exceeds ~ 50 cd m-2.

Visual range is of central importance to flight operations, as the values of the RM and RA determine whether or not aircraft may land or take-off. Hence it is important to remove all ambiguities in determining and reporting its value. To avoid confusion between RM and RA, just one quantity viz., Runway Visual Range or "RVR" (R) is presently reported [Ref.2, World Meteorological Organization (WMO)]. As defined by the WMO, R takes on the value of the greater of RM and RA in daylight, and the value of RA at night. Thus only the single value R, as defined above, is reported to the pilot as the RVR, thereby eliminating ambiguity.

In daylight it thus become important to delineate the boundary between the regimes RM > RA and RM < RA to make the correct choice of RM or RA to represent RVR. Qualitatively, it is well known that RM > RA under conditions of high atmospheric or "background" brightness (B), and/or low runway edge light intensity (I). Under such conditions, runway markings can be perceived at greater distances than can runway lights. In this communication, we develop an explicit criterion to determine the boundary of the regime RM > RA so that the choice of RM or RA to be reported to the pilot can be made automatically by the instrument, without the introduction of human error.

As recently reported [Pichamuthu, 2005], the MOR varies in different directions according to the atmospheric brightness in the direction of view. The criterion for RM to be greater than RA is given in Eq. (1). The effects of directional variations of brightness on RM. are incorporated in the equation. The derivation of Eq. (1) will be presented in the paper. The visual range at the boundary of the regime RM > RA is represented by Req, and given by the criterion:

Req = {(ICâ"/Et)1/2 (1)

Req is the visual range at which Req = RM = RA and thus delineates the boundary separating the two regimes. The anisotropy of atmospheric brightness is represented by the factor â"â|ânân If, as in the classic Koschmieder theory, uniform atmospheric brightness is assumed, ânâ"ânâânâ░â|ânThe contrast between the background and the object being viewed is C, I is the runway edge or center line light intensity and Et the illuminance threshold of the eye. Equation (1) thus provides a quantitative basis for demarcating the regime RM > RA. It also provides a simple test of the software to verify that RM or RA is correctly selected as the RVR according to the prevailing conditions of background brightness and runway edge light intensity. Because â" > 1, the anisotropy increases Req, while it reduces RM. Thus the changeover from RA to RM as the RVR will occur at higher values of background brightness (which means higher values of Et) than if the atmosphere were to be uniform. REFERENCES: 1. Middleton, W.E.K. (1952), Vision through the atmosphere, University of Toronto Press, Canada 2. Secretariat of the World Meteorological Organization (1983), Guide to meteorological instruments and methods of observation, 5th Ed., WMO document 8( World Meteorological Organization, Geneva, Switzerland, 1983, Chaps 10 and 16, pp. 10.1 -10.15, 16.4- 16.7) 3. Pichamuthu, J. P. (2005), "Directional variation of visual range due to anisotropic atmospheric brightness ", Applied Optics (to be published), Vol. 44, No. 8