Advisory Circular (AC) No. 601-004

Subject: CL-810 Mid-point Lights, Azimuth Intensity Distribution

Issuing Office: Civil Aviation, Standards
Document No.: AC 601-004
Classification File No.: Z 5000-34
Issue No.: 01
RDIMS No.: 14250222-v7
Effective Date: 2020-02-15

Table of contents

List of figures

1.0 Introduction

  • (1) This Advisory Circular (AC) is provided for information and guidance purposes. It describes an example of an acceptable means, but not the only means, of demonstrating compliance with regulations and standards. This AC on its own does not change, create, amend or permit deviations from regulatory requirements, nor does it establish minimum standards.

1.1 Purpose

  • (1) The purpose of this Advisory Circular (AC) is to provide a specification for the azimuth [horizontal] intensity distribution of specialized obstacle lights used for mid-point lighting of windturbines. These lights are installed from the inside, through the wall of the mast, and would not have a 360 degree distribution. However, these lights can be considered as meeting CL-810 in that individually they render a portion of the required 360 distribution.

1.2 Applicability

  • (1) This AC is applicable to manufacturers of obstacle lights, lighting designers and owners of wind turbines of more than 150 m in height. This AC is also available for information purposes to the aviation industry.

1.3 Description of changes

  • (1) Not applicable.

2.0 References and requirements

2.1 Reference documents

  • (1) It is intended that the following reference materials be used in conjunction with this document:
    • (a) Standard 621 - Obstruction Marking and Lighting.

2.2 Cancelled documents

  • (1) Not applicable.

2.3 Definitions and abbreviations

  • (1) The following abbreviations are used in this document:
    • (a) cd: candela
    • (b) CL-810: low intensity red light fixture identified in Standard 621.

3.0 Background

  • (1) Standard 621 stipulates that, for windturbines of more than 150 m in height, at least three CL-810 light fixtures are installed around the mid-point level of the mast.
  • (2) The CL-810 requirement is for a 360 degree horizontal distribution. That is, omnidirectional output in azimuth. The output over this range, and 2.5 to 12.5 degrees vertical, is to be more than a minimum of 32 cd.
  • (3) As shown in Figure 1 below, these specialized lights are intended for installation from the inside, by means of a hole drilled in the wall of the mast. Obviously, it is impossible for such lights to have a 360 degree horizontal distribution.

    Figure 1 – Through-the-wall light

    source: Quantec Networks
    Figure 1 – Through-the-wall light - Text version

    Figure 1 shows a "through the wall" light unit. The light unit is cylindrically shapped so that it can be pushed through a hole drilled in the wall of the wind turbine mast. The left-hand portion is the LED source which will be exposed on the outside of the wind turbine mast. The mid-portion shows two rubber O-rings which serve as seals within the wall. The right-hand portion is the electrical box which is on the inside of the mast and to which supply wiring is connected.

  • (4) Standard 621 specifies characteristics of the light signal as seen by the pilot. In as much as there are 3 lights equally spaced around the mid-point of the mast, each produces a portion of the 360 degree signal to the pilot.

4.0 Discussion

  • (1) If the light fixtures, as shown in Figure 2, are co-located [with no separation], the output at the edges of ±60 degrees for adjacent lights, can be added to meet the minimum requirement. Thus the measured output at the defined edges [± 60 degrees] of the light beam can be a minimum of 16 cd, such that 16 cd + 16 cd = 32 cd.

    Figure 2 – Sum of emissions

    Figure 2 – Sum of emissions - Text version

    Figure 2 is a diagram showing how the light output from 3 lights, located coincident [as a group] and each orientated at 120 degrees around a centre point, can add together so as to produce the minimum intensity requirement of 32 candela over 360 degrees. For example, if at the 60 degree edges of two lights each produce 16 candela, these can be added to produce the minimum of 32 candela to the pilot.

  • (2) If the lights are not co-located, as illustrated in Figure 3, the distance "d" can be such that the lights are resolved [seen separately] by the pilot eye. If resolved, the outputs at the edges cannot be added and therefore at least the 32 cd minimum is required at the edges of the light distribution of each fixture.

    Figure 3 – Light separation

    Figure 3 – Light separation - Text version

    Figure 3 shows the same 3 lights of Figure 2, but instead of being coincident at a centre point, they are separated by the windturbine mast around which these lights are installed. The 60 degree edges of each light are now separated by a dimension "d" which will be dependent upon the diameter of the mast.

  • (3) Given an angle of resolution of 1.5 minutes of arc and an acquisition distance of 1900 m, as illustrated in Figure 4, the lights would be resolved when the distance of separation is greater than 0.83 m or a mast diameter of more than 0.96 m.

    Figure 4 – Minimum separation distance

    Figure 4 – Minimum separation distance - Text version

    Figure 4 is a plan view of a line drawing showing the projected width at 1900 meters of a viewing angle of 1.5 minutes for an installation of 3 lights which are spaced at 120 degrees around an hypothetical wind turbine mast. The projected width is calculated as 0.83 meters or diameter of 0.96 meters. The 0.96 meters is equal to 0.83 meters divided by the sine of 60 degrees. Since 1.5 minutes is the minimum angle of resolution for the pilot eye, Figure 4 shows that for diameters of mast in excess of 0.96 meters the lights would be resolved by the pilot eye and thus their edge intensities cannot be added. Therefore, the intensities at the 60 degree edges must then be at least the minimum of 32 candela.

  • (4) In as much a typical windturbine mast has a diameter in excess of 0.96 m, it can be anticipated that the mid-point lights would always be resolved.
  • (5) Because fixtures installed through-the-wall of the mast may not be orientated precisely or may move after installation, a tolerance of ±10 degrees is applied.

5.0 Summary / Conclusion

  • (1) Through-the-wall light fixtures installed around the mid-point of the windturbine mast would be acceptable as meeting the specified requirements of CL-810, if the output at ±70 degrees horizontal and +2.5 to +12.5 degrees vertical is more than the minimum of 32 cd.

6.0 Information management

  • (1) Not applicable.

7.0 Document history

  • (1) Not applicable.

8.0 Contact us

For more information, please contact: Flight Standards, AARTA


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Robert Sincennes
Director, Standards
Civil Aviation