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(b) Flashing Beacons. The number of beacon levels may be obtained from Appendix 2, Figs, 1-3.
1. Normally, structures not more than 450 feet (135m) in beacons. height do not need intermediate levels of flashing
2. If the structure exceeds 450 feet (135m) two or more flashing beacons should be installed on each level and installed in a manner to allow unobstructed viewing of at least one beacon.
h. Prominent Buildings and Similar Extensive Obstructions. If individual objects within a group of obstructions are approximately the same overall height above ground, or water if so situated, and are located not more than 150 feet (45m) apart, the group of obstructions may be considered an extensive obstruction. Prominent buildings and follows: similar extensive obstructions should display steady burning obstruc- tion lights in a manner to indicate the extent of the obstruction as
(1) Direction. Structures Not More Than 150 Feet (45m) in Either Horizontal If the structure/extensive obstruction is not more than 150 feet (45m) in either horizontal dimension, at least one steady burning obstruction light should be displayed on the highest point of each end of the major axis of the obstruction.
If this method of lighting is impracticable because of the shape of the obstruction, then two lights may be displayed in the center of the highest point.
(2) Structures More Than 150 Feet (45m) in at Least One Horizontal one steady burning obstruction light should be displayed for each 150 feet (45m) in either or both horizontal dimensions, at least Direction. If the structure/extensive obstruction is more than 150 feet (45m), or fraction thereof, of the overall length of the major axis of the obstruction. At least one of these lights should be displayed on the highest point of each end of the obstruction. Additional lights, as required, should be displayed at approximately equal intervals not to exceed 150 feet (45m) on the highest points or edge between the end lights. If there are two or more edges of the same height on an obstruction located lighted. near a landing area, the edge nearest the landing area should be
(3) lieu of steady burning obstruction lights in the event early or Page 18 Chap. 5 Par. 16
displayed on the highest points of an extensive obstruction at intervals not exceeding 3,000 feet (900m). At least three beacons should be displayed on one side of the extensive obstruction to indicate a line of lights.
(4) If individual objects within a group of obstructions are not the same height and are spaced more than 150 feet (45m) apart, the prominent objects within the group should be lighted in accordance with the specifications for individual obstructions of a corresponding height, In addition, at least one rotating beacon should be installed at the top of a prominent center obstruction or on a special tower located near the center of the group of obstructions.
(5) Obstructions More Than 150 Feet (45m) AGL. Steady burning obstruction lights should be installed at the top as specified in paragraphs (1) or (2) above. At selected intermediate levels, steady burning lights should be displayed for each 150 feet (45m), or fraction thereof. The position of these lights on the vertical plane should be equidistant between the top lights and the ground level as the shape and type of obstruction will permit. One such light should be displayed at each outside corner on each level with the remaining lights evenly spaced between the corner lights.
i. Alternate Method of Displaying Obstruction Lights. In lieu of installing lights on the obstruction, lights may be placed on poles of slightly greater height than the obstrućtion and installed on or adjacent to the structure.
Chap. 5 Par. 16 Page 19 (and 20)
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17. CHAPTER 6. HIGH INTENSITY OBSTRUCTION LIGHTING STANDARDS
STANDARDS FOR LIGHTING OBSTRUCTIONS TO AIR NAVIGATION WITH HIGH INTENSITY WHITE OBSTRUCTION LIGHTS. The general standards which follow apply to the obstruction lighting of structures with capacitor discharge obstruction lighting systems. The overall height of a structure including all appurtenances such as rods, antennas, obstruction lighting, etc., are used to determine whether the object is an obstruction to air navigation in accordance with FAR, Part 77, Objects Affecting the Navigable Airspace.
However, the overall height of the main structure excluding all appurtenances, is used to determine the number of light levels recommended on a structure. In addition, a white omnidirectional beacon should be displayed on the highest portion of any antenna or other appurtenance, exceeding 20 feet (6m) supported by the main structure. These standards do not require modification of the marking and lighting on existing structures; however, the FAA may recommend that marking and lighting displayed on an existing structure be modified if that structure is altered or replaced by a similar structure.
High Intensity Obstruction Light, The lighting systems referred to in the following standards should conform with the applicable provisions of the latest issuance of FAA Advisory Circular 150/5345-43, FAA/DOD Specification L-856, High Intensity Obstruction Lighting Systems.
Light Specifications. The following are the basic recommended standards for high intensity obstruction lighting systems applied to structures. However, the intensity, flash rate, sequence and placement of light units are unique in the case of supporting structures of overhead transmission lines. (See paragraph 17.j.)
Effective Intensity. Day Mode - no less than 200,000 candelas (100,000 candelas for transmission line supporting structures);
twilight mode - approximately 20,000 candelas; night mode - approximately 4,000 candelas.
Intensity Step Changing. The systems should automatically change intensity steps when the north sky illumination on a vertical surface is as follows:
* (a) Day to Twilight. This should not occur before the illumination drops to 60 footcandles (645.8 lux), but should occur before it drops below 30 footcandles (322.9 lux).
The illuminance sensing device should, if practical, face the north sky.
(b) 1wilight to Night. This should not occur before the illumination drops to five footcandles (53.8 lux), but should occur before it drops below two footcandles (21.5 lux). * Chap. 6 Par. 17 Page 21
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(c) Night to Day. The intensity changes listed in (a) and (b) above should be reversed in transitioning from the night to day mode.
(3) Flash Rate. All light units should flash simultaneously at 40 pulses per minute. (See paragraph 17.j. for flash rate and unique sequencing of catenary systems.)
Beam Spread. A relatively narrow vertical beam spread is specified to provide full light intensity at possible collision altitudes with the structure while persons on the ground or at altitudes above the structure will receive only stray light.
Antenna or Similar Appurtenance Light. The specifications for the high intensity lighting system provide for a small omnidirectional white light, similar in size to the 300 mm red flashing beacon, for installation on top of antennas or similar appurtenances. This light should operate 24 hours a day, flash in synchronism with the high intensity lighting system, be of approximately the same intensity as the twilight mode during daylight and reduce in intensity to the night mode simultaneously with the remainder of the system. This appurtenance light should only be used as an adjunct to the high intensity lighting system.
Flashtube Replacements. The flashtubes in a light unit should be replaced when the peak effective daytime intensity falls below 200,000 candelas (100,000 candelas for systens installed on the supporting structures of overhead transmission lines).
d. Installation Guidance. The manufacturing specifications provide for installation would be for all light units to be installed at zero degrees elevation. Where terrain, nearby residential areas or other situations dictate, it may be desirable to use louvers and/or elevate the light beam above the horizontal, The beam of the light at the lower level should not strike the ground closer than three statute miles (5km) from the structure to prevent irritation to nearby residents. Should elevating the light beam be necessary, it is recommended that the beam be elevated (in degrees above the horizontal) as shown below. If additional adjustments are necessary, each light may be adjusted upward, in one degree increments, starting at the bottom. The top light should normally remain at zero degrees to the horizontal.
Chap. 6 Page 22 Par. 17
Level Number of Levels of Lights Installed of Light 1 2 TOP 5 4 3 1 2 2 1 2 2 2 BOTTOM 2 2 3 3 Figures indicate degrees of elevation above the horizontal. * e • High Intensity Light Units. The high intensity capacitor discharge light units known to date which meet the FAA/DOD specification L-856 are not omnidirectional, except for the light used on antennas or similar appurtenances. See paragraph 17.b.(5). Therefore, more than one light unit is required in order to obtain 200,000 candelas for 360 degrees of coverage about a structure to ensure that the system is visible from aircraft at any normal angle of approach. A light unit may contain one or more flashtubes.
(1) Number of Levels of Lights. The number of levels of lights used is dependent upon the overall height of the structure above ground level. The lights should be installed at the levels indicated in Appendix 2, Figs. 4-6, for chimneys and similar solid structures. On structures having an antenna or similar appurtenance extending more than 20 feet (6m) above the main structure, such as a radio or television antenna, the lights should be installed at the levels indicated in Appendix 2, E188. 7-9. The light levels recommended for other structures,
i.e., cooling towers, supporting structures of catenaries and buildings, may be found under the appropriate subject paragraph.
(2) Number of Light Units Per Level. The number of light units recommended per level (except for the supporting structures of transmission lines and buildings) is dependent upon the average outside diameter of the specific structure:
(a) Structures 20 feet (6m) or less - three light units per level.
Chap. 6 Para. 17 Page 23
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(b) Structures exceeding 20 feet (6m) but not more than 100 feet (30m) - four light units per level.
(c) Structures exceeding 100 feet (30m) but not more than 200 feet (60m) - six light units per level.
(d) Structures exceeding 200 feet (60m) - eight light units per level.
Relocation or Omission of Light Units. Light units should not be installed in such a manner or elevation on a structure that the intended purpose would be derogated.
(1) Lowest Level, The lowest level of light units may be installed at a higher elevation than normal for the height of the structure if the lights, by installing them at the prescribed eleva- tion, would be lower than surrounding terrain, trees or adjacent building(s). See Appendix 1, Fig. 7. In certain instances, as lowest level may be permissible. determined by an FAA aeronautical study, the elimination of the Iwo Adjacent Structures. Where two structures are situated within 200 feet (60m) of each other and the light units will be installed at the sane levels, the sides of the structures facing each other need not be lighted; therefore, the inboard lights ment of the lights to either or both of the structure's inter- flash in snychronism. Minor adjustments in the vertical place- may be eliminated; however, all lights on both structures must mediate levels of lights may be made in order to place the lights on the same horizontal plane. Where one structure is higher than the other, complete level(s) of lights should be installed on that part of the higher structure which extends above the top of the lower structure. If the structures are of such heights that the levels of lights cannot be placed in identical horizontal planes, then the light units should be placed such that the center of the horizontal beam patterns do not face toward the adjacent structure. For example, structures situated north and south of each other should have the light units on both structures installed on a northwest/ Figs. 6, 7 and 8. southeast and northeast/southwest orientation. See Appendix 1, Three or More Adjacent Structures. The treatment of a cluster of structures as individaul structures or a complex will be spacing between structures. determined by the FAA as the regult of an aeronautical study, taking into consideration the structure's location, heights and Page 24 Chap. 6 Par. 17
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g. Chimneys, Flare Stacks and Similar Solid Structures. The number of levels of lights • commended depends on the height of the structure and may be obtained from Appendix 2, Figs. 4-6. The amount of light units recommended for each level is dependent on the outside diameter of the structure but should in no case be less than three.
The light units should be installed in a manner to ensure unobstructed viewing of the system from aircraft at any normal angle of approach.
Normally, the top level of light units are installed on the highest point of a structure. However, in the case of shimneys, the top level of lights may be installed from five (1.5m) to 10 feet (3m) below the top. This is to avoid the obscurant effects of the deposits generally emitted by this type structure.
h. Radio and Television Towers and Similar Skeletal Structures.
Mounting of Lights. The number of levels recommended depends on the height of the structure, excluding antennas and similar appurtenances, and may be obtained from Appendix 2, Figs. 7-9.
At least three lights should be installed on each level and mounted so that the effective. intensity over the full beam is not impaired by the structural members. They should be mounted in a manner to ensure unobstructed viewing from aircraft at any normal angle of approach.
(2) Ice Shields. Where climatic conditions dictate, metal grates or similar protective ice shields should be installed directly over each light unit to prevent falling ice accumulation from damaging the light units.
Top Level of Lights. One level of lights should be installed at the highest point of the structure. If the highest point is a rod or antenna incapable of supporting a lighting system then the top level of lights should be installed at the highest portion of the main skeletal structure. If the rod or antenna is more than 20 feet (6m) above the main structure, a white capacitor discharge omnidirectional light should be mounted on the highest point.
If the appurtenance (such as a whip antenna) is incapable of supporting the omnidirectional light, one or more beacons should be installed adjacent to the appurtenance and to a height within 20 feet (6m) of the tip of the appurtenance so as to permit unobscured viewing of at least one beacon.
(4) Intermediate Levels of Lights. The intermediate levels of lights should be installed at approximately the same heights shown in Appendix 2, Figs. 7-9.
i. Hyperbolic Cooling Towers.
Mounting of Obstruction Lights. The light units should be installed in a manner to ensure unobstructed visibility of at least two lights from aircraft at any normal angle of approach.
Chap. 6 Par. 17 Page 25
AC 70/7460-1D 4/18/75 More than one light unit should be visible to provide the required intensity about the structure and to indicate to pilots the magnitude of the structure.
(2) Top Level of Obstruction Lights. The number of light units recommended depends on the diameter of the structure at the top. See paragraph 17.e.(2).
(3) Structures Exceeding 600 feet (180m) AGL. Structures greater than 600 feet (180m) AGL should have one additional level of light units installed approximately mid-point on the structure and in line with the top level of lights.
j. Supporting Structures of Overhead Transmission (Catenary) Lines.
(1) Purpose. The purpose of marking the supporting structures of lighting system is to warn pilots of the presence of the support overhead transmission lines with a unique, sequentially flashing catenary line. structures themselves and to alert pilots that they support a
(2) Mounting of Obstruction Lights. A system of three levels of sequentially flashing light units should be installed on each supporting structure (or adjacent terrain) of an overhead transmission line. One level should be installed at the top of the structure; one at the height of the lowest point in the catenary; and one level at approximately midway between the of 50 feet (15m) from the other two levels. If the installation The middle level should normally be a minimum of the top level of lights presents a potential danger to maintenance personnel, or is necessary for lightning protection, such lights may be mounted from five (1.5m) to 10 feet (3n) The flash sequence should be middle level, top level and bottom level with all lights on the same level flashing simultaneously. The time delay between flashes of the levels is designed to present a unique system display similar to a flash of lightning.
(b) Flash Rate. Each series of flashes is repeated 60 times every minute.
(c) Horizontal Coverage. Two or more light units should be installed at each light level and directed on a horizontal plane such as to provide 180 degrees of coverage centered on the transmission line. Where a catenary crossing is situated near a bend in a river, canyon, etc., the direction Page 26 Chap. 6 Par. 17
of the horizontal beam pattern should be directed to provide the most effective light coverage to warn pilots approaching from either direction of the presence of the transmission lines.
Variation. The vertical and horizontal arrangements of the lights may be subject to the structural limits of the towers and/or adjacent terrain and a tolerance of 20 percent from uniform spacing of the bottom and middle light is allowed, If the base of the supporting structure(s) is higher than the lowest point in the catenary, such as a canyon crossing, one or more lights should be installed on the adjacent terrain at the level of the lowest point in Special Cases. Where lighting systems are installed on supporting structures located near vehicular bridges, waterways, ensure that the lights do not cause a hazard to motorists' In airport approach areas, etc., caution should be exercised to vessel operators or pilots on an approach to an airport.
these cases, shielding or an adjustment to the vertical or horizontal light direction may be necessary. This adjustment should not derogate the intended purpose of the lighting system.
Area Surrounding Support Structures. The area in the immediate vicinity of the base of the supporting structures should be cleared of all structures and objects of natural growth that could interfere with the line-of-sight between a pilot and the
(5) Omission of Standard Marking. When the high intensity obstruction lighting system is used to identify a catenary line crossing, the following marking and lighting may be omitted:
(a) Spherical markers on the lines.
(b) The marking of the supporting structures for the surface area having the same exposure covered by this lighting system.
(c) The installation of red obstruction lights on the supporting structures serving the same exposure covered by the high intensity obstruction lighting system.
Total Elimination of Marking and Red Obstruction Lights.
Transmission line supporting structures having a catenary extending in one direction only and obstruction lighted as out-.
lined under paragraph 17.j. (2) may omit the standard marking and red obstruction lighting provided:
Chap. 6 Par. 17 Page 27
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(a) At least three light units are installed on the top level and situated so as to provide 360 degrees coverage about the structure. Four lights should be used if any light is installed more than 20 feet (6m) from another light on the top level. In addition, four light units may be required to effectively cover the approach to the structure dependent upon the beam pattern of the light unit and possible directions of approach, In certain situations, it may be desirable to have three lights on the top level of one supporting structure and four on the companion structure to provide adequate obstruction lighting coverage.
Structures exceeding 500 feet (150m) above ground, or water if so situated, have at least three light units installed at the middle level to provide 360 degrees coverage.
(c) All lights at each level flash simultaneously.
Three or More Supporting Structures. Where a transmission line crossing requires three or more supporting structures, the inner structures should be equipped with three or four light units per level to provide 360 degrees coverage. The number will depend upon the bean pattern of the particular manufacturer's light and the area desired to be covered as determined by the possible directions of approach. Four lights should be used if any light is installed more than 20 feet (6m) from another light on any level. Structures equipped with a lighting system providing 360 degrees of coverage about the structure may delete the standard marking and red obstruction lighting system.
Prominent Buildings, Observation Towers and Similar Extensive Obstructions. Prominent obstructions should have lights installed at the highest portion or edge of the obstruction, on the same horizontal plane and provide 360 degrees of coverage about the obstruc- They should be installed in a manner to indicate the extent of the obstruction and to ensure unobstructed visibility of at least two light units from aircraft at any normal angle of approach.
(1) If the obstruction is not more than 200 feet (60m) in either horizontal dimension, there should be installed at the highest portion of the structure three or more light units in a manner to ensure that at least one light is visible from aircraft at any normal angle of approach. They may be mounted on a single pedestal at or near the center of the obstruction. If the light units are placed more than 20 feet (6m) from a central point, a minimum of four light units should be used.
Page 28 Par. 17 Chap. 6
4/18/75 AC 70/7460-1D * (2) If the obstruction is more than 200 feet (60m) in one horizontal dimension, but not more than 200 feet (60m) in the other, two light units should be placed on each of the shorter sides.
These light units may either be installed adjacent to each other at the mid-point of the edge of the obstruction or at or near each corner and directed horizontally such as to give 180 degrees of coverage to each end of the obstruction. One or more light units should be installed along the overall length of the major axis. These lights should be installed at approximately equal intervals not to exceed a distance of 100 feet (30m) from the corners or from each other.
(3) If the obstruction is more than 200 feet (60m) in both horizontal dimensions, there should be installed one light unit for each 100 feet (30m) or fraction thereof of the overall perimeter of the obstruction.
Chap. 6 Par. 17 Page 29 (and 30)
CHAPTER 7. DUAL LIGHTING SYSTEMS
18. DUAL LIGHTING SYSTEMS. Red (nighttime) and high intensity white (daytime) obstruction lighting systems may be utilized under certain conditions when it might not be feasible to operate the high intensity white lighting systems at night. The use of the high intensity white lights for daytime obstruction lighting precludes the painting recommendations.
The systems should be installed and operated as specified in Chapters 4, 5 and 6. The light units should be installed as though each system was the only system being used. However, the levels of lights recommended for the high intensity lighting systems may be adjusted, if necessary, to coincide with the nearest platform used for the red obstruction lights.
The lights should be installed as specified in Appendix 2, Figs. 10-12.
Chap. 7 Par. 18 Page 31 (and 32)