CHAPTER 6. MEDIUM INTENSITY FLASHING WHITE OBSTRUCTION LIGHT SYSTEMS
60. PURPOSE Medium intensity flashing white (L-865) obstruction lights may provide conspicuity both day and night.
Recommendations on lighting structures can vary depending on terrain features, weather patterns, geographic location, and in the case of wind turbines, number of structures and overall layout of design.
61. STANDARDS The medium intensity flashing white light system is normally composed of flashing omnidirectional lights.
Medium intensity flashing white obstruction lights may be used during daytime and twilight with automatically selected reduced intensity for nighttime operation. When this system is used on structures 500 feet (153m) AGL or less in height, other methods of marking and lighting the structure may be omitted.
Aviation orange and white pa int is always required for daytime marking on structures exceeding 500 feet (153m) AGL. This system is not normally recommended on structures 200 feet (61m) AGL or less.
The use of a 24-hour medium intensity flashing white light system in urban/populated areas in not normally recommended due to their tendency to merge with background lighting in these areas at night. This makes it extremely difficult for some types of aviation operations, i.e., med-evac, and police helicopters to see these structures. The use of this type of system in urban and rural areas often results in complaints. In addition, this system is not recommended on structures within 3 nautical miles of an airport.
62. RADIO AND TELEVISION TOWERS AND
SIMILAR SKELETAL STRUCTURES
a. Mounting Lights . The number of levels recommended depends on the height of the structure, including antennas and similar appurtenances.
1. Top Levels . One or more lights should be installed at the highest point to provide 360-degree coverage ensuring an unobstructed view.
2. Appurtenances 40 feet (12m) or less . If a rod, antenna, or other appurtenance 40 feet (12m) or less in height is incapable of supporting the medium intensity flashing white light, then it may be placed at the base of the appurtenance. If the mounting location does not allow unobstructed viewing of the medium intensity flashing white light by a pilot, then additional lights should be added.
3. Appurtenances Exceeding 40 feet (12m ). If a rod, antenna, or other appurtenance exceeds 40 feet (12m) above the tip of the main structure, a medium intensity flashing white light should be placed within 40 feet (12m) from the top of the appurtenance. If the appurtenance (such as a whip antenna) is incapable of supporting the light, one or more lights should be mounted on a pole adjacent to the appurtenance.
Adjacent installations should not exceed the height of the appurtenance and be within 40 feet (12m) of the tip to allow the pilot an unobstructed view of at least one light.
b. Intermediate Levels. At intermediate levels, two beacons (L-865) should be mounted outside at diagonally or diametrica lly opposite positions of intermediate levels. The lo west light level should not be less than 200 feet (61m) AGL.
c. Lowest Levels . The lowest level of light units may be installed at a higher elevation than normal on a structure if the surrounding terrain, trees, or adjacent building(s) would obscure the lights. In certain instances, as determined by an FAA aeronautical study, the lowest level of lights may be eliminated.
d. Structures 500 Feet (153m) AGL or Less . When white lights are used during nighttime and twilight only, marking is required for daytime. When operated 24 hours a day, other methods of marking and lighting are not required.
e. Structures Exceeding 500 Feet (153m) AGL .
The lights should be used during nighttime and twilight and may be used 24 hours a day. Marking is always required for daytime.
f. Ice Shields. Where icing is likely to occur, metal grates or similar protective ice shields should be installed directly over each light unit to prevent falling ice or accumulations from damaging the light units.
The light should be mounted in a manner to ensure an unobstructed view of at least one light by a pilot approaching from any direction.
63. CONTROL DEVICE The light intensity is controlled by a device that changes the intensity when th e ambient light changes.
The system should automatically change intensity steps when the northern sky illumination in the Northern Hemisphere on a vertical surface is as follows:
a. Twilight-to-Night. This should not occur before the illumination drops below five foot-candles (53.8 Chap 6 17
2/1/07AC 70/7460-1K CHG 2 lux) but should occur before it drops below two footcandles (21.5 lux).
b. Night-to-Day. The intensity changes listed in subparagraph 63a above s hould be reversed when changing from the night to day mode.
64. CHIMNEYS, FLARE STACKS, AND SIMILAR
SOLID STRUCTURES
a. Number of Light Units . The number of units recommended depends on the diameter of the structure at the top. Normally, the top level is on the highest point of a structure. However, the top level of chimney lights may be installed as low as 20 feet (6m) below the top to minimize deposit build-up due to emissions. The number of lights recommended are the minimum. When the structure diameter is:
1. 20 Feet (6m) or Less. Three light units per level.
2. Exceeding 20 Feet (6m) But Not More Than 100 Feet (31m). Four light units per level.
3. Exceeding 100 Feet (31m) But Not More Than 200 Feet (61m). Six light units per level.
4. Exceeding 200 Feet (61m). Eight light units per level.
65. GROUP OF OBSTRUCTIONS When individual objects within a group of obstructions are not the same height and are spaced a maximum of 150 feet (46m) apart, the prominent objects within the group should be lighted in accordance with the standards for individual obstructions of a corresponding height. If the outer structure is shorter than the prominent, the outer structure should be lighted in accordance with the standards for individual obstructions of a corresponding height. Light units should be placed to ensure that the light is visible to a pilot approaching from any direction. In addition, at least one medium intensity flashing white light should be installed at the top of a prominent center obstruction or on a special tower locate d near the center of the group.
66. SPECIAL CASES Where lighting systems are installed on structures located near highways, waterways, airport approach areas, etc., caution should be exercised to ensure that the lights do not distract or otherwise cause a hazard to motorists, vessel operators, or pilots on an approach to an airport. In these cases, shielding may be necessary.
This shielding should not derogate the intended purpose of the lighting system.
67. PROMINENT BUILDINGS AND SIMILAR
EXTENSIVE OBSTRUCTIONS When objects within a group of obstructions are approximately the same overall height above the surface and are located a maximum of 150 feet (46m) apart, the group of obstructions may be considered an extensive obstruction. Install light units on the same horizontal plane at the highest portion or edge of prominent obstructions. Light units should be placed to ensure that the light is visible to a pilot approaching from any direction. Lights should be displayed to indicate the extent of the obstruction as follows:
a. Structures 150 Feet (46m) or Less in Any Horizontal Direction. If the structure/extensive obstruction is 150 feet (46m) or less horizontally, at least one light should be displayed on the highest point at each end of the major axis of the obstruction. If this is impractical because of the overall shape, display a double obstruction light in the center of the highest point.
b. Structures Exceeding 150 Feet (46m) in at Least One Horizontal Direction . If the structure/extensive obstruction exceeds 150 feet (46m) horizontally, display at least one light for each 150 feet (46m) or fraction thereof, of the overall length of the major axis.
At least one of these lights should be displayed on the highest point at each end of the obstruction.
Additional lights should be displayed at approximately equal intervals not to exceed 150 feet (46m) on the highest points along the edge between the end lights.
If an obstruction is located near a landing area and two or more edges are the same height, the edge nearest the landing area should be lighted.
c. Structures Exceeding 150 Feet (46m) AGL .
Lights should be installed on the highest point at each end. At intermediate levels, lights should be displayed for each 150 feet (46m), or fraction thereof. The vertical position of these lights 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.
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CHAPTER 7. HIGH INTENSITY FLASHING WHITE OBSTRUCTION LIGHT SYSTEMS
70. PURPOSE Lighting with high intensity (L-856) flashing white obstruction lights provides the highest degree of conspicuity both day and night. Recommendations on lighting structures can vary depending on terrain features, weather patterns, geographic location, and in the case of wind turbines, number of structutes and overall layout of design.
71. STANDARDS Use high intensity flashing white obstruction lights during daytime with automatically selected reduced intensities for twilight and nighttime operations.
When high intensity white lights are operated 24 hours a day, other methods of marking and lighting may be omitted. This system should not be recommended on structures 500 feet (153m) AGL or less unless an FAA aeronautical study shows otherwise.
72. CONTROL DEVICE Light intensity is controlled by a device that changes the intensity when the ambient light changes. The use of a 24-hour high intensity flashing white light system in urban/populated areas is not normally recommended due to their tendency to merge with background lighting in these areas at night. This makes it extremely difficult for some types of aviation operations, i.e., med-evac, and police helicopters to see these structures. The use of this type of system in urban and rural areas often results in complaints.
The system should automatically change intensity steps when the northern sky illumination in the Northern Hemisphere on a vertical surface is as follows:
a. Day-to-Twilight. This should not occur before the illumination drops to 60 foot-candles (645.8 lux), but should occur before it drops below 35 foot-candles
(376.7 lux). The illuminance-sensing device should, if practical, face the northern sky in the Northern Hemisphere.
b. Twilight-to-Night. This should not occur before the illumination drops below five foot-candles (53.8 lux), but should occur before it drops below two footcandles (21.5 lux).
c. Night-to-Day . The intensity changes listed in subparagraph 72 a and b a bove should be reversed when changing from the night to day mode.
73. UNITS PER LEVEL One or more light units is needed to obtain the desired horizontal coverage. Th e number of light units recommended per level (except for the supporting structures of catenary wires and buildings) depends upon the average outside diameter of the specific structure, and the horizontal beam width of the light fixture. The light units should be installed in a manner to ensure an unobstructed view of the system by a pilot approaching from any direction. The number of lights recommended are the minimum. When the structure diameter is:
a. 20 Feet (6m) or Less. Three light units per level.
b. Exceeding 20 Feet (6m) But Not More Than 100 Feet (31m). Four light units per level.
c. Exceeding 100 Feet (31m). Six light units per level.
74. INSTALLATION GUIDANCE Manufacturing specifications provide for the effective peak intensity of the light beam to be adjustable from zero to 8 degrees above the horizon. Normal installation should place the top light at zero degrees to the horizontal and all other light units installed in accordance with Table 2:
Light Unit Elevation Above the Horizontal Height of Light Unit Above Terrain Degrees of Elevation Above the Horizontal Exceeding 500 feet AGL 0 401 feet to 500 feet AGL 1 301 feet to 400 feet AGL 2 300 feet AGL or less 3 TBL 2
a. Vertical Aiming . Where terrain, nearby residential areas, or other situations dictate, the light beam may be further elevated above the horizontal.
The main beam of light at the lowest level should not strike the ground closer than 3 statute miles (5km) from the structure. If additional adjustments are necessary, the lights may be individually adjusted upward, in 1-degree increments, starting at the bottom.
Excessive elevation may reduce its conspicuity by raising the beam above a collision course flight path.
b. Special Cases. Where lighting systems are installed on structures located near highways, waterways, airport approach areas, etc., caution should be exercised to ensure that the lights do not distract or otherwise cause a hazard to motorists, vessel operators, or pilots on an approach to an airport. In these cases, Chap 7 19
AC 70/7460-1K CHG 2 2/1/07 shielding or an adjustment to the vertical or horizontal light aiming may be necessary. This adjustment should not derogate the intended purpose of the lighting system. Such adjustments may require review action as described in Chapter 1, paragraph 5.
c. Relocation or Omission of Light Units . Light units should not be installed in such a manner that the light pattern/output is disrupted by the structure.
1. Lowest Level . The lowest level of light units may be installed at a higher elevation than normal on a structure if the surrounding terrain, trees, or adjacent building(s) would obscure the lights. In certain instances, as determined by an FAA aeronautical study, the lowest level of lights may be eliminated.
2. Two Adjacent Structures . Where two structures are situated within 500 feet (153m) of each other and the light units are installed at the same levels, the sides of the structures facing each other need not be lighted. However, all lights on both structures must flash simultaneously, except for adjacent catenary support structures. Adjust vertical placement of the lights to either or both structures’ intermediate levels 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 that 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 pattern s do not face toward the adjacent structure. For example, structures situated north and south of each ot her should have the light units on both structures installed on a northwest/southeast and northeast/southwest orientation.
3. Three or More Adjacent Structures . T h e treatment of a cluster of structures as an individual or a complex of structures will be determined by the FAA as the result of an aeronauti cal study, taking into consideration the location, heights, and spacing with other structures.
75. ANTENNA OR SIMILAR APPURTENANCE LIGHT When a structure lighted by a high intensity flashing light system is topped with an antenna or similar appurtenance exceeding 40 feet (12m) in height, a medium intensity flashing white light (L-865) should be placed within 40 feet (12m) from the tip of the
appurtenance. This light should operate 24 hours a day and flash simultaneously with the rest of the lighting system.
76. CHIMNEYS, FLARE STACKS, AND SIMILAR
SOLID STRUCTURES The number of light levels depends on the height of the structure excluding appurtenances. Three or more lights should be installed on each level in such a manner to ensure an unobstructed view by the pilot.
Normally, the top level is on the highest point of a structure. However, the top level of chimney lights may be installed as low as 20 feet (6m) below the top to minimize deposit build-up due to emissions.
77. RADIO AND TELEVISION TOWERS AND
SIMILAR SKELETAL STRUCTURES
a. Mounting Lights . The number of levels recommended depends on the height of the structure, including antennas and similar appurtenances. At least three lights should be installed on each level and mounted to ensure that the effective intensity of the full horizontal beam coverage is not impaired by the structural members.
b. Top Level. 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. When guy wires come together at the top, it may be necessary to install this level of lights as low as 10 feet (3m) below the top. If the rod or antenna exceeds 40 feet (12m) above the main structure, a medium intensity flashing white light (L-865) should be mounted on the highest point. If the appurtenance (such as a whip antenna) is incapable of supporting a medium intensity light, one or more lights should be installed on a pole adjacent to the appurtenance.
Adjacent installation should not exceed the height of the appurtenance and be within 40 feet (12m) of the top to allow an unobstructed view of at least one light.
c. Ice Shields. Where icing is likely to occur, metal grates or similar protective ice shields should be installed directly over each light unit to prevent falling ice or accumulations from damaging the light units.
78. HYPERBOLIC COOLING TOWERS Light units should be installed in a manner to ensure an unobstructed view of at least two lights by a pilot approaching from any direction.
a. Number of Light Units . The number of units recommended depends on the diameter of the structure Chap 7 20
at the top. The number of lights recommended in the following table are the minimum. When the structure diameter is:
1. 20 Feet (6m) or Less. Three light units per level.
2. Exceeding 20 Feet (6m) But Not More Than 100 Feet (31m). Four light units per level.
3. Exceeding 100 Feet (31m) But Not More Than 200 Feet (61m). Six light units per level.
4. Exceeding 200 Feet (61m). Eight light units per level.
b. Structures Exceeding 600 Feet (183m) AGL .
Structures exceeding 600 feet (183m) AGL should have a second level of light units installed approximately at the midpoint of the structure and in a vertical line with the top level of lights.
79. PROMINENT BUILDINGS AND SIMILAR
EXTENSIVE OBSTRUCTIONS When objects within a group of obstructions are approximately the same overall height above the surface and are located not more than 150 feet (46m) apart, the group of obstructions may be considered an extensive obstruction. Install light units on the same horizontal plane at the highest portion or edge of prominent obstructions. Light units should be placed to ensure that the light is visible to a pilot approaching from any direction. These lights may require shielding, such as louvers, to ensure minimum adverse impact on local communities. Extreme caution in the use of high intensity flashing white lights should be exercised.
a. If the Obstruction is 200 feet (61m) or Less in Either Horizontal Dimension, install three or more light units at the highest portion of the structure in a manner to ensure that at least one light is visible to a pilot approaching from any direction. Units may be mounted on a single pedestal at or near the center of the obstruction. If light units are placed more than 10 feet (3m) from the center point of the structure, use a minimum of four units.
b. If the Obstruction Exceeds 200 Feet (61m) in One Horizontal Dimension , but is 200 feet (61m) or less in the other, two ligh t units should be placed on each of the shorter sides. These light units may either be installed adjacent to each other at the midpoint of the edge of the obstruction or at (near) each corner with the light unit aimed to provide 180 degrees of coverage at each edge. 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 (31m) from the corners or from each other.
c. If the Obstruction Exceeds 200 Feet (61m) in Both Horizontal Dimensions , light units should be equally spaced along the overall perimeter of the obstruction at intervals of 100 feet (31m) or fraction thereof.
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CHAPTER 8. DUAL LIGHTING WITH RED/MEDIUM INTENSITY FLASHING WHITE SYSTEMS
80. PURPOSE This dual lighting system includes red lights (L-864) for nighttime and medium intensity flashing white lights (L-865) for daytime and twilight use. This lighting system may be used in lieu of operating a medium intensity flashing white lighting system at night. There may be some populated areas where the use of medium intensity at night may cause significant environmental concerns. The use of the dual lighting system should reduce/mitigate those concerns.
Recommendations on lighting structures can vary depending on terrain features, weather patterns, geographic location, and in the case of wind turbines, number of structutes and overall layout of design.
81. INSTALLATION The light units should be installed as specified in the appropriate portions of Chap ters 4, 5, and 6. The number of light levels needed may be obtained from Appendix 1.
82. OPERATION Lighting systems should be operated as specified in Chapter 3. Both systems should not be operated at the same time; however, there should be no more than a 2- second delay when changing from one system to the other. Outage of one of two lamps in the uppermost red beacon (L-864 incandescent unit) or outage of any uppermost red light shall cause the white obstruction light system to operate in its specified ”night” step intensity.
83. CONTROL DEVICE The light system is controlled by a device that changes the system when the ambient light changes. The system should automatically change steps when the northern sky illumination in the Northern Hemisphere on a vertical surface is as follows:
a. Twilight-to-Night. This should not occur before the illumination drops below 5 foot-candles (53.8 lux) but should occur before it drops below 2 foot-candles
(21.5 lux).
b. Night-to-Day. The intensity changes listed in subparagraph 83 a above should be reversed when changing from the night to day mode.
84. ANTENNA OR SIMILAR APPURTENANCE LIGHT When a structure utilizing this dual lighting system is topped with an antenna or similar appurtenance exceeding 40 feet (12m) in height, a medium intensity flashing white (L-865) and a red flashing beacon (L-
864) should be placed within 40 feet (12m) from the tip of the appurtenance. The white light should operate during daytime and twilight and the red light during nighttime. These lights should flash simultaneously with the rest of the lighting system.
85. OMISSION OF MARKING When medium intensity white lights are operated on structures 500 feet (153m) AGL or less during daytime and twilight, other methods of marking may be omitted.
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CHAPTER 9. DUAL LIGHTING WITH RED/HIGH INTENSITY FLASHING WHITE SYSTEMS
90. PURPOSE This dual lighting system includes red lights (L-864) for nighttime and high intensity flashing white lights (L-856) for daytime and twilight use. This lighting system may be used in lieu of operating a flashing white lighting system at night. There may be some populated areas where the use of high intensity lights at night may cause significant environmental concerns and complaints. The use of the dual lighting system should reduce/mitigate those concerns.
Recommendations on lighting structures can vary depending on terrain features, weather patterns, geographic location, and in the case of wind turbines, number of structutes and overall layout of design.
91. INSTALLATION The light units should be installed as specified in the appropriate portions of Chap ters 4, 5, and 7. The number of light levels needed may be obtained from Appendix 1.
92. OPERATION Lighting systems should be operated as specified in Chapters 4, 5, and 7. Both systems should not be operated at the same time; however, there should be no more than a 2-second delay when changing from one system to the other. Outage of one of two lamps in the uppermost red beacon (L-864 incandescent unit) or outage of any uppermost red light shall cause the white obstruction light system to operate in its specified “night” step intensity.
93. CONTROL DEVICE The light intensity is controlled by a device that changes the intensity when the ambient light changes.
The system should automatically change intensity steps when the northern sky illumination in the Northern Hemisphere on a vertical surface is as follows:
a. Day-to-Twilight. This should not occur before the illumination drops to 60 foot-candles (645.8 lux) but should occur before it drops below 35 foot-candles
(376.7 lux). The illuminance-sensing device should, if practical, face the northern sky in the Northern Hemisphere.
b. Twilight-to-Night. This should not occur before the illumination drops below 5 foot-candles (53.8 lux) but should occur before it drops below 2 foot-candles
(21.5 lux).
c. Night-to-Day. The intensity changes listed in subparagraph 93 a and b a bove should be reversed when changing from the night to day mode.
94. ANTENNA OR SIMILAR APPURTENANCE LIGHT When a structure utilizing this dual lighting system is topped with an antenna or similar appurtenance exceeding 40 feet (12m) in height, a medium intensity flashing white light (L-865) and a red flashing beacon (L-864) should be placed within 40 feet (12m) from the tip of the appurtenance. The white light should operate during daytime and twilight and the red light during nighttime.
95. OMISSION OF MARKING When high intensity white lights are operated during daytime and twilight, other methods of marking may be omitted.
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Chap 9 26
CHAPTER 10. MARKING AND LIGHTING OF CATENARY AND CATENARY SUPPORT STRUCTURES
100. PURPOSE This chapter provides guidelines for marking and lighting catenary and catenary support structures. The recommended marking and lighting of these structures is intended to provide day and night conspicuity and to assist pilots in identifying and avoiding catenary wires and associated support structures.
101. CATENARY MARKING STANDARDS Lighted markers are available for increased night conspicuity of high-voltage (69KV or greater) transmission line catenary wires. These markers should be used on transmission line catenary wires near airports, heliports, across rivers, canyons, lakes, etc. The lighted markers should be manufacturer certified as recognizable from a minimum distance of 4,000 feet (1219m) under nighttime conditions, minimum VFR conditions or having a minimum intensity of at least 32.5 candela. The lighting unit should emit a steady burning red light. They should be used on the highest energized line. If the lighted markers are installed on a line other than the highest catenary, then markers specified in paragraph 34 should be used in addition to the lighted markers. (The maximum distance between the line energizing the lighted markers and the highest catenary above the lighted marker should be no more than 20 feet (6m).)
Markers should be distinctively shaped, i.e., spherical, cylindrical, so they are not mistaken for items that are used to convey other information. They should be visible in all directions from which aircraft are likely to approach. The area in the immediate vicinity of the supporting structure’s base should be clear of all items and/or objects of natural growth that could interfere with the line-of-sight between a pilot and the structure’s lights. Where a catenary wire crossing requires three or more supporting structures, the inner structures should be equipped with enough light units per level to provide a full coverage.
a. Size and Color. The diameter of the markers used on extensive catenary wires across canyons, lakes, rivers, etc., should be not less than 36 inches (91cm).
Smaller 20-inch (51cm) markers are permitted on less extensive power lines or on power lines below 50 feet (15m) above the ground and within 1,500 feet (458m) of an airport runway end. Each marker should be a solid color such as aviation orange, white, or yellow.
b. Installation.
1. Spacing. Lighted markers should be spaced equally along the wire at intervals of approximately 200 feet (61m) or a fraction thereof. Intervals between markers should be less in critical areas near runway ends, i.e., 30 to 50 feet (10m to 15m). If the markers are installed on a line other than the highest catenary, then markers specified in paragraph 34 should be used in addition to the lighted markers. The maximum distance between the line energizing the lighted markers and the highest catenary above the markers can be no more than 20 feet (6m). The lighted markers may be installed alternately along each wire if the distance between adjacent markers meets the spacing standard. This method allows the weight and wind loading factors to be distributed.
2. Pattern. An alternating color scheme provides the most conspicuity against all backgrounds. Mark overhead wires by alternating solid colored markers of aviation orange, white, and yellow. Normally, an orange marker is placed at each end of a line and the spacing is adjusted (not to exceed 200 feet (61m)) to accommodate the rest of the markers. When less than four markers are used, they should all be aviation orange.
102. CATENARY LIGHTING STANDARDS When using medium intensity flashing white (L-866), high intensity flashing white (L-857), dual medium intensity (L-866/L-885) or dual high intensity (L- 857/885) lighting systems, operated 24 hours a day, other marking of the support structure is not necessary.
a. Levels. A system of three levels of sequentially flashing light units should be installed on each supporting structure or adjacent terrain. Install one level at the top of the structure, one at the height of the lowest point in the catenary and one level approximately midway between the other two light levels. The middle level should normally be at least 50 feet (15m) from the other two levels. The middle light unit may be deleted when the distance between the top and the bottom light levels is less than 100 feet (30m).
1. Top Levels . One or more lights should be installed at the top of the structure to provide 360- degree coverage ensuring an unobstructed view. If the installation presents a potential danger to maintenance personnel, or when necessary for lightning protection, the top level of lights may be mounted as low as 20 feet (6m) below the highest point of the structure.
2. Horizontal Coverage . The light units at the middle level and bottom level should be installed so as to provide a minimum of 180-degree coverage centered perpendicular to the flyway. Where a catenary crossing is situated near a bend in a river, canyon, etc., or is not perpendicular to the flyway, the Chap 10 27
2/1/07AC 70/7460-1K CHG 2 horizontal beam should be directed to provide the most effective light coverage to warn pilots approaching from either direction of the catenary wires.
3. Variation. The vertical and horizontal arrangements of the lights may be subject to the structural limits of the towers and/or adjacent terrain.
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 the span. These lights should be installed on the structure or terrain at the height of the lowest point in the catenary.
b. Flash Sequence . The flash sequence should be middle, top, and bottom with all lights on the same level flashing simultaneously. The time delay between flashes of levels is designed to present a unique system display. The time delay between the start of each level of flash duration is outlined in FAA AC 150/5345-43, Specification for Obstruction Lighting Equipment.
c. Synchronization. Although desirable, the corresponding light levels on associated supporting towers of a catenary crossing need not flash simultaneously.
d. Structures 500 feet (153m) AGL or Less. When medium intensity white lights (L-866) are operated 24 hours a day, or when a dual red/medium intensity system (L-866 daytime & twilight/L-885 nighttime) is used, marking can be omitted. When using a medium intensity while light (L-866) or a flashing red light (L-
885) during twilight or nighttime only, painting should be used for daytime marking.
e. Structures Exceeding 500 Feet (153m) AGL.
When high intensity white lights (L-857) are operated 24 hours a day, or when a dual red/high intensity system (L-857 daytime and twilight/L-885 nighttime) is used, marking can be omitted. This system should not be recommended on structures 500 feet (153m) or less unless an FAA aeronautical study shows otherwise. When a flashing red obstruction light (L-
885), a medium intensity (L-866) flashing white lighting system or a high intensity white lighting system (L-857) is used for nighttime and twilight only, painting should be used for daytime marking.
103. CONTROL DEVICE The light intensity is controlled by a device (photocell) that changes the intensity when the ambient light changes. The lighting system should automatically change intensity steps when the northern sky illumination in the Northern Hemisphere on a vertical surface is as follows:
a. Day-to-Twilight (L-857 System). This should not occur before the illumination drops to 60 foot-candles
(645.8 lux), but should occur before it drops below 35 foot-candles (376.7 lux). The illuminant-sensing device should, if practical, face the northern sky in the Northern Hemisphere.
b. Twilight-to-Night (L-857 System). This should not occur before the illumination drops below 5 footcandles (53.8 lux), but should occur before it drops below 2 foot-candles (21.5 lux).
c. Night-to-Day. The intensity changes listed in subparagraph 103 a. and b. above should be reversed when changing from the night to day mode.
d. Day-to-Night (L-866 or L-885/L-866). This should not occur before the illumination drops below 5 foot-candles (563.8 lux) but should occur before it drops below 2 foot-candles (21.5 lux).
e. Night-to-Day. The intensity changes listed in subparagraph d. above s hould be reversed when changing from the night to day mode.
f. Red Obstruction (L-885) . The red lights should not turn on until the illumination drops below 60 footcandles (645.8 lux) but should occur before reaching a level of 35 foot-candles (367.7 lux). Lights should not turn off before the illuminance rises above 35 footcandles (367.7 lux), but should occur before reaching 60 foot-candles (645.8 lux).
104. AREA SURROUNDING CATENARY SUPPORT
STRUCTURES The area in the immediate vicinity of the supporting structure’s base should be clear of all items and/or objects of natural growth that could interfere with the line-of-sight between a pilot and the structure’s lights.
105. THREE OR MORE CATENARY SUPPORT
STRUCTURES Where a catenary wire crossing requires three or more supporting structures, the inner structures should be equipped with enough light units per level to provide a full 360-degree coverage.
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