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Codes and Standards |
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Chapter 6. Medium Intensity Flashing White Lighting Systems Standards 60. PURPOSE. Medium intensity flashing white (1---865) obstruction lights may provide conspicuity both day and night. 61. STANDARDS. The medium intensity flashing white light system is normally composed of flashing omnidirectional lights. This system is not normally recommended on structures less than 200 feet AGL. 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. ( See APPENDIX 1. ) 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, ff 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. lntermediate Levels. At intermediate levels, two beacons (L-865) should be mounted outside at diagonally or diametrically opposite positions of intermediate levels. The lowest light level should not be less than 200 feet (6lin) 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 ( See APPENDIX 1. ) 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. Stntctures 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. (See CHAFFER 2. and CHAPTER 3. ) 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 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: at. Twilight--to-Night This should not occur before the illumination drops below five footcandles (53.8 lux) but should occur before it drops below two footcandles (21.5 lux). b. Night--to.-Day. The intensity changes listed in (a) above should 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 in the following table are the rain/mum. When the structure diameter is:
2. Exceeding 20 Feet (6m) But Not More Than 100 Feet (3lin). Four light units per level. 3. Exceeding 100 Feet (3lin) But Not More Than 200 Feet (61m). Six light units per level. 4. Exceeding 200 Feet (6lin). Eight light units per level 65. GROUP OF OBSTRUCTIONS. When individual objects within a group of obstructions are not the same heightand 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. 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 located 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. (Also see paragraph 47.) 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 fxaction 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 obslruction will permit. One such light should be displayed at each outside comer on each level with the remaining lights evenly spaced between the comer lights. |
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