Towers, poles, tanks, smokestacks and similar obstructions which are so grouped as to present a common hazard to air navigation should be lighted in accordance with the following specifications:

(1) Specification "H-1."

(a) This specification applies to a group of closely spaced towers, poles, tanks, smokestacks or similar obstructions of approximately the same over-all height above ground, or water if so situated, in which the spacing between the individual structures does not exceed 150 feet.

(b) The group may be considered an extensive obstruction and lighted in accordance with the "E" specifications for prominent buildings and similar extensive obstructions.

(2) Specification "H-2."

(a) This specification applies to a group of closely spaced towers, poles, tanks, smokestacks and similar obstructions, which may or may not be of the same over-all height, in which the spacing between the individual structures is not in all cases equal to or less than 150 feet.

(b) Each prominent object within the group should be lighted in accordance with the specifications hereinbefore prescribed for individual towers, poles, and similar obstructions or a corresponding over-all height above ground, or water if so situated.

(c) In addition, there 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, at least one rotating beacon producing aviation red flashes. The frequency of its flashes should be such as hereinbefore specified for rotating beacons.