(1) The purpose of lighting an obstruction which presents a hazard to air commerce is to warn airmen during the hours of darkness and during periods of limited daytime light intensity of the presence of such an obstruction. To accomplish this objective, it is necessary to provide adequate lighting on the obstruction in a manner which assures visibility of such lighting from aircraft at any normal angle of approach. In determining the proper amount of obstruction lighting to adequately mark an obstruction, the mean elevation of the top of the building in closely built-up areas may be used as the equivalent of the ground level.

(2) The top light, or lights, displayed on an obstruction should be installed so as to mark the points or edges of the obstruction highest in relation to an obstruction determining surface, except that when no obstruction determining surface is involved, such top light, or lights, should be installed on the points or edges of the obstruction highest in relation to the ground, or water if so situated. If two or more edges of an extended obstruction located near a landing area are of the same height, the edge nearest the landing area should be lighted.

(3) When the upper part of only a portion of a building or similar extensive object projects above an obstruction determining surface, that portion only need be obstruction lighted and the point or edge of it highest in relation to the obstruction determining surface should be regarded as the "top of the obstruction." In certain cases, however, such as when the obstruction determining surface concerned is an approach or transition surface (i.e., sloping) this point or edge highest in relation to the obstruction determining surface may not be the highest above a horizontal plane passing through the base of the object. In such cases, additional obstruction lights should be placed on the highest part of the object as well as on the point or edge highest in relation to the obstruction determining surface.

(4) If a light, or lights, which is installed on an obstruction is shielded in any direction by an adjacent object, additional lights should be mounted on that object in such a way as to retain the general definition of the obstruction, the shielded light, or lights, being omitted if it does not contribute to the definition of the obstruction.

(5) Obstruction lights and hazard beacons should be operated at all times when the center of the sun's disc is 6° or more below the horizon and during periods of restricted visibility. They may also be operated at such other times as considered desirable. For the purpose of this standard, the term "sunset to sunrise" shall be generally regarded as that period when the center of the sun's disc is 6° or more below the horizon.