more than 150 feet (45 meters) above the level of the surrounding ground, an additional light will be added for each additional 150 feet (45 meters) or fraction thereof. Additional lights will be equally spaced between the top light and the ground level. Any such additional light position shielded in all directions by other objects shall have the shielded lights omitted.

(2) Horizontal Arrangement of Obstruction Lights. The arrangement and number of obstruction lights in the horizontal plane passing through the elevation to be marked shall be such that the obstruction is defined from every angle of azimuth.

(3) Horizontal Spacing of Obstruction Lights

a. Horizontally extended obstructions having a projected length parallel to the nearest edge of the landing area of 75 feet (23 meters) or less, above an obstruction marking surface as defined in the Section on landing areas, shall have the top marked at approximately the center of the point or edge highest above the obstruction marking surface. If more than one edge is of equal height above the obstruction marking surface, the edge nearest the landing area shall be selected for marking.

b. Horizontally extended obstructions having a projected length parallel to the nearest edge of the landing area of more than 75 feet (23 meters), or an actual length of more than 300 feet (90 meters) above an obstruction marking surface as defined in the Section on landing areas shall have the top marked at each end of the length on the edge highest above the obstruction marking surface and, in addition, shall have an obstruction light placed at the highest point above the obstruction marking surface between the end lights if any such intermediate point of greater height exists. If more than one edge is of equal height above the obstruction marking surface, the edge nearest the landing area shall be selected for marking. An additional obstruction light shall be added for each 150 feet (45 meters), or fraction thereof, the projected length exceeds 150 feet (45 meters), or for each 300 feet (90 meters), or fraction thereof, the actual length exceeds 300 feet (90 meters), whichever results in the greatest number of lights. Such additional obstruction lights shall be spaced approximately at equal intervals between end lights positioned as above.

c. Intermediate lights required by the Paragraph on vertical spacing above shall be installed to comply with the horizontal arrangement required by the Paragraph on horizontal spacing above, and shall be, in so far as practicable, in the plane perpendicular to the nearest edge of the usable landing area that passes through the corresponding top.light.

(d) Lighting of Wind Direction Indicator. At least one wind direction indicator shall be lighted or illuminated at each landing area. (e) Lighting of Landing Direction Indicator. The landing direction indicator shall be lighted or illuminated.

(f) Outlining the Landing Strip. The landing strips or runways, but not both, of landing areas shall be delineated by parallel rows of fixed lights placed in pairs on perpendiculars to the center line of the landing strip, unless the entire usable area is delineated by boundary

lights as hereinafter provided, in which case the landing strip need not be outlined. Light units may be installed both around the landing strip and around the runway, but both installations shall not be operated at one time.

(1) Airfield and Land Airport Strip Lights. The rows of lights shall be not more than 500 feet (150 meters) apart and the lights in the rows shall be spaced approximately 300 feet (90 meters) apart, starting at the corners of the strip. Four threshold lights symmetrically placed with respect to the center line of the strip shall be placed at 50-foot (15-meter) intervals across each end of the strip. The lights alongside the strip shall show aviation white from the approach end to a point approximately 1,500 feet (450 meters) from the opposite end, and shall show aviation yellow from that point to the end. The threshold lights shall show aviation green from all azimuths.

(2) Airfield and Land Airport Runway Lights. Parallel rows of the lights shall be located in the surface of the runway not less than 150 feet (45 meters) apart. The lights in the rows shall be spaced approximately 200 feet (60 meters) apart starting approximately 200 feet (60 meters) from the end of the runway. Threshold lights shall be placed at each corner of the ends of the runway and at uniformly spaced positions approximately 50 feet (15 meters) apart except that for runways less than 150 feet (45 meters) wide, the spacing shall be decreased to allow a total of four threshold lights to be used. The lights alongside the runway shall show aviation white from the approach end to a point approximately 1,500 feet (450 meters) from the opposite end, and shall show aviation yellow from that point to the end. The threshold lights shall show aviation green in all azimuths.

(3) Water Airport Strip Lights. The rows of lights alongside the landing strip shall be not more than 1,000 feet (300 meters) apart and the lights in the rows shall be not more than 1,000 feet (300 meters) apart starting not more than 1,000 feet (300 meters) from the end of the strip. Threshold lights shall be placed at each corner of the ends of the strip and in at least two uniformly spaced positions across the ends of the strip between the corner lights. The lights alongside the strip shall show aviation green in all azimuths. The threshold lights shall show aviation yellow in all azimuths.

(g) Outlining the Landing Area. The landing area shall be delineated by boundary lights unless the landing strips are delineated as herein before provided, in which case the landing area need not be outlined.

(1) Airfield and Land Airport Boundary Lights. The lights used shall be aviation white boundary lights spaced approximately 300 feet (90 meters) apart, except that where a preferred landing path crosses the boundary, the white boundary lights shall be omitted and aviation green boundary range lights spaced 50 feet (15 meters) apart, symmetrically set on a line perpendicular to the center line of the path substituted therefor.

When more than one landing path exists, the boundary range light spacing at the extremities of these paths shall be numerically coded, using a decreased spacing if necessary. The coding assigned

will be dependent upon the desirability of the landing path; the most desirable path shall be coded with the greatest number of lights.

(2) Water Airport Boundary Lights. The lights used shall be fixed aviation green boundary lights spaced not over 1,000 feet (300 meters) apart, except that where a preferred landing channel crosses the boundary, the green boundary lights shall be omitted and aviation yellow boundary range lights on lesser spacing symmetrically set on a line perpendicular to the center line of the path substituted therefor. When more than one landing channel exists, the boundary range light spacing at the extremities of those channels shall be numerically coded. The coding assigned will be dependent upon the desirability of the landing channel; the most desirable channel shall be coded with the greatest number of lights.

(h) Illumination of Landing Area. The usable landing area or the part thereof on which landings should be made may be illuminated by a floodlight or floodlight system used in conjunction with the strip, runway, or boundary lights.

(i) Lighting of Taxiways. Taxiways may be outlined by parallel rows of fixed aviation blue lights placed, where feasible, in pairs or perpendiculars to the center lines of the taxiway.

(j) Lighting of Approachways. The approachways to the landing area may be indicated by light lanes. When only one line of lights is used in an approach light lane, it shall be located along an extension of the left line of contact lights when approaching the landing area.

(1) Airfield and Land Airport Approach Lights. The color of airfield and land airport approach lights shall be aviation red except for the light or lights at the junction of the approach area and the end of the landing strip, which shall be aviation green.

(2) Water Airport Approach Lights. The color of water airport approach lights shall be alternately aviation red and aviation green in progression toward the boundary of the landing area. The first and last lights in each line shall be aviation red.

SECTION V

Ground Aids to Instrument Operation

16 [9]. General. A complete system of aids to instrument operation shall consist of:

(a) Instrument aids to navigation

(b) Instrument aids to final approach and landing

(c) Instrument aids to taxiing

(d) Instrument aids to take-off

(e) Instrument aids to collision warning

(f) Instrument aids to search and rescue operation

(g) Instrument aids to air traffic control

17 [10]. Instrument Aids to Navigation. Short and long distance radio navigational aids and two-way communication facilities to provide guidance along the course of the airway, to provide for separation of air traffic, to mark points on the airway, and to provide references for direction finding, shall be provided to enable aircraft to be navigated along the airway on instruments.

18 [11]. Instrument Aids to Final Approach and Landing. Instrument aids to final approach affording, if possible, means whereby aircraft can safely be landed by instruments, shall be provided to serve at least one runway of each airport at which they are required by this Annex. The aids provided shall at least afford:

(a) An indication in azimuth of the appropriate direction of approach

(b) An indication in elevation of the appropriate direction of approach

(c) An indication, receivable at a distance of not less than 4 miles (6.4 kilometers), of the distance from the landing area at two or more points along the approach path.

19 [12]. Instrument Aids to Taxiing. Instrument aids to taxiing, designed to meet operating requirements, shall be provided when required by this Annex.

20 [13]. Instrument Aids to Take-Off. Instrument aids to take-off, complementary to those for final approach, shall be provided to serve at least one runway of each airport at which they are required by this Annex. The aids provided shall afford:

(a) An indication in azimuth of the appropriate direction of take-off

(b) An indication in elevation of the appropriate flight path after take-off

21 [14]. Instrument Aids to Collision Warning. Instrument aids to indicate to the pilot and the air traffic control service the possibility of collision shall be provided as and when specified to meet operational requirements.

22 [15]. Instrument Aids to Search and Rescue Operation. Instrument aids to assist the staff at an air traffic control center in the expeditious conduct of search and rescue operations shall be provided as required in this Annex. The requirement is for direction finding service and rapid communication between centers and with any agency capable of contributing to the search or rescue proceedings.

23 [16]. Instrument Aids to Air Traffic Control. Instrument aids to air traffic control shall be provided in airport traffic control towers and air traffic control centers as required by this Annex. The requirement is for apparatus that accurately records the progress of all international flights and enables traffic to be regulated in an orderly manner in the interests both of regularity and safety of operation.

ANNEX B

COMMUNICATIONS PROCEDURES AND SYSTEMS

SECTION I
Definitions

(a) Aeronautical Ground Radio Station. A radio station operated for the purpose of providing point-to-point and air-ground communications in connection with operation of aircraft. See Land Station. (b) Aeronautical Radio Service. A radio service carried on between aircraft stations and land stations and by aircraft stations among themselves. This term shall also apply to special radio services intended to insure the safety of aerial navigation. See Mobile Service.

(c) Aeronautical Radio Station. A land station carrying on a service with aircraft stations. This may be a fixed station assigned also to communicate with aircraft stations; in this case it shall be considered as an aeronautical station only for the duration of its service with aircraft stations.

(d) Aerophare. A radiobeacon utilized in aeronautical services for air navigation purposes.

(e) Aircraft Radio Station. A radio station on board any aircraft.

See Mobile Station.

(f) Air-Ground Radio Frequency. A frequency specified or agreed upon for transmissions from an aircraft station to an aeronautical ground station. See Ground-Air Radio Frequency.

(g) Air Navigation Radio Aids. Aeronautical ground stations, aeronautical radio beacons, and aeronautical direction-finding and similar aids.

(h) Airport Traffic Control Tower. A facility established to provide adequate supervision of air traffic within an airport control sector.

(i) Air Traffic. Aircraft in motion anywhere in the air space and aircraft on the usable portion of a landing area.

(j) Air Traffic Clearance. Approval of a flight or portion thereof by an air traffic control center or airport traffic control tower.

(k) Air Traffic Control Center. A facility established to provide adequate supervision of air traffic within a specified control area.

(1) Air Traffic Control Service. See Introductory Section to Annex D, "Air Traffic Control Practices".

(m) Airway. A designated path through the navigable air space, identified by an area of a specified width on the surface of the earth. (n) Alternate Landing Area. A landing area specified in the flight plan to which a flight may proceed when a landing at the intended destination becomes inadvisable.

(0) Arrival Message. A message, in a specified form, containing relevant items of information on the arrival of an aircraft at any point.