and the condenser discharge circuits. Each of these circuits will be covered separately.

Runway Edge Lights

Runway edge lighting is designed to show the width and length of the usable landing area; there are two rows of lights - one row on each side - that run the length of the runway. The light they give off is aviation white (clear). The edge lights are installed not more than 10 feet from the edge of the full-strength runway paving. Both lines of lights will be the same distance from the runway center line. It is best if the lines of lights are located as close to the runway as the base mounting for the lights allows. The lights are equally spaced along the runway at distances not to exceed 200 feet. (See fig. 6-35.)

The runway lighting controls are set up so that the lights on intersecting runways cannot be on at the same time. Also. the controls must turn all the light systems of one runway on at the same time. The runway edge lights are controlled so all the lights are the same brightness. In a high-intensity system, threshold lights are one step higher than the runway edge lights except when the runway edge lights are at full brightness. At this time. both the runway edge lights and the threshold lights are at full brightness. In a medium-intensity runway lighting system, all lights (runway edge lights and threshold lights) are the same brightness.

In some instances, it is a good practice to use runway edge light fixtures and lamps for the threshold lights, so that the difference is noticeable when the threshold lighting configuration has to be stepped up one brightness higher than the runway edge lights. To determine the number of circuits required for runway edge lights, you need to determine the length of the runway. You determine the number of lights on one circuit by considering not only the number of lights connected to the circuit but also the voltage loss for the circuit cables and the feeder cables from the vault to the runway. If this distance is long, you may need to adjust the number of lights in the circuit.

Do not load the regulator less than one half of its rated kilowatt (kW) output. If more than one regulator is required, each regulator should be equally loaded.

Each light circuit will be fed by a series loop. The current leaves one terminal of the CCR, goes through the circuit to each light unit, and returns to the other terminal of the regulator.

Taxiway Lights

Taxiway lights are used to show the pilot the width and direction of the "taxiing route." The lights are aviation blue in color. They are basically the same as runway lighting circuits.

Approach Lights

Runway approach light systems are used on high- intensity-equipped runways. The system starts at the threshold and extends outward for 3,000 feet. When the full length of the land cannot be used, the greatest length possible is used. Condenser discharge (strobe) lights that put out a high-intensity, bluish white light start at the 3,000-foot mark and flash in sequence toward the threshold. The system is used to help the pilots land under low-visibility conditions. The condenser discharge lights are discussed in more detail later in this chapter.

The lights of the approach system are located on an imaginary line that extends from the runway center line. Each light bar is centered on this imaginary line and spaced the same distance apart for the entire 3,000 feet.

The supply and control circuits of the approach lighting systems are installed underground and are usually in conduit; however, in some cases, the last 2,000 feet of the approach lights can be above ground. In some cases, the supply cable from the series circuit can be direct burial.

Above ground circuits may be used for approach lighting when the cables do not present a hazard to vehicular traffic or are not accessible to unauthorized personnel and animals. The cable must be installed, normally, a minimum of 22 feet above ground. Where the area is completely closed off (fenced): a lower ground clearance is acceptable. Control circuits may use a small-size conductor when it is supported by a messenger cable. DO NOT USE ALUMINUM CONDUCTORS. Use standard overhead construction practices for series circuits. Use lightning arresters when the cables go from underground to overhead. Connect the ends of the circuits in the same way as underground cables.

Besides the basic runway light configuration, there are other airfield lighting aids to help the pilot in landing and takeoff operations. Four such aids for landing and taking off are the visual approach slope indicators (VASI), the Fresnel

^® lens optical loading system (FLOLS), the runway distance markers, and the threshold lights.