alternate double-peaked white flashes and a single green flash to identify the airfield as a military facility. The size of the unit is about 24 inches; a rigid drum duplex type with a clear double-flasher spread-light lens on one end and a plain green lens on the other. There is an automatic built-in lamp change in case of lamp burnout. An illustration of a typical airport beacon is shown in figure 6-38. Beacon lights may be manually controlled from the tower or from the lighting vault. If the facility is not operated on a 24- hour basis, an automatic control is possible with a photoelectric control that turns the unit ON or OFF automatically.

The identification beacon, or code beacon, identifies an airfield where the airport beacon is more than 5,000 feet away from the airfield or where two or more airfields are close enough to use the same airport beacon. This nonrotatable unit can be seen from all directions and is equipped with a flasher switch operating at 40 flashes per minute with a range adjustment. The beacon has white lenses with green filters and is manually controlled from the tower but may be controlled automatically.

The third beacon, the hazard, or obstruction beacon, furnishes visual identification of natural features or structures that are 150 feet above airfield elevation for on-station or off-station hazards; that is, tanks, towers, stacks, and so forth. The beacon uses white lenses with red filters and is manually controlled from the tower. When automatic controls are desirable, a photoelectric control system may be used. Since the beacon does not rotate, a flashing system is used-flashing 26 times per minute. The beacon lamps and motor require 120 volts for operation. Most of the time, this unit is fed by a 120/240-volt or 120/208-volt, three-wire service. You can use a 120-volt, two-wire service, but it is not recommended. When the lighting vault is less than 800 feet away from the beacon, a low- voltage service can be used. When the vault is more than 800 feet away, high voltage (2,400 volts) from the lighting vault is used to supply a distribution transformer at the base ofthe beacon. You also can run a control wire from the vault to the beacon to operate a relay that, in turn, switches on the power from a local source near the beacon. The last method works best when the beacon is at a remote location from the airfield.

Because of the extreme hazard to life, an alternate low-voltage source near the beacon is usually required.

TYPES OF FIXTURES AND LAMPS

To meet different system requirements, you must have different intensities of lighting. Along with these systems, you need different kinds of fixtures to meet

Figure 6-38. - Beacon with one door open and base pan dropped.