The majority of airports have some type of lighting for night operations, and the variety and type of lighting systems depends on the volume and complexity of operations at a given airport. We’re going to be examining these types today with help from the FAA’s Pilot’s Handbook of Aeronautical Knowledge.
Airport beacons help a pilot identify an airport at night. The beacons are operated from dusk till dawn. The beacon has a vertical light distribution to make it most effective from 1–10° above the horizon, although it can be seen well above or below this spread. The combination of light colors from an airport beacon indicates the type of airport. (Figure 1) Some of the most common beacons are:
- Flashing white and green for civilian land airports;
- Flashing white and yellow for a water airport;
- Flashing white, yellow, and green for a heliport; and
- Two quick white flashes alternating with a green flash identifying a military airport.
Approach light systems are intended to provide a means to transition from instrument flight to visual flight for landing. The system configuration depends on whether the runway is a precision or nonprecision instrument runway. Some systems include sequenced flashing lights, which appear to the pilot as a ball of light traveling toward the runway at high speed. Approach lights can also aid pilots operating under VFR at night.
Visual glidescope indicators provide the pilot with glidepath information that can be used for day or night approaches. By maintaining the proper glidepath as provided by the system, a pilot should have adequate obstacle clearance and should touch down within a specified portion of the runway.
VASI (Visual Approach Slope Indicator) installations are the most common visual glidepath systems in use. The VASI provides obstruction clearance within 10° of the runway extended runway centerline, and to four nautical miles (NM) from the runway threshold.
The VASI consists of light units arranged in bars. There are 2-bar and 3-bar VASIs. The 2-bar VASI has near and far light bars and the 3-bar VASI has near, middle, and far light bars. Two-bar VASI installations provide one visual glidepath which is normally set at 3°. The 3-bar system provides two glidepaths, the lower glidepath normally set at 3° and the upper glidepath 1⁄4 degree above the lower glidepath.
The basic principle of the VASI is that of color differentiation between red and white. Each light unit projects a beam of light, a white segment in the upper part of the beam and a red segment in the lower part of the beam. The lights are arranged so the pilot sees the combination of lights shown in Figure 2 to indicate below, on, or above the glidepath.
Other Glidepath Systems
A precision approach path indicator (PAPI) uses lights similar to the VASI system except they are installed in a single row, normally on the left side of the runway. (Figure 3)
A tri-color system consists of a single light unit projecting a three-color visual approach path. Below the glidepath is indicated by red, on the glidepath is indicated by green, and above the glidepath is indicated by amber. (Figure 4)
Pulsating visual approach slope indicators normally consist of a single light unit projecting a two-color visual approach path into the final approach area of the runway upon which the indicator is installed. The on glidepath indication is a steady white light. The slightly below glidepath indication is a steady red light. If the aircraft descends further below the glidepath, the red light starts to pulsate. The above glidepath indication is a pulsating white light. The pulsating rate increases as the aircraft gets further above or below the desired glideslope.
We’ll see more from our CFI on Thursday!
