Visual Glide Slope Indicators
Visual Approach Slope Indicator (VASI):
The VASI is a system of lights so arranged to provide visual descent guidance information during the approach to a runway
VASI, think vertical
Consists of either 2, 4, 6, 12, or 16 light units arranged in bars referred to as near, middle, and far bars
Most VASI installations consist of 2 bars, near and far, and may consist of 2, 4, or 12 light units
Some VASIs consist of three bars, near, middle, and far, which provide an additional visual glide path to accommodate high cockpit aircraft
This installation may consist of either 6 or 16 light units. VASI installations consisting of 2, 4, or 6 light units are located on one side of the runway, usually the left
Where the installation consists of 12 or 16 light units, the units are located on both sides of the runway
Visible 3-5 miles during the day and up to 20 or more at night
Provides safe obstruction clearance within plus or minus 10° of the extended runway centerline and to 4 NM from the threshold
Glideslope guidance is only accurate when the aircraft is visually aligned with the runway
Descent, using the VASI, should not be initiated until the aircraft is visually aligned with the runway
Lateral course guidance provided by the runway or runway lights
In certain circumstances, the safe obstruction clearance area may be reduced by narrowing the beam width or shortening the usable distance due to local limitations, or the VASI may be offset from the extended runway centerline
This will be noted in the Chart Supplement U.S. and/or applicable Notices to Airmen (NOTAMs)
Although normal glide path angles are 3°, angles at some locations may be as high as 4.5° for obstacle clearance but remember, higher glide slopes increase runway length required for landing and roll-out
Two bar VASI provide 1 glide slope typically 3°
Three bar VASI provide 2 glide slopes
Lower provided by the near and middle bar usually provides a 3° glide slope
Upper provided by the middle and far bars typically set 1/4° higher for high cockpit aircraft to provide safe threshold crossing heights
The principle is color differentiation between the white and red lamps
Each light unit projects a beam of light having a white segment in the upper part of the beam and red segment in the lower part of the beam
The light units are arranged so that the pilot using the VASIs during an approach will see the combination of lights shown below
Rules of thumb:
Red over red, you're dead
Red over white, you're alright
White over white, fly all night
Precision Approach Path Indicator (PAPI):
Similar to a VASI, the precision approach path indicator (PAPI) is installed in a row of 2 or 4 light unit providing the same guidance criteria [Figure 4]
Best effective range is 5 miles during the day and 20 miles at night
Provides safe obstruction clearance within plus or minus 10° of the extended runway centerline and to 3.4 SM from the threshold
Glideslope guidance is only accurate when the aircraft is visually aligned with the runway
Lateral course guidance provided by the runway or runway lights
In certain circumstances, the safe obstruction clearance area may be reduced by narrowing the beam width or shortening the usable distance due to local limitations, or the VASI may be offset from the extended runway centerline
This will be noted in the Chart Supplement U.S. and/or applicable NOTAMs
Usually installed on the left side of the runway
Pulsating Systems:
Pulsating visual approach slope indicators normally consist of a single light unit projecting a two color visual approach path [Figure 6]
Useful range is 4 miles during the day and 10 miles at night
During descent, if below glideslope, the pulsating rate increases as the aircraft gets further above or below the desired glide slope
As with the tri-color system, caution must be exercised not to confuse this system with other adjacent light.
Stand-Alone Final Approach Runway Occupancy Signal (FAROS):
The standalone FAROS system is a fully automated system that provides runway occupancy status to pilots on final approach to indicate whether it may be unsafe to land
When an aircraft or vehicle is detected on the runway, the Precision Approach Path Indicator (PAPI) light fixtures flash as a signal to indicate that the runway is occupied and that it may be unsafe to land
The standalone FAROS system is activated by localized or comprehensive sensors detecting aircraft or ground vehicles occupying activation zones
The standalone FAROS system monitors specific areas of the runway, called activation zones, to determine the presence of aircraft or ground vehicles in the zone [Figure 8]
These activation zones are defined as areas on the runway that are frequently occupied by ground traffic during normal airport operations and could present a hazard to landing aircraft
Activation zones may include the fulllength departure position, the midfield departure position, a frequently crossed intersection, or the entire runway
Pilots can refer to the airport specific FAROS pilot information sheet for activation zone configuration
Clearance to land on a runway must be issued by Air Traffic Control (ATC)
ATC personnel have limited control over the system and may not be able to view the FAROS signal
Operating Characteristics:
If an aircraft or ground vehicle occupies an activation zone on the runway, the PAPI light fixtures on that runway will flash
The glide path indication is not affected, i.e. the configuration of red and white PAPI lights observed by the pilot on approach does not change
The standalone FAROS system flashes the PAPI lights when traffic occupies an activation zone whether or not there is an aircraft on approach
Pilot Observations:
A pilot on approach to the runway observes the PAPI lights flashing if there is traffic on the runway activation zones and notices the PAPI lights cease to flash when the traffic moves outside the activation zones
A pilot on departure from the runway should disregard any observations of flashing PAPI lights
Pilot Actions:
When a pilot observes a flashing PAPI at 500 feet above ground level (AGL), the pilot must look for and attempt to acquire the traffic on the runway
At 300 feet AGL, the pilot must contact ATC for resolution if the FAROS indication is in conflict with the clearance [Figure 8]
If the PAPI lights continue to flash and the pilot cannot visually determine that it is safe to land, the pilot must execute an immediate "go around"
As with operations at nonFAROS airports, it is always the pilot's responsibility to determine whether or not it is safe to continue with the approach and to land on the runway.