Top

Visual Glide Slope Indicators

Introduction:

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 while some VASIs consist of three bars, near, middle, and far, which provide an additional visual glide path to accommodate high cockpit aircraft
    • VASI installations consisting of 2, 4, or 6 light units are located on one side of the runway, usually the left but when 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 Airport/Facility Directory and/or applicable Notices to Airmen (NOTAMs)
  • Angles 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
  • Although normal glide path angles are three degrees, angles at some locations may be as high as 4.5° to give proper obstacle clearance
  • Pilots of high performance aircraft are cautioned that use of VASI angles in excess of 3.5° may cause an increase in runway length required for landing and rollout

Visual Approach Slope Indicator (VASI)
Figure 1: Visual Approach Slope Indicator (VASI)
Visual Approach Slope Indicator (VASI)
Figure 2: Visual Approach Slope Indicator (VASI)
Visual Approach Slope Indicator (VASI)
Figure 3: Visual Approach Slope Indicator (VASI)

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 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 Airport/Facility Directory and/or applicable NOTAMs
  • Usually installed on the left side of the runway
Precision Approach Path Indicator (PAPI)
Figure 4: Precision Approach Path Indicator (PAPI)

Tri-Color System:

  • Consists of a single light unit projecting a three-color visual approach path into the final approach area of the runway upon which the indicator is installed [Figure 5]
  • Typically useful from 1/2 to 1 mile during the day and up to 5 miles at night
  • Caution must be exercised not to confuse this system with other adjacent lights
  • When the aircraft descends from green to red, the pilot may see a dark amber color during the transition
Tri-Color Visual Glide Slope System
Figure 5: Tri-Color System

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 lights
Pulsating Visual Glide Slope System
Figure 6: Pulsating System

Alignment of Elements Systems:

  • Low cost painted plywood panels, normally black and white or fluorescent orange [Figure 7]
  • Some are lighted for night use
  • Useful range is approximately 3/4 of a mile
  • System works by illusions of the plywood where the plane is positioned
Alignment of Elements Visual Glide Slope System
Figure 7: Alignment of Elements System

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
FAROS Activation Zones
Figure 8: FAROS Activation Zones
  • 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
FAROS Glide Slope Action Points
Figure 9: FAROS Glide Slope Action Points
    • Pilots should inform the ATCT when they have executed a go around due to a FAROS indication that is in conflict with ATC instructions

Conclusion:

  • At this time, the stand­alone FAROS system is not widely implemented and is used for evaluation purposes
  • Runway Status Lights (RWSLs) may be implemented with FAROS
    • RWSL is an independent system and does not substitute for an ATC clearance to enter, cross, takeoff from, land on, or operate on a runway which must still come from ATC; however, query ATC if the lights are in confliction with a clearance

References: