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Visual Glide Slope Indicators

Introduction:

Visual Approach Slope Indicator:

  • The Visual Approach Slope Indicator, or VASI, is a system of lights so arranged to provide visual descent guidance information during the approach to a runway
  • 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 [Figure 1/2/3]
  • 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
  • Lateral Guidance:

    • Lateral course guidance is generally provided by the runway or runway lights
  • Vertical Guidance:

    • The VASI consists of light units arranged in bars
    • There are two-bar and three-bar VASIs:
      • Two bar VASI consists of a near and far light which provides a single glideslope, typically 3°
      • Three bar VASI consists of near, middle and far light bars which provide two glideslopes
        • 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
    • Although normal glide path angles are 3°, angles at some locations may be as high as 4.5° for obstacle clearance
      • Pilots of high performance aircraft are cautioned that use of VASI angles in excess of 3.5 degrees may cause an increase in runway length required for landing and rollout
    • In addition to the runway however, VASI's provide 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
    • 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)
  • Rules of thumb:

    • Red over red, you're dead
    • Red over white, you're alright
    • White over white, fly all night
  • To differentiate between a VASI and PAPI, think V, vertical
Visual Approach Slope Indicator (VASI)
Visual Approach Slope Indicator (VASI)
Visual Approach Slope Indicator (VASI)
Visual Approach Slope Indicator (VASI)
Visual Approach Slope Indicator (VASI)
Visual Approach Slope Indicator (VASI)

Precision Approach Path Indicator:

  • Similar to a VASI, the Precision Approach Path Indicator, or 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
  • Usually installed on the left side of the runway
  • Lateral Guidance:

    • Lateral course guidance provided by the runway or runway lights
  • Vertical Guidance:

    • 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
      • 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 PAPI may be offset from the extended runway centerline
      • This will be noted in the Chart Supplement U.S. and/or applicable NOTAMs
Precision Approach Path Indicator (PAPI)
Precision Approach Path Indicator (PAPI)

Tri-Color System:

  • Tri-color visual approach slope indicators normally 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
  • Below the glidepath is indicated by red, on the glidepath is indicated by green, and above the glidepath is indicated by amber
  • 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
    • Pilots should not mistake this area for an "above the glidepath" indication
Tri-Color Visual Glide Slope System
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
  • The on glide path indication is a steady white light
  • The slightly below glide path indication is a steady red light
  • If the aircraft descends further below the glide path, the red light starts to pulsate
  • The above glide path indication is a pulsating white light
  • 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
Pulsating System

Alignment of Elements Systems:

  • Alignment of elements systems are installed on some small general aviation airports
  • 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 aircraft is positioned
Alignment of Elements Visual Glide Slope System
Alignment of Elements System

Stand-Alone Final Approach Runway Occupancy Signal (FAROS):

  • FAROS alerts arriving pilots that the approaching runway is occupied by flashing of the Precision Approach Path Indicator (PAPI)
  • While the FAROS provides no glide slope, it displays critical information to a pilot on final
  • To learn more about the system and what it means to a pilot on final approach, visit our airport lighting page

Conclusion:

  • VASI's are the most common approach light system
  • Although only regulatory in Class D airspace according to FAR 91.129, each pilot operating an airplane approaching to land on a runway served by a visual approach slope indicator should maintain an altitude at or slightly above the glide path until a lower altitude is necessary for a safe landing

References: