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Visual approach slope indicator

The Visual Approach Slope Indicator (VASI) is a system of lights positioned on the left side of the runway threshold (or both sides for larger installations) that provides pilots with immediate visual descent guidance during the final approach to landing, helping to maintain a safe glide path angle typically around 3 degrees to avoid obstacles and ensure proper touchdown.^[1] These systems use precisely angled light projectors to create overlapping red and white beams: white lights indicate the aircraft is flying too high (above the glide path), red lights signal too low (below the glide path), and a balanced "pink" or mixed display shows alignment on the proper path, with visibility extending 3-5 miles during daylight and up to 20 miles or more at night.^[1] VASIs come in various configurations to suit different runway lengths and aircraft types, including 2-bar systems (with 2, 4, or 12 lights for a single glide path), 3-bar systems (with 6 or 16 lights offering a lower 3-degree path and an upper 3.25-degree path for high-cockpit aircraft like large jets), and installations of up to 16 lights per side for extended coverage.^[1] The system ensures safe obstacle clearance within ±10 degrees of the runway centerline and up to 4 nautical miles from the threshold, though pilots must verify local glide angles via the Chart Supplement or NOTAMs, as steeper angles over 3.5 degrees can extend landing rollout distance.^[1] Descent using VASI guidance should only commence once the runway is visually acquired and the aircraft is laterally aligned.^[1] While the VASI remains in use at many airports, it is one of several visual glideslope indicators standardized by the FAA, including the more precise Precision Approach Path Indicator (PAPI)—a single row of 2 or 4 lights providing similar red/white cues but with finer angular resolution and now the current standard VGSI, with the FAA planning to replace remaining VASI installations with PAPIs—and lower-cost alternatives like tricolor VASIs (using red, green, and amber from a single unit, visible up to 5 miles at night), pulsating VASIs (a single flashing light unit with varying pulse rates for deviation feedback, effective to 10 miles at night), and basic alignment-of-elements systems (non-electric painted panels for short-range guidance at small fields).^[1]^[2] These tools collectively enhance safety under visual flight rules by reducing the risk of controlled flight into terrain or runway excursions, particularly at non-precision approaches.^[1]

Introduction

Definition and Purpose

The Visual Approach Slope Indicator (VASI) is a system of lights arranged on the side of a runway threshold to provide pilots with visual descent guidance during the final approach to landing.^[1] It consists of light bars or units that project beams using red and white lamps, visible to approaching aircraft, to indicate the aircraft's position relative to the desired glide path.^[3] These components are typically installed on the left side of the runway or on both sides, ensuring lateral guidance aligns with runway markings during visual flight rules (VFR) operations.^[1] The primary purpose of the VASI is to assist pilots in maintaining a safe glide path, typically set at a 3-degree angle, to avoid obstacles and achieve a proper touchdown point on the runway.^[1] By providing immediate color-coded feedback—white lights indicating the aircraft is above the path, red below, and a combination on path—it enables precise vertical guidance during approach.^[3] This system is visible from 3 to 5 miles during the day and up to 20 miles or more at night, supporting effective use in varying conditions.^[1] Key benefits of the VASI include enhanced safety through obstruction clearance within ±10 degrees of the runway centerline and up to 4 nautical miles from the threshold, reducing risks from shallow or steep approaches that could lead to unsafe landings.^[1] Primarily employed for VFR approaches, it offers pilots straightforward, real-time path correction to minimize landing errors and promote consistent, obstacle-free descents.^[3]

History

The development of the Visual Approach Slope Indicator (VASI) emerged in the post-World War II era amid rapid aviation growth and rising concerns over approach and landing accidents. The Federal Aviation Administration (FAA), established in 1958 from its predecessor agencies, recognized the limitations of emerging instrument landing systems in marginal visibility and initiated research into reliable visual aids to bridge the gap between instrument and visual approaches.^[4] In the late 1950s, FAA engineers developed the foundational VASI concept through patented innovations for automated glide path indication using colored lights.^[5] Development continued into the early 1960s, with operational installations at airports. The system received ICAO approval for the red-white VASI in 1961.^[6] By the mid-1960s, the FAA had standardized the VASI to enhance safety at general aviation and commercial airports. This addressed the inadequacies of non-precision approaches in low-visibility conditions, reducing controlled flight into terrain risks.^[4] Key milestones included the standard two-bar VASI, which used rows of red and white lights to signal positions relative to the glide path. By the 1970s, variants such as the pulsating VASI were introduced, featuring a single flashing light unit to provide deviation feedback, particularly at smaller fields. These evolutions reflected ongoing FAA efforts to refine visual guidance amid increasing air traffic, replacing earlier rudimentary visual aids. The shift to LED technology for VASI systems began in the 2000s, driven by energy efficiency and reduced maintenance needs compared to incandescent bulbs. FAA approvals supported retrofits at airports through the 2010s and into the 2020s.

Operational Principles

Light Configurations and Guidance

The Visual Approach Slope Indicator (VASI) conveys glide path information to pilots through distinct light configurations that rely on the aircraft's vertical position relative to the runway threshold. The system employs red and white lights arranged in horizontal bars, with the observed color depending on the pilot's line of sight intersecting the light beams at specific angles. When the aircraft is on the nominal 3-degree glide path, the pilot sees an equal number of red and white lights, typically appearing as red over white in a two-bar configuration, indicating alignment with the safe descent angle. If the aircraft is above the glide path, the configuration shows predominantly or all white lights, signaling the need to descend more steeply; conversely, below the path, predominantly or all red lights appear, prompting a shallower descent to avoid terrain or obstacles. This color-based logic ensures intuitive guidance, with the transition between colors occurring over a narrow angular band of approximately 0.5 degrees to maintain precision.^[1] VASI configurations generally feature two or three bars of lights positioned along the runway edge, containing 2 to 16 light units per bar, divided into near and far segments for basic systems. The near bar projects a beam at a slightly higher angle than the far bar, creating an optical overlap zone where the red lower beam from the near bar transitions to the white upper beam from the far bar, defining the on-path condition at the desired glide slope. For instance, in a standard two-bar setup, the near bar might be aimed at around 3.5 degrees while the far bar at 3.0 degrees, though exact angles vary by installation to achieve the 3-degree nominal path while providing obstacle clearance. Three-bar systems extend this by adding a middle bar for dual paths, but the principle remains the same: segmented beams ensure the color mix directly correlates to angular deviation from the threshold.^[7] The angular principles of VASI rely on basic optics and trigonometry to project beams that intersect the pilot's eye level at precise elevations. Each light unit emits a fan-shaped beam with a sharp color transition, calibrated so the glide slope angle θ is determined by θ ≈ arctan(Δh / d), where Δh represents the vertical height difference between the light source and the pilot's eye at threshold crossing, and d is the horizontal distance from the lights to the runway threshold. This derivation stems from the trigonometric relationship in the right triangle formed by the beam path, where the tangent of the angle equals the opposite (height) over adjacent (distance) sides, ensuring the system guides aircraft to cross the threshold at a safe height, typically 25 to 55 feet depending on aircraft type. Pilots interpret these configurations directly from the cockpit, offering real-time vertical guidance independent of instrument systems, visible up to 3-5 miles during the day and over 20 miles at night under clear conditions.^[1]

Visibility and Coverage

The Visual Approach Slope Indicator (VASI) provides visibility typically ranging from 3 to 5 miles during the day and up to 20 miles or more at night, depending on atmospheric conditions and light intensity.^[1] This range is achieved through light units ensuring detectability for approaching aircraft.^[6] Coverage is designed for horizontal angles of ±10 degrees from the extended runway centerline, extending up to 4 nautical miles from the runway threshold, with provisions for wider angles up to 30 degrees in some installations to accommodate varying airport layouts.^[1] Vertically, the beam width spans 1 to 2 degrees, with a narrow transition zone of approximately 0.5 degrees between red and white light indications, optimized for pilot eye heights between 25 and 50 feet above the ground to provide precise glide path guidance.^[8] Environmental factors significantly influence VASI performance; fog, rain, or snow can reduce visibility by scattering or absorbing light, potentially limiting effective range to less than 1 mile in adverse weather.^[1] At non-towered airports, intensity is adjustable via pilot-controlled lighting (PCL), allowing pilots to activate or increase brightness through radio clicks for better detection in varying conditions.^[1] VASI systems are limited in heavy weather where visual cues become unreliable, serving primarily as a supplement to instrument landing systems (ILS) rather than a replacement, and are intended for visual flight rules (VFR) operations.^[1]

Types of VASI Systems

Standard VASI

The Standard VASI is the most prevalent configuration of the Visual Approach Slope Indicator system, featuring rows of lights arranged in bars perpendicular to the runway centerline, typically consisting of two or three bars with white upper beams and red lower beams emitted from each light unit.^[1] These split-beam lights provide angular guidance by projecting a narrow white beam above the intended glide path and a broader red beam below it, with the transition zone producing a pinkish appearance where the beams overlap over approximately 0.25 degrees.^[8] Developed as a straightforward visual aid, the Standard VASI has been an FAA-approved system since the 1960s, offering reliable descent guidance for visual flight rules operations.^[9] In operation, the Standard VASI indicates the aircraft's position relative to the glide path through color combinations: all white lights signal the aircraft is above the path (fly lower), all red lights indicate below the path (fly higher), and a mix of red on the near bar(s) with white on the far bar(s) confirms alignment on the path.^[1] The pink transition effect during the overlap serves as a precise on-path cue, enhancing pilot awareness in the critical approach phase.^[8] Visibility extends up to 3-5 nautical miles during the day and over 20 nautical miles at night, covering a 20-degree horizontal field either side of the runway centerline.^[1] Common configurations include the two-bar VASI, which provides a single glide path nominally set at 3 degrees and suits most applications with 2, 4, or 12 light units total—such as the widely used 4-unit setup (two lights per bar) for balanced coverage.^[1] Three-bar systems, with 6 or 16 units, offer dual glide paths (e.g., 3.25 degrees on the outer path for aircraft with high cockpits and 3 degrees on the inner), extending guidance for varied aircraft types while maintaining the standard red/white scheme.^[8] These setups are scalable up to 16 units for broader eye height coverage, ensuring safe obstruction clearance within 4 nautical miles of the threshold.^[1] The Standard VASI's advantages lie in its simplicity and cost-effectiveness, requiring minimal infrastructure compared to precision instrument systems, which makes it ideal for general aviation fields and smaller airports handling diverse aircraft.^[8] It delivers coarse vertical guidance suitable for non-precision approaches, promoting consistent landings without the need for advanced electronics, though newer installations have largely shifted to PAPI systems since the 1990s.^[1]

Tri-color VASI

The tri-color visual approach slope indicator (VASI) is a compact variant of the VASI system, consisting of a single light unit that projects a three-color visual approach path into the final approach area of the runway. This design uses optical filters to produce distinct colors based on the pilot's vertical position relative to the desired glide path: red when below the path, green when on the path, and amber when above the path. Developed as a simpler alternative to multi-light bar systems, it provides basic descent guidance while minimizing installation complexity.^[1] In operation, the single steady light unit changes color through angularly selective filters as the aircraft's approach angle varies, offering pilots an intuitive indication without the need for multiple aligned lights. The transition between colors, particularly from green to red, may appear as a dark amber hue due to overlapping filter effects, requiring pilots to carefully identify the signal to avoid confusion with other airport lights. Visibility for the tri-color VASI extends approximately 0.5 to 1 mile during the day and up to 5 miles at night under clear conditions, making it suitable for general aviation and smaller airfields.^[1] One key advantage of the tri-color VASI is its reduced footprint, as the single-unit setup requires far less runway-adjacent space compared to standard multi-bar configurations, which can span hundreds of feet. This makes it particularly useful for space-constrained runways at general aviation airports or locations with terrain or obstacle limitations. The system's simplicity also lowers installation and maintenance costs, though it provides less precise guidance than systems with multiple light bars.^[1]

Pulsating VASI

The pulsating visual approach slope indicator (PVASI), also known as the pulse light approach slope indicator (PLASI), is a simplified variant of the VASI system designed for basic visual descent guidance. It consists of a single light unit that projects a two-color visual approach path into the final approach area of the runway, utilizing red and white lights to convey the aircraft's position relative to the desired glide slope. Unlike multi-light configurations, this design employs dynamic intensity variations through pulsation to indicate deviations, with the light remaining steady when the aircraft is on the correct path and pulsing at increasing rates as the deviation grows. The system typically emits a steady white or alternating red and white light when the aircraft is on the glide path, a steady red light for positions slightly below the path, a pulsating red light for further below, and a pulsating white light for positions above the path.^[1] The operation of the PVASI relies on the pilot's observation of the light's behavior to adjust the descent angle, with the pulsation rate serving as a key differentiator from steady airport lighting such as runway threshold lights. For instance, the pulsating white signal above the glide path increases in frequency as the aircraft climbs higher, providing an urgent cue for descent, while the pulsating red below the path similarly accelerates to warn of excessive low altitude. This intensity-varying mechanism was developed by the Federal Aviation Administration (FAA) in the mid-20th century to distinguish the guidance signal from other steady-burning lights on or near the runway, reducing potential pilot confusion during approach. The effective visual range of the PVASI is approximately 4 nautical miles during the day and up to 10 nautical miles at night, offering sufficient coverage for most general aviation operations while maintaining simplicity.^[1] One primary advantage of the PVASI is its low installation and maintenance costs, making it suitable for small general aviation airports, remote locations, or temporary runway setups where more complex systems like standard VASIs are impractical. The single-unit construction minimizes infrastructure needs, requiring only basic electrical and mounting support, which aligns with FAA recommendations for cost-effective visual aids in low-traffic environments. Despite its limitations in precision compared to multi-bar systems, the PVASI effectively supports safe obstruction clearance within ±10 degrees of the runway centerline up to 4 nautical miles, prioritizing accessibility over advanced features. Pilots are advised to confirm the PVASI signal to avoid misinterpretation from extraneous lights.^[1]

Dual VASI

The Dual VASI, also known as the three-bar VASI, is a specialized variant of the visual approach slope indicator system designed to provide two distinct glide paths for airports accommodating aircraft of varying sizes, particularly long-bodied jets with elevated pilot eye positions. It consists of three horizontal rows of lights—referred to as the upwind (near), middle, and downwind (far) bars—each containing multiple light units arranged perpendicular to the runway centerline, typically at intervals of approximately 700 feet from the threshold. This configuration allows for a lower glide path using the middle and downwind bars for smaller aircraft, while the upwind and middle bars form an upper glide path tailored for larger aircraft, such as the Boeing 747 or Lockheed C-5A, ensuring compatibility with eye heights ranging from 50 to 100 feet above the runway.^[10] In operation, the Dual VASI employs the standard red-over-white light sequence to indicate the pilot's position relative to the intended glide path: all white lights signal above the path, a mix of red and white indicates on path, and all red signifies below path. For long-bodied aircraft, pilots follow the upper glide path, which intersects the runway threshold at a higher altitude (typically around 75 feet) to account for the greater distance from the cockpit to the main landing gear, thereby aligning the touchdown point appropriately and reducing the risk of "high sink" maneuvers where the aircraft descends too steeply near the runway. This dual-path approach provides stepped guidance, with the system often combining elements of standard two-bar VASIs but extended for precision in diverse operations. The aiming angles are set to achieve nominal 3-degree glide paths, with the upwind bar at approximately 3.25 degrees, the middle at 3.0 degrees, and the downwind at 2.75 degrees, adjustable based on site-specific needs.^[10]^[11] Developed in the 1970s to address the challenges posed by the introduction of wide-body jets, the Dual VASI was evaluated through flight tests at the National Aviation Facilities Experimental Center in 1970, involving 34 pilots and phototheodolite tracking to compare configurations like the three-bar system against alternatives. Its primary advantages include enhanced safety for large aircraft by minimizing undershoot risks and providing clear visual cues for high-cockpit operations, making it suitable for major international airports handling heavy jets. Although no longer installed as new systems by the FAA, existing Dual VASI installations continue to support operations at select locations with long-bodied aircraft traffic.^[11]^[10]

Installation and Siting

Location and Alignment Requirements

The location of a Visual Approach Slope Indicator (VASI) system is determined by the need to provide a precise visual glide path for approaching aircraft, typically positioned along one side of the runway to avoid interference with operations. For standard installations, the system is sited approximately 700 to 1,000 feet from the runway threshold, with the exact distance calculated using the formula D_1 = \text{TCH} \times \cot(\theta), where TCH is the threshold crossing height (often 50 feet for light aircraft) and \theta is the glide path angle of the lowest on-course signal (typically around 3 degrees, yielding about 950 feet).^[7] The lateral offset places the inboard light unit at least 50 feet from the runway edge (reducible to 30 feet for small general aviation runways), ensuring clearance from aircraft movements while maintaining visibility.^[7] Alignment requirements ensure the VASI projects a nominal 3-degree glide path that intersects the runway approximately 50 feet above the threshold at the runway reference point, adjustable to coincide with instrument landing system (ILS) or other precision approach paths within ±30 feet.^[7] The light units must be positioned symmetrically about the runway centerline, with aiming parallel to the centerline within ±0.5 degrees, and spaced 20 to 30 feet apart to form the required bars for glide path indication.^[7] Obstacles are accounted for through clear zone calculations, potentially increasing the glide slope angle or shifting the installation farther from the threshold to maintain safe obstruction clearance.^[7] Terrain factors, particularly sloping runways, necessitate adjustments to the installation to preserve the intended glide path. For runways with a longitudinal gradient, the distance is modified using d = \frac{\text{TCH}}{\tan(\theta)} \pm S, where S accounts for the slope adjustment factor derived from the runway gradient, ensuring the projected path intersects appropriately despite elevation changes.^[7] Graphical or mathematical methods are employed site-specifically to project the intersection point, with aiming angles adjusted accordingly.^[7] Installation requirements vary by airport and runway type, with visual glideslope indicators (such as VASI or PAPI) required for precision instrument runways supporting vertically guided approaches with landing minimums below 250 feet decision height or visibility below 3/4 statute mile, and recommended for Category I, II, and III operations to support low-visibility operations and aligned with the approach glide path angle.^[12] For non-precision instrument runways, it is recommended when visibility minimums are below 1 statute mile, while optional for visual runways serving VFR operations, though encouraged for enhanced safety on runways accommodating larger aircraft.^[12]

Calibration and Testing

The calibration of a Visual Approach Slope Indicator (VASI) system involves precise adjustment of light unit angles to ensure the intended glidepath guidance, typically set at 3 degrees but adjustable for obstacle clearance. Ground personnel use survey equipment such as clinometers, aiming bars, calibration bars, and levels to measure and set vertical aiming angles for each light bar, with the upwind bar aimed higher than the downwind bar to create the desired slope transition. Adjustments to mirrors or lamps are made to achieve tolerances of ±0.1 degrees for commissioning and ±0.2 degrees for ongoing operations, ensuring the beam angles align with published specifications in the Airport/Facility Directory.^[13]^[14] Testing procedures for VASI systems combine ground and flight methods to verify performance. Ground checks employ theodolites positioned near the runway threshold to measure beam angles and confirm horizontal alignment parallel to the runway centerline within ±0.5 degrees, while also assessing light intensity uniformity across units. Flight tests involve aircraft approaches along the runway centerline at various distances, from 1 to 4 nautical miles, to observe color transitions—white above the glidepath, red below, with a pink transition zone of 0.25 to 0.5 degrees—ensuring safe obstruction clearance within ±10 degrees of the centerline and coincidence with any electronic glidepath within 0.2 degrees. These tests confirm that all light boxes transition correctly without deviations exceeding operational tolerances.^[13]^[14] The Federal Aviation Administration (FAA) mandates compliance with Advisory Circular (AC) 150/5340-30J for VASI design and installation standards and AC 150/5340-26C for maintenance, which includes requirements for intensity settings (all lamps operating at equal relative brightness) and color verification to prevent misleading signals. Tools such as digital multimeters, volt-ohm-milliampmeters, and theodolites must be calibrated annually to National Institute of Standards and Technology (NIST) traceable standards for accurate measurements in the 2.8–20 ampere range. Photometric testing for intensity and color is performed before and after adjustments to validate effectiveness.^[13]^[7] Calibration and testing occur initially upon installation during commissioning inspections to establish baseline performance, and subsequently after any repairs or modifications to detect and correct drift in angles or intensity. Periodic flight inspections for VGSI facilities like VASI are conducted in conjunction with inspections of associated navigational aids, with surveillance performed during other runway inspections as required, and special inspections required for changes in glidepath angle or equipment to maintain ongoing accuracy and safety. Records of all angular settings and test results must be logged per FAA guidelines to document compliance.^[13]^[14]

Maintenance and Regulations

Maintenance Procedures

Maintenance of Visual Approach Slope Indicators (VASI) is essential to ensure reliable guidance for aircraft during landing approaches, preventing deviations from the intended glide path.^[15] Procedures outlined in FAA Advisory Circular (AC) 150/5340-26C emphasize routine inspections and timely repairs to maintain operational integrity, with tolerances allowing no more than one lamp out per light box.^[15] Inspection routines for VASI systems follow a scheduled cadence to detect issues early. Daily visual checks involve verifying that all lamps are operational and exhibit equal brightness, with immediate replacement of any burned-out or darkened bulbs to avoid signal degradation.^[15] Monthly inspections require examining the system for physical damage, corrosion, debris such as water or insects, and alignment using a VASI aiming bar, while cleaning lamps and filters to preserve light output; aiming angles must be recorded during these checks.^[15] Quarterly assessments focus on confirming the obstacle clearance surface plane remains unobstructed.^[15] Semi-annual tests measure insulation resistance of cables and grounding system resistance to identify potential electrical weaknesses.^[15] Annual full system tests, per FAA AC 150/5340-26C, include detailed lens cleaning, vertical aiming verification (e.g., downwind bar at ½° below glide path, within ±6 minutes operating tolerance), and horizontal alignment parallel to the runway centerline within ±½°.^[15] Repair protocols prioritize safety and precision to restore functionality without compromising performance. Lamp replacement begins with de-energizing the circuit, checking fuses, and installing compatible spares; traditional halogen lamps are often upgraded to light-emitting diode (LED) equivalents during these procedures to enhance longevity and energy efficiency, as part of the FAA's broader transition from incandescent technologies.^[15]^[16] Optics and lenses are cleaned monthly or as needed to remove contaminants that could distort the light beam.^[15] Vertical aiming adjustments use a VASI aiming bar and small machinist's level or clinometer to ensure compliance with glide path angles.^[15] All repair actions, including dates, times, and technician remarks, must be logged in maintenance records to track compliance and history.^[15] Common issues affecting VASI reliability include weather-related damage such as corrosion, misalignment from soil moisture changes or equipment like mowers and snowplows, and electrical faults like burned wiring or scorched lightning arresters.^[15] Preventive measures mitigate these risks through installation of surge suppressors and lightning arresters for electrical protection, proper grounding systems, and efforts to deter rodent activity that could damage cabling.^[15] Tools essential for VASI maintenance include multimeters for testing voltage, current, and circuit integrity, as well as elevation and alignment tools such as aiming bars, levels, and clinometers to verify beam angles accurately.^[15] These procedures ensure ongoing calibration integration during deeper maintenance, supporting safe airport operations.^[15]

Standards and Guidelines

The Federal Aviation Administration (FAA) establishes standards for Visual Approach Slope Indicator (VASI) systems primarily through Advisory Circular (AC) 150/5340-25, which outlines requirements for design, installation, and maintenance of various VASI configurations, including simplified abbreviated VASI (SAVASI), two-bar VASI, and multi-bar systems.^[17] These standards apply to airports certified under 14 CFR Part 139 that serve scheduled air carrier operations, where VASIs are required on runways supporting turbine-powered aircraft under Parts 121, 135, or certain Part 91 operations to provide vertical guidance aligned with instrument procedures.^[12] The nominal glide path angle specified is 3 degrees, though site-specific adjustments up to 4.5 degrees may be used for obstacle clearance, with typical configurations providing paths around 3 to 3.25 degrees, ensuring safe descent within ±10 degrees of the runway centerline out to 4 nautical miles.^[18]^[1] The International Civil Aviation Organization (ICAO) provides equivalent guidance in Annex 14, Volume I, which specifies VASIS types such as T-VASIS (true visual approach slope indicator system) and abbreviated T-VASIS (AT-VASIS) for instrument runways, harmonizing closely with FAA criteria while permitting regional adaptations. However, as of 1 January 2020, ICAO Annex 14 recommends the discontinuation of T-VASIS and AT-VASIS as standard visual approach slope indicator systems in favor of alternatives like PAPI. ICAO standards mandate a VASIS or equivalent (e.g., PAPI) on precision approach runways to support safe visual descent, with a standard glide path of 3 degrees and provisions for steeper angles up to 3.5 degrees based on aerodrome characteristics.^[19] Compliance is required for runways serving international operations, particularly those with Category I precision approaches, ensuring alignment with global obstacle clearance surfaces. VASI systems must meet mandatory compliance for precision and non-precision approaches at certificated airports, with ongoing FAA updates incorporating LED technology, initiated in the 2010s, to comply with the Energy Independence and Security Act of 2007, reducing energy consumption by up to 75% compared to incandescent systems while maintaining photometric standards (as of 2025).^[16] Unlike the Precision Approach Path Indicator (PAPI), which has become the preferred system due to its compact design and sharper color transitions, VASI installations are being phased out at many U.S. sites, though existing systems remain compliant if they meet AC 150/5340-25 criteria.^[20] Internationally, VASI systems gained ICAO approval for variants like T-VASIS in the 1970s, facilitating global adoption on runways serving turbojet traffic, but usage has declined post-1990s in favor of PAPI due to its ICAO-preferred status for new installations and simpler maintenance.^[6]^[19] Today, VASIs persist at select sites worldwide, particularly in regions with legacy infrastructure, but must adhere to updated Annex 14 provisions for LED integration and energy efficiency.

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