Precision approach path indicator
The Precision Approach Path Indicator (PAPI) is a visual glide slope indicator system installed at airports to provide pilots with precise vertical guidance during the final approach phase of landing, helping to prevent controlled flight into terrain and runway excursions.[1] It consists of a single row of four light units (or two for abbreviated versions) mounted perpendicular to the runway edge, typically on the left side when approaching, near the landing threshold.[2] Each unit projects a beam divided horizontally into an upper white sector and a lower red sector, creating a color pattern that indicates the aircraft's position relative to a nominal 3-degree glide path.[3] The PAPI's light configuration is interpreted sequentially from the pilot's perspective: four white lights signal the aircraft is too high on the approach; three white and one red indicate slightly high; two white and two red denote on the correct glide path; one white and three red mean slightly low; and four red lights warn of being too low.[2] This system ensures safe obstruction clearance within ±10 degrees of the runway centerline and up to 3.4 nautical miles from the threshold, with visibility ranging from approximately 5 miles during daylight to 20 miles at night.[2] Installation requires frangible mounting to minimize hazards in case of aircraft contact, and the lights must operate reliably in extreme conditions, including temperatures from -67°F to 131°F and winds up to 100 mph.[3] Developed in the late 1970s by the United Kingdom's Royal Aircraft Establishment as an improvement over earlier visual aids like the Visual Approach Slope Indicator (VASI), the PAPI was adopted as an international standard by the International Civil Aviation Organization (ICAO) in 1981.[4] The U.S. Federal Aviation Administration (FAA) evaluated and incorporated it through rigorous testing in the early 1980s, establishing detailed specifications in Advisory Circular 150/5345-28, with updates incorporating LED technology for enhanced reliability and efficiency.[5][3] Today, LED-based PAPIs extend lamp life to 40,000 hours from 2,000 hours in traditional incandescent models while reducing energy use, making it the predominant visual descent guidance system at runways worldwide.[1]Overview
Purpose and function
The Precision Approach Path Indicator (PAPI) is an airport lighting system designed to provide pilots with precision vertical guidance during the final approach phase of landing.[3] It consists of a row of lights positioned alongside the runway threshold, offering visual cues to ensure safe alignment with the runway.[1] The primary function of the PAPI is to assist pilots in maintaining a nominal 3° glide slope, which helps prevent runway excursions, undershoots, overshoots, and controlled flight into terrain incidents.[3] By providing immediate feedback on the aircraft's vertical position, it promotes stabilized descents and reduces the risk of hard landings or deviations from the intended touchdown point.[1] In low-visibility conditions, such as fog or reduced daylight, the PAPI serves as a critical supplement to instrument-based approaches, enhancing pilot situational awareness and supporting safer transitions to visual flight.[3] This visual aid is particularly essential for non-precision approaches where electronic guidance may be limited.[3] The PAPI achieves its guidance through color-coded lights that indicate the aircraft's position relative to the ideal glide path, with white lights denoting above-path, red below-path, and an equal mix signaling on-path alignment.[3][1]Basic components
The Precision Approach Path Indicator (PAPI) system consists of four identical light units, designated as Type L-880 by the Federal Aviation Administration (FAA), arranged horizontally perpendicular to the runway edge to provide visual glide slope guidance.[6] Each light unit functions as a self-contained assembly, typically weighing no more than 100 pounds (45 kg) and standing up to 40 inches (1 m) in height when mounted.[6] At the core of each unit are the lamps, which can be either incandescent bulbs rated for a minimum 1,000-hour life or light-emitting diodes (LEDs) for modern installations, arranged to project the required beam pattern.[6] These lamps illuminate through a combination of optical elements, including precision reflectors that direct the light, color filters to achieve the red lower beam (using red-transmissive filters compliant with MIL-C-7989 Class B standards), and clear upper sections for the white beam, along with converging lenses made of heat-resistant glass to focus the output.[3] The dual-beam design within each unit creates a sharp horizontal transition: the beam appears red to the pilot when below the optimal glide path and white when above, enabling intuitive vertical position assessment.[6] The housings enclosing these components are constructed from corrosion-resistant materials, commonly anodized aluminum for durability against environmental exposure, with external surfaces painted in International Orange (Federal Standard 595 Color 12197) for visibility and UV-resistant plastics for any transparent or protective elements.[3][7] For runways with space constraints, an optional Abbreviated PAPI (A-PAPI) variant uses only two light units (Type L-881), maintaining the same core components but scaled for abbreviated guidance.[6]Design and Operation
Light configuration and angles
The Precision Approach Path Indicator (PAPI) employs a standard configuration of four light units arranged horizontally in a single row, oriented perpendicular to the runway centerline and typically installed on the left side of the runway. These units are spaced 9 meters apart, with the nearest unit positioned 15 meters laterally from the runway edge, and the entire array located approximately 300 meters from the runway threshold to align with a threshold crossing height of around 50-60 feet for most aircraft categories. This geometric layout ensures uniform visibility and precise angular coverage for pilots on final approach.[8] To guide aircraft along a nominal 3° glide path, the light units are optically aligned at specific elevation angles from the horizontal: the innermost (nearest to the runway) unit at 3°30', the second at 3°10', the third at 2°50', and the outermost at 2°30'. These settings position the color transition points symmetrically around the 3° path angle, with the on-glide path occurring midway between the second and third units, providing a balanced indication of two red and two white lights when the aircraft is correctly aligned. The angles are adjustable within a range of 2° to 8° during installation to accommodate variations in runway elevation or obstacle clearance requirements.[9][3] Each light unit produces a dual-color beam with a sharp transition from red in the lower sector to white in the upper sector, where the red indicates the aircraft is below the intended path and white indicates above. The transition angle between these colors is limited to ≤3 arc minutes (0.05°) at the beam center and ≤5 arc minutes at the edges, ensuring minimal overlap and high precision in visual guidance; this narrow transition width is critical for maintaining the integrity of the glide path indication over distances up to 3 nautical miles. The beams themselves have a vertical spread of approximately 4° to 10° and a horizontal spread of 20° to 30°, optimized for cockpit visibility without excessive light spill.[3]Signal interpretation
The Precision Approach Path Indicator (PAPI) provides pilots with real-time visual guidance on their vertical position relative to the desired glide path through a row of four lights, where each light displays either red or white based on the aircraft's eye height relative to the light's beam transition point.[2] A configuration of two white and two red lights indicates the aircraft is on the correct 3-degree glide path.[2] Three white and one red lights signify the aircraft is slightly above the path, while four white lights indicate the aircraft is dangerously high and requires immediate correction to avoid excessive descent later.[2] Conversely, one white and three red lights mean the aircraft is slightly below the path, and four red lights indicate the aircraft is too low, with all red also appearing when the aircraft is on the ground or at extremely low altitudes below all beam transitions.[2] Pilots adjust their approach based on these signals to maintain the stabilized glide path: if more white lights are visible (too high), reduce power to increase the descent rate and fly a lower path; if more red lights are visible (too low), increase power to shallow the descent and fly a higher path.[2] These adjustments ensure the aircraft crosses the runway threshold at the appropriate height while adhering to safe obstruction clearance within 10 degrees of the runway centerline and up to 3.4 nautical miles from the threshold.[2] The interpretation of PAPI signals accounts for aircraft-specific factors, particularly the Minimum Eye Height over Threshold (MEHT), which varies by aircraft category to ensure the on-path indication aligns with the pilot's eye position at touchdown.[10] For smaller aircraft in Category A, MEHT is typically 25 feet, while for larger aircraft in Category C or Height Group 4 (such as the Boeing 747 or Concorde, with cockpit-to-wheel heights exceeding 25 feet), it increases to around 50-75 feet, often requiring a "slightly high" indication (three white, one red) for safe threshold crossing.[10] This customization prevents smaller aircraft from flying excessively low paths on runways designed for heavy jets. PAPI systems may be activated by air traffic control (ATC) at towered airports or by pilots at non-towered fields using pilot-controlled lighting (PCL) via keyed microphone clicks on the Common Traffic Advisory Frequency (CTAF), typically seven clicks for low intensity, five for medium, and three for high.[2][6] This remote activation ensures the lights are available when needed, with visibility ranging from 5 miles daytime to 20 miles at night.[2]Installation and Calibration
Site requirements and placement
The Precision Approach Path Indicator (PAPI) system is typically installed on the left side of the runway, approximately 300 meters beyond the landing threshold, to provide optimal visual glide slope guidance aligned with the runway centerline.[11] This placement ensures the lights are visible to approaching pilots within a nominal range of approximately 5 miles during daylight and up to 20 miles at night, while maintaining compatibility with instrument landing systems (ILS) by positioning the lights at the same distance from the threshold as the ILS glide path touchdown point, with a tolerance of ±10 meters.[2][6] For runways with specific constraints, such as obstructions or operational needs, the system may be placed on the right side or configured as a dual installation, though left-side placement remains the standard unless deemed impracticable.[12] Laterally, the light units are positioned a minimum of 15 meters from the runway edge for runway codes 3 and 4, to avoid interference with aircraft operations and ensure frangible mounting for safety.[6] The units are spaced 6 to 10 meters center-to-center, mounted at a low height of about 1 meter above ground level on level terrain, and oriented parallel to the runway to appear horizontal to pilots. Frangible mounting structures are required to break away under impact with minimal hazard to aircraft.[3][13] PAPI is suitable for precision approach runways (Categories I, II, or III) used by turbojet aircraft or in environments requiring enhanced visual guidance, such as over water or featureless terrain; for shorter runways or non-precision operations, alternative systems like VASI may be considered instead.[11] Site selection requires a flat approach zone with no significant terrain variations that could distort the glide path angle, nominally set at 3 degrees (adjustable between 2 and 3.5 degrees based on aircraft type and local conditions).[11] Obstructions within the light signal clearance surface—extending from the light units at 1 degree below the lowest signal angle, laterally ±14 degrees, and up to 8 nautical miles—must be mitigated, particularly within 4 nautical miles of the threshold where full obstacle clearance is mandatory.[12] According to FAA and ICAO standards, a comprehensive site survey is required prior to installation, evaluating obstruction-free zones, alignment with the runway centerline, elevation tolerances, and potential penetrations of approach or transitional surfaces; any obstacles within 3,000 meters must be removed, marked, lighted, or otherwise addressed to comply with aerodrome obstacle limitation requirements.[6][11]Calibration procedures
Calibration of the Precision Approach Path Indicator (PAPI) involves precise alignment of the light units to ensure the visual glide path provides accurate descent guidance for approaching aircraft. The process begins with determining the appropriate glide path angle, typically between 2° and 4.5°, based on the minimum eye height over the threshold (MEHT) for the expected aircraft category, as specified in international standards. MEHT values range from 10 meters to 60 meters depending on the aerodrome reference code and aircraft type, with nominal values often between 12 and 16 meters for compatibility with instrument landing systems. Adjustments are made using data from aircraft manufacturers, such as eye height tables for various models, to set the angles so that the transition from red to white signals occurs at the desired height above the runway threshold.[11] The alignment procedure requires each of the four PAPI light units to be positioned parallel to the runway centerline and aimed vertically using specialized tools, such as digital inclinometers or aiming devices accurate to within ±3 minutes of arc. The inner two units are set to define the on-glide path, with the third unit's angle serving as the reference for MEHT calculation (angle B minus 2 minutes). For different aircraft types, custom adjustments account for variations in pilot eye height, wheel loading, and approach speeds; for example, higher MEHT settings may be used for larger turbojet aircraft to ensure adequate obstacle clearance. Tolerances for the glide path slope are maintained at ±0.1°, and the color transition sector must be within 3 minutes of arc to avoid misleading signals.[3][11] Verification of the calibration is conducted through aircraft flyover checks or ground-based optical methods, such as theodolites, to confirm the transition points where the lights change from all white (above glide path) to all red (below glide path). During flyovers, pilots or survey aircraft approach along the runway extended centerline to observe the signal at specific distances, ensuring the 2 red/2 white configuration aligns with the targeted MEHT. Ground surveys measure beam intensity, spread, and color at multiple points within the isocandela diagram, with intensity not falling below 50% of specified values. These checks must verify that the system provides safe wheel clearance over the threshold, typically 1.5 to 9 meters based on aircraft category.[3][11] Calibration is performed initially during installation and periodically thereafter, with ICAO standards requiring at least annual flight checks or equivalent verifications to maintain accuracy, especially after any maintenance or environmental changes. In-field measurements, including photometric assessments, are recommended twice yearly for embedded lights and once for elevated units, adjusted for factors like airport traffic volume and equipment condition. For runways serving diverse aircraft fleets, recalibration may be needed to accommodate varying operational requirements, ensuring the PAPI remains a reliable visual aid without compromising safety margins.[11]Technical Specifications
Optical and electrical characteristics
The Precision Approach Path Indicator (PAPI) employs specific optical parameters to ensure reliable visual glide path guidance for aircraft. The light intensity for white lights typically ranges from 5,000 to 30,000 candela, while red lights maintain a minimum of 2,500 candela (up to 15,000 candela), with adjustments for day and night operations to prevent dazzling pilots.[14][15] These intensities are measured across elevation angles from -4° to +4°, ensuring consistent visibility within the approach sector.[14] The beam configuration features a transition width of 10 minutes of arc (approximately 0.167°) at the beam center (sharpening to 3 minutes at center and 5 minutes at edges when viewed from 1,000 feet), providing pilots with precise slope indication. The beam center is adjustable between 2° and 8° above the horizontal to align with standard glide paths. Horizontally, the beam extends at least 10° on either side of the centerline, up to ±10° azimuth, forming a fan-shaped pattern for broad coverage.[15][14] Color coordinates adhere to aviation standards, with white lights falling within CIE 1931 chromaticity boundaries of x=0.320–0.440 and y=0.292–0.433, and red lights between x=0.620–0.720 and y=0.290–0.350 (with y ≤0.320 at full intensity).[15][16]| Color | Chromaticity Coordinates (CIE 1931) | Standard Reference |
|---|---|---|
| Aviation White | x: 0.320–0.440, y: 0.292–0.433 | FAA EB 67D[16] |
| Aviation Red | x: 0.620–0.720, y: 0.290–0.350 | SAE AS-25050; FAA EB 67D; ICAO Annex 14[14][16][15] |