PAPI is a visual landing aid for pilots that shows whether an aircraft is above, below, or on the correct glide path using a row of red and white lights.
The Precision Approach Path Indicator (PAPI) is one of aviation’s most important visual landing aids, helping pilots maintain a safe, stabilized approach to the runway during the critical phase of landing. Found at airports large and small, as well as at heliports and military fields, the PAPI is a globally standardized system that provides unambiguous, easily-interpreted visual guidance on an aircraft’s glide slope. This comprehensive guide explores every facet of PAPI: from its design and operation to its impact on flight safety, recent technological developments, and best practices for installation and maintenance.
A PAPI is a row of bright lights, typically four, installed perpendicular to the runway centerline, about 300 meters (980 feet) beyond the runway threshold and 15 meters (49 feet) from the edge. Each unit projects a beam with a sharp transition between red (lower) and white (upper) light. When a pilot is on the correct approach path, two lights appear red and two appear white. Deviations above or below the ideal glide slope are instantly shown with a change in the color pattern.
Key Features:
When approaching the runway, pilots observe the PAPI’s four lights:
The transition between red and white is sharp and unmistakable, thanks to precision optics and calibration. The system’s design ensures that, from far out on final approach up to the runway threshold, the pilot’s perspective on the color pattern changes predictably with vertical position.
Mnemonic:
Red on white, you’re all right; red on red, you’re dead; white on white, check your height.
Each PAPI light unit is a rugged, weatherproof box housing a high-intensity lamp or LED array, precision lenses, and a red filter. Each unit is set at a slightly different vertical angle, corresponding to a specific approach slope. Modern systems use:
Mounting Angles for a 3° Glide Slope:
This stagger creates the five distinct PAPI indications.
Proper PAPI installation is governed by ICAO Annex 14 and FAA standards. The lights are usually placed on the left side of the runway (right side if necessary for site reasons), at a standard offset and distance from the threshold. The units are spaced 9 meters (30 feet) apart, perpendicular to the runway.
Critical factors:
The service volume is the area where PAPI guidance is valid and obstacle clearance is ensured:
Pilots should use PAPI only within this area to guarantee safe obstacle clearance.
| Lights Displayed | Glide Path Position |
|---|---|
| 4 White | Well above slope |
| 3 White, 1 Red | Slightly above slope |
| 2 White, 2 Red | On correct slope |
| 1 White, 3 Red | Slightly below slope |
| 4 Red | Well below slope |
The system’s sharp transitions support quick, reliable corrections.
| Feature | PAPI (4-Unit) | VASI (2/3-Bar) | Tri-Color VASI |
|---|---|---|---|
| Indications | 5 | 3 | Continuous |
| Precision | High | Moderate | Low |
| Usage | Global Std. | Less common | Rare |
PAPI provides finer resolution, easier interpretation, and is now the gold standard worldwide.
Calibration must be performed regularly using precision theodolites or laser instruments to maintain the correct transition angles.
Maintenance Activities:
LED PAPI systems reduce maintenance with their long life and resilience.
PAPIs are built to withstand:
Some have heated lenses for snow/frost prevention. Pilots are trained to cross-check PAPI with other approach aids, especially under poor visibility.
Standard PAPI calibration assumes a typical pilot eye height at threshold. Large jets (e.g., B747, A380) or small aircraft may perceive the lights differently. Correction tables are available to adjust for cockpit height variations.
Regular maintenance and redundant approach aids are vital.
PAPIs have been shown to significantly reduce Controlled Flight Into Terrain (CFIT) and unstable approach accidents. Their widespread use is a cornerstone of global aviation safety, supporting both professional and general aviation operations.
The Precision Approach Path Indicator is a simple, robust, and universally recognized visual aid that has transformed approach safety for generations of pilots. Its sharp, unambiguous guidance is trusted by aviation professionals worldwide—whether flying a jumbo jet into a major hub, a turboprop into a regional field, or a helicopter to a hospital pad.
For airport operators, investing in modern PAPI technology—especially LED or solar-powered systems—delivers tangible improvements in safety, reliability, and operational efficiency.
Looking to upgrade or install a PAPI system? Contact us or schedule a demo to see how we can help you achieve world-class runway safety.
This guide is maintained by aviation safety and airfield lighting experts. For questions or technical consultations, please reach out via our contact page.
PAPI provides clear visual cues using red and white lights, showing pilots if they're above, below, or on the correct glide path. This helps maintain a safe descent profile, avoiding obstacles and ensuring a stabilized approach to the runway.
While both are visual approach slope indicators, PAPI uses four light units for more precise five-stage guidance, while VASI uses two or three bars and offers three-stage (above, on, or below slope) indications. PAPI is now the global standard due to its accuracy and clarity.
PAPI systems must be regularly calibrated to maintain accuracy, typically at intervals prescribed by ICAO or FAA. Maintenance includes lamp or LED checks, cleaning, alignment verification, and prompt repair of any faults.
PAPIs are designed to be robust against weather, but visibility can be reduced in fog, heavy rain, or snow. Heated lenses and weather-resistant construction help maintain performance, but pilots must always cross-check with instruments in adverse conditions.
Contact us to learn how advanced PAPI systems can improve runway safety and efficiency. Explore our solutions for modern airfields, from LED upgrades to solar-powered systems.
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