Four small light units beside a runway can tell a pilot something extraordinarily useful without transmitting a radio signal or calculating the aircraft’s position: whether the aircraft is above, below or approximately on the visual approach path. The system is called PAPI—Precision Approach Path Indicator—and it works by projecting sharply defined red and white light sectors into the approach. What the pilot sees depends on the viewing angle.[1][2]
The short answer
A standard four-unit PAPI is normally arranged beside the runway. When the aircraft is on the intended visual glide path, the pilot sees two white lights and two red. More white indicates the aircraft is high; more red indicates it is low. Four white means significantly high, while four red means significantly low.[1]
The lights do not change colour electronically
The clever part is that the units project different colours at different vertical angles. Each optical unit creates a white sector above a transition region and a red sector below it. As the aircraft’s viewing angle changes, the apparent colour changes.
The PAPI therefore does not need to track the aircraft. The pilot’s position relative to the fixed light beam determines the indication.
Why there are four units
The four light units are set to slightly different vertical angles. Their combined indications create a stepped display. On the nominal approach path, two units appear white and two red. As the aircraft rises above that path, additional units become white. As it descends below, additional units become red.
This gives more information than a simple “high/low” lamp and allows the pilot to recognise trends visually.
What two white and two red actually means
Two white and two red means the pilot is within the central angular sector defined by the installation. It does not mean the aircraft is guaranteed to touch down at one exact point. Aircraft geometry, pilot eye height, flare, threshold crossing height and local installation all influence the relationship between PAPI indication and actual wheel path.
PAPI is a visual approach aid, not an automatic landing system.
The typical visual glide path
Many PAPI installations are set around a three-degree visual approach slope, but local terrain, obstacles, runway geometry or operational requirements can lead to different settings. Pilots therefore use published information for the specific runway rather than assuming every PAPI represents exactly the same angle.
FAA information states that the PAPI visual glide path typically provides obstacle clearance within a defined area extending to approximately 3.4 nautical miles from the runway threshold, subject to local limitations.[1]
Why PAPI is normally beside the runway
Placing the units to one side keeps them clear of the landing surface while allowing the pilot to view all units as a horizontal bar. The FAA notes that the row is normally installed on the left side, although airport geometry and approved local design can differ.[1]
The system provides vertical guidance only; runway alignment or other visual and instrument references provide lateral guidance.
Why the transition between red and white has to be sharp
If the colour boundary were broad and vague, pilots could not determine their vertical position accurately. PAPI optical systems therefore use lenses, filters and careful alignment to create a controlled transition zone.
FAA AC 150/5345-28 specifies performance standards for PAPI equipment, while installation standards address siting and aiming.[2][3]
PAPI is calibrated, not merely pointed upward
Each unit has a defined aiming angle. Technicians use precision equipment to align the light boxes and verify their angular settings. Small angular errors matter because the indication is based entirely on geometry.
Maintenance therefore includes more than replacing failed lamps; alignment and optical condition are safety-relevant.
Why the units can move out of alignment
Ground settlement, maintenance disturbance, impact, vibration and environmental conditions can alter an installation. Airports inspect and calibrate visual aids to ensure the projected path remains where the published procedure expects it to be.
ICAO aerodrome material similarly treats PAPI calibration as part of the status of visual aids for navigation.[4]
PAPI versus ILS glideslope
An Instrument Landing System glideslope is a radio-navigation signal used by aircraft instruments or autopilot systems. PAPI is an external visual indication seen directly by the pilot. The two systems can be designed to provide compatible approach guidance, but they are not the same technology.
A pilot can therefore see two red and two white while also monitoring an electronic glidepath indication in the cockpit.
Why the two paths may not appear perfectly identical
The PAPI is observed from the pilot’s eye position, while radio-navigation antennas and aircraft receivers use different geometric reference points. Large aircraft have significant vertical distance between the cockpit and main landing gear. Installation design considers these relationships, but a visual indication and an instrument glideslope should not be imagined as one infinitely thin line occupying exactly the same physical path.
Eye-to-wheel height
On a large transport aircraft, the pilot’s eyes may be many metres above and ahead of the main landing gear. If the cockpit follows a given visual path, the wheels pass lower relative to that eye path. Airports therefore consider the aircraft population and threshold-crossing geometry when siting visual approach slope indicators.
This is one reason crews should not treat PAPI as a universal wheel-clearance guarantee for every aircraft outside published operational assumptions.
Why pilots should be aligned with the runway
The PAPI’s obstacle-clearance assessment applies within a defined region around the extended runway centreline. FAA guidance specifically cautions that descent using PAPI should not begin until the aircraft is visually aligned with the runway.[1]
Seeing the lights from far off to one side does not mean the same protected geometry applies.
Why four red lights demand attention
Four red indicates the aircraft is below the intended PAPI path. That does not automatically mean terrain impact is imminent, but it tells the pilot the visual approach is lower than the designed indication. Stabilised-approach criteria and operator procedures determine the required response.
PAPI provides information; pilots and procedures determine what action is appropriate.
Why four white lights also matter
Four white means the aircraft is above the nominal visual path. Being high can create its own operational problems: excessive descent rate, unstable approach, long touchdown or insufficient landing distance if the crew tries to force the aircraft onto the runway.
A go-around can be the correct outcome when a stable approach cannot be achieved within operator criteria.
PAPI and stabilised approaches
Airlines use stabilised-approach policies that typically consider flight path, speed, configuration, descent rate and power. PAPI is one visual source that can help the crew assess path during visual flight, but it is not the only criterion.
A perfect two-red/two-white indication cannot compensate for excessive speed or an otherwise unstable aircraft state.
PAPI at night
At night the red-and-white pattern can be visible at substantial distance because there is less background brightness. Light intensity is controlled so the units remain visible without excessive glare. FAA air traffic procedures include intensity settings for remotely controlled five-step PAPI systems.[5]
Some installations use photoelectric control and operate continuously, while others can be controlled from the tower.
Why brightness changes
A light intensity suitable for bright daytime conditions could be dazzling at night. Conversely, a low night setting may be difficult to see in sunlight. Variable intensity allows the same optical system to remain usable across changing ambient conditions.
Fog and visibility
PAPI is a visual aid, so its usefulness ultimately depends on the pilot being able to see it. Fog, heavy precipitation and low cloud can reduce acquisition range. Instrument procedures and runway lighting provide other layers of guidance when visibility is restricted.
PAPI does not turn a visual approach into an all-weather instrument approach.
Why approach lights and PAPI do different jobs
Approach-light systems help pilots identify runway alignment, centreline and distance cues during the transition from instruments to visual flight. PAPI primarily gives vertical path information. The systems complement one another but should not be confused.
PAPI and displaced thresholds
If a runway has a displaced landing threshold, the PAPI is sited and aimed in relation to the usable landing geometry and obstacle-clearance requirements. Pilots should therefore use the visual aid associated with the published landing threshold rather than assuming the physical beginning of pavement is the touchdown reference.
Why terrain can change an installation
Hills, buildings, roads or other obstacles in the approach area can require a steeper visual path, restricted usable sector or other approved adjustment. FAA information notes that usable PAPI distance or beam width can be reduced by local limitations and that such information is published where applicable.[1]
This is why local aeronautical information takes precedence over generic descriptions.
Why a road near a runway can matter
Visual glidepath siting must consider obstacle clearance not only over buildings and terrain but over vehicles and other transient objects where relevant. Airports assess the approach surface and protected visual-aid area so the published path does not direct aircraft toward unacceptable obstacles.
What happens if PAPI is unserviceable
An unserviceable PAPI is reported through the applicable aeronautical-information and NOTAM system. Whether operations can continue depends on runway facilities, weather, approach type, operator procedures and regulatory requirements.
A PAPI outage does not automatically close every runway, but crews need to know the aid is unavailable before relying on it.
Maintenance access
Because PAPI units sit close to an active runway, maintenance work has to be coordinated with aerodrome operations and air traffic control. Technicians may need controlled access to inspect lamps, lenses, foundations, electrical supplies and alignment.
Airport visual aids are infrastructure systems with maintenance requirements, not passive objects left untouched after installation.
Electrical redundancy and monitoring
Visual-aid electrical systems can include dedicated circuits, regulators and monitoring depending on the airport and operational category. Failure detection allows the airport or control tower to identify when a lighting system is degraded.
The precise architecture varies by installation and national standard.
PAPI versus VASI
Visual Approach Slope Indicator systems, or VASI, predate PAPI at many airports and use a different arrangement of light bars. Both provide red/white visual glidepath information, but their display geometry and equipment differ. PAPI’s compact four-unit display has become extremely widespread.
APAPI
Some aerodromes use an abbreviated PAPI, commonly called APAPI, with fewer units. The basic red/white angular principle remains, but the indication has less granularity than a standard four-unit PAPI.
Pilots use the system installed and published for that runway rather than assuming every visual slope indicator has four lights.
Why pilots still look at the runway itself
PAPI tells the pilot about vertical path, but landing requires a much richer visual picture: runway alignment, aiming point, apparent runway shape, drift, flare height and touchdown-zone position. The lights are one component of the visual scan.
As the aircraft reaches the flare, pilots transition from approach-path control toward the visual cues needed to reduce descent rate and touch down.
Why PAPI is such an elegant system
The system converts three-dimensional geometry into a pattern the human eye can interpret almost instantly. No cockpit pairing is required. No data link is required. Any appropriately positioned pilot who can see the lights receives the indication.
That simplicity is precisely why optical accuracy, siting and maintenance matter so much.
The bigger lesson
PAPI does not “know” where the aircraft is. Each light simply projects red below a precisely aimed boundary and white above it. Four units set to slightly different angles turn that optical geometry into a five-step vertical-position display: four white, three white/one red, two white/two red, one white/three red, or four red.
For pilots, that tiny row of lights provides an immediate cross-check of approach path. For airport engineers, it is a precision optical instrument whose foundations, aiming angles, electrical supply and protected approach geometry all have to remain within defined standards for the indication to mean what pilots expect it to mean.
Sources / Technical References
- [1] FAA, Aeronautical Information Publication, Airport Lighting Aids — https://www.faa.gov/air_traffic/publications/atpubs/aip_html/chap2_section_1.html
- [2] FAA, AC 150/5345-28H, Precision Approach Path Indicator Systems — https://www.faa.gov/airports/resources/advisory_circulars/
- [3] FAA, AC 150/5340-30, Design and Installation Details for Airport Visual Aids — https://www.faa.gov/airports/engineering/design_standards
- [4] ICAO, Aerodrome Design Manual Part 4 — Visual Aids — https://store.icao.int/en/aerodrome-design-manual-part-4-visual-aids-doc-9157-part-4
- [5] FAA, Air Traffic Control, PAPI operation and intensity — https://www.faa.gov/air_traffic/publications/atpubs/atc_html/chap3_section_4.html
- [6] UK CAA, CAP 637 Visual Aids Handbook — https://www.caa.co.uk/data-and-publications/publications/documents/content/cap-637/
Disclaimer: Cockpit King provides general aviation education and reference information. PAPI angles, protected areas, serviceability, approach procedures and visual guidance are runway-specific and must be verified using current official aeronautical publications and approved operator procedures. This article is not flight instruction.