# Aerodrome Visual Aids for Navigation: PAPI, ALS, and Runway Lighting

> Understand visual navigation aids at aerodromes, including PAPI indications, approach lighting systems, and runway centreline light color transitions.

- **Subject:** 010 — Air Law
- **Section:** 010-09 — Aerodromes
- **Topic code:** 010-09-04
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/air-law/aerodromes/visual-aids-for-navigation/

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## Approach Slope Indicators (PAPI/APAPI)
The **PAPI** (Precision Approach Path Indicator) consists of 4 units (lights) in a wing bar, usually on the left side of the runway.

![Comparison of visual landing systems (PAPI)](https://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Comparison_visual_landing_systems.svg/640px-Comparison_visual_landing_systems.svg.png)
*Diagram showing PAPI indications (right) for correct, high, and low glide path.*

**PAPI Indications:**
*   **On slope:** 2 Red (near runway) / 2 White (far runway).
*   **Slightly high:** 1 Red / 3 White.
*   **Too high:** 4 White.
*   **Slightly low:** 3 Red / 1 White.
*   **Too low:** 4 Red.

> Mnemonic: "Red on White, you're alright" (though in PAPI they are side by side, the mix logic serves).

## Approach Lighting Systems (ALS)
![Runway Diagram](https://upload.wikimedia.org/wikipedia/commons/thumb/e/e9/Runway_diagram.svg/640px-Runway_diagram.svg.png)
*Typical runway elements: Threshold, Touchdown Zone, and Centerline.*

*   **Simple Approach Lighting System:**
    *   Row of lights on the extended centerline (minimum 420 m).
    *   Crossbar at 300 m from the threshold.
    *   Mandatory for **non-precision** approach runways (where physically practicable).
*   **Precision Approach Cat I:**
    *   Extended centerline up to 900 m.
    *   Crossbar at 300 m (30 m long).
*   **Cat II and III:**
    *   Require a more complex system with side rows and multiple crossbars (150 m and 300 m).

## Runway and Taxiway Lights
*   **Runway Centre Line:**
    *   Variable **White** from threshold to 900 m from the end.
    *   Alternate **Red/White** from 900 m to 300 m from the end.
    *   **Red** for the last 300 m.
*   **Taxiway Edge:** Fixed **Blue** lights.
![No Entry Sign (Mandatory) and Location Sign (Information)](https://upload.wikimedia.org/wikipedia/commons/thumb/0/05/Airport_no_entry_sign.svg/200px-Airport_no_entry_sign.svg.png) ![Location Sign](https://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Airport_location_sign.svg/200px-Airport_location_sign.svg.png)
*Left: Mandatory instruction sign (Red background). Right: Location sign (Black background/Yellow letter).*

*   **Taxiway Centre Line:** Generally green (specific coding in ILS sensitive areas).

## Signs and Markings
*   **Mandatory Instruction Signs:**
    *   **Red background with White inscription.**
    *   Include: NO ENTRY, Runway Designation, Holding Positions (Cat I/II/III).
    *   Identify locations that **must not be crossed without ATC clearance**.
*   **Road-holding position:** Stop sign for vehicles, white on red background.
*   **Runway Designation Marking:** Located at the threshold.

## Markers
*   **Boundary Markers:** Cones (usually orange or red/white) used to delimit the landing area when **there is no paved runway**.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.