As an airplane descends for landing, passengers are often treated to a mesmerizing view: a perfectly ordered pattern of lights emerging from the darkness. This display is far from decorative. It is a precise visual language, meticulously designed to guide a multi-ton aircraft safely to the ground. Each light, with its specific color, intensity, and position, conveys critical information to the pilot.

This essential infrastructure is a major component of an airport’s overall airfield ground lighting systems. For pilots, these lights are the primary visual reference during the most demanding phases of flight, especially at night or in poor weather. Understanding this "language of light" reveals the incredible precision that underpins modern aviation safety.

The creation and implementation of these systems are a core responsibility of airport engineering. This specialized field ensures that every runway is equipped with a standardized, fault-tolerant lighting configuration that meets strict international regulations. This uniformity allows pilots to fly into any major airport in the world and instantly understand the visual cues presented to them.

From guiding the aircraft’s final approach to marking the very edge of the pavement, different types of runway lighting work together to create a safe operational environment. Let’s break down the key systems that illuminate the path for every takeoff and landing.

Approach Lighting System (ALS)

Before a plane even reaches the runway, the pilot must transition from flying on instruments to flying visually. The Approach Lighting System (ALS) facilitates this critical phase. It consists of a series of high-intensity light bars and strobes extending outward from the start of the runway, sometimes for thousands of feet.

• Function: The ALS acts as a visual funnel, helping the pilot align the aircraft with the runway centerline long before they are over the pavement. It provides roll guidance (ensuring the wings are level) and helps the pilot judge their height and distance from the landing threshold.
• Components: Different configurations exist, but most include steady-burning white lights that form a distinct pattern and sequenced flashing lights (often called "the rabbit") that appear to "run" toward the runway, drawing the pilot's eye to the landing area.

Visual Glideslope Indicators

Proper vertical alignment is just as important as horizontal alignment. A pilot must descend at a precise angle to touch down in the designated zone. Visual glideslope indicators are ground-based systems that give pilots an simple, intuitive reference for their glide path.

Precision Approach Path Indicator (PAPI)

This is the modern standard at most airports. It consists of a single row of four light units placed to the side of the runway. Each unit projects a beam of light that is white in its upper portion and red in its lower portion.

• How it works: If the pilot is on the correct glideslope, they will see two red lights and two white lights. If they are too high, they will see more white lights (three or four). If they are too low, they will see more red lights. The mnemonic is simple: "Four white, you're high as a kite. Four red, you're dead."

Visual Approach Slope Indicator (VASI)

An older system, VASI works on a similar principle but typically uses two bars of lights, a near bar and a far bar. If both bars appear red, the pilot is too low. If both appear white, they are too high. One red and one white indicates the correct path. While being phased out in favor of PAPI, VASI systems are still in use at many smaller airfields.

Runway Threshold Lighting

The beginning of the usable landing surface is marked by a distinct set of lights to provide an unmistakable visual cue.

• Runway End Identifier Lights (REIL): This system consists of a pair of high-intensity, synchronized flashing lights (strobes) located on either side of the runway threshold. They are especially useful in identifying a runway surrounded by other lights, such as in an urban environment.
• Threshold Lights: A row of steady green lights spanning the width of the runway marks the very beginning of the safe landing area. When viewed from the landing direction, they signal "go" or "safe to land here."

Runway Edge Lights (HIRL, MIRL, LIRL)

These are the most familiar lights to passengers, as they outline the entire length of the runway. They consist of a line of elevated white lights spaced approximately every 200 feet along both sides of the pavement.

• Intensity Levels: The system is classified by its intensity: High-Intensity Runway Lights (HIRL), Medium-Intensity Runway Lights (MIRL), or Low-Intensity Runway Lights (LIRL). Airports with precision instrument approach capabilities use HIRL, which can be turned up to maximum brightness to cut through dense fog.
• Color Coding: While primarily white, the last 2,000 feet of the runway edge lights change to yellow. This provides a visual warning to pilots that the end of the runway is approaching, which is critical during takeoff runs or long landing rolls.

Runway Centerline and Touchdown Zone Lights

For precision operations, particularly in low visibility, lights embedded directly into the pavement are essential. These fixtures are built to be run over by aircraft and provide crucial guidance when edge lights are not visible.

• Runway Centerline Lighting System (RCLS): A single line of white lights is spaced every 50 feet along the runway's centerline. They provide powerful directional control, helping pilots stay perfectly straight during takeoff and landing. As the pilot approaches the end of the runway, these lights alternate between red and white, and the final 1,000 feet are solid red.
• Touchdown Zone Lights (TDZL): This system consists of two rows of white light bars arranged symmetrically about the runway centerline. They begin 100 feet from the threshold and extend for 3,000 feet. They create a "landing carpet," clearly marking the ideal zone for the aircraft's wheels to make contact with the pavement.

Runway End and Stopway Lights

Just as the beginning of the runway is clearly marked, so is the end. A line of red lights spanning the width of the runway faces the aircraft, indicating the end of the usable surface. When viewed from the opposite direction (facing an approaching aircraft), these same fixtures appear as the green threshold lights. This clever bi-directional coloring ensures the correct signal is always displayed based on the direction of travel.

Conclusion

The various types of runway lighting systems are more than just illumination; they are a critical safety system. Each component, from the approach lights miles away to the final red end lights on the pavement, is designed to provide clear, unambiguous information. By working together, they form a robust visual network that allows for safe and efficient airport operations around the clock and in nearly all weather conditions. The next time you see these lights, you can appreciate the complex engineering that keeps the world of aviation moving safely.