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What is ALS in Aviation? (Approach Lighting System)

Updated: February 17, 2024

The Importance of Approach Lighting System (ALS)

Aviation is a complex and highly regulated industry, with safety being the top priority. One crucial component of ensuring safe landings is the Approach Lighting System (ALS). The ALS is a series of lights that provide visual guidance to pilots during the final stages of landing. It plays a vital role in enhancing visibility and helping pilots align the aircraft with the runway. This article explores the significance of ALS in aviation and explains its various components and functionalities.

The Components of Approach Lighting System

The Approach Lighting System consists of several components that work in harmony to assist pilots in making precise landings. These components include:

1. Precision Approach Path Indicator (PAPI)

The Precision Approach Path Indicator, commonly known as PAPI, is a visual aid that helps pilots maintain the correct glide slope during the landing approach. It consists of a row of four lights, usually arranged in a two-by-two formation, located on the left side of the runway. The lights emit a combination of red and white colors, indicating whether the aircraft is too high, too low, or on the correct glide path.

The PAPI system is highly beneficial for pilots, as it allows them to make adjustments to their descent angle in real-time. By ensuring they are following the correct glide slope, pilots can avoid the risk of landing too short or too far down the runway, reducing the chance of accidents or incidents.

2. Runway End Identifier Lights (REIL)

The Runway End Identifier Lights (REIL) are a pair of synchronized flashing lights located at the end of the runway. These lights help pilots identify the runway during low visibility conditions or at night. The REIL system consists of two sets of lights, one on each side of the runway, and they flash in a synchronized manner to provide clear runway identification.

REIL is particularly useful during instrument approaches when pilots heavily rely on the guidance provided by instruments. By illuminating the runway threshold, REIL assists pilots in identifying the correct runway and aligning the aircraft accordingly.

3. Touchdown Zone Lights (TDZL)

The Touchdown Zone Lights (TDZL) are a series of lights that are installed to provide visual guidance to pilots during the landing approach. These lights are typically installed in two rows along the centerline of the runway and extend a certain distance from the threshold. TDZL helps pilots identify the touchdown zone, which is the portion of the runway where the aircraft should ideally touch down.

The TDZL system uses high-intensity lights that are visible even in adverse weather conditions. By providing a clear visual reference of the touchdown zone, pilots can make precise landings and avoid landing too early or too late on the runway.

The Functionality of Approach Lighting System

The Approach Lighting System (ALS) serves multiple functions that contribute to the safety and efficiency of landings. These functions include:

1. Enhancing Visibility

One of the primary functions of the Approach Lighting System is to enhance visibility for pilots during the landing approach. By providing a well-lit visual path from the final approach fix to the runway threshold, ALS helps pilots maintain situational awareness and make accurate judgments about their position and altitude.

The ALS's bright lights make it easier for pilots to identify the runway and align the aircraft with precision. This is especially crucial during low visibility conditions, such as fog or heavy rain, where visual cues may be limited. The enhanced visibility offered by ALS significantly reduces the risk of runway incursions or misalignments, ensuring a safe landing.

2. Facilitating Safe Landings

The Approach Lighting System plays a key role in facilitating safe landings by providing pilots with clear visual references and aiding in proper aircraft alignment. The precision and accuracy offered by ALS enable pilots to make smooth and controlled descents, reducing the chances of hard landings or runway excursions.

With the help of PAPI, pilots can maintain the correct glide slope and ensure a stabilized approach. The REIL system assists in runway identification, especially during low visibility conditions, while the TDZL system guides pilots to touchdown at the appropriate zone. Together, these components of ALS provide pilots with the necessary information to execute safe landings.

3. Adhering to Regulatory Standards

The aviation industry operates under strict regulatory standards to ensure the highest level of safety. The installation and maintenance of Approach Lighting Systems are regulated by aviation authorities to ensure compliance with these standards.

For example, the Federal Aviation Administration (FAA) in the United States outlines specific requirements for ALS installations, including the type and positioning of lights, their intensity, and the minimum visibility range they should provide. These standards are designed to ensure consistency and uniformity across different airports, enabling pilots to rely on consistent visual cues regardless of their location.

Adhering to regulatory standards for ALS installations is essential for airports and aviation authorities to maintain operational safety. It also helps pilots familiarize themselves with the consistent layout and functionality of ALS systems worldwide, contributing to a safer and more efficient global aviation network.


The Approach Lighting System (ALS) is an integral part of aviation infrastructure that greatly enhances the safety and efficiency of landings. Its components, such as the Precision Approach Path Indicator (PAPI), Runway End Identifier Lights (REIL), and Touchdown Zone Lights (TDZL), work together to provide pilots with visual guidance during the final stages of landing.

By enhancing visibility, facilitating safe landings, and adhering to regulatory standards, the ALS ensures that pilots can make precise landings in various weather conditions and low visibility situations. The ALS is a testament to the aviation industry's commitment to safety and the continuous improvement of landing procedures.

For more information on Approach Lighting System and its components, you can visit the Federal Aviation Administration's website.

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