What is ALS in Aviation? (Approach Lighting System)

Approach lighting system (ALS) is an integral part of the infrastructure at airports that ensures the safe landing of aircraft. It provides pilots with visual cues and assists them in aligning their aircraft with the runway during approach and landing. ALS is a critical component of aviation operations, enhancing the safety and efficiency of aircraft movements.

The ALS consists of a series of lights that are strategically placed along the approach path to guide pilots and provide valuable information about the runway environment. These lights are designed to be highly visible, even in adverse weather conditions, and help pilots maintain the correct glide path and runway alignment.

The ALS is activated when an aircraft is approximately five nautical miles from the runway threshold. As the aircraft approaches the runway, the lights become increasingly bright and clearly visible to the pilot. The ALS serves as a visual reference, allowing pilots to judge their altitude, distance to the runway, and vertical glide path.

The Components of the Approach Lighting System

The ALS is composed of various lighting elements that work together to provide pilots with essential visual cues. These components include:

1. Runway Alignment Indicator Lights (RAILs)

The Runway Alignment Indicator Lights (RAILs) are the first set of lights that pilots encounter during the approach. These lights are typically located at the beginning of the approach path and provide a visual indication of the runway’s alignment. The RAILs are white lights that extend horizontally across the runway, helping pilots establish an accurate approach.

When the pilot sees the RAILs perfectly aligned, it indicates that the aircraft is on the correct approach path and helps the pilot maintain the desired alignment throughout the landing process.

2. Touchdown Zone Lights (TDZLs)

The Touchdown Zone Lights (TDZLs) are a set of lights that are placed in the vicinity of the runway threshold. These lights are white and extend in a row parallel to the runway centerline. The TDZLs provide pilots with a visual reference to determine the precise touchdown point on the runway.

The TDZLs are especially useful during nighttime and low-visibility conditions, as they allow pilots to accurately align their aircraft with the runway and ensure a safe touchdown.

3. Approach Lighting Structure (ALSf)

The Approach Lighting Structure (ALSf) is a key component of the ALS, located near the runway threshold. It consists of a series of lights arranged in a specific configuration to guide pilots during the final stages of approach and landing.

The ALSf may employ various light configurations, such as the Simple Approach Lighting System (SALS) or the Medium Approach Lighting System (MALS). These configurations include a combination of white, red, and sometimes green lights.

The ALSf provides pilots with vital information about their position relative to the runway, aiding in accurate alignment and allowing them to gauge the proper touchdown point.

The Importance of Approach Lighting System

The approach lighting system plays a crucial role in aviation due to its numerous benefits. Let’s explore the importance of the ALS:

Enhanced Visibility

One of the primary advantages of the approach lighting system is its ability to enhance visibility for pilots during approach and landing. The bright and distinct lights guide pilots, even in low-visibility conditions such as fog, rain, or darkness.

The ALS ensures that pilots have a clear visual reference of the runway and its alignment, allowing them to make precise adjustments to their aircraft’s position and glide path. This enhanced visibility greatly improves the safety of aircraft operations and reduces the risk of incidents or accidents during landing.

Precise Approach and Landing

The ALS provides pilots with essential visual cues that enable them to conduct a precise approach and landing. By following the lights’ guidance, pilots can align their aircraft with the correct glide path and ensure a smooth touchdown at the intended touchdown zone on the runway.

The accuracy offered by the ALS is particularly crucial in low-visibility conditions, as it helps pilots maintain awareness of their position relative to the runway and make necessary corrections. This precision enhances operational efficiency and reduces the likelihood of runway incursions or deviations from the desired approach path.

Improved Safety and Confidence

With the ALS in place, pilots can approach and land with a higher level of safety and confidence. The visual cues provided by the lights allow pilots to make informed decisions and maintain better situational awareness, mitigating the risk of accidents or runway excursions.

Moreover, the presence of a reliable ALS reduces the cognitive workload on pilots, as they can rely on the system for accurate guidance. This, in turn, enhances the overall safety of the approach and landing process, benefiting both the flight crew and passengers.


The approach lighting system (ALS), with its strategically placed lights along the approach path, plays a pivotal role in aviation. It ensures the safe landing of aircraft by providing pilots with visual cues and assisting them in aligning their aircraft with the runway. The ALS enhances visibility, enables precise approach and landing, and improves safety and confidence for pilots. Its importance in aviation cannot be understated, as it contributes significantly to the overall safety and efficiency of aircraft operations.

For More: What is WAT in Aviation? (“Weight)