Approach Lighting System (ALS)
An Approach Lighting System (ALS) is a landing aid consisting of a series of navigation and signal lights that help pilots approach and align with a runway , especially in poor visibility. This system is essential for safe landings in commercial and military aviation.
1. Function of the ALS
✅ Provides visual guidance → Helps pilots approach the runway correctly .
✅ Increases visibility → Essential in fog, rain or night landings.
✅ Works with other landing systems → Supports Instrument Landing Systems (ILS) .
✅ Can have different lengths → 300 to 900 meters for precision instrument landings.
2. Components of an Approach Lighting System
An ALS consists of several types of lights and markings :
✈ 1. Approach Light Bar System
- Regular rows of white lights → Placed in front of the runway, at fixed distances .
- Provides depth perception and alignment → Helps the pilot correctly estimate descent .
✈ 2. Sequenced Flashing Lights (SFL) - 'Rabbit'
- Bright flashing lights → Indicate the landing direction .
- Placement → Starts well before the runway and guides the aircraft to the beginning of the runway.
✈ 3. Precision Approach Path Indicator (PAPI)
-
Four lights next to the runway → Indicate the correct descent angle :
- Two red, two white → Correct approach.
- More red than white → Too low.
- More white than red → Too high.
✈ 4. Runway Alignment Indicator Lights (RAIL)
- Additional flashing lights → Indicate the exact alignment with the track .
✈ 5. Runway End Identifier Lights (REIL)
- White flashing lights on both sides of the runway → Indicate the beginning of the touchdown zone .
3. Types of ALS systems
| Type ALS | Usage | Features |
|---|---|---|
| ALSF-1 (Approach Lighting System with Sequenced Flashing Lights-1) | Precision landings | White strip lights + flashing lights |
| ALS-2 / ALSF-2 | Low visibility landings | Additional light structure and flashing sequences |
| MALSR (Medium Intensity Approach Lighting System with RAIL) | Medium visibility | Cheaper and less intense lighting system |
| ODALS (Omnidirectional ALS) | Small airports | Shorter system with 5-7 lights |
4. Operation of ALS in combination with other landing aids
🚦 ALS + ILS (Instrument Landing System) → Radio- and light-based guidance in poor visibility.
🔴 ALS + PAPI (Precision Approach Path Indicator) → Ensures correct descent angle during the day and night.
🌫 ALS + RVR (Runway Visual Range Sensor) → Measures the visibility of landing lights for pilots.
💡 In short:
The Approach Lighting System (ALS) is a critical visual aid for pilots during nighttime or low-visibility landings . It combines white and red landing lights, flashing lights, and alignment indicators to ensure safety and precision .