Learn how runway approach lighting systems guide pilots, what the colours and flashers mean, and how ALS differs from PAPI, runway lights and ILS.
In real-world aviation, a runway approach lighting system is a fixed pattern of bright lights extending outward from the threshold. It gives pilots rapid visual cues for runway alignment, roll attitude, distance and closure rate, helping them transition from instrument guidance to a safe visual landing in darkness or reduced visibility.
What does an approach lighting system show the pilot?
The fixed geometry of an approach lighting system, or ALS, creates a recognisable perspective leading directly to the runway threshold. When the aircraft is correctly aligned, the centreline appears straight and the pattern is balanced; lateral displacement, bank or a rapidly expanding sight picture becomes easier to detect.
- Centreline lights or barrettes extend the runway centreline into the approach area and reveal left-or-right displacement.
- Crossbars provide a horizontal roll reference and known distance cues within the pattern.
- Sequenced flashers fire from the outer end towards the runway, drawing the pilot's eye to the threshold. Pilots often call this moving-light effect the rabbit.
- Red side-row or terminating lights appear in the inner portion of certain precision systems and provide an additional close-in visual reference.
The array does not track the aircraft or calculate its position. Alignment and closure cues come from perspective and optical flow as the fixed pattern grows in the windscreen. It does not provide obstacle clearance or a reliable vertical glide path.
What do the colours and sequenced flashers mean?
Most lights in an approach array appear white, while some precision layouts add red elements near the threshold. The apparent movement of sequenced flashers always leads towards the runway; it is an attention and alignment cue, not a command to descend.
The green threshold lights, white runway-edge lights and red runway-end lights are normally classified as runway lighting rather than the approach array. Two synchronised flashing lights beside the threshold are runway end identifier lights, or REILs, and do not by themselves constitute a full approach lighting system.
What are the main types of runway approach lighting?
Approach-light systems fall into precision, medium-intensity and simplified families, but their names and exact layouts vary between national standards.
| System family | Typical features | Operational role |
|---|---|---|
| High-intensity precision | Long, dense centreline and crossbar array, often with sequenced flashers and red close-in elements | Supports precision approaches and the lowest authorised visibility minima when the complete installation and runway are approved |
| Medium-intensity | Shorter or less dense array; variants such as MALSR add runway-alignment flashers | Common with Category I ILS and other instrument approaches |
| Simplified or alignment system | Fewer steady lights, short barrettes, flashers or an omnidirectional arrangement | Helps pilots acquire and align with the runway but usually provides less operational credit |
Examples include the ALSF, HIALS or Calvert, MALS/MALSR, SSALS/SSALR, SALS and ODALS families. An acronym alone does not determine the landing minimum: the published procedure, system serviceability, aircraft approval and governing regulations do.
Who controls the approach lights?
At a towered aerodrome, air traffic control normally selects the lighting and intensity level, and pilots can request an adjustment. Some non-towered aerodromes have pilot-controlled lighting activated through radio transmissions; others use local controls, timers or light sensors.
Steady lamps are arranged in electrical circuits with selectable brightness, while a controller fires the sequenced strobes or LED units in order. Flashers may be controlled separately, so an array can remain partly illuminated when one component is unserviceable.
How is ALS different from PAPI, runway lights and ILS?
ALS primarily provides runway acquisition and horizontal alignment before the threshold; the other systems answer different questions.
| System | Location | Primary information |
|---|---|---|
| Approach lighting system | Extends outward from the threshold | Runway direction, alignment, roll reference, distance and visual transition |
| PAPI or VASI | Beside the runway, usually near the touchdown area | Vertical approach path through colour combinations; our explanation of red-and-white PAPI glide-path indications covers how to read them |
| Runway lighting | On or immediately beside the landing surface | Threshold, edges, centreline, touchdown zone and runway end |
| ILS | Radio transmitters serving the runway | Electronic lateral and vertical guidance before visual references are acquired; see how localizer and glideslope navigation guidance works |
A mistake we see constantly is treating the approach lights as a glideslope display. If the runway appears centred but every PAPI light is red, the aircraft can still be dangerously low; do not descend simply because the ALS is visible.
Can a pilot land using only the approach lights?
Seeing an approach-light array may permit limited descent below an instrument approach decision altitude or minimum descent altitude, but it is not blanket authorisation to land.
The required visual references depend on the jurisdiction and procedure. Under US operating rules, for example, approach lights alone generally permit descent only to 100 feet above touchdown-zone elevation; below that, specified additional references such as red terminating bars or red side-row lights must be distinctly visible. Other authorities phrase and apply the rule differently, so pilots must use the applicable regulations and approach documentation.
If the required visual references are not acquired at the decision point, or are lost before landing, the correct response is normally a missed approach. An ALS outage does not automatically close the runway, but it can raise the permitted minima, remove operational credit or make a low-visibility approach unavailable. Crews must check aerodrome notices and apply the relevant inoperative-equipment tables.
Why are approach lights hard to see in a flight simulator?
In a flight simulator, missing or incorrect approach lights usually result from airport scenery, lighting logic, viewing angle or rendering settings rather than the aircraft's navigation equipment.
- Verify the runway installation. Check that the selected runway end actually has an ALS. Reciprocal ends can use completely different systems, and a displaced threshold can make the geometry look unexpected.
- Separate ALS from PAPI and REIL. A PAPI sits beside the runway, while REILs are a pair of threshold flashers. Neither should be mistaken for the centreline approach array.
- Test the viewing conditions. Approach lights use directional optics and can look weak from well above, far off-centre or behind the fixtures. Test on a normal final approach with suitable ambient light and the airport lighting activated.
- Isolate scenery conflicts. Duplicate airport packages can place two layouts together or suppress the stock lights. Understanding how scenery add-ons override simulator assets helps identify which package is controlling the airport.
- Check visual settings. Object distance, lighting quality, exposure, bloom and terrain can hide or wash out fixtures. Setting names vary by simulator, so change one lighting-related option at a time and retest the same approach.
Tuning an ILS will not repair missing approach lights because visual scenery and radio navigation data are separate systems. Once the array is displaying correctly, our guide to using runway cues during a visual approach explains how to combine it with the aiming point, runway perspective and glide-path indicators.