How a runway approach lighting system works: HIALS, barrettes, crossbars, flashers, red side-row lights, landing rules and simulator fixes.
An approach lighting system (ALS) is a fixed array of lights extending outward from a runway threshold. It helps a pilot find the runway, judge alignment, roll and closing distance, then transition from instrument guidance to the visual references needed for landing at night or in poor visibility; it does not provide a glideslope.
What does ALS mean in aviation, and what does the pattern show?
In Aviation & Real-World Flying, ALS means approach lighting system: the organised pattern commonly called runway approach lights. The fixtures remain fixed; they do not track the aircraft or calculate its position.
Instead, the pilot interprets the array's geometry and optical flow. A centred, balanced pattern helps reveal runway alignment, while its increasing apparent width and spacing provide closing-distance cues. Because each light station occupies a known place, the pattern also helps the pilot recognise the threshold through darkness, rain, haze or low cloud.
- Centreline lights extend the runway centreline into the approach area, making lateral displacement and drift easier to detect.
- Barrettes are compact transverse rows of closely spaced lights that appear as short bars from the air.
- Crossbars run across the extended centreline, providing roll, perspective and known-distance references.
- Sequenced flashers fire in order towards the runway. Pilots often call the resulting moving-light effect the rabbit.
- Red close-in lights appear in certain precision systems and can have specific operational significance.
An ALS provides useful visual perspective, but no dependable vertical guidance or obstacle clearance. A recognisable centreline pattern does not prove that the aircraft is on the correct glidepath.
What is a barrette approach lighting system?
A barrette approach lighting system uses groups of three or more closely spaced lights arranged in short transverse lines. From approach distance, each group appears as one illuminated bar rather than several individual lamps.
Barrettes may form the centreline, crossbars or red side rows. The phrase does not identify one universal system: ALSF, HIALS and other precision layouts can all incorporate barrettes, with spacing and colours determined by the applicable national standard.
What do runway crossbar lights and sequenced flashers mean?
A runway approach crossbar is a horizontal row perpendicular to the extended centreline, while sequenced flashers attract attention and lead the eye towards the threshold. Neither is a command to descend or a touchdown marker.
Crossbars make bank, offset and perspective easier to recognise, especially when the horizon is obscured. The flash sequence always progresses in the landing direction, but it supplies no course corrections of its own. The pilot must still follow the instrument procedure or maintain a safe visual flight path.
What are red terminating barrettes and red side-row barrettes?
Red terminating barrettes and red side-row barrettes are close-in elements found in certain high-intensity precision approach systems. They identify the inner part of the array and are not the same as red runway-end lights at the far end of the landing surface.
In FAA terminology, red terminating bars form a short red transverse reference near the threshold, while red side-row lights form paired rows alongside the inner white centreline. The terms “bar” and “barrette” are sometimes used interchangeably because each visible bar is assembled from several lamps.
These lights do not mean stop, runway closed or below glidepath. Under certain US approach rules they are qualifying visual references for descent below 100 ft above touchdown-zone elevation; the exact rule is covered below. Other jurisdictions may classify and use them differently.
What is a HIALS approach lighting system?
HIALS means High Intensity Approach Lighting System, but the acronym describes a family or capability rather than one worldwide layout. A HIALS normally provides a long, conspicuous and intensity-adjustable pattern for a precision runway.
Some installations follow a Calvert-style centreline-and-crossbar pattern; others use centreline barrettes, side rows and sequenced flashers. “High intensity” also does not mean the lights always operate at maximum output—controllers usually select an intensity suited to the visibility and background lighting.
| Runway ALS family | How it is recognised | Typical role |
|---|---|---|
| HIALS or Calvert/barrette precision systems | Long high-intensity centreline with multiple crossbars or barrettes; exact pattern depends on the national standard | Supports visual transition on precision approaches and low-visibility operations when the runway and procedure are approved |
| ALSF-1 and ALSF-2 | US high-intensity systems with sequenced flashing lights; ALSF-2 includes additional close-in elements such as red side rows | ALSF-1 is associated with Category I support; ALSF-2 is configured for Category II/III operations |
| MALS, MALSF and MALSR | Medium-intensity centreline and crossbar array; flasher variants add sequenced or runway-alignment lights | Common on Category I ILS and other instrument runways |
| SSALS, SSALR and SALS | Shorter, simplified steady-light pattern; an R suffix in FAA names denotes runway-alignment indicator lights | Runway acquisition and alignment where a full precision array is unnecessary or unavailable |
| ODALS | A sparse omnidirectional flashing arrangement rather than a dense barrette array | Basic runway identification and alignment, generally with less operational credit |
A long HIALS can extend roughly 900 m from the threshold, although terrain, land availability and national standards can produce shorter installations. The name printed on a chart does not by itself establish the landing minimum or approach category; procedure design, equipment serviceability, aircraft approval and operating rules all matter.
Who controls the approach lights?
At a towered aerodrome, air traffic control normally selects the approach-light system and intensity, and a pilot can request a brighter or dimmer setting. Maximum intensity can be distracting in clear conditions, while a higher setting may help penetrate low visibility.
Some non-towered aerodromes use pilot-controlled lighting triggered by timed radio transmissions. The required transmission count, activation period and lights controlled vary by installation, so pilots must use the published aerodrome information rather than assume one universal sequence.
Steady lights and sequenced flashers may use separate controls and circuits. An array can therefore remain partly illuminated when its flashers, side rows or another component are unavailable.
How is ALS different from PAPI, runway lights, REIL and ILS?
An approach lighting system helps the pilot acquire and align with the runway, while PAPI, runway lighting and ILS provide different information.
| System | Location | What it tells the pilot |
|---|---|---|
| Approach lighting system | Extends outward from the landing threshold | Runway direction, lateral alignment, roll reference, closing distance and visual transition |
| PAPI or VASI | Beside the runway, normally near the touchdown area | Vertical path through red-and-white indications; our guide explains reading PAPI colours and correcting the glidepath |
| Runway lighting | On or immediately beside the landing surface | Threshold, edges, centreline, touchdown zone and runway end; see our explanation of the different runway-light colours and their uses |
| REIL | One flashing light on each side of the threshold | Rapid identification of the runway end; a REIL pair alone is not a complete ALS |
| ILS | Radio transmitters serving the approach | Electronic lateral and vertical guidance before sufficient visual references are acquired; this is separate from how localiser and glideslope guidance work |
Green threshold lights, white runway-edge lights and red runway-end lights belong to the runway-lighting system rather than the approach array. A mistake we see constantly is treating any bright group near the threshold as “the approach lights”, which can lead to confusion between ALS, PAPI and REIL.
How should a pilot use runway approach lights on final?
A pilot should use the approach-light pattern to confirm runway identity and alignment while maintaining the instrument or visual flight path with separate vertical guidance.
- Remain on the cleared procedure. Follow the localiser, approach course, glideslope or other authorised guidance until the rules and published minimum permit a visual transition.
- Identify the complete pattern. Confirm that the lights lead to the intended runway threshold rather than another runway, taxiway or road.
- Check alignment and roll. Use the centreline and crossbars to detect lateral displacement, drift and bank. In a crosswind, remember that aircraft heading and ground track may differ.
- Confirm the vertical path separately. Use the instrument glidepath, PAPI or VASI where available. An apparently centred ALS can still be seen from a dangerously low or high approach.
- Acquire the required runway references. Identify the threshold, touchdown area or other references required by the applicable rule before continuing below the relevant limit.
- Go around when the picture is inadequate. A few indistinct lights, a lost visual reference or uncertainty about runway identity is not a reason to continue descending.
Can a pilot land using only the approach lights?
White approach lights alone are generally a transition reference, not sufficient visual information for continuing all the way to touchdown. The legal limit depends on the jurisdiction, operation and approach being flown.
Under US rules, when the approach-light system is the only qualifying visual reference, a pilot generally may descend no lower than 100 ft above touchdown-zone elevation. Descent below that point requires the red terminating bars or red side-row lights to be distinctly visible and identifiable, or another specified runway reference to be continuously visible.
This is not a universal international rule. Other authorities and operators define required visual references differently, and decision altitude, minimum descent altitude and required visibility must be taken from the applicable procedure and operating rules. Our simulator-focused explanation of applying approach and landing minima covers the decision process without treating first sight of the lights as automatic landing clearance.
An ALS outage does not necessarily close the runway. It can increase the required approach visibility, remove credit for a lower minimum or make a particular low-visibility operation unavailable. Crews must check notices, equipment status and the appropriate inoperative-components table before commencing the approach.
Why are runway approach lights missing or too dim in a flight simulator?
Missing or incorrect approach lights in a flight simulator usually result from airport scenery, activation logic, viewing geometry or rendering settings rather than the aircraft's ILS equipment.
- Verify the runway end. Check that the selected end actually has an ALS and identify its published type. Reciprocal ends often have completely different lighting, and a displaced threshold can make the array appear to stop beyond the start of the pavement.
- Distinguish ALS from PAPI and REIL. A pair of threshold flashers is probably REIL, while four lights beside the runway are likely PAPI. Neither indicates that a centreline approach array should exist.
- Activate the airport lighting. Test at dusk or night and use ATC or pilot-controlled lighting if the simulator models it. Some simulators simplify automatic visibility and time-of-day logic.
- Approach from the designed direction. Many fixtures use directional optics and can appear weak from far above, well off-centre or behind the lamps. Test from a stable final rather than from an external overhead camera.
- Isolate scenery conflicts. Temporarily remove duplicate airport packages and determine whether stock or add-on scenery is drawing the airport. Package priority can suppress lights, combine two incompatible layouts or leave old fixtures at a moved threshold.
- Check scenery against navigation data. Updated procedure data can point to a runway whose visual model is older. Tuning the correct ILS frequency will not create missing 3D light objects because radio navigation and airport scenery are separate systems.
- Change one graphics option at a time. Lighting quality, object distance, exposure, bloom and fog rendering can wash out or hide small fixtures. Repeat the same final approach after each change so the result is meaningful.
The same diagnostic sequence applies broadly to Microsoft Flight Simulator 2020 and 2024, X-Plane, FSX and Prepar3D, although setting names and airport fidelity differ. If the correct runway, lighting state and scenery package are confirmed but a published HIALS still lacks crossbars, flashers or red side-row barrettes, the airport model itself is probably incomplete; changing avionics settings will not repair it.