PAPI vs VASI lights explained: learn how to read their red-and-white glidepath signals, compare layouts and precision, and avoid common mistakes.
In real-world aviation, PAPI and VASI are visual glidepath aids that help pilots judge vertical position on final approach. PAPI normally uses four lights in one horizontal row, giving five red-and-white indications. A standard two-bar VASI uses separate near and far bars, giving three basic indications: high, on slope or low.
PAPI means Precision Approach Path Indicator, while VASI means Visual Approach Slope Indicator. Both change colour according to the pilot's viewing angle. They provide vertical guidance only: neither indicates runway alignment, traffic, landing clearance or wind.
How do PAPI lights work?
A normal PAPI has four light units arranged in a horizontal row, usually beside the left side of the runway, although installations vary. Each unit projects white above its transition angle and red below it, with the four units aimed at slightly different angles.
| PAPI indication | Aircraft position |
|---|---|
| Four white | High above the approach path |
| Three white, one red | Slightly high |
| Two white, two red | On the indicated path |
| One white, three red | Slightly low |
| Four red | Low below the approach path |
The useful rule is to count the colours: more white means high, while more red means low. An abbreviated PAPI, or APAPI, has two units and therefore provides only high, on-path and low indications.
For a practical simulator hardware example, our Arduino-controlled PAPI display for MSFS shows how the four-unit logic produces these five indications.
How do VASI lights work?
A standard two-bar VASI places a near bar and a far bar at different positions along the runway edge. Their apparent vertical arrangement produces three basic signals:
- White over white: the aircraft is high.
- Red over white: the aircraft is on the approach slope.
- Red over red: the aircraft is low.
In the on-path indication, the far bar is red and appears above the white near bar, giving the familiar phrase “red over white, you're all right”. VASI installations are not all identical. A three-bar VASI can provide separate lower and upper paths, with the upper path intended for aircraft whose cockpits sit well above their landing gear.
Which is more precise, PAPI or VASI?
PAPI gives finer visual deviation information because its usual four-light display distinguishes slightly high and slightly low conditions. A standard two-bar VASI only distinguishes high, on slope and low.
| Feature | Four-unit PAPI | Standard two-bar VASI |
|---|---|---|
| Layout | One horizontal row | Near and far bars |
| Normal indications | Five | Three |
| On-path signal | Two white and two red | Red over white |
| Deviation detail | Shows slightly high or slightly low | Shows high or low |
This does not mean a maintained VASI defines its intended slope inaccurately. PAPI simply communicates smaller deviations more clearly. Nor does the word “precision” make a PAPI-equipped approach a precision instrument approach.
Can PAPI or VASI replace ILS glide-slope guidance?
PAPI and VASI cannot replace an ILS, RNAV vertical path or other approved instrument guidance during the instrument portion of an approach. They are visual references used after the required visual conditions have been established.
The visual path may not coincide perfectly with an electronic glide slope because the systems can use different siting, threshold-crossing and reference geometry. PAPI guides the pilot's eye; the wheels are below and behind that reference, which matters particularly in large aircraft. The indicated angle is often near three degrees, but pilots must not assume every installation uses exactly that angle.
- Do not dive after an all-white indication. Re-establish a stable path gradually or go around if a safe correction is no longer possible.
- Treat all red as a serious low-path warning. Continuing to descend will not correct it.
- Do not chase the lights over the threshold. The indication becomes increasingly sensitive at short range and may change as the aircraft passes the installation.
- Do not assume lateral obstacle clearance. The protected visual sector is finite; being on the colours while displaced from the runway centreline is not a guarantee of terrain clearance.
If visual and instrument guidance disagree materially, verify the runway, altimeter setting, selected approach and aid status. Do not average the two indications; discontinue the approach if the correct path is uncertain.
Why can PAPI or VASI look wrong in a flight simulator?
Incorrect simulator indications usually come from the camera position, airport scenery or a mismatch between the modelled visual and instrument paths. Diagnose the problem in this order:
- Confirm the runway and system type. The opposite runway end may use a different aid or none at all.
- Use the normal cockpit viewpoint. An external camera or altered eyepoint changes the viewing angle and therefore the colours.
- Check for scenery conflicts. Duplicate airport packages, a moved threshold or incorrect runway elevation can place the lights on the wrong path. Isolate the add-on and retest with the base airport.
- Compare from a stabilised final approach. At extreme range, resolution, exposure and atmospheric effects can blend the individual lights.
Our FSX visual approach and landing walkthrough demonstrates how both PAPI and VASI indications should change during a properly established approach.