VASI vs. PAPI: Visual Glide Path Lights

VASI and PAPI lights help pilots answer one important question on final approach: am I too high, too low, or on a usable glide path?

They are visual glide path systems installed near runways. You do not need special cockpit equipment to use them. If you can see the lights and understand the colors, they can help you fly a more stable approach.

What VASI Means

VASI stands for Visual Approach Slope Indicator. A common VASI uses two light bars near the runway. The lights appear red or white depending on your position relative to the glide path.

The classic memory aid is:

"Red over white, you're all right."

If both bars appear white, you are high. If both bars appear red, you are low. If the near bar is white and the far bar is red, you are on the intended path.

Some airports use three-bar VASI systems. These provide different glide paths for smaller aircraft and larger aircraft with higher cockpit-to-wheel geometry. If you are flying a normal training airplane, you usually use the lower path as published and briefed.

There are also tri-color VASI systems, which use a single light unit showing different colors. These are less common but still based on the same idea: color tells you vertical position.

What PAPI Means

PAPI stands for Precision Approach Path Indicator. A typical PAPI has four lights in a row, usually on the left side of the runway.

For a four-light PAPI:

• Four white: too high.
• Three white, one red: slightly high.
• Two white, two red: on glide path.
• One white, three red: slightly low.
• Four red: too low.

The common phrase is:

"White over white, high as a kite. Red over white, you're all right. Red over red, you're dead."

That last line is memorable, but do not let it replace judgment. If the indication shows low, correct promptly and smoothly. If the approach becomes unstable, go around.

Similarities

Both VASI and PAPI provide visual vertical guidance. Both help pilots avoid being too low on approach, especially at night or over areas with fewer visual cues.

Both are typically designed around a published visual glide path, often near three degrees, but always check the charted information for the runway.

Both depend on visibility. If weather, sun angle, snow, terrain, or runway lighting makes the lights hard to see, you must rely on the rest of your approach briefing and be ready to go around.

Differences

PAPI gives more detailed information than a basic two-bar VASI. With four lights, it can show slightly high or slightly low, not just high, on path, or low.

VASI systems may provide guidance over a somewhat different distance and can include multiple configurations. PAPI systems are common at many airports because they are compact and precise.

From a student pilot standpoint, PAPI is usually easier to interpret quickly: aim for two white and two red. VASI is also simple, but the two-row layout requires you to remember which bar should be which color.

How to Use Them Properly

Do not chase every flicker. Small color changes can happen as the airplane moves through the glide path. Make smooth pitch and power corrections.

Use the lights as part of a complete approach picture:

• Airspeed
• Runway alignment
• Descent rate
• Aiming point
• Wind correction
• Aircraft configuration
• Stabilized approach criteria

If the lights say you are on path but your airspeed is wrong, the approach is not automatically good. If the lights look right but you are drifting off centerline, fix the drift or go around.

Common Mistakes

One mistake is staring at the lights and forgetting the runway. The lights support the landing, but you still need to fly the airplane.

Another mistake is diving to regain a low indication or pulling aggressively to correct a high indication. Use coordinated, stable corrections.

Students also sometimes continue below a safe path because the runway looks close. Trust your training. Low indications near the ground deserve immediate attention.

Which One Is Better?

PAPI is more precise for most pilots, but VASI is still useful and effective. The better system is the one installed on the runway you are using and the one you correctly brief before final.

VASI and PAPI do not land the airplane for you. They give you visual guidance so you can make better decisions. Use them early, cross-check them with the rest of the approach, and go around if the picture stops making sense.

For related runway and landing cues, see Runway Lights Explained and How to Improve Your Landings.

Official References

• FAA AIM 2-1-2, Visual Glideslope Indicators
• FAA Aeronautical Information Manual
• 14 CFR Part 91 General Operating Rules
• FAA Aeronautical Chart User's Guide