What Is a PAPI? Precision Approach Lights Explained

A PAPI (Precision Approach Path Indicator) is a row of lights beside the runway that tells you, with red and white colors, whether you’re on the correct descent angle to the touchdown zone. Two white and two red means you’re on the glide path; more red means you’re low, more white means you’re high. It’s one of the simplest, most reliable tools you’ll use on every approach — and once you learn to read it, you’ll never want to land without it. The PAPI gives you a visual glide path so you don’t have to guess your descent angle by eye alone, especially at night or over dark terrain where depth perception gets tricky.

Let’s break down exactly how it works, how to read it, and why it matters from your very first solo.

What does PAPI stand for and what does it do?

PAPI stands for Precision Approach Path Indicator. It’s a system of lights — almost always a single horizontal row of four — installed beside the runway, usually on the left side near the touchdown zone. The PAPI projects beams of light that appear red or white to a pilot on approach depending on whether the aircraft is above or below the correct descent angle. Per the FAA’s Aeronautical Information Manual (AIM 2-1-2), the PAPI gives you visual glide path guidance during the approach.

Each light unit contains a sharp red-over-white split. When you’re looking up through the beam (because you’re below the path), the unit shows red. When you’re looking down through it (because you’re above the path), it shows white. Because the four units are aimed at slightly different angles, the row transitions one light at a time as you move up or down through the glide path — and that’s what gives you the fine resolution you need.

You’ll find PAPI guidance covered in the FAA Pilot’s Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25C) under airport operations, and you’ll see it on most paved runways that handle anything from trainers to airliners.

How do you read a PAPI on approach?

Reading a PAPI is all about counting reds and whites in that single row of lights. Two white and two red means you are on the correct glide path. Three or four red lights mean you are too low and should reduce your descent. Three or four white lights mean you are too high and should increase your descent. The classic memory aid is “red over white, you’re alright” — but with a PAPI, think of it as the row sliding toward red as you sink below the path.

Here’s the simplest way to lock it in: as you descend below the glide path, more lights turn red, so more red equals more low. As you climb above the path, more turn white, so more white equals more high. Don’t overthink which light flips first — just count the colors. The goal is to split the row evenly, two and two, and fly it down to the touchdown zone.

A quick word of caution: “four red” near the ground is a serious flag. It means you’re well below the protected path and potentially close to obstacles or terrain. Don’t chase the runway down with a steep dive — add power, level your descent, and reintercept the path from below.

What’s the difference between a PAPI and a VASI?

A PAPI and a VASI both give you visual glide path guidance, but they’re built differently. A VASI (Visual Approach Slope Indicator) uses two or more horizontal bars stacked at different distances down the runway, and you read it by the red-and-white relationship between the bars. A PAPI uses a single horizontal row of lights, and you read it by counting reds and whites in that one row. The PAPI’s single-row design gives finer resolution on small deviations.

With a two-bar VASI, the memory aid is “red over red, you’re dead; white over white, you’re high as a kite; red over white, you’re alright.” The near bar (lower in your view) and far bar (higher) change color to bracket the path. It works, but it’s coarser — you’re essentially reading three states.

A PAPI, by contrast, walks through five distinct indications (4 white through 4 red), so you catch a deviation earlier and correct sooner. That’s why newer installations and most precision runways use PAPIs. Both systems are described in AIM 2-1-2, and both provide safe obstruction clearance within plus or minus ten degrees of the extended runway centerline out to about four miles from the threshold — but always check the Chart Supplement for any noted restrictions at a specific airport.

What glide path angle does a PAPI use?

Most PAPI installations are set to a 3.0-degree glide path to the runway. That’s the same angle as a standard ILS glideslope, which is no accident — it gives you a stable, predictable descent that clears obstacles and delivers you to the touchdown zone at a comfortable rate. Some airports use a steeper angle (3.5 degrees or more) when terrain, obstacles, or noise abatement require it, so it’s worth a glance at the Chart Supplement before you go somewhere new.

Why does 3.0 degrees matter to you as a student? Because flying the PAPI builds the exact muscle memory you’ll lean on later for instrument flying. When you keep that row pinned at two-and-two, you’re flying roughly the same descent profile an instrument pilot flies down the glideslope. Learn it visually now and the transition gets a lot smoother.

One practical note: the PAPI tells you about your vertical path, not your airspeed or your alignment with the centerline. It’s one instrument in your scan, not the whole picture. Fly the airplane first, use the PAPI to fine-tune the descent, and keep your eyes moving.

Why does following the PAPI keep you safe?

Following the PAPI keeps you safe because it removes the guesswork from your descent angle, and a stabilized, on-path descent is the foundation of every good landing. The PAPI guarantees you obstacle clearance along the approach path and prevents you from getting dangerously low — the kind of situation that leads to landing short or hitting terrain you can’t see. This matters most at night, in haze, or over dark water and unlit terrain where your eyes will lie to you about your height.

Human depth perception is unreliable on approach. Over a wide runway you’ll think you’re high; over a narrow one you’ll think you’re low. Upsloping terrain fools you into a low approach; a black-hole night approach over water can make you descend far too early. The PAPI cuts through all of that with hard color data that doesn’t care what your eyes think.

It also reinforces the stabilized approach concept the FAA hammers in the Airplane Flying Handbook (AFH, FAA-H-8083-3C): a constant, on-path descent at a controlled airspeed sets you up for a safe touchdown. Chasing the runway with big power and pitch changes does the opposite. If you keep the PAPI happy, you’re most of the way to a stabilized approach already.

If you want to build this kind of judgment from the ground up — reading the airport environment, flying a stabilized approach, and making every landing a non-event — that’s exactly what we drill in the Angle of Attack Private Pilot Ground School. It’s built to make you day-one ready, not just checkride ready.

A night into a dark Alaska strip

I’ll tell you when the PAPI earned my respect for good. I was flying a 172 into a small strip in Alaska as the light was draining out of the sky — that long, blue Alaskan dusk where the mountains turn to silhouettes and the ground goes flat and featureless. No moon yet. Just a dark valley and a thin line of runway lights ahead.

Everything in my eyeballs was telling me I was high. The runway looked small, the terrain was rising toward it, and instinct wanted me to push the nose down and get on it. But the PAPI was sitting there showing me one white and three red. I was already low — instinct had it backwards. The terrain illusion had me convinced I needed to descend when I actually needed to hold what I had and add a touch of power.

I trusted the lights over my gut. I brought the power up, nudged the descent shallow, and watched the row settle back to two white and two red. We crossed the threshold right where we should have, and the landing was a non-event. If I’d flown what my eyes wanted, I’d have been scraping in low over rising terrain in fading light.

That’s the lesson I drill into every student: on a dark or unfamiliar approach, the PAPI is more honest than your eyes. Build the habit now — find the lights, fly them down to two-and-two — and it’ll be there for you on the night you really need it.

How do you use a PAPI on the practical test?

On your practical test, the examiner expects you to identify and use the PAPI (or VASI) when it’s available, and to fly a stabilized approach on the indicated glide path. The Private Pilot Airman Certification Standards (ACS) require you to maintain a stabilized approach and a recommended approach airspeed, and using available visual glide path guidance like a PAPI is part of demonstrating that. You won’t pass by ignoring the lights and diving for the numbers.

In practice, this means: call out the PAPI when you have it (“two white, two red, on glide path”), make small, smooth corrections to keep it centered, and don’t fixate on it at the expense of your airspeed and centerline. The examiner wants to see judgment — that you’re integrating the PAPI into a complete scan, not staring at it.

Knowing the system also helps on the oral and the written. Expect to explain the color code, the standard 3.0-degree path, and how a PAPI differs from a VASI. If you can teach it back in plain language, you own it.

If you want every one of these airport-environment concepts laid out the way it’ll actually show up on your knowledge test and checkride, start free with the Total Student Pilot course — it’s the on-ramp we built for brand-new students.

PLT Study Guide

The FAA tags knowledge-test questions with PLT (learning statement) codes. Here are the codes that actually map to PAPI and the broader airport-lighting environment, with the FAA’s own wording translated into plain study points.

Plain-English study points:

• PLT147 (visual glide path indicators): This is the one to nail for PAPI questions. Memorize “two white, two red, on glide path.” More red = too low; more white = too high. Know that the standard glide path is 3.0 degrees and that a PAPI is a single row while a VASI uses stacked bars.
• PLT145 (runway lighting): Understand the lights that surround the PAPI — white runway edge lights, green threshold lights, and the red/white relationship that defines the visual approach. Questions here test the overall lighting picture, not just the glide path.
• PLT141 (markings / signs / lighting): Be ready to identify visual aids as a set. The PAPI is one piece; expect questions that mix it with signs, markings, and other lighting so you can prove you read the whole airport environment.

Frequently Asked Questions

FROM CHRIS

Reading a PAPI is a small skill that pays off on every single approach — and it’s a perfect example of how the airport environment is talking to you the whole time, if you know how to listen. Learn the color code cold, trust the lights over your gut on a dark night, and keep that row pinned at two white and two red. Do that, and you’ve taken one more piece of guesswork out of landing the airplane.

Throttle On!

Chris Palmer

Founder & Chief CFI, Angle of Attack — Two-Time Master Aviation Educator and Gold Seal CFI