If you're a pilot, you've probably used Precision Approach Path Indicator (PAPI) lights during your final approach to landing. But do you know how they work? Here's what you need to know...

Are You On Glide Path?

PAPI lights are a great visual reference for flying a stable final approach. Whether you're flying VFR or IFR, following the PAPI is a good idea.

A series of 4 horizontal lights will show 2 red, 2 white when you're flying on the established glide path (typically 3 degrees).

The more red lights you see, the lower you are on glide path.

• If you're slightly low on glide path, you'll see 3 red and 1 white (2.8 degrees).
• If you're low on glide path, you'll see 4 red and 0 white (less than 2.5 degrees).

The more white lights you see, the higher you are on glide path.

• If you're slightly high on glide path, you'll see 1 red and 3 white (3.2 degrees).
• If you're high on glide path, you'll see 0 red and 4 white (more than 3.5 degrees).

Trying to remember what the lights mean? Use the phrase "All white, you're high as a kite. All red, you're dead."

PAPI System Design

Your aircraft's position (and thus your line of sight) determines which color lights you'll see outside. A series of static lenses and color filters on the PAPI lights make this happen, and they're designed to help you maintain a constant glide path angle. As you dip below glide path, the lights (from right to left) will fade from white to red. The reverse is true of flying high on glide path.

Most of the time, PAPI lights are located on the left side of the runway. The non-standard location is on the right side of the runway.

In 2008, some PAPIs began a modernization phase with LED lights replacing incandescent lamps.

PAPI vs. VASI

While we're focusing on PAPIs in this article, you'll also find VASIs around the country too. According to the AIM, the Visual Approach Slope Indicator (VASI) is a system of lights arranged to provide visual descent guidance information during the approach to a runway. These lights are visible from 3-5 miles during the day and up to 20 miles or more at night. The visual glide path of the VASI provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 4 NM from the runway threshold.

If you see two red lights over two white lights, you're on glide path. Although normal glide path angles are 3 degrees, VASI lights at some airports may be as high as 4.5 degrees to give proper obstacle clearance.

The Precision Approach Path Indicator (PAPI) uses light units similar to the VASI, but are installed in a single row of either two or four light units. These lights are visible from about 5 miles during the day and up to 20 miles at night. The visual glide path of the PAPI typically provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 3.4 NM from the runway threshold.

Again... Two white lights and two red lights mean you're on the established glide path.

PAPIs In Use

According to the FAA, "there are 938 PAPIs in the National Airspace System. PAPIs are used to replace Visual Approach Slope Indicator Lights (VASI), and to support Commercial Aviation Safety Team (CAST) and Land and Hold Short Operations (LASHO) new requirements."

Here's what a row of PAPI lights looks like from the ground...

Exceptions Are Out There...

At most airports equipped with visual guidance to the runway, you'll fly a standard glide path of 3 degrees. But this isn't the case everywhere. Some airports adjust the PAPI glide path for terrain or obstacles.

At Steamboat Springs, Colorado, the 2 light PAPI for Runway 32 is situated at 4 degrees due to rising terrain.

Pilots are prohibited from referencing the 4 light PAPI at Molokai's Airport (PHMK) beyond 1.8 NM from the landing threshold due to rapidly rising terrain. At Molokai, the PAPI is situated at a 4 degree glide path and is installed as a reference for straight-in approach traffic.

Fly Your Approach Using The PAPI

In almost every case, following the PAPI will give a single-engine piston aircraft more than enough room to land and stop well before the end of the runway. It's much more likely for a single-engine piston to land short when aiming for the threshold than to overrun the runway after touching down near the aiming points.

The runway aiming points (commonly called the 1000 foot markers) are a perfect target to descend towards, and you should plan to touchdown within 200 feet of them. If landing performance allows, having some runway distance prior to your landing point will ensure that you don't land short. There's rarely a time when landing on the numbers is safer than landing near the aiming point.

Configuration changes, tailwinds, and stop-and-go landings are a few reasons why you might plan to land before the aim point markers, to ensure you have enough usable runway left. Because of that, it's not a firm rule to follow every time you land. But in the majority of cases, using vertical guidance to land in the touchdown zone of the runway is the safest option.

Do you use the PAPI as a visual reference when you fly? Tell us in the comments below.