Flight Operations

PAPI and VASI Lights: How to Read the Glide Path Lights, and Where They Sit Among the Other Airport Lights

Red over white, you're all right. What PAPI and VASI lights tell an arriving pilot, how the two systems differ, and why a remote pilot near an airport should know where that glide path runs.

Jordan Reed·September 27, 2026·8 min read

Key takeaway

PAPI and VASI lights mark a glide path of about 3 degrees, which puts arriving aircraft below 400 feet above the runway for roughly the last 1.25 nautical miles. That low final approach is where a drone near an airport is most likely to meet one.

PAPI lights are a row of four lights beside the runway that show an arriving pilot whether they are on the glide path. Two white and two red means on the path, normally 3 degrees. More white means high, more red means low. VASI lights do the same job with bars of lights: red over white, you're all right.

Both systems are visual glideslope indicators, the lights a pilot uses to fly a stable descent to the runway without any instruments. You will never use one to fly a drone. You still need to understand them, because they show you exactly where arriving aircraft will be, and on the last mile of that path they are down in the same few hundred feet you work in.

A runway diagram showing threshold bars, the runway designator, aiming point markings, displaced-threshold chevrons pointing toward the usable runway, and a taxiway holding position marking with its mandatory sign. — The glide path lights guide an aircraft down to this pavement. A displaced threshold means the aircraft crosses the start of the concrete already low and descending.

What are PAPI lights, and what do they mean?

PAPI stands for precision approach path indicator. The FAA describes it as the current standard visual glide slope indicator: four light boxes in a single row at right angles to the runway edge, normally on the left side of the runway. Some smaller installations use two boxes.

Each box projects a beam that is white in its upper part and red in its lower part. The boxes are aimed at slightly different angles, so what you see changes with how high you are. The pilot counts the white lights.

What the pilot sees	What it means	Approximate angle
4 white	High	More than 3.5°
3 white, 1 red	Slightly high	3.2°
2 white, 2 red	On glide path	3.0°
1 white, 3 red	Slightly low	2.8°
4 red	Low	Less than 2.5°

The angles in that table are for a standard 3-degree installation. The AIM says a PAPI is visible from about 5 miles by day and up to 20 miles at night. It gives safe obstruction clearance within 10 degrees either side of the extended runway centerline, out to 3.4 nautical miles from the threshold.

What are VASI lights, and what does "red over white" mean?

VASI stands for visual approach slope indicator, the older system. Its light units use the same red-and-white split beam, but they are laid out in bars set one behind the other along the runway: a near bar and a far bar, and sometimes a middle bar too.

On a two-bar VASI, the near bar marks the bottom edge of the glide path and the far bar marks the top. You read them as a pair:

Far bar	Near bar	What it means
Red	White	On glide path. Red over white, you're all right
White	White	Above glide path
Red	Red	Below glide path. Red over red, you're dead

The memory aids are crude, and they work. A two-bar VASI gives one glide path, normally 3 degrees.

A three-bar VASI gives two glide paths. The lower one uses the near and middle bars and is normally 3 degrees. The upper one uses the middle and far bars and sits about a quarter of a degree higher. It is meant for aircraft with high cockpits, whose wheels would otherwise cross the threshold too low. The AIM says VASI angles at some airports go as high as 4.5 degrees to clear obstacles.

VASI lights are visible from 3 to 5 miles by day and up to 20 miles or more at night. They give obstruction clearance within 10 degrees of the centerline out to 4 nautical miles.

What is the difference between VASI and PAPI lights?

Both give the same answer: high, on path, or low. They differ in how they show it and how precisely.

	PAPI	VASI
Layout	One row of 2 or 4 boxes	2 or 3 bars, near to far
How you read it	Count the white lights	Compare near bar with far bar
Steps of information	Five (high to low)	Three (high, on, low)
Day range (AIM)	About 5 miles	3 to 5 miles
Obstacle clearance out to	3.4 NM	4 NM
Status	Current FAA standard	Older, still in service at many fields

The PAPI can tell you "slightly high" and "slightly low", which a two-bar VASI cannot. That is why the FAA calls it the standard system. Many VASIs are still in service, so the exam can ask about either one.

Two rarer types also appear in the AIM. A tri-color VASI is a single light: amber means high, green means on path and red means low. It is only useful from about half a mile to a mile by day. A pulsating VASI is also a single light. It shows steady white on path, steady red slightly below, and pulsing red further below. Above the path it pulses white, and the pulse gets faster the further off you are.

Is flying the glide path a rule or just advice?

At a towered airport it is a rule. Under 14 CFR 91.129(e)(3), an airplane approaching a runway served by a visual approach slope indicator must stay at or above the glide path until a lower altitude is needed to land safely. The same paragraph allows normal small corrections above and below the path.

This matters to you because it means arriving airplanes are supposed to be predictable. They will be on or just above the path, not creeping in low across the fields. It does not guarantee anything. A pilot in a slightly low approach is still legal and still lower than you expect.

Where do PAPI and VASI sit among the other airport lights?

The glide path lights are one system among several. The airport beacon marks the field itself, and the rest mark the pavement. From the AIM:

Light	Color and meaning
Runway edge lights	White. On instrument runways, yellow for the last 2,000 feet or half the runway, whichever is less
Runway end lights	Green seen from the approach (threshold), red seen from the runway (end)
Runway end identifier lights (REIL)	A pair of synchronized flashing lights either side of the threshold
Runway centerline lights	White, then alternating red and white from 3,000 feet remaining, then all red for the last 1,000 feet
Touchdown zone lights	Steady white bars from 100 feet to 3,000 feet past the threshold
Taxiway edge lights	Blue
Taxiway centerline lights	Green, alternating green and yellow where they lead on or off a runway

The pattern is the same as with airport signs and runway markings. White belongs to the runway, blue and green to the taxiways, and red means stop or the end.

At many non-towered fields the pilot turns these lights on from the cockpit. Seven clicks of the microphone within five seconds gives the highest intensity, five gives medium and three gives low. The lights then stay on for 15 minutes. The AIM notes that the activation frequency is often the CTAF, but not always. It is listed in the Chart Supplement, and it is not shown on the sectional. The CTAF vs UNICOM guide covers where to find it.

How do you know if a runway has a PAPI or VASI?

Check the runway data in the Chart Supplement. Visual glideslope indicators are listed with a code for the type, the number of lights and the side of the runway. For example, PAPI(P2R) means a two-light PAPI on the right side of the runway. P4L means four lights on the left. The Chart Supplement guide walks through a full entry.

The same entry tells you if a PAPI or VASI is offset from the centerline, or if its clearance area is reduced by local obstacles. The AIM says those limitations appear in the Chart Supplement or in NOTAMs.

What this means when you are actually working

A 3-degree glide path drops about 318 feet per nautical mile. That comes from 6,076 feet in a nautical mile multiplied by the tangent of 3 degrees. Work it backward, and an aircraft on the path is below 400 feet above the runway for roughly the last 1.25 nautical miles, about 7,600 feet. A slightly low aircraft is lower still, and the touchdown point is usually some way past the threshold, which lowers the path again.

In other words, the last mile and a quarter of every approach runs through the airspace a drone uses. That is the practical reason for the rules that keep you out of it:

In controlled airspace, you need authorization under 14 CFR 107.41, usually through LAANC. The zero-foot grid squares on the facility maps usually sit on the extended runway centerlines, which is where this glide path lies.
Near a non-towered field, there is no authorization to get. 14 CFR 107.43 says you may not interfere with operations and traffic patterns at any airport, heliport or seaplane base. The traffic pattern guide shows where the final approach leg lies.
In all cases, you give way. 14 CFR 107.37 makes you yield right of way to all aircraft. An airplane on short final is committed and slow to maneuver. You are neither.

If you can see a PAPI from your launch site, work out which runway it serves and draw the extended centerline in your head. Do not fly in that line. If you are set up near a field at dusk and the runway lights suddenly come on, somebody has just clicked the mic, and they are probably within a few miles.

Frequently asked questions

What do the PAPI lights mean?

A PAPI is a row of four lights, usually on the left of the runway. Count the white lights. Two white and two red means on the glide path, normally 3 degrees. Three or four white means high, and three or four red means low. Four red means the aircraft is below about 2.5 degrees and needs to climb or level off.

What is the difference between PAPI and VASI lights?

Both show a pilot whether they are high, on path or low. A PAPI uses a single row of two or four lights and gives five steps of information. A VASI uses two or three bars set one behind the other, read as a pair, and gives three steps. The FAA describes the PAPI as the current standard system.

What does PAPI stand for in aviation?

PAPI stands for precision approach path indicator. VASI stands for visual approach slope indicator. Both belong to a family the FAA calls visual glide slope indicators, lights that give descent guidance to a pilot flying a visual approach. They give no left-right guidance. The runway and its lights do that.

What does "red over white, you're all right" mean?

It is the memory aid for a two-bar VASI. The far bar should look red and the near bar white, which means the aircraft is on the glide path. Both white means too high. Both red, "red over red, you're dead", means too low and at risk of hitting obstacles short of the runway.

Do drone pilots need to know PAPI and VASI lights?

You will not use them to fly, but they show where arriving aircraft will be. A 3-degree glide path puts them below 400 feet above the runway for roughly the last 1.25 nautical miles. Airport lighting also falls within the airport operations material the Part 107 knowledge test draws on.

How far away can you see PAPI lights?

The AIM puts a PAPI's visibility at about 5 miles by day and up to 20 miles at night, with obstruction clearance guaranteed out to 3.4 nautical miles and within 10 degrees of the centerline. A VASI is visible from 3 to 5 miles by day and 20 miles or more at night.

Want to see how well airport operations questions stick? The free readiness assessment mixes them in with the other knowledge areas, or drill the terms with the flashcards.

Sources

Put this guide into practice.

Take the free assessment to see how this topic fits into your current exam readiness.

Start free practice

Editorial note: FAA rules and guidance may change. Confirm current operational requirements with official FAA resources before every flight. Updated September 27, 2026.