On a clear final approach you can fly a perfect glidepath without any instruments at all, just by reading a row of coloured lights beside the runway. The PAPI and the older VASI turn your height on the approach into red and white that you can read at a glance, and they are among the most useful, and most testable, visual aids in flying.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
The idea: white above, red below
Both systems work on one elegant principle: each light unit projects a beam that is white in its upper part and red in its lower part, split at a precise angle. If your eye is above that angle, you see white; if your eye is below it, you see red. By setting several units at slightly different angles across the approach, the system turns your exact position on the glidepath into a pattern of white and red. The single sentence to carry away is white above, red below: more white means you are too high, more red means you are too low.
The systems are defined internationally in ICAO Annex 14 and described for pilots in the FAA AIM. They give a typical 3-degree visual glidepath, similar to an ILS glideslope, so they let you fly a stable, normal descent to the touchdown zone visually.
PAPI: the four-light bar
The precision approach path indicator (PAPI) is the modern standard: a single row of four lights, usually on the left side of the runway, set at four slightly different angles. Reading them is just counting white and red:
- four white: well above the glidepath;
- three white, one red: slightly high;
- two white, two red: on the glidepath;
- one white, three red: slightly low;
- four red: well below the glidepath, dangerously low.
The symmetry is the beauty of it: the on-slope indication, two and two, sits in the middle, and you simply move toward more red if you are high and more white if you are low. Because there are four lights, a PAPI also tells you how far off you are, not just which side, which is what makes it precise.
VASI: the bars
The older visual approach slope indicator (VASI) uses bars of lights rather than a single row, and you read the relationship between a near bar and a far bar.
On a two-bar VASI:
- both bars white: too high;
- near bar white, far bar red (red over white): on the glidepath;
- both bars red: too low.
A three-bar VASI adds a third bar to provide two glidepaths: a lower one for normal aircraft and a higher one for aircraft with a high cockpit, such as widebody jets, whose eyes sit far above the wheels. The crew uses the pair of bars appropriate to their aircraft. Some countries use further variants, such as the T-VASIS, but PAPI and VASI are the two you will meet most.
The mnemonics that keep them straight
Generations of pilots have remembered these with rhymes, and they are worth knowing because they work under pressure:
- For the VASI: "red over white, you're all right"; "white over white, you're high as a kite"; "red over red, you're dead".
- For the PAPI, the same idea: more white is high, more red is low, and "red, red, red, red, you're dead" for the all-red, far-too-low case.
The thread through both is identical to the underlying physics: white is high, red is low, and a balanced indication is on the path.
Using it with the rest of the approach
A visual glidepath system pairs naturally with everything else on the approach. Flown to its indication, it produces a stable, continuous descent of the kind our guide to stabilised approaches describes, with no need to dive or drag. On an instrument approach broken off visually near the minimum, the PAPI gives you an independent confirmation of your glidepath as you transition from the ILS glideslope to a visual landing. And because it is set to the touchdown zone, it keeps you clear of obstacles and on a profile that lands you in the right place, provided you read it correctly.
Cautions worth knowing
A visual glidepath is reliable, but a few traps deserve respect. The indication is only valid for the runway it serves, so reading the PAPI for the wrong runway, or being lined up on the wrong runway entirely, gives a meaningless picture. The system is set for a defined eye-to-wheel height, so a much larger or smaller aircraft than the system was designed for can be slightly off; the three-bar VASI exists precisely to handle that. And the glidepath clears obstacles only when followed, so chasing the lights into a steep correction close in is worse than a smooth return to the path. Read it, trust it, but fly it smoothly.
The angle, the threshold crossing and obstacle clearance
A visual glidepath is not just a height aid; it is an obstacle-clearance aid, and that is why it must be flown, not merely glanced at. The system is aligned to give a defined threshold crossing height, so that an aircraft holding the on-slope indication crosses the threshold at a safe wheel height and touches down in the touchdown zone. It is also set so that the approach path it defines clears the obstacles in the approach area, but only for an aircraft that stays on the indicated slope. Drift low, into the reds, and that obstacle protection is no longer guaranteed.
The standard angle is about 3 degrees, but some runways publish a steeper visual glidepath to clear terrain or obstacles or to meet a noise requirement, and that non-standard angle is noted in the aerodrome information. A steeper slope means a higher rate of descent for the same groundspeed, so it is worth knowing before you arrive rather than discovering it on short final. The practical rule that ties it together: the lights promise a safe, obstacle-free path to the touchdown zone when you follow them, so the on-slope two-and-two is not only the comfortable picture, it is the protected one.
A worked example
You roll out on final and pick up the PAPI on the left of the runway showing three white and one red. That is slightly high, so you ease the descent to bring yourself back down toward the path. A few seconds later it shows two white and two red: you are now on the glidepath, so you hold that picture, adjusting gently to keep the two-and-two as you descend.
Closer in, a gust pushes you down and the PAPI flicks to one white, three red. You are now slightly low, so you arrest the descent with a touch of power and reduce your rate until the two-and-two returns. Had it gone to four red, you would have been well below the path and dangerously low, the "red, red, red, red" case, calling for a prompt correction or a go-around. Throughout, you never needed an instrument: the lights told you your glidepath the whole way down, white for high, red for low, two-and-two for on the slope.
Common pitfalls
- Forgetting which colour means what. White is high, red is low; a balanced indication is on the path.
- Reading the wrong runway's system. A PAPI or VASI only tells the truth for the runway it serves.
- Chasing the lights. Smooth, small corrections back to the on-slope picture beat steep grabs at the glidepath near the ground.
- Ignoring the eye-height effect. A very large aircraft may sit slightly off a system set for smaller ones; the three-bar VASI addresses this.
- Treating four red as recoverable at leisure. All red means dangerously low; correct promptly or go around.
In Pilot EFB
Pilot EFB is a study and planning companion for the visual aids you will use on the approach, helping you learn what the PAPI and VASI indications mean before you meet them for real, alongside the approach chart and the rest of your offline-first briefing. It does not read the lights, fly the glidepath, or judge your height, and what you act on is what you see out of the window and your official source of record. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and fly the approach from your official sources.