There is a sentence every student pilot in the world learns by heart, usually while sweating through their first night approach: red over white, you’re alright. Say it out loud a few times. It rhymes, which is useful, because you are about to stake a 30-ton aircraft and all the lives inside it on a row of coloured lights the size of car headlights mounted alongside the runway.

Those lights are called PAPI — Precision Approach Path Indicator — and they are one of the quiet, brilliant bits of 20th-century engineering that have been saving pilots’ lives every night, at almost every serious airport on earth, for 50 years. Once you understand how they work, you’ll spot them out of airliner windows forever.

Four Lights, One Answer

PAPI is almost embarrassingly simple. Four light boxes sit in a row alongside the runway threshold, usually on the left side as the pilot approaches. Each box shows either white or red depending on the angle you are looking at it from. No software, no radio, no pilot input. The aircraft’s position in space does all the work.

The magic is inside each box. Each unit contains a brilliant halogen or LED lamp, a lens, and a horizontal coloured filter — red on the bottom half, clear (white) on the top. Look at the light from above a precise angle and you see the clear half: white. Look at it from below that angle and the red filter cuts across the beam: red. Look at it from exactly at the transition line, and you see a sharp cut between the two colours.

Each of the four units is aimed at a slightly different angle. The left-most box is aimed highest. The right-most is aimed lowest. Between them they carve the sky above the runway into five horizontal slices.

The Five Slices of Sky

As you fly your approach, you pass through each of these slices in turn, and the combination of red and white lights you see tells you exactly where you are relative to the ideal 3-degree glide path:

Four white lights. You are well above glide path. The runway is slipping away from you. You are going to overshoot or land long. Standard pilot response: reduce power, lower the nose, or initiate a go-around if the deviation is extreme.

Three white, one red. Slightly high. Not dangerous, but not perfect. Small nose-down correction usually solves it.

Two white, two red. Bang on. You are on the ideal glide path — typically 3 degrees down. This is the hymn every pilot is singing in their head. Keep doing what you’re doing.

One white, three red. Slightly low. The aircraft is approaching obstacles or terrain faster than ideal. A small power addition and slight pitch-up brings it back.

Four red lights. You are dangerously low. The runway threshold is higher than where you are pointed. In the worst case, you are flying into the approach lighting towers or the blast fence. Standard pilot response: immediate go-around. There is no conversation to have with four red lights. You add power and climb.

This is where the ugly mnemonic comes from: white on white, too high as a kite. Red on red, you’re dead. It rhymes because it has to stick.

Who Invented the Thing

PAPI was developed in the early 1970s at the Royal Aircraft Establishment at Bedford, in the United Kingdom, by a team led by Tony Smith and David Johnson. The problem they were trying to solve was that the earlier system — VASI, the Visual Approach Slope Indicator — used only two bars of lights and told the pilot essentially three things: too high, right, too low. Modern jet aircraft, flying steeper stabilised approaches at higher speeds, needed something with more resolution.

Smith and Johnson’s breakthrough was the four-unit arrangement combined with a sharper optical cut-off. Where VASI’s colour transition was gradual and fuzzy, PAPI’s was precise to within a few hundredths of a degree. Pilots could now see deviations they could actually do something about, not just confirm a problem after it had become an emergency.

The first operational PAPI installation went live in the late 1970s. By the mid-1990s it was the ICAO international standard. Today, nearly every runway in the world capable of handling anything larger than a light twin uses PAPI — or its slightly different cousin, the Canadian-designed APAPI (abbreviated to two units, used at smaller airports).

Why You Can See Them From So Far Away

A single PAPI unit puts out around 10,000 candela. That is roughly as bright as a car headlight, but narrowly focused. From a PAPI installation designed for jets, you can pick out the constellation of four points from about 20 nautical miles on a clear night — long before the runway itself is visible. This is partly why PAPI installations use so much electricity and why they are one of the first things to get upgraded when an airport modernises its approach aids. LED PAPI, now rolling out globally, uses about a fifth of the power and last three times as long.

When the Lights Don’t Match the Instruments

Every pilot eventually faces a moment on approach where the PAPI is telling them one thing and the instrument landing system glideslope is telling them something slightly different. It is almost always the PAPI that is right. The instrument glideslope can be bent by terrain, truck traffic near the transmitter, or atmospheric ducting. The PAPI is photons in a straight line — physics, essentially, with a filter glued to the front.

This is why the standard procedure in gusty, marginal, or partial-instrument conditions is to fly the instruments for navigation and the PAPI for the last mile of glide path. You end up on the ideal touchdown point on the runway because you are flying the most honest guidance the system offers. Four small halogen boxes, each aimed a fraction of a degree differently, telling you where to go without ever saying a word.

Red over white, you’re alright. Red on red, you’re dead. Fifty years old and still the cleanest safety system in aviation.

Sources: Boldmethod, FAA Aeronautical Information Manual, Royal Aeronautical Society archives, ICAO Annex 14.