Those red and white lights beside US runways tell pilots if they're coming in too high or too low; seeing 2 red and 2 white means the approach is on target

If you fly at night, look at the runway as you land. You may spot a short row of red and white lights beside it. They are not decoration. The Federal Aviation Administration’s Aeronautical Information Publication refers to this as a precision approach path indicator, or PAPI. The system uses a single row of two or four light units. It works like a height indicator for pilots. It doesn't say stop or go. It says “too high,” “too low,” or “just right.”

What the lights are

The FAA's Navigation Services office describes PAPI as the current standard visual glide slope indicator. Its page describes four light boxes that project a pattern of red and white lights down the desired descent path to the touchdown point. Why red and white? The publication notes that PAPI uses light units similar to those in an older system known as VASI. In VASI, each unit transmits white light from the top of its beam and red light from the bottom. So the color depends on the aircraft's altitude relative to the light as seen by the pilot.


How pilots read the pattern

Here is the pattern for a four-light PAPI, as laid out by SKYbrary, the aviation safety website. Two white lights and two red lights mean the plane is on the right glide path. Three red lights mean it's below the path, and four red lights mean it is well below. Three white lights mean it is above the course, and four white lights mean it is well above the course. More white means too high. More red means too low. An even split means the approach is on target.

Why lights are important

These systems are meant to reduce the risk of controlled flight into terrain, along with landing overruns and underruns, the FAA's PAPI page explains. They do this by helping pilots establish a stabilized descent. In simple terms, that means avoiding a crash to the ground or a landing that is too long or too short. It also means the plane comes down on a steady path.

How far away you can see them

The Aeronautical Information Publication puts the daytime range of PAPI lights at about 5 miles. At night, that can reach up to 20 miles. The light path typically provides safe obstruction clearance within 10 degrees on either side of the extended runway centerline. That extends out to 3.4 nautical miles from the runway threshold. The runway threshold is the point where the runway begins for landing. A nautical mile is a bit longer than a regular mile.

What PAPI does not do

PAPI only helps with the height. Side-to-side guidance comes from the runway or the runway lights. Pilots should not start descending on PAPI until the aircraft is visually aligned with the runway, the publication adds.

Where the lights sit

Usually, the PAPI row is on the left side of the runway. SKYbrary notes it can sit on the right instead, depending upon the layout of the runway and taxiways. Sometimes, the safe area may be smaller, or the lights may be offset from the runway centerline. In those cases, the details appear in the Chart Supplement or Notices to Airmen, which are official alerts for pilots.

Other systems you might hear about

PAPI isn't the only option. The same publication also lists VASI systems with 2, 4, 6, 12, or 16 light units, arranged in bars called near, middle, and far. There are also tri-color systems. They use a single light that shows red below the track, amber above it, and green on it. Their range is about half a mile to one mile by day and up to five miles at night. Pulsing systems also use one light. A steady white light, or a light alternating red and white, shows the plane is on the path. Steady red means a little below. A pulsing red light is further down. A pulsing white light is up. Its range is about four miles by day, and as much as ten miles by night. So next time you land after dark, look for that line of lights along the side of the runway. You know the code now. An even mix of red and white is the goal.