Definition
VASI and PAPI are ground-based light systems that give a pilot a visual glide path to the runway on approach, and the FAA Aeronautical Information Manual describes them in its airport visual-aids chapter. Both work on the same optical principle: each light unit projects a beam that appears white when seen from above a set angle and red when seen from below it. By arranging several units so their transition angles differ slightly, the system as a whole tells the pilot whether the aircraft is high, on, or low relative to the intended descent path, most commonly a 3-degree angle to the runway.
The Visual Approach Slope Indicator is the older system. The common two-bar VASI has a near bar and a far bar of light units, and the pilot reads the pair together. When the near bar shows white and the far bar shows red, the aircraft is on the glide path; both bars white means the aircraft is too high, and both bars red means it is too low. Generations of pilots remember this as "white over white, high as a kite; red over white, you're all right; red over red, you're dead." VASI installations provide a safe glide path down to the runway threshold and are usable from several miles out by day and considerably farther at night.
The Precision Approach Path Indicator has largely superseded VASI at newer installations. A PAPI consists of a single row of four light units, normally on the left side of the runway. On the correct glide path the pilot sees two red lights and two white lights; three white and one red indicates slightly high, and four white indicates well above the path; three red and one white indicates slightly low, and four red indicates well below it. The single-row layout gives finer, more intuitive above-and-below resolution than the two-bar VASI, which is why it is now the standard visual glide-path aid at most airports serving instrument approaches.
Using these aids well is central to a stabilized approach. A pilot who flies the indicated path arrives at a consistent descent angle, at an appropriate speed and configuration, which reduces the risk of landing long and running off the end or of dropping below a safe path and striking terrain or obstacles short of the runway. The systems also help maintain adequate obstacle clearance on the final segment, and following the visual glide path keeps the aircraft in a predictable place within the traffic pattern flow.
While the red-over-white principle and the 3-degree standard reflect internationally harmonized ICAO practice, the specific U.S. configurations, tolerances, and light-count meanings described here follow the Aeronautical Information Manual. Reading VASI and PAPI correctly is a required knowledge and skill area from the earliest stages of flight training.
Why It Matters for Flight Schools
For a flight school, VASI and PAPI are a standard part of teaching landings and stabilized approaches, and they give instructors an objective reference for coaching approach path control. A student who consistently sees all-white on short final is being taught to recognize and correct a high, fast approach before it turns into a long landing and a possible runway excursion; a student seeing all-red learns to recognize a dangerously low path. Because these aids are present at most training airports, they become a daily teaching tool rather than an occasional topic.
The systems also connect to a school's approach-and-landing safety focus. Unstable approaches are a well-documented contributor to landing accidents, and a school that trains pilots to use visual glide-path guidance and to go around when the path cannot be salvaged is directly managing that risk. Tracking approach and landing proficiency, and any stabilized-approach exceedances, is part of a mature training and safety program.
How Aviatize Handles This
Aviatize helps a school teach and track approach and landing proficiency. The Training Management module records each learner's progress through the landing and approach elements of the syllabus, so an instructor can log a student's ability to fly a stabilized approach using visual glide-path guidance and confirm that skill before solo. Ground Training & Checking supports the briefings where the VASI and PAPI light interpretations are taught.
Where a school runs an approach-and-landing safety program, the Safety Management module gives it a place to record unstable-approach events or landing incidents and track the training or standard-operating-procedure changes that follow. Keeping the training record and the safety record together lets a chief instructor connect a pattern of unstable approaches back to the specific coaching that addresses it.
Frequently Asked Questions
- How do you read a PAPI?
- A PAPI is a single row of four lights, usually left of the runway. Two red and two white means on the glide path; three white and one red means slightly high, four white means well high; three red and one white means slightly low, four red means well below the path. On the correct path you see an even split of red and white.
- What is the difference between VASI and PAPI?
- VASI is the older system, typically two bars of lights read together, where white over white is high, red over white is on path, and red over red is low. PAPI uses a single row of four lights that gives finer above-and-below resolution and has become the standard visual glide-path aid at most airports serving instrument approaches.
- What glide path angle do VASI and PAPI indicate?
- Both systems are most commonly set to a 3-degree descent angle to the runway, which provides a safe approach path with adequate obstacle clearance. The precise angle at a given runway is published, but 3 degrees is the standard reference pilots plan around for a stabilized approach.