VASI versus PAPI: what is the difference?
Two bars compared by colour against one bar read by counting. The same white-above, red-below idea, in a different layout.
The VASI and the PAPI both show a pilot whether the aircraft is above, on or below the approach slope, with white for high and red for low. They differ in layout and in how the pilot reads them.
A VASI has two or three bars placed at different distances along the runway. The pilot compares the colour of the far bar with the colour of the near bar: red over white is on slope. A PAPI has one bar of four units. The pilot reads how many are red or white: two and two is on slope. The PAPI also gives more steps between all white and all red.
| Feature | VASI | PAPI |
|---|---|---|
| Layout | 2 or 3 bars, at different distances | 1 bar of 4 units (APAPI: 2 units) |
| Pilot reads | Colours of the near and far bars | Number of red and white lights |
| On slope | Far red · near white | 2 white · 2 red |
| Steps, high to low | 3 (two-bar system) | 5 |
| Glide paths | 3-bar version gives a lower and upper path | One approach slope |
| Equipment and space | More units, spread along the runway | Less equipment, one position |
| Status | Older FAA-described system, many still in service; not an ICAO Annex 14 system | ICAO and FAA standard; common choice for new installations |
Because a PAPI needs only one bar, it uses less equipment and less space than a VASI, and it has become the usual choice for new work. Many VASI systems are still in service, and they have to be maintained and aligned correctly until they are replaced.
What is the difference between PAPI and VASI?
Both show a pilot whether the aircraft is above, on or below the approach slope, with white for high and red for low. They differ in layout and in how they are read. A VASI has two or three bars at different distances along the runway, and the pilot compares the colour of the far bar with the near bar; far red over near white is on slope. A PAPI has one bar of four units, and the pilot counts how many are red or white; two white and two red is on slope. A PAPI gives more steps between all white and all red, uses less equipment and space, and is the usual choice for new installations, while many VASI systems remain in service.
Frequently Asked Questions
Q1What is the main difference between a VASI and a PAPI?
A VASI uses two or three bars of light units placed at different distances along the runway, and the pilot compares the colours of the bars. A PAPI uses one bar of four units, and the pilot reads how many of them are red or white.
Q2How does a pilot read each system on the slope?
On a two-bar VASI, the far bar is red and the near bar is white. On a PAPI, two lights are white and two are red. Above the slope both systems show more white, and below it they show more red.
Q3Why has the PAPI replaced the VASI on new installations?
A PAPI needs one bar instead of two or three, so it uses less equipment and less space along the runway. It also gives more steps between high and low. ICAO Annex 14 specifies the PAPI and the APAPI, while the two- and three-bar VASI is an FAA-described system that ICAO deleted from Annex 14 in 1995. Many older VASI systems remain in service until they are replaced.
Q4Does a PAPI have a three-bar version?
No. A three-bar VASI gives the pilot a choice between a lower and a higher path. A PAPI gives one approach slope, and the setting angles of its units are chosen for the aircraft and the approach.
Q5Can I choose between them from this page?
No. The choice depends on the runway, the aircraft that use it and the applicable standard. Use this page to see how the two systems differ, and take the design decision from ICAO Annex 14, the FAA advisory circulars and the manufacturer's data.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.