How do the visual guidance systems compare?
Five slope indicators side by side: what the pilot sees above, on and below the slope, and how each one is read.
PAPI, APAPI, VASI, PVASI and T-VASI all do the same job: they tell the pilot, by light, whether the aircraft is above, on or below the approach slope. They differ in how many units they use and in how the pilot reads the signal.
The PAPI and APAPI are read by counting the red and white lights. The VASI is read from the colours of two bars, and the T-VASI from the shape of the pattern. The PVASI uses a single unit and adds a pulse to its red and white signal. The more units a system has, the more it costs to install, align and maintain.
| System | Layout | Pilot reads | On slope | Typical role |
|---|---|---|---|---|
| PAPI | 4 units in one bar | Number of red and white lights | 2 white · 2 red | Common choice for new installations |
| APAPI | 2 units in one bar | Number of red and white lights | 1 white · 1 red | Simpler layout, for smaller aircraft and runways |
| VASI | 2 or 3 bars of units | Colours of the near and far bars | Far red · near white | Older system, many still in service |
| PVASI | 1 unit | Colour, and steady or pulsing | Steady white | Single-unit installations where a full bar is not used |
| T-VASI | Wing bars and a stem | Shape of the pattern | Wing bar only | More units, shows the size of the error by shape |
Two other entries in this section do different jobs and are not slope indicators. Runway lead-in lighting marks the track over the ground along an approach path. Docking guidance guides the aircraft onto the stand after landing. Neither one shows the approach slope.
Frequently Asked Questions
Q1What do the visual guidance systems have in common?
They are all visual aids that the pilot reads by eye. PAPI, APAPI, VASI, PVASI and T-VASI show whether the aircraft is above, on or below the approach slope. Lead-in lighting marks the track over the ground, and docking guidance guides the aircraft onto the stand.
Q2What is the main difference between the slope indicators?
How the signal is read. A PAPI and an APAPI are read by counting how many units show red. A VASI is read from the colours of its near and far bars. A PVASI is read from the colour of one light and whether it pulses. A T-VASI is read from the shape the lights make.
Q3Why is PAPI more common than VASI on new installations?
A PAPI uses a single bar of units and gives a clear change from white to red at the slope, so it needs less equipment and space than a VASI with several bars. Many older VASI systems are still in service, and some are being replaced.
Q4Which system shows how far the aircraft is from the slope?
Most of them show it in steps. A PAPI shows more red or more white as the error grows. A T-VASI adds more fly-down or fly-up lights, and a PVASI changes from steady to pulsing. The steps and angles depend on the system and the standard.
Q5Can I choose a system from this page?
No. The choice depends on the runway, the aircraft that use it, the approach and the applicable standard. Use this page to learn how the systems differ, and take the design decision from ICAO Annex 14, the FAA advisory circulars and the manufacturer's data.
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