PAPI VS VASI
Two visual glidepath systems, two very different indication logics. PAPI's four-light row gives the pilot finer graduation of vertical position; VASI's traditional two-bar geometry gives a simpler colour read. Both exist to answer one operational question — is the aircraft on the correct visual approach slope?
At a Glance
| Feature | PAPI | VASI |
|---|---|---|
| Full form | Precision Approach Path Indicator | Visual Approach Slope Indicator |
| Main purpose | Vertical approach-path guidance | Vertical approach-path guidance |
| Light units | 4 (2 in the abbreviated APAPI / L-881 form) | 2, 4, 6, 12 or 16 units in 2 or 3 bars |
| Typical arrangement | One horizontal row | Near / far bars |
| Visual indications | 5 states — on path is 2 white + 2 red | 3 states (2-bar): white over white, red over white, red over red |
| Path indications | More precise | Simpler |
| Pilot interpretation | Number of white/red lights | Colour combination |
| Location | Normally the left side of the runway | One side (2, 4 or 6 units) or both sides (12 or 16 units) |
| Glidepath information | Yes | Yes |
| Azimuth guidance | No | No |
| Replaces ILS | No | No |
What Is PAPI & What Is VASI?
PAPI
PAPI consists of four light units installed in a horizontal row, normally on the left side of the runway. Each unit produces a white indication when the aircraft is above the designed approach path and a red indication when it is below it.
Pilot interpretation
The pilot reads position from the number of white and red lights across the row — more white means higher, more red means lower.
VASI
VASI traditionally uses two light bars — a near bar and a far bar. The pilot sees different colour combinations between the two bars depending on the aircraft's position relative to the approach path. Installations use 2, 4, 6, 12 or 16 light units; three-bar versions add a second, slightly higher glide path.
Pilot interpretation
The pilot reads position from the colour relationship between the two bars — white over white, red over white, or red over red, read as the far bar over the near bar.
How Each System Signals Position
Both systems mark high, on-path and low — but PAPI does it with a light count, VASI does it with a bar-colour relationship.
PAPI
VASI
Every Indication State
PAPI — 5 Principal States
| 4 White | Too high |
| 3 White + 1 Red | Slightly high |
| 2 White + 2 Red | On approach path |
| 1 White + 3 Red | Slightly low |
| 4 Red | Too low |
VASI — 3 Principal States
| White / White | Too high |
| Red / White | On approach path |
| Red / Red | Too low |
Colours are read far bar over near bar — the familiar “red over white” is the on-path indication. Three states versus PAPI's five means VASI gives the pilot a coarser read of vertical position.
Number of Lights vs Colour Relationship
PAPI
The pilot determines position from the number of white and red lights across a single row — four units, more graduation.
VASI
The pilot determines position from the colour relationship between the near and far bars — simpler, coarser read.
Both systems answer the same question: is the aircraft on the correct visual approach slope? PAPI just gives a more finely graded answer.
What PAPI & VASI Do Not Provide
Neither system provides lateral navigation like a localizer, range information like DME, full precision guidance like an ILS, runway centreline guidance, or horizontal course deviation. They primarily provide vertical approach-path information.
PAPI and ILS can be installed on the same runway. The ILS localizer gives lateral guidance and the glideslope gives vertical guidance; PAPI provides an important visual cross-check once the runway environment becomes visible.
Common Circuit Architecture
Both systems are relatively simple compared with an ALS or ILS, and share a similar simplified electrical arrangement: power supply → CCR / series circuit → isolation transformers → PAPI or VASI light units. This is the series-circuit (CCR-fed) form; voltage-powered variants also exist — for example, FAA PAPI Style A uses a power and control unit instead of a CCR, while Style B is the series-circuit version.
- Intensity control
- Monitoring and fault indication
- Remote control
- ALCMS interface, depending on the installation
Keeping Each System On-Path
PAPI Maintenance
- Optical cleanliness and lens condition
- Light-source condition
- Unit alignment, elevation and azimuth
- Structural condition
- Electrical connections, transformer/cable condition
Alignment is particularly important — even with all four units illuminated, incorrect alignment can produce an incorrect approach-path indication.
VASI Maintenance
- Light-source condition and lens cleanliness
- Optical and bar alignment
- Elevation angle
- Structural condition
- Electrical connections, transformer/cable condition
The two-bar geometry must remain correctly aligned to the designed visual approach path.
Myths vs Facts
No. PAPI primarily provides vertical approach-path guidance, not lateral or centreline guidance.
No. PAPI is a visual aid; ILS is a radio-navigation system with its own localizer and glideslope.
No — four red means the aircraft is below the designed approach path.
No. It indicates the designed vertical approach path. Lateral alignment is a separate consideration.
No. They have different configurations, visual indications, design criteria and installation requirements.
PAPI vs VASI — Engineering Parameters
| Parameter | PAPI | VASI |
|---|---|---|
| Light units | 4 (APAPI: 2) | 2 to 16, in 2 or 3 bars |
| Arrangement | Single row | Two (or three) bars |
| Primary guidance | Vertical | Vertical |
| Indication | White/red count | White/red bar combination |
| Main on-path indication | 2 White + 2 Red | Red over white (far bar red, near bar white) |
| Vertical discrimination | Higher | Lower |
| Optical setting | Multiple unit angles | Bar-based optical geometry |
| Alignment criticality | Very high | Very high |
| Electrical complexity | Low / moderate | Low / moderate |
| Maintenance | Optical + alignment | Optical + alignment |
PAPI vs VASI
PAPI is not simply a modern replacement for VASI — it is a finer-graduation visual glidepath system. Both answer the same operational question, but PAPI's four-unit row gives the pilot more discrimination of vertical position than VASI's two-bar colour read.
Neither system replaces an ILS, provides lateral guidance, or confirms centreline alignment — both are purely vertical visual references, meant to be read together with the rest of the approach environment.
PAPI has largely superseded VASI in new installations, but VASI installations remain in service at many airports — always verify the actual system installed at a given runway rather than assuming. ICAO Annex 14 no longer carries a specification for the two-/three-bar VASIS.
Engineering & Expert Notes
Check Settings Against the Design
AlignmentRecord each unit's angle.
Verify Siting
SitingCheck distances and positions.
Measure Photometry and Colour Transition
PhotometryVerify both.
Consider Replacement Planning for Older Systems
LifecyclePlan upgrades before parts become hard to find.
Frequently Asked Questions
PAPI uses a single row of four light units read as a white/red count, giving five possible indications. VASI uses two bars read as a colour relationship, giving three possible indications — so PAPI provides finer vertical discrimination.
No. Both PAPI and VASI provide vertical approach-path guidance only — neither gives lateral navigation, range information or centreline guidance.
Yes. They perform different functions — the ILS gives radio-based lateral and vertical guidance, while PAPI gives the pilot a visual cross-check once the runway environment is in sight.
It means the aircraft is on the designed approach path — the standard on-path indication for a typical PAPI installation.
Because the indication depends entirely on the optical setting of each unit or bar. Even with every light illuminated, incorrect alignment produces an incorrect approach-path indication to the pilot.
PAPI has largely become the standard for new installations, but VASI systems remain in service at a number of airports — the applicable system should always be verified for the specific runway.
PAPI uses a single wing bar of two or four light units, each showing red or white depending on the pilot's angle, with a sharp transition. VASI uses two or more bars of lights placed at different distances from the threshold and is read by the red and white pattern between the bars. PAPI is more compact, gives a more precise indication and is the standard choice for new installations.
Governing Documents
For an actual project design, always verify the current edition of the applicable documents.
ICAO Annex 14, Volume I
Aerodrome Design and Operations, §5.3.5 — visual approach slope indicator systems: PAPI/APAPI configuration, siting and photometric requirements. The two-/three-bar VASIS specification is no longer carried in Annex 14.
DGCA CAR Section 4, Series B, Part I
India's aerodrome design and operations requirements, implementing ICAO Annex 14 — the primary regulatory reference for visual aids on Indian airport projects. Read alongside ICAO Doc 9157 Part 4 (Visual Aids) for design guidance.
FAA AC 150/5345-28 & AC 150/5340-30
PAPI equipment specification (L-880 four-box, L-881 two-box; Style A voltage-powered, Style B series-circuit) and airport visual-aid installation details including PAPI siting. For existing VASI installations, rely on the original installation documents and the airport's approved design.
IS 12290 & IS 7785 (BIS)
IS 12290 covers isolating transformers for airport lighting systems; IS 7785 is the multi-part series for elevated-type aerodrome lighting fittings. Neither is an equipment specification for PAPI or VASI light units — for those units, follow ICAO Annex 14 / Doc 9157 Part 4 or the FAA PAPI specification, as the project requires.
Final Engineering Note
Do not design a PAPI or VASI installation from a generic web diagram. The final siting and alignment must always be developed from the current governing standard, approved airport design and project-specific requirements.
PAPI is not a "better VASI" in every sense — it is a different optical logic that happens to give finer graduation. Either system, correctly aligned and maintained, exists to answer the same question for the pilot on approach.
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