PAPI VS AVASI
A four-unit precision system versus a stripped-down, two-unit visual slope indicator. PAPI's single row of four lights gives the pilot five graduated indications of vertical position; AVASI abbreviates the older VASI concept down to a single light unit in each of the near and far bars, giving three basic indications. Neither is "more precise" in the ILS sense — both are visual approach-slope indicators, sized very differently for the installation.
At a Glance
| Feature | PAPI | AVASI |
|---|---|---|
| Full form | Precision Approach Path Indicator | Abbreviated Visual Approach Slope Indicator |
| Light units | 4 | 2 |
| Arrangement | One horizontal row | Near bar + far bar (1 unit each) |
| Primary function | Vertical approach-path guidance | Vertical approach-path guidance |
| Normal on-path indication | 2 White + 2 Red | 1 White + 1 Red |
| Visual information | More detailed | Simpler |
| System complexity | Higher | Lower |
| Maintenance | More units | Fewer units |
| Power / field wiring | More | Less |
| Azimuth guidance | No | No |
| Lateral guidance | No | No |
What Is PAPI & What Is AVASI?
PAPI
A PAPI normally consists of four light units arranged in a horizontal row. Each unit is optically set to a slightly different elevation angle, so the pilot sees different combinations of white and red as the aircraft moves above or below the desired approach path.
AVASI
AVASI (Abbreviated Visual Approach Slope Indicator) is a simplified version of the older VASI concept, keeping the same near-bar / far-bar principle but reducing each bar to a single light unit instead of several. The near unit and far unit are set to slightly different elevation angles, so the pilot reads their combined colour as the aircraft moves above or below the designed path, giving a coarser, more compact visual reference than a multi-unit VASI bar.
How Each System Signals Position
PAPI reads as a white/red count across a single row of four units. AVASI is read differently — as a near-bar/far-bar combination, one unit set nearer the threshold and one set farther along the approach, rather than a side-by-side row — with fewer graduated states.
PAPI
AVASI
Vertical Discrimination
| Parameter | PAPI | AVASI |
|---|---|---|
| Number of units | 4 | 2 |
| Optical settings | 4 distinct unit indications | 2 unit indications |
| Vertical discrimination | Greater | Lower |
| On-path indication | 2 White + 2 Red | White + Red |
| High indication | 4 White | 2 White |
| Low indication | 4 Red | 2 Red |
| Intermediate indication | Yes | No |
The missing intermediate states — 3W/1R and 1W/3R on a PAPI — are the main reason AVASI provides less detailed information than a full four-unit installation.
Fewer Units, Same Principle
AVASI abbreviates the older VASI concept down to a single light unit in each of the near and far bars — instead of the multiple units per bar used in a full multi-unit VASI installation — for fewer fixtures, less wiring, lower installation complexity and a lower maintenance burden. Compared with a full four-unit PAPI, it also means coarser visual information: three states instead of five.
"Abbreviated" refers to the configuration — the number of light units — not to the quality or reliability of the fixtures themselves.
Shared Architecture, Different Scale
Both systems follow the same basic electrical concept: power supply → CCR / series circuit → isolation transformers → light units. AVASI simply terminates at two units instead of four, so it generally requires fewer fixtures and associated components.
- Intensity control
- Monitoring and fault indication
- Remote control
- ALCMS interface, depending on the installation
Fewer Fixtures, Same Alignment Discipline
PAPI
Four units require inspection of light source, lens, optical system, alignment, elevation setting, azimuth, housing, electrical connections and transformer/cable condition.
AVASI
The same types of checks apply, but with only two fixtures — so AVASI carries a lower fixture-maintenance burden overall.
Reducing four units to two does not make alignment unimportant. Correct elevation angle, azimuth, unit height and installation geometry remain fundamental — incorrect alignment on either system can produce a misleading indication.
PAPI/AVASI Choice Does Not Set the Approach Category
The runway's operational requirements, approach procedure, obstacle-clearance considerations and applicable regulatory criteria determine the appropriate installation — not the other way around.
Avoid this oversimplification: "PAPI = precision approach, AVASI = non-precision approach." Both PAPI and AVASI are visual approach slope indicators; neither designation by itself determines the approach category.
When an Abbreviated System Is Attractive
An AVASI may be appropriate where a full four-unit PAPI installation is not necessary, or where an abbreviated installation is specified by the applicable design criteria.
- Smaller installation footprint
- Fewer light units and less equipment
- Lower wiring requirement
- Reduced maintenance burden
The actual selection must follow the applicable airport design criteria rather than simply choosing the cheaper option.
A Legacy Term, Not a Current ICAO Configuration
This is an important point for an aviation engineering comparison. The simple two-bar VASI family — of which AVASI is the abbreviated, single-unit-per-bar form — was removed from ICAO Annex 14 as a specified visual approach slope indicator system some time ago; today Annex 14 specifies only the PAPI/APAPI family and the T-VASIS/AT-VASIS family (with the latter itself recommended for discontinuation as a standard system from 1 January 2020). Where "AVASI" appears in current use, it is generally as a legacy installation, a national/FAA-side term for an abbreviated VASI configuration, or informal shorthand — not a system a new ICAO-compliant design would specify.
For a new installation, treat PAPI/APAPI as the current standard family. Any existing AVASI or simple-VASI installation, and any reference to it in a design brief, should be checked against the current governing standard and national regulator's position before being carried into new project documentation.
Myths vs Facts
No. AVASI is an abbreviated form of the older VASI system, not a stripped-down PAPI — it keeps VASI's near-bar/far-bar reading logic rather than PAPI's single-row count, and has its own defined optical and installation requirements.
No. PAPI has additional intermediate indications that AVASI's two-unit arrangement does not provide.
No. Its primary function is vertical approach-path guidance only.
No — 4 Red means too low, not on path.
No. "Abbreviated" refers to the configuration — the number of light units — not the quality of the lights.
No. Current ICAO Annex 14 specifies the PAPI/APAPI family and the T-VASIS/AT-VASIS family; the simple VASI family that AVASI abbreviates is not part of the current specified system set. Existing AVASI installations exist, but a new design would not specify AVASI against current ICAO provisions.
Three Visual Slope Systems, Side by Side
| Feature | PAPI | AVASI | Traditional VASI |
|---|---|---|---|
| Units | 4 | 2 | 2 bars |
| Arrangement | One row | Near + far bar (1 unit each) | Near + far bar (multiple units each) |
| Main indication | White/red count | Near/far bar colour relationship | Bar colour relationship |
| On-path indication | 2W/2R | W/R | W/R |
| Vertical discrimination | High | Moderate | Simpler |
| Primary purpose | Visual approach slope | Visual approach slope | Visual approach slope |
PAPI vs AVASI
AVASI is not a lower-precision PAPI — it is an abbreviated, near/far-bar descendant of the older VASI system. PAPI's four units, in a single row, give five graduated visual indications; AVASI's two units, one in the near bar and one in the far bar, give a simpler, three-state read in a more compact installation.
Neither PAPI nor AVASI replaces an ILS, and the choice between them should follow the applicable airport design criteria and operational requirements — not be treated as a shortcut for approach category.
For a new installation, note that AVASI is a legacy configuration rather than part of the current ICAO-specified system set — see Section 10.
Both systems answer the same operational question — is the aircraft on the correct visual approach slope — PAPI simply answers it with finer graduation.
Engineering & Expert Notes
Match the System to Runway Use
SelectionCheck the requirement and the aircraft using the runway.
Check Aiming and Setting Angles
AlignmentRecord angles and verify after maintenance.
Verify Position Against the Design
SitingCheck distance and lateral position.
Verify Photometry
PhotometryConfirm intensity and colour.
Frequently Asked Questions
PAPI uses four light units in a single row for five graduated indications; AVASI abbreviates this to two units — one in a near bar, one in a far bar — for three indications, using the same white/red optics but the older VASI near/far reading logic rather than a row count.
No. PAPI vs AVASI selection does not by itself determine the approach category — that depends on the runway's operational requirements, procedures and applicable regulatory criteria.
PAPI's on-path indication is 2 White + 2 Red across its row. AVASI's on-path indication is the near unit showing red and the far unit showing white.
No. Correct elevation angle, azimuth and installation geometry remain fundamental on both systems — incorrect alignment can produce a misleading indication regardless of unit count.
Where a full four-unit installation is not necessary, or where the applicable airport design criteria specify an abbreviated system — the selection should follow those criteria rather than cost alone.
No. Current ICAO Annex 14 specifies the PAPI/APAPI family and the T-VASIS/AT-VASIS family. The simple VASI family AVASI abbreviates is not part of the current specified system set, so a new design would specify PAPI/APAPI rather than AVASI.
No. Both are visual vertical approach-path aids. An ILS provides radio-based lateral guidance (localizer) and vertical guidance (glideslope) — a different function entirely.
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 — visual approach slope indicator systems. The current edition specifies PAPI/APAPI configuration, siting and photometric requirements; the simple two-bar VASI family that AVASI abbreviates is not part of the current specified system set (T-VASIS/AT-VASIS remain specified but are recommended for discontinuation as standard systems from 1 January 2020). Verify the current edition for the exact provisions.
FAA AC 150/5345-28, Current Edition
Precision Approach Path Indicator (PAPI) Systems (equipment designations L-880/L-881) — FAA equipment standards covering PAPI unit geometry, siting, electrical systems and photometric criteria. AVASI-type installations are addressed under the FAA's separate VASI equipment guidance.
IS 7785 / IS 12290 / IS 12291
Indian standards for aeronautical ground lighting equipment: IS 7785 (Part 4/Sec 1) for visual approach slope indicator fittings, IS 12290 for isolating transformers and IS 12291 for constant current regulators, applicable to PAPI/AVASI installations on Indian airport projects.
Final Engineering Note
Do not select PAPI or AVASI from a generic web diagram. The final siting, unit count and alignment must always be developed from the current governing standard, approved airport design and project-specific requirements.
AVASI is an abbreviated, single-row visual slope system — not a lower-quality PAPI. Either system, correctly aligned and maintained, gives the pilot a reliable visual reference for the designed approach slope.
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