PAPI VS PVASI
Two visual glidepath systems built on completely different optical logic. PAPI reads position from the count of white and red lights across four units; PVASI normally uses a single light unit and reads position from colour plus pulsation. Both exist to answer the same operational question — is the aircraft on the correct visual approach slope?
At a Glance
| Feature | PAPI | PVASI |
|---|---|---|
| Full form | Precision Approach Path Indicator | Pulsating Visual Approach Slope Indicator |
| Typical light units | 2 or 4 | Normally 1 |
| Arrangement | Single row | Single projector |
| Main indication | Steady red/white combination | Steady or pulsating red/white |
| On glide path | 2 White + 2 Red (4-unit PAPI) | Steady white, or alternating red/white on some systems |
| Slightly low | 1 White + 3 Red | Steady red |
| Further below | 4 Red | Pulsating red |
| Above path | Increasing white indication | Pulsating white |
| Interpretation | Count white/red lights | Colour + pulsation |
| Visual information | More detailed | Simpler |
| Typical installation | One side of runway | One side of runway |
| Main advantage | Precise, intuitive indication | Simple, compact system |
What Is PAPI & What Is PVASI?
PAPI
PAPI consists of light units — typically four — installed in a horizontal row, normally on one side of the runway. Each unit has a slightly different elevation cut-off, producing 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.
PVASI
PVASI normally uses a single light unit that projects a two-colour visual approach path, rather than a row of separate boxes. The pilot's cue comes from both the colour of the light and whether it is steady or pulsating.
Pilot interpretation
The pilot reads position from colour plus pulsation — steady white on path, steady red slightly low, pulsating red further low, pulsating white above the path.
How Each System Signals Position
Both systems mark high, on-path and low — but PAPI does it with a light count across a row, while PVASI does it with a single lamp's colour and pulsation rate.
PAPI
PVASI
On PVASI, the pulsation rate increases the farther the aircraft moves above or below the desired glide path. Some installations use an alternating red/white indication for the on-path condition instead of steady white.
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 |
PVASI — 4 Principal States
| Pulsating white | Above glide path |
| Steady white | On glide path |
| Steady red | Slightly below glide path |
| Pulsating red | Further below glide path |
The pilot's cue is colour combined with pulsation rate, rather than a count of separate light units.
Number of Lights vs Colour + Pulsation
PAPI
The pilot asks: "How many lights are white and how many are red?" Position is read from the count across a row of separate units.
PVASI
The pilot asks: "What colour is the light, and is it steady or pulsating?" Position is read from a single lamp's colour and behaviour, not a count.
Both systems answer the same question: is the aircraft on the correct visual approach slope? PAPI answers it with light count; PVASI answers it with colour and pulsation.
Multiple Elevation Cut-offs vs a Single Projector
PAPI
Four light units each have a slightly different elevation cut-off angle. The aircraft's vertical position determines how many of those units appear white and how many appear red.
Physically, this means multiple light units mounted in a row — a larger footprint than a single-lamp system.
PVASI
A single projector produces different indications depending on the aircraft's position relative to the visual glide path, with pulsation providing the additional deviation cue.
Normally just one light unit, which makes PVASI a considerably more compact installation than a four-box PAPI.
Typical Visibility Range
Indicative values only, not guaranteed operational ranges — actual visibility depends on conditions and installation.
~20 mi night
~10 mi night
Different Circuit Architecture
PAPI
Power → CCR / power supply → series circuit → isolation transformers → four PAPI units. A relatively simple, well-established arrangement.
PVASI
Power → power supply → PVASI control unit → single light unit. Fewer light units overall, but the control system needed to generate the pulsating indication is an important part of the design.
Keeping Each System On-Path
PAPI Maintenance
- Lamp/LED and lens condition
- Optical alignment, elevation and azimuth
- Housing and structural condition
- Electrical connections, transformers and cabling
Four separate units means more physical components to inspect and align.
PVASI Maintenance
- Pulsation function and correct colour
- Optical alignment
- Steady/pulsating transition behaviour
- Light-source condition and electrical/control system
Fewer physical light units, but the pulsation control system adds its own maintenance check.
Myths vs Facts
No. PVASI normally uses one light unit and relies on colour plus pulsation, not a count of separate units.
No. Standard PAPI indications are based on steady red/white projections, not the pulsating principle used by PVASI.
No — pulsating white means above the desired path. Steady white is the on-path indication in the basic PVASI system.
No. Like PAPI, it provides vertical approach-slope guidance only.
No — 4 Red means too low, not on path.
Neither Replaces an ILS
PAPI and PVASI are purely visual vertical approach-slope references. Neither provides lateral navigation, range information, or the full precision guidance of an ILS.
ILS provides two different functions: the localizer gives lateral guidance (left/right) and the glideslope gives vertical guidance (up/down) by radio. PAPI or PVASI can supplement this with a purely visual vertical cross-check once the runway environment is in sight.
PAPI vs PVASI — Engineering Parameters
| Parameter | PAPI | PVASI |
|---|---|---|
| Light units | Typically 4 | Normally 1 |
| Arrangement | Single row | Single projector |
| Primary guidance | Vertical | Vertical |
| Indication logic | White/red count | Colour + pulsation |
| On-path indication | 2 White + 2 Red | Steady white (or alternating red/white on some systems) |
| Principal states | 5 | 4 |
| Physical footprint | Larger — multiple units | Compact — single unit |
| Control complexity | Lower | Higher — needs pulsation control |
| Current FAA status | Standard VGSI | Recognized but less common/legacy-style |
PAPI vs PVASI
PVASI is not a smaller PAPI — it is a different optical logic entirely. PAPI achieves finer visual gradation by adding light units in a row; PVASI achieves a compact installation by using a single lamp and letting colour plus pulsation carry the deviation information.
The FAA currently identifies PAPI as the standard Visual Glide Slope Indicator, while PVASI remains a recognized but far less common system. Neither replaces an ILS, and the applicable system installed at a given runway should always be verified rather than assumed.
PAPI is the current standard for new installations, but PVASI installations remain recognized in FAA aeronautical information and charts — always confirm the actual system in service at a specific runway.
Engineering & Expert Notes
Know How the Indication Is Read
PrincipleDifferent systems use different cues. Be clear about what each shows.
Verify Alignment and Settings
AlignmentRecord angles for each unit.
Check Colour and Pulsation Behaviour
PhotometryConfirm each system's indications.
Check Which System Is Specified
ComplianceVerify the requirement.
Frequently Asked Questions
PAPI uses multiple light units — typically four — read as a white/red count. PVASI normally uses a single light unit and is read by colour plus whether the lamp is steady or pulsating.
Pulsating white indicates the aircraft is above the desired glide path — it is not the on-path indication.
In the basic PVASI system, steady white indicates on the glide path. Some installations use an alternating red/white indication instead for the on-path condition.
Yes. The pulsation rate increases as the aircraft moves farther above or below the desired glide path, giving an additional cue about the size of the deviation.
PAPI is the FAA's current standard Visual Glide Slope Indicator for new installations, but PVASI remains a recognized VGSI type in FAA aeronautical information and charts, though it is much less common than PAPI.
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, §5.3.5 — visual approach slope indicator systems: PAPI/APAPI (and legacy T-VASIS/AT-VASIS). PVASI is not an Annex 14 system, so its use on an ICAO-aligned project should be confirmed with the aerodrome regulator.
FAA AIM §2-1-2, AC 150/5345-28 & AC 150/5340-30
The AIM describes PVASI colour and pulsation behaviour and range; the ACs cover PAPI equipment (L-880/L-881) and installation. For any PVASI installation, verify equipment and siting requirements against the applicable authority and manufacturer documentation.
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 PVASI 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 select PAPI or PVASI from a generic web diagram. The final siting, unit configuration and alignment must always be developed from the current governing standard, approved airport design and project-specific requirements.
PAPI and PVASI answer the same operational question with different optical logic. Either system, correctly aligned and maintained, gives the pilot a reliable visual reference for the designed approach slope.
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