PAPI VS T-VASIS
A compact 4-light row against a 20-light T-shaped array. PAPI condenses vertical approach-path guidance into a simple white/red count; T-VASIS spreads it across a much larger wing-bar and fly-up/fly-down arrangement. Both are ICAO-recognised visual approach slope systems — but only one of them is still the current standard.
At a Glance
| Feature | PAPI | T-VASIS |
|---|---|---|
| Full name | Precision Approach Path Indicator | T Visual Approach Slope Indicator System |
| Basic layout | 4 units in one row | T-shaped array |
| Number of light units | 4 | 20 |
| Runway centreline relationship | Normally one side of runway | Symmetrical about centreline |
| Main indication | White/red combination | Wing-bar + fly-up/fly-down indications |
| Deviation indications | 4 discrete indications | 7 discrete indications |
| On-path indication | 2 White + 2 Red | Wing-bar indication |
| Physical footprint | Compact | Large |
| Installation complexity | Lower | Higher |
| Current ICAO status | Standard | Discontinuation as standard recommended from 1 Jan 2020 |
What Is PAPI & What Is T-VASIS?
PAPI
PAPI consists of four light units arranged in a single horizontal row, generally on one side of the runway. Each unit is set at a different vertical aiming angle, so the aircraft's position produces a different white/red combination.
Pilot interpretation
The pilot counts how many units are white and how many are red across the row — more white means higher, more red means lower.
T-VASIS
T-VASIS is a large T-shaped arrangement of 20 light units, symmetrical about the runway centreline: two wing bars of four units each (8 units), each bisected by a longitudinal line of six units — three fly-down and three fly-up (12 units).
Pilot interpretation
The pilot reads the wing-bar and fly-up/fly-down light pattern — a different logic from simply counting coloured lights.
Compact Row vs T-Shaped Array
This is one of the biggest engineering differences between the two systems — PAPI is a single row, T-VASIS is a large symmetrical array built from wing bars and a centreline stem.
PAPI Layout
Four units, single horizontal row, one side of the runway.
T-VASIS Layout
Two 4-unit wing bars, one either side of the centreline, each bisected by a 6-unit longitudinal line (3 fly-down + 3 fly-up) — 8 + 12 = 20 units in total.
Counting Lights vs Reading the T-Pattern
PAPI reduces vertical position to a light count. T-VASIS uses different optical light-unit types — wing-bar, fly-up and fly-down units — to build a richer picture of vertical deviation.
PAPI
T-VASIS
Above path: one, two or three fly-down lights show above the wing bar — the more lights, the higher the aircraft.
On path: the wing bar alone is visible.
Below path: one, two or three fly-up lights show below the wing bar; when well below, the wing bar and all three fly-up lights turn red.
4 Discrete Indications vs 7
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 |
One normal approach-path indication plus four discrete deviation indications.
T-VASIS — More Discrete States
Under the ICAO Annex 14 specification, T-VASIS provides one normal approach-path indication plus seven discrete deviation indications, built from the wing-bar plus fly-up/fly-down light units: one, two or three fly-down lights above the path (3), one, two or three fly-up lights below it (3), and the all-red “well below” indication (1).
Seven deviation states versus PAPI's four means T-VASIS can convey a finer-grained picture of vertical deviation — at the cost of a much larger installation.
Number of Lights vs T-Shaped Pattern
PAPI
The pilot essentially counts the white and red lights: "how many are white, how many are red?"
T-VASIS
The pilot interprets the wing-bar and fly-up/fly-down light pattern: "what does the T-shaped indication tell me about my vertical position?"
Both answer the same operational question — is the aircraft on the desired vertical approach path? PAPI answers it with a light count; T-VASIS answers it with a pattern read.
What Neither System Replaces
PAPI and T-VASIS both provide visual vertical approach-slope guidance only. Neither replaces an ILS, and neither provides lateral guidance on its own.
PAPI and T-VASIS can complement an ILS. The localizer gives lateral guidance and the glide slope gives vertical radio guidance; the visual system is designed so its indications can correlate with the non-visual glide path signal.
Series Circuit vs Multiple-Circuit Array
PAPI Electrical System
CCR / power → series circuit → isolation transformers → 4 PAPI units.
- Constant-current supply
- Series circuit
- Isolation transformers
- Light-source monitoring, intensity control, alignment
T-VASIS Electrical System
CCR / power → multiple circuits → isolation transformers → 20 T-VASIS light units.
- Substantially more light units and optical components
- More cabling and isolation transformers
- More structural elements and maintenance points
Maintenance Burden Compared
| Maintenance item | PAPI | T-VASIS |
|---|---|---|
| Light-source inspection | ✓ | ✓ |
| Lens / optics inspection | ✓ | ✓ |
| Alignment | Critical | Critical |
| Structural inspection | ✓ | ✓ |
| Cable & transformer inspection | ✓ | ✓ (much larger scope) |
| Number of fixtures | 4 | 20 |
| Optical-unit types | Relatively simple | Multiple types (wing bar, fly-up, fly-down) |
| Maintenance footprint | Smaller | Much larger |
T-VASIS generally carries a significantly larger maintenance burden simply on account of unit count and optical-type variety, even though the per-unit maintenance tasks are similar in kind.
Myths vs Facts
No. T-VASIS uses a different indication logic entirely — wing-bar and fly-up/fly-down cues rather than a white/red light count.
No. ICAO has recommended discontinuing T-VASIS and AT-VASIS as standard visual approach-slope systems from 1 January 2020. PAPI remains a current standard system.
T-VASIS's 20 units do provide more discrete deviation indications than PAPI's 4, but that comes with a much larger physical footprint, higher installation complexity and a heavier maintenance burden.
No. The normal approach slope is commonly 3°, but another slope can be selected for a specific runway depending on considerations such as obstacle clearance or harmonisation with a non-visual glide path.
No. They have different configurations, visual indication logic, design criteria and installation requirements — one is not a drop-in substitute for the other.
PAPI vs T-VASIS — Engineering Parameters
| Parameter | PAPI | T-VASIS |
|---|---|---|
| Light units | 4 | 20 |
| Layout | Single horizontal row | Large T-shaped system |
| Location | Normally one side | Symmetrical about centreline |
| Indication | White/red combinations | Wing-bar + fly-up/fly-down |
| Normal path indication | 2W + 2R | Wing-bar indication |
| Deviation indications | 4 | 7 |
| Physical footprint | Small | Large |
| Electrical complexity | Lower | Higher |
| Maintenance | Lower | Higher |
| Installation | Simpler | More complex |
| Current ICAO status | Standard | Legacy / discontinuation recommended |
PAPI vs T-VASIS
T-VASIS is not simply an older PAPI — it is a different optical logic that trades a compact footprint for finer deviation resolution. Both answer the same operational question, but T-VASIS spreads that answer across 20 units and seven deviation states instead of PAPI's 4 units and four states.
Given the larger footprint, higher installation complexity and heavier maintenance burden, ICAO has moved away from T-VASIS as a standard system in favour of PAPI's compact, simpler-to-maintain design.
T-VASIS installations remain in service at some airports, but ICAO recommended discontinuing it as a standard system from 1 January 2020 — always verify the actual system installed at a given runway rather than assuming.
Engineering & Expert Notes
Plan for Legacy Maintenance
LegacySpares and expertise can be scarce.
Verify Alignment Unit by Unit
AlignmentMore units mean more points to check.
Check Requirements Before Retaining
ComplianceConfirm acceptability.
Document Decisions
RecordsKeep a record of choices.
Frequently Asked Questions
PAPI uses 4 light units in a single row, read as a white/red count. T-VASIS uses 20 light units in a T-shaped array symmetrical about the runway centreline, read as a wing-bar plus fly-up/fly-down pattern.
ICAO has recommended discontinuing T-VASIS and AT-VASIS as standard visual approach-slope systems from 1 January 2020. PAPI remains a current standard.
T-VASIS provides one normal approach-path indication plus seven discrete deviation indications, versus four for PAPI — so it can convey finer vertical deviation detail, at the cost of a much larger installation.
T-VASIS's 20 units are arranged as two 4-unit wing bars (one either side of the runway centreline), each bisected by a 6-unit longitudinal line — 8 + 12 = 20 units, versus PAPI's single compact 4-unit row.
No. Both are purely vertical visual approach-slope references. Neither provides lateral navigation, range information or the full precision guidance of an ILS.
Yes. Both are designed so their visual indications can correlate with a non-visual glide-path signal, giving the pilot a visual cross-check once the runway environment is in sight.
Not necessarily. A 3° slope is common, but another angle can be selected for a specific runway based on considerations like obstacle clearance or harmonisation with a non-visual glide path.
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 T-VASIS/AT-VASIS configuration, siting, and the recommendation to discontinue T-VASIS/AT-VASIS as standard systems from 1 Jan 2020.
ICAO Doc 9157, Aerodrome Design Manual Part 4
Visual Aids — design guidance for PAPI and T-VASIS light-unit types, optical cut-off characteristics and deviation-indication counts.
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 and installation details (L-880 four-box, L-881 two-box). T-VASIS is specified through ICAO Annex 14 and Doc 9157 rather than by FAA equipment ACs.
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 T-VASIS light units — for those units, follow ICAO Annex 14 / Doc 9157 Part 4 or the FAA PAPI specification, as the project requires.
Do not design a PAPI or T-VASIS installation 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.
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