PAPI VS AT-VASIS
Two visual glidepath systems built on completely different optical logic. PAPI reads as a simple white/red count across four units in a single row; AT-VASIS reads as a wing-bar plus fly-up/fly-down pattern across ten units arranged in a one-sided "T". Both exist to answer the same operational question — is the aircraft on the correct visual approach slope? — but AT-VASIS is now a legacy system.
At a Glance
| Feature | PAPI | AT-VASIS |
|---|---|---|
| Full name | Precision Approach Path Indicator | Abbreviated T Visual Approach Slope Indicator System |
| Light units | 4 | 10 |
| Arrangement | Single horizontal row | T-shaped, one side of runway |
| Location | Normally one side of runway | One side of runway |
| Main indication logic | White/red combination | Wing bar + fly-up/fly-down lights |
| Normal on-path indication | 2 White + 2 Red | Wing bar visible, no fly cues |
| Discrete deviation indications | 4 | 7 |
| Physical footprint | Compact | Larger |
| Optical principle | Different elevation settings per unit | Different optical cut-off characteristics |
| Installation complexity | Lower | Higher |
| Lateral/roll guidance | No | No |
| Current ICAO status | Current standard | Legacy — discontinuation recommended from 1 Jan 2020 |
What Is PAPI & What Is AT-VASIS?
PAPI
PAPI consists of four light units installed in a single horizontal row, normally on one side of the runway. Each unit has its own elevation setting, producing a white indication when the aircraft is above the designed approach path and red when it is below.
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.
AT-VASIS
AT-VASIS is an abbreviated, one-sided version of T-VASIS. ICAO specifies it as ten light units on one side of the runway — a four-light wing bar plus a six-light longitudinal line running back from it, resembling one arm of a "T".
Pilot interpretation
The pilot reads the wing-bar and fly-up/fly-down pattern along the longitudinal line — not a simple colour count.
AT-VASIS as Half of Full T-VASIS
The full T-VASIS uses 20 light units arranged symmetrically as wing bars on both sides of the runway centreline, each with its own longitudinal line, forming a complete "T" pattern that a pilot can read from either side. AT-VASIS is the abbreviated, one-sided derivative of that system — it keeps a single wing bar and a single longitudinal line, for a total of 10 units, rather than duplicating the arrangement on both sides.
AT-VASIS = one-sided, abbreviated version of T-VASIS. Full T-VASIS uses 20 units on both sides; AT-VASIS uses 10 units on one side only.
How Each System Signals Position
PAPI and AT-VASIS solve the same problem with fundamentally different optical logic — one is a light count, the other is a pattern read along a line.
PAPI
AT-VASIS
This is simplified for illustration — the actual AT-VASIS indication sequence along the six-unit longitudinal line is more sophisticated than a single high/on-path/low state, which is exactly why it yields more discrete deviation indications than PAPI.
PAPI's 4 vs AT-VASIS's 7 Deviation States
This is one of the more important technical distinctions between the two families. Beyond the single on-path indication, ICAO guidance associates each system with a different number of discrete deviation indications:
| System | Discrete Deviation Indications |
|---|---|
| APAPI | 2, plus the on-path indication |
| PAPI | 4, plus the on-path indication |
| T-VASIS / AT-VASIS | 7, plus the on-path indication |
AT-VASIS provides more discrete vertical-deviation information than PAPI — its ten-unit wing-bar-plus-line arrangement simply has more graduation to read than PAPI's four-unit row, even though PAPI became the more widely adopted system.
Compact Row vs One-Sided "T"
PAPI
Four units sit in a single, compact horizontal row. It is the smallest-footprint visual glide slope arrangement in common use, which keeps installation and alignment straightforward.
AT-VASIS
A four-unit wing bar sits at the base, with a six-unit longitudinal line extending back from it. The line makes AT-VASIS considerably more extensive on the ground than PAPI, though — unlike full T-VASIS — it is not duplicated on both sides of the runway centreline.
Elevation Settings vs Cut-Off Characteristics
PAPI
Each of the four units is set to a slightly different elevation angle. Where the aircraft sits relative to those overlapping angle boundaries determines how many units appear white versus red.
AT-VASIS
Units are designed with different optical cut-off characteristics rather than a simple elevation stack. This is what produces the wing-bar and fly-up/fly-down cues instead of a red/white count.
PAPI = four elevation settings read as a colour count. AT-VASIS = T-VASIS-family cut-off optics read as a bar-and-cue pattern.
Common Circuit Architecture, Different Scale
Both systems share a broadly similar simplified electrical arrangement — power supply, CCR / series circuit, isolation transformers, then the light units themselves. The difference is one of scale: AT-VASIS simply has more than double the light units of PAPI, which carries through to more isolation transformers, more circuit branches, more optical components and a larger installed footprint to wire and maintain.
- Intensity control
- Monitoring and fault indication
- Remote control
- ALCMS interface, depending on the installation
Keeping Each System On-Path
| Maintenance Item | PAPI | AT-VASIS |
|---|---|---|
| Light-source inspection | ✓ | ✓ |
| Lens / optics condition | ✓ | ✓ |
| Alignment | Critical | Critical |
| Electrical inspection | ✓ | ✓ |
| Transformer inspection | ✓ | ✓ |
| Number of units | 4 | 10 |
| Optical complexity | Lower | Higher |
| Physical area | Smaller | Larger |
| Overall maintenance burden | Lower | Higher |
With more than double the light units and a larger physical spread, AT-VASIS naturally carries a greater maintenance and alignment burden than PAPI — more units to inspect, more optics to keep clean, and a longer line to keep correctly aligned to the designed approach slope.
What Neither System Provides
Neither PAPI nor AT-VASIS provides the same function as an ILS. The ILS localizer gives lateral guidance and the glideslope gives vertical radio guidance; PAPI and AT-VASIS give the pilot a visual vertical approach-slope reference only, once the runway environment is in sight. Neither gives lateral navigation, range information, or runway centreline guidance.
Where an ILS or MLS is present, the visual system's siting and elevation settings are intended to make the visual approach slope conform as closely as possible to the relevant non-visual glide path — the normal approach slope is commonly 3°, though the actual selected slope can differ to account for obstacles or to correlate with a specific glide path.
PAPI or AT-VASIS can coexist with an ILS on the same runway. They serve as an independent visual cross-check, not a replacement for radio-based guidance.
Current Standard vs Legacy System
This is an important point for an aviation engineering comparison. ICAO recommended that the use of T-VASIS and AT-VASIS as standard visual approach-slope indicator systems be discontinued from 1 January 2020, with PAPI and APAPI remaining the current standard family.
No. The recommendation was against continuing AT-VASIS as a standard system for new installations — it does not mean an existing, correctly maintained AT-VASIS installation suddenly stops functioning.
No. AT-VASIS actually offers finer deviation graduation (7 states) than PAPI (4 states). PAPI became the standard mainly because of its smaller footprint, simpler optics and lower installation/maintenance burden — not because AT-VASIS was operationally inferior.
No. They differ in configuration, number of units, indication logic and installation footprint, and any conversion or new design must follow the current governing standard.
PAPI vs AT-VASIS — Engineering Parameters
| Parameter | PAPI | AT-VASIS |
|---|---|---|
| Units | 4 | 10 |
| Arrangement | Single row | T-shaped, one side |
| Parent system | PAPI family | T-VASIS family |
| Normal indication | 2 White + 2 Red | Wing bar visible |
| Deviation indications | 4 | 7 |
| Colour-count interpretation | Yes | No |
| Fly-up / fly-down concept | No | Yes |
| Physical footprint | Small | Large |
| Installation complexity | Lower | Higher |
| Maintenance | Lower | Higher |
| Centreline symmetry | No | No |
| ICAO status | Current standard | Legacy / discontinuation recommended |
PAPI vs AT-VASIS
AT-VASIS is not a cruder ancestor of PAPI — it is actually the more finely graduated of the two. Its ten-unit, wing-bar-plus-line arrangement yields seven discrete deviation indications against PAPI's four. PAPI overtook it as the standard mainly on practical grounds: a quarter of the units, a fraction of the footprint, simpler pilot interpretation, and a lower installation and maintenance burden.
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's dominance is a practicality story, not a performance story — AT-VASIS installations still in service should be assessed and maintained on their own terms, not treated as an inferior predecessor.
Engineering & Expert Notes
Treat Legacy Systems as Maintenance Challenges
LegacySpares and expertise may be limited. Plan ahead.
Check the Current Requirement Before Retaining
ComplianceConfirm that the retained system is still acceptable.
Verify Alignment on Every Unit
AlignmentMulti-unit systems have more points to check.
Document Any Replacement Plan
RecordsKeep a record of the decision and timeline.
Frequently Asked Questions
PAPI uses four light units in a single row, read as a white/red count. AT-VASIS uses ten units — a four-unit wing bar plus a six-unit longitudinal line — read as a wing-bar and fly-up/fly-down pattern rather than a colour count.
Because it is derived from the T-VASIS family, which uses a longitudinal line of units to give finer graduation of vertical position — seven discrete deviation indications compared with PAPI's four.
ICAO recommended that T-VASIS and AT-VASIS as standard visual approach-slope indicator systems be discontinued from 1 January 2020, with PAPI and APAPI remaining the current standard family. This affects their status as a standard for new installations — it does not mean existing systems are automatically decommissioned.
Full T-VASIS uses 20 units arranged symmetrically on both sides of the runway. AT-VASIS is the abbreviated, one-sided version, using just 10 units — a single wing bar and longitudinal line.
No. Both systems provide vertical approach-path guidance only — neither gives lateral navigation, range information or centreline guidance.
Mainly practical reasons: PAPI needs only four units in a compact footprint, has simpler electrical and optical systems, is easier to align, and is simpler for pilots to interpret — even though AT-VASIS offers more discrete deviation indications.
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 system configurations, including PAPI, APAPI, T-VASIS and AT-VASIS siting and photometric requirements.
ICAO Doc 9157 — Aerodrome Design Manual, Part 4
Guidance on visual aids design, including the number of discrete deviation indications associated with PAPI, APAPI and T-VASIS/AT-VASIS.
FAA AC 150/5345-28, Current Edition
Precision Approach Path Indicator (PAPI) Systems (equipment designations L-880/L-881) — FAA equipment standards covering unit geometry, siting, electrical systems and photometric criteria. T-VASIS/AT-VASIS are ICAO Annex 14 systems not part of the current FAA visual-aid family, and are not addressed by this AC.
Final Engineering Note
Do not design a PAPI or AT-VASIS installation from a generic web diagram. The final configuration, siting and alignment must always be developed from the current governing standard, approved airport design and project-specific requirements.
AT-VASIS is not simply "an old PAPI" — it is a different member of the visual glide slope family, with its own unit count, optical logic and installation footprint. Either system, correctly designed, aligned and maintained, exists to answer the same question for the pilot on approach.
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