WHY APPROACH LIGHTING IS IMPORTANT
Approach lighting helps the pilot establish visual reference with the runway environment. Important information can include:
- Runway alignment
- Longitudinal perspective
- Roll (bank) reference from the crossbars
- Threshold identification
- Lateral reference
- Transition from instrument to visual conditions
MAIN FAA APPROACH LIGHTING TERMS
ALSF-1
Approach Lighting System with Sequenced Flashing Lights, Category I configuration. A high-intensity approach lighting configuration in FAA terminology, associated with Category I approaches. Typical length 2,400–3,000 ft.
ALSF-2
Approach Lighting System with Sequenced Flashing Lights, Category II configuration. FAA describes it as a high-intensity system supporting Category II/III instrument approaches. Typical length 2,400–3,000 ft.
SSALR
Simplified Short Approach Lighting System with Runway Alignment Indicator Lights. A simplified arrangement with runway alignment indication. FAA describes it as medium intensity and associated with Category I approaches; at some runways the ALSF-2 can also operate in an SSALR mode. Typical length 1,400 ft.
MALSR
Medium Intensity Approach Lighting System with Runway Alignment Indicator Lights. Installed in runway approach zones along the extended centreline; combines steady-burning light bars with sequenced flashers (RAIL). Associated with Category I approaches; typical length 1,400 ft.
ALSF = Approach Lighting System with Sequenced Flashing Lights
MALSR = Medium-intensity system + runway alignment indicator lights
SSALR = Simplified Short approach system + runway alignment indicator lights
RAIL = Runway Alignment Indicator Lights (sequenced flashers on the extended centreline)
Other FAA systems
- MALS: medium-intensity approach lighting system. MALSF adds sequenced flashers; MALSR adds runway alignment indicator lights (RAIL).
- SALS: short approach lighting system, a short-length system for runways that do not need a full precision approach lighting system.
- ODALS: omnidirectional approach lighting system, omnidirectional sequenced flashing lights, typically for non-precision runways.
Indian airport example
Current Indian AIP publications show that aerodromes use different approach lighting configurations depending on the runway and operational characteristics. For example, current AAI eAIP data includes runway entries showing CAT I approach systems, CAT II approach systems, PAPI and touchdown-zone lighting at different aerodromes. Never assume every airport uses the same ALS configuration. In the AIP, look at the aerodrome section AD 2.14 (approach and runway lighting), which gives the type, length and intensity of approach lighting for each runway.
ICAO ANNEX 14 APPROACH LIGHTING
Indian aerodromes follow ICAO-based provisions, so the FAA names above (ALSF, MALSR and so on) are not normally what you will see in an AAI AIP entry. ICAO describes approach lighting by category rather than by system name. The values below are the standard Annex 14 layouts; always confirm against the applicable AIP or airport specification.
| ICAO system | Typical layout |
|---|---|
| Simple approach lighting | Centreline row of lights extending not less than 420 m from the threshold wherever possible, with a crossbar at 300 m; for non-instrument and non-precision runways |
| Precision approach CAT I | Centreline barrettes extending 900 m from the threshold wherever possible, with a crossbar at 300 m; white lights. A shorter system may be unavoidable at some sites but can limit operating minima |
| Precision approach CAT II/III | Centreline 900 m wherever possible, crossbars at 300 m and 150 m, and red side-row barrettes over the inner 270 m |
For CAT II/III the inner 300 m of approach lighting also has the shortest secondary-power switch-over (1 s under Annex 14 Table 8-1, check the current edition); see Note 17.
Related notes: 16 CAT I, II & III AGL · 18 ICAO Annex 14 for AGL · 19 FAA AC 150/5340 Series.
PAPI: PRECISION APPROACH PATH INDICATOR
PAPI provides visual approach slope guidance. FAA describes it as normally consisting of two or four lights installed in a single row, normally to the left of the runway. The two-light version is called APAPI (Abbreviated PAPI). The common four-light interpretation:
| Pilot sees | Meaning |
|---|---|
| 4 White | Too high |
| 3 White + 1 Red | Slightly high |
| 2 White + 2 Red | On slope |
| 1 White + 3 Red | Slightly low |
| 4 Red | Too low |
Try it: tap a position
Viewed from the approach: the left light is the unit farthest from the runway and the right light is the unit nearest the runway, so the first light to turn red when you drop below the slope is the one nearest the runway.
Easy memory
- MORE WHITE = HIGH
- MORE RED = LOW
- 2 WHITE + 2 RED = ON PATH
Two-light APAPI
| Pilot sees | Meaning |
|---|---|
| 2 White | Above slope |
| 1 White + 1 Red | On slope |
| 2 Red | Below slope |
APAPI gives only three indications, so it is less precise than a four-light PAPI; it is typically used where the aircraft mix does not justify a full PAPI. See Study Note 11: PAPI & APAPI for siting and aiming.
VASI: VISUAL APPROACH SLOPE INDICATOR
VASI also provides vertical approach slope guidance. FAA describes a common two-bar interpretation:
| Pilot sees | Meaning |
|---|---|
| White/White | Above path |
| Red/White | On path |
| Red/Red | Below path |
Read the indication as far bar / near bar: red over white means on path. Three-bar VASI arrangements can provide two visual glide paths (a lower one for most aircraft and a higher one for aircraft with a high pilot eye height).
PAPI VS VASI
| Feature | PAPI | VASI |
|---|---|---|
| Full form | Precision Approach Path Indicator | Visual Approach Slope Indicator |
| Common arrangement | 4 lights | 2 bars |
| Main function | Vertical approach guidance | Vertical approach guidance |
| Typical on-path indication | 2 White + 2 Red | Red/White |
| Main concept | Colour balance | Colour relationship |
| Two-light version | APAPI | None |
| Standard status | Used under ICAO and FAA | FAA legacy; not an ICAO standard system |
APPROACH & VISUAL AIDS REVISION TABLE
| System | Remember |
|---|---|
| ALSF-1 / ALSF-2 | High intensity with sequenced flashers; ALSF-1 is CAT I, ALSF-2 supports CAT II/III (FAA) |
| SSALR | Simplified short system with RAIL, medium intensity, CAT I (FAA) |
| MALSR | Medium intensity with RAIL along the extended centreline (FAA) |
| ICAO CAT I | Centreline 900 m, crossbar at 300 m |
| ICAO CAT II/III | Centreline 900 m, crossbars at 300 m and 150 m, red side rows over inner 270 m |
| PAPI / APAPI | 4 lights: 2 white + 2 red = on slope. APAPI (2 lights): 1 white + 1 red = on slope |
| VASI | Two bars; red/white = on path. FAA legacy, not an ICAO standard system |
APPROACH LIGHTING REVISION QUESTIONS
What is the purpose of approach lighting?
To give pilots runway alignment, height perception and horizontal reference as they move from instrument to visual conditions.
What does ALSF-2 support?
Category II/III instrument approaches in FAA terminology (high-intensity system with sequenced flashers).
What does RAIL stand for?
Runway Alignment Indicator Lights: sequenced flashers along the extended centreline, used in MALSR and SSALR.
What does a pilot see on a four-light PAPI when on the glide slope?
Two white and two red lights.
What does a pilot see when slightly high on a PAPI?
Three white and one red.
What is the ICAO CAT I approach lighting length?
Centreline extending 900 m from the threshold, with a crossbar at 300 m.
What is the ICAO CAT II/III approach lighting layout?
Centreline of 900 m wherever possible, crossbars at 300 m and 150 m, and red side-row barrettes over the inner 270 m.
What does a two-light APAPI show when on the approach slope?
One white and one red light. Two white lights mean above the slope and two red lights mean below it.
Is VASI an ICAO standard visual approach slope indicator?
No. VASI is an FAA legacy system. ICAO Annex 14 specifies PAPI and APAPI; Annex 14 recommends that T-VASIS and AT-VASIS be discontinued as standard systems from 1 January 2020; the simple two-bar and three-bar VASIS were deleted from Annex 14 in 1995.
Why should FAA system names not be assumed for an Indian aerodrome?
Indian aerodromes follow ICAO-based provisions and AIP entries describe approach lighting by category, not by FAA system name.
What is a simple approach lighting system in ICAO terms?
A centreline row of lights extending at least 420 m from the threshold wherever possible, with a crossbar at 300 m, used for non-instrument and non-precision runways.
Key points for exams
- PAPI = Precision Approach Path Indicator
- PAPI common 4-light on-path indication = 2 White + 2 Red
- More White = Above Path
- More Red = Below Path
- ALSF-2 = High-intensity approach lighting associated with CAT II/III FAA applications
- MALSR = Medium-intensity approach system with runway alignment indicator lights
- APAPI (2 lights): 1 White + 1 Red = on slope
- VASI = FAA legacy; ICAO uses PAPI/APAPI
ENGINEERING & EXPERT NOTES
Know Which Cue Each Element Gives
ConceptCentreline, crossbars and flashers each give the pilot different information. Understand what is lost when one is missing.
Aim and Level Matter Along a Long Pattern
AlignmentSmall errors repeated along a line are visible from the cockpit. Verify aiming at commissioning and after impacts.
Walk or Drive the Pattern at Night
InspectionDim, misaligned or missing units show up in darkness when they can be missed by day.
Do Not Mix ICAO and FAA Terms Without Care
StandardsTerminology and configurations differ. Say which framework you are using.
REFERENCES & FURTHER READING
These notes are a study aid. Verify details against the primary sources below.
| # | Source | Use it for |
|---|---|---|
| 1 | ICAO Annex 14, Volume I – Aerodrome Design and Operations | Chapter 5 (Visual Aids for Navigation): approach lighting systems by category and visual approach slope indicators (PAPI). |
| 2 | ICAO Doc 9157 – Aerodrome Design Manual | Part 4 (Visual Aids): approach lighting and PAPI design guidance. |
| 3 | FAA AC 150/5340-30J – Design and Installation Details for Airport Visual Aids | FAA approach lighting (ALSF-1/2, SSALR, MALSR) and PAPI/VASI installation details. |
| 4 | FAA Aeronautical Information Manual (AIM) – Chapter 2, Section 1: Airport Lighting Aids | FAA descriptions of approach lighting systems, PAPI and VASI indications. |
| 5 | FAA AC 150/5345-28 – Precision Approach Path Indicator (PAPI) Systems | Equipment specification for PAPI. Use the current edition. |
| 6 | FAA AC 150/5345-52 – Generic Visual Glideslope Indicators (GVGI) | Specification for generic visual glideslope indicators. Use the current edition. |
| 7 | FAA AC 150/5340-26C – Maintenance of Airport Visual Aid Facilities | Maintenance of PAPI, VASI and medium intensity approach lighting systems. |
| 8 | AAI AIP India / eAIP and DGCA CAR Section 4, Series B, Part I – Aerodrome Standards | Indian approach lighting and PAPI entries (aerodrome section AD 2.14); confirm the category and layout in the applicable AIP. |
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