Aerodrome Visual Aids for Navigation: PAPI, ALS, and Runway Lighting
Approach Slope Indicators (PAPI/APAPI)
The PAPI (Precision Approach Path Indicator) consists of 4 units (lights) in a wing bar, usually on the left side of the runway.
Diagram showing PAPI indications (right) for correct, high, and low glide path.
PAPI Indications:
- On slope: 2 Red (near runway) / 2 White (far runway).
- Slightly high: 1 Red / 3 White.
- Too high: 4 White.
- Slightly low: 3 Red / 1 White.
- Too low: 4 Red.
Mnemonic: "Red on White, you're alright" (though in PAPI they are side by side, the mix logic serves).
Approach Lighting Systems (ALS)
Typical runway elements: Threshold, Touchdown Zone, and Centerline.
- Simple Approach Lighting System:
- Row of lights on the extended centerline (minimum 420 m).
- Crossbar at 300 m from the threshold.
- Mandatory for non-precision approach runways (where physically practicable).
- Precision Approach Cat I:
- Extended centerline up to 900 m.
- Crossbar at 300 m (30 m long).
- Cat II and III:
- Require a more complex system with side rows and multiple crossbars (150 m and 300 m).
Runway and Taxiway Lights
Runway Centre Line:
- Variable White from threshold to 900 m from the end.
- Alternate Red/White from 900 m to 300 m from the end.
- Red for the last 300 m.
Taxiway Edge: Fixed Blue lights.
Left: Mandatory instruction sign (Red background). Right: Location sign (Black background/Yellow letter).Taxiway Centre Line: Generally green (specific coding in ILS sensitive areas).
Signs and Markings
- Mandatory Instruction Signs:
- Red background with White inscription.
- Include: NO ENTRY, Runway Designation, Holding Positions (Cat I/II/III).
- Identify locations that must not be crossed without ATC clearance.
- Road-holding position: Stop sign for vehicles, white on red background.
- Runway Designation Marking: Located at the threshold.
Markers
- Boundary Markers: Cones (usually orange or red/white) used to delimit the landing area when there is no paved runway.