VASI, PAPI, AVASI and APAPI

VASIS = umbrella term for approach slope indicators (TC AIM AGA 7.6.1):

• VASI
• AVASI (abbreviated VASI)
• PAPI
• APAPI (abbreviated PAPI)

Basics:

• Visible from about 4 NM.
• Show desired approach slope, usually 3°.
• PAPI = current Canadian standard. Some airports still run older VASI.
• Full (four-unit) VASI and PAPI: left side of runway only.
• Strip too narrow: AVASI or APAPI (two units) installed instead.
• Do not start descending on the indication until visually aligned with runway centreline.

EWH (eye-to-wheel height):

• In-flight vertical distance from eye path to wheel path at Vref, max certified landing weight.
• VASIS categorized by EWH; one published for an EWH category usable by all aircraft in that group unless stated otherwise (AGA 7.6.4.1).

Light-unit layout:

PAPI reads by count of red units, counted from runway edge inward:

APAPI (two units, coarser 3-step read):

VASI reads by bar, not unit count:

• Well below slope: two bars' lights merge into one red signal.

Contaminated lens:

• Frost or fog on PAPI/APAPI lens can mix red and white beams: false, mostly white signal.
• Suspected: disregard display, use other cues (runway perspective).

Trap: fogged lens reads falsely white, mistaken for "fly down" when actually on or below slope.

Practice