Approach Lighting can be broken down into two separate systems, the actual Approach Light System (ALS) and Visual Glideslope Indicators.
As a generalization, Approach Light Systems provide lateral or horizontal guidance to align an aircraft with the runway centerline, and Visual Glideslope Indicators provide vertical guidance to the runway or a specified landing point on the runway.
Approach Light System (ALS)
Approach Light Systems are specific to instrument approach equipped runways, and help a pilot transition from instrument flight rules to visual flight rules.
There are several different types/setups of the lighting systems, each unique to the level of guidance required for the instrument approach. In general, the light system will begin at the runway threshold and extend from 1,500' to 3,000' before the runway on the approach end (farther distances are for precision approaches while shorter systems are for non-precision approaches).
The basic systems generally consist of fixed lights, while more intensive systems may include sequenced flashing lights that appear to the pilot as though the lights are moving forward toward the runway and guiding the aircraft in with them.
Specifics for the ALS at individual airports can be found by identifying the light system codes/images on the instrument approach plate airport diagram and then referring to the legend of the Digital Terminal Procedures Supplement for specifics on what the light codes indicate.
Visual Glideslope Indicators
There are three main types of Visual Glideslope Indicators, all of which can be further broken down into more specific instruments within the category of indicator. The main terms you will hear for the glideslope indicators are Visual Approach Slope Indicator (VASI), Precision Approach Path Indicator (PAPI), and Tri-Color Systems. All of these systems serve the same purpose, which is to align an aircraft vertically with the runway
Visual Approach Slope Indicator (VASI)
VASIs come in multiple configurations and arrangements to allow for use by multiple sizes of aircraft. The base configuration is a 2 bar VASI, which consists of 2 bars of lights, near and far, ad project one glideslope. 3 bar VASIs incorporate a middle bar as well, and allow for two glideslopes to be projected.
A VASI is typically set to provide a glideslope descent angle of 3 degrees, although there are some that indicate higher descents for obstacle clearance purposes. A VASI has an intensity that can be seen roughly 3-5 miles from the air in daylight hours and up to 20 miles at night.
The base concept behind the operation of a VASI is that depending on the angle you perceive the light, it will appear either white or red. On a two bar VASI, a proper glidepath will be indicated by the near bar showing white lights and the far bar showing red lights. Being above glideslope is shown by both the near and far bars indicating white, and below glideslope is indicated by both the near and far bars indicating red.
A handy trick to remembering this is the phrase "White over White, you're high as a kite; Red over White, you're alright; Red over Red, you're dead."
On a three bar VASI, all aircraft other than high cockpit aircraft (high cockpit refers only to certain very large aircraft) will utilize the lower glide path. This means you refer to the near and middle bar to distinguish your position on the glideslope and read it the way you would a two bar VASI system.
A three bar VASI
Precision Approach Path Indicator (PAPI)
A PAPI utilizes the same concept as a VASI, only the lights are laid out in a single row rather than multiple bars.
The four lights will appear red or white depending on the aircraft's angle relative to the glideslope. Due to the layout of the lights, more precision is allowed for in the representation of aircraft position.
Similarly to a VASI, the PAPI intensity is such that it should be visible 3-5 miles from the airport during daylight hours and up to 20 miles at night.
Tri color systems are comprised of a single light that projects three different colors depending on the angle from which it is viewed with reference to the proper glideslope. Above glideslope is indicated by an Amber light, on glideslope is a Green light, and below glideslope is a Red light. Caution should be used when flying an approach using a tri-color system since a band of amber light is sometimes mistakenly seen during the transition from green to red if a pilot descends below glideslope.
The useful range of a tri-color system is only one half to one mile during the day and up to 5 miles at night.