Visual Approach Slope Indicator (VASI)
A VASI system normally consists of two light bars located on the left or right side of the approach end of the runway. An on glide path indication is provided when the top or far bar is red, while the bottom or closer bar is white. Red over red is a below glide path indication, and white over white is an above glide path indication.
At some locations, a three bar VASI system is used. With these types of VASI's, the near and middle bars provide on glide path angle, while the upper two bars provide a steeper descent angle. Use of a three bar VASI involves choosing whether you want to use the near and middle bars or descent on the steeper angle provided by the top two bars. Treat the two bars you choose as a two bar VASI.
Usable range for a VASI system is 3-5 miles during the day and up to 20 miles during the night.
When operating in Class B, C, or D airspace, pilots are required to stay on or above the glide path projected by a visual approach slope indicator, unless a lower altitude is necessary to make a safe landing.
Although the minimum altitude rule is listed in the section for Class D airspace, the sections for operations in Class B and C airspaces state 91.129 must still be complied with. As a result, this regulation applies to operations in Class B, C, and D airspace areas.
Precision Approach Path Indicator (PAPI)
A PAPI system consists of four lights located on the left or right side of the approach end of the runway. The four lights appear red or white in different combinations to indicate glide path to the pilot.
If all four lights are white, the PAPI is giving the pilot an above glide path indication. One red and three white lights indicate slightly high. Two red and two white lights indicate the aircraft is on glide path. One white and three red lights indicate slightly low. Finally, if all four lights are red, the PAPI is indicating the aircraft is very below the glide path.
Useable range of a PAPI is about 5 miles during the daytime and up to 20 miles at night.
Tri-color Glide Path Indication Systems
Tri-color glide path indication systems normally consist of a single light projection unit, which projects three different colors to indicate glide path to the pilot. Green is an on glide path indication. Red is a below glide path indication, and amber is an above glide path indication.
Useful range for these types of systems is about 1/2 mile during the daytime and up to five miles at night.
Pulsating Glide Path Indication Systems
A pulsating glide path system usually consists of a single light unit, which appears differently to the pilot depending on the aircraft's glide path. If the light appears as steady white, this is an on glide path indication. Steady red is a slightly below glide path indication. If the aircraft continues to descend below glide path, the light will begin to pulse red. The further below glide path the aircraft gets, the more rapid the red light pulsates.
Similarly, as the aircraft strays on the high side of the glide path, the white light begins to pulsate. The higher above glide path the aircraft is, the faster the white light pulsates.
Useful range of these systems is about four miles during the day and up to ten miles at night.
Element Alignment Systems
One simple and cheap method for providing pilots visual glide path indications is to place painted or lit plywood panels adjacent to the approach end of the runway. When the different elements of such a system are in alignment, it indicates to the pilot that the aircraft is on the proper glide path.
Useful range of these systems is about 3/4 of a mile.
VASI and PAPI systems provide obstruction clearance within 10 degrees of the extended runway centerline out to a distance of 4 nm from the runway threshold.