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The '''Precision Approach Path Indicator (PAPI)''' is a [[light]]/[[radio]] system positioned beside the runway that consists of two or four boxes of lights that provide a visual indication of an airplane's position on the [[glidepath]] for the associated [[runway]]. The [[FAA]] standard for the PAPI is the same as the [[ICAO]]'s standard Visual Glide Slope Indicator.
1) A [[Localizer]]
2) A [[Glidescope]]
3) An [[Outer Marker]]
4) An [[Middle Marker]]
5) An [[Inner Marker]]
The Glidescope is a set of two or four boxes of lights that provide a visual indication of an airplane's position on the [[glidepath]] for the associated [[runway]]. These lights are directional lights at 3 degrees angle.
Each box of lights is equipped with an [[optics|optical]] apparatus that horizontally splits light output into two phases: red and white. Depending on the viewing (and thus the descent) angle, a pilot will see either the red or the white portion.
A normal glidepath is indicated by an even split in red and white lights. If an airplane is beneath the glidepath, more red lights visible to the pilots; if an airplane is above the glidepath, more white lights are visible.
During aircraft [[descent]], this system, along with other airport lights, may be activated by the [[pilot]] by keying the airplane [[microphone]] with the airplane's communication radio tuned to the [[CTAF]].
The glidescope helps the pilot to maintain an appropriate safe descent andle during landing.
A Localizer helps the pilot to approach the landing strip and be on runway centerline, marked by dashed white lines. The localizer is placed is such a way that if the aircraft deviates from the runway centerline the pilot would hear distinct tones. One indicating that he's offsetted towards the left of the runway centerline and the other indicating that he's offsetted towards the right of the runway centerline. These two tones are at different frequencies.
The outer marker [[beacon]] is placed some miles before the runway.
The middle marker [[beacon]] is placed some feet on the runway towards the other end.
The inner marker [[beacon]] is placed some feet ahead of the middle marker.
These marker beacons transmit directional RF ([[Radio Frequency]]) with [[Amplitude Modulation]] (AM) vertically up. The pilot when crosses these marker beacons he hears a distinct beep tone. The pilot has with him a flight plan which tells him that when approaching a runway/landing strip maintaing a correct altitude he would hear the outer marker at an a priori know altitude so on and so forth with the middle and inner marker beacons.
So when the pilot hears the beep tone he compares his altitude with that given in the flight plan, if his altitude is lesser then he's lower than the specified altitude and he pulls up to make a safe landing. If his altitude is higher then he's higher than the specified altitude and he pulls up to make a safe landing.
All these are used in conjunction with [[ILS]]/([[Instrument Landing System]]).
== Sources ==
* [http://av-info.faa.gov/terps/Directives_files/8260.49.pdf FAA TERPS directive]
* An [[FAA]] Aircraft Accident Report of Federal Express Flight 1478
[[Category:Aviation]]
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