Precision approach path indicator: Difference between revisions

Revision as of 15:48, 1 March 2023

Revision as of 15:52, 1 March 2023

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==Interpretation==

[[File:comparison_visual_landing_systems.svg|thumb|250px|Comparison of [[Precision approach path indicator|PAPI]], [[Visual approach slope indicator|VASI]], and [[Optical landing system|OLS]] meatball and datum lights {{nowrap|(not to scale)}}]]

The ratio of white to red lights seen is dependent on the angle of approach to the runway. Above the designated [[glide slope]] a pilot will observesee more white lights than red; at approaches below the ideal angle more red lights than white will be seen. At the optimum approach angle the ratio of white to red lights will be equal, for most aircraft.

The greater number of red lights visible compared with the number of white lights visible in the picture means that the aircraft is flying below the glide slope. To use the guidance information provided by the aid to follow the correct glide slope a pilot would manoeuvre the aircraft to obtain an equal number of red and white lights.

Student pilots in initial training may use the mnemonic

Line 46 ⟶ 44:

The light characteristics of all light units are identical. In good visibility conditions the guidance information can be used at ranges up to {{convert|5|mi|km}} by day and night. At night the light bars can be seen at ranges of at least {{convert|20|mi|km}}.

Each light unit consists of one or more light sources, red filters and lenses. Each light unit emits a high-intensity beam. The lower segment of the beam is red, and the upper part is white. The transition between the two colours must take place over an angle not greater than three minutes of arc. This characteristic makes the color change very conspicuous, a key feature of the PAPI signal. To form the PAPI guidance signal, the color transition boundaries of the four units are fixed at different angles. The lowest angle is used for the unit furthest from the runway, the highest for the unit nearest to the runway. The designated glideslope is midway between the second and third light unit settings. Depending on the position of the aircraft relative to the specified angle of approach, the lights will appear either red or white to the [[aviator|pilot]]. The pilot will have reached the normal glidepath (usually 3 degrees) when there is an equal number of red and white lights. If an aircraft is beneath the glidepath, red lights will outnumber white; if an aircraft is above the glidepath, more white lights are visible.

PAPI systems are readily available from airfield lighting manufacturers worldwide. PAPI is normally operated by [[air traffic control]] (ATC). If ATC services are not normally provided at an aerodrome, PAPI along with other airport lights may be [[pilot-controlled lighting|activated by the pilot]] by keying the aircraft microphone with the aircraft's communication radio tuned to the [[CTAF]] or dedicated pilot controlled lighting (PCL) frequency.