PAPI and VASI

A visual approach slope indicator is a group of light units beside a runway that shows a pilot on final approach whether the aircraft is above, on or below a defined approach slope. In the PAPI and the VASI, each unit projects toward the approach a beam that is white in its upper part and red in its lower part, so the color the pilot sees depends only on the angle at which the eye views the unit. The precision approach path indicator (PAPI) is the current standard; the VASI, which carries the family's generic name, is its predecessor, still found at some aerodromes.

These lights matter because the eye judges an approach angle badly: at night, over featureless ground or toward an unusual runway, the picture suggests the wrong height, usually producing a low approach. ICAO Annex 14 standardizes the systems and chapter 2 of the FAA's AIM describes those in US use (see aerodrome lighting and visual aids). ICAO speaks of visual approach slope indicator systems (VASIS) and the approach slope; the FAA calls the family visual glideslope indicators (VGSI) and speaks of the glide path.

Purpose and the family of indicators

A slope indicator gives vertical guidance only. Lateral guidance comes from the runway and its lights, and the FAA says descent on a VASI or PAPI should not begin until the aircraft is visually aligned with the runway. Its obstacle protection is limited too: the FAA's PAPI typically provides safe obstruction clearance within 10° either side of the extended centerline out to 3.4 NM from the threshold, a VASI out to 4 NM. Either may be offset or have its usable range shortened, which the Chart Supplement or NOTAMs then state.

The indicators also appear in the rules. Under EASA rules a visual glide slope indicator is one of the references that allow an approach to continue below a Category I decision height or the MDH, and 14 CFR 91.175 lists it among those that permit descent below DA or MDA. Under 14 CFR 91.129(e)(3), written for operations to an airport in Class D airspace and applied to Class C and B airspace by 91.130 and 91.131, the pilot of an airplane approaching to land on a runway served by a visual approach slope indicator must stay at or above its glide path until a lower altitude is necessary for a safe landing; normal bracketing above and below the path is allowed. AIPs add local rules: at Lyon the runway 35L and 35R PAPIs are mandatory for approaches without vertical guidance, because of an obstacle, and at Brest the VPT to runway 07R is prohibited for turbofans with the PAPI unserviceable. France's national rules, in a difference filed to Annex 14, do not require a PAPI or APAPI on runways used by turbojet airplanes; European rules follow ICAO.

PAPI: four lights and their meaning

ICAO defines the PAPI as a wing bar of four uniformly spaced light units, normally on the left of the runway. The units are set at slightly different angles, the one nearest the runway highest, so as the aircraft descends through the slope they turn from white to red one at a time, starting next to the runway.

On the slope the two units nearest the runway show red and the two outer ones white; seen from the approach with the bar on the left, the picture reads white, white, red, red. One more red light means lower, one fewer means higher. The correction should be gentle, back toward two red and two white, never a dive at the runway.

The FAA says a PAPI is visible from about 5 miles by day and up to 20 at night. An ICAO-format AIP such as France's lists it in each aerodrome's section AD 2.14 with its angle, in degrees and as a gradient, and its MEHT; the visual landing chart labels it, as in PAPI 3.2° (5.6%) for Nice's runway 22L. The FAA Chart Supplement gives a glide angle and threshold crossing height instead, as in PAPI(P4L)—GA 3.0° TCH 45' at Battle Mountain, and FAA airport sketches mark a PAPI with a circled P.

APAPI

The abbreviated PAPI (APAPI) has two units. One red and one white is on the slope, two white above it, two red below it. The classic error is to read it like a four-light PAPI, on which one red light means slightly high: on an APAPI that is the on-slope picture, and two red lights mean below the slope, not on it. The FAA likewise describes PAPIs of two or four units.

VASI, T-VASIS and other systems

FAA VASI installations have 2, 4, 6, 12 or 16 light units arranged in near, middle and far bars. Those of up to six units stand on one side of the runway, usually the left; those of 12 or 16 units on both sides. Each unit is white in the upper part of its beam and red in the lower.

A two-bar VASI gives one glide path, normally 3°. On it the near bar shows white and the far bar red, seen as red over white; both white means too high, and both red too low, calling for a shallower descent until the near bar turns white. A three-bar VASI gives two paths: the lower, from the near and middle bars, normally 3°, and the upper, from the middle and far bars, normally 1/4° higher. The upper path is for high-cockpit aircraft only, to give them a sufficient threshold crossing height; on it the near and middle bars show white and the far bar red. The FAA quotes a VASI's range as 3 to 5 miles by day and 20 miles or more at night.

ICAO's standard systems are the PAPI and APAPI and, where still in service, the T-VASIS and its abbreviated form, the AT-VASIS, whose lights form a T pattern that includes a horizontal wing bar; the AT-VASIS stands on the left of the runway only. ICAO recommended phasing out T-VASIS and AT-VASIS as standard systems from 1 January 2020, and France's national rules do not allow them. The two-bar VASI, common in the United States, is not an Annex 14 standard system.

The FAA also describes single-light systems. A tri-color indicator shows red below the path, amber above and green on it, and is usable from about 1/2 to 1 mile by day and 5 miles at night. A pulsating indicator shows steady white, or alternating red and white, on the path, steady red slightly below, pulsating red further below and pulsating white above. Either can be confused with other lights. Alignment of elements systems are painted panels the pilot keeps in line, usable from about 3/4 mile.

Eye-to-wheel height and threshold crossing

A slope indicator guides the pilot's eye; the wheels follow below and behind it. The eye-to-wheel height is the vertical distance between the pilot's eye and the lowest part of the main wheels on the approach. It is small in a light airplane and several meters in a widebody: even on the ground the eye sits about 3.5 m up in a DC-9 against about 8.7 m in a Boeing 747.

The minimum eye height over the threshold (MEHT) is the lowest height of the pilot's eye above the threshold at which the PAPI still shows the on-slope picture. It is not a decision height. The wheels cross the threshold at roughly the MEHT minus the eye-to-wheel height.

Worked example. Lyon's runway 35L has a 3.0° PAPI with an MEHT of 59 ft, about 18 m. An airplane whose eye-to-wheel height is 12 m therefore crosses the threshold with its wheels only about 6 m up, and the AIP says exactly that: for airplanes with an eye-to-wheel height greater than 12 m the threshold crossing margin is less than 6 m, and their crews are to disregard the PAPI signal.

The geometry explains the size of the MEHT. A 3° slope is a 5.2% gradient (tan 3° = 0.0524), so a slope that meets the runway 400 m beyond the threshold, where the aiming point marking stands on runways with 2,400 m or more of landing distance, crosses the threshold at about 400 × 0.0524 ≈ 21 m (69 ft). Where very large aircraft use a runway, the PAPI may be set for them: Toulouse-Blagnac's four 3° PAPIs, with MEHTs of 68 and 75 ft, are calibrated for the threshold overflight of the Boeing 747. The three-bar VASI's upper path does the same job for high cockpits.

Approach slope angle and the ILS

The approach slope a PAPI defines is normally 3°, but obstacles or terrain can demand more. French AIPs list, among others, Nice's runway 22 PAPIs at 3.2° (5.6%, MEHT 67 ft), Ajaccio's runway 20 at 3.7° (6.5%, MEHT 63 ft), Cannes at 4.0° (7.0%) and Chambéry's runway 18 at 4.5° (7.8%, MEHT 46 ft). The FAA notes that angles may reach 4.5° for obstacle clearance and warns pilots of high-performance aircraft that VASI angles above 3.5° may increase the runway length needed for landing and rollout.

A steeper slope needs a higher rate of descent. The rate in ft/min is the gradient in percent times the groundspeed in knots, plus about 1% (a knot is 101 ft/min): on a 3° slope at 140 kt about 740 ft/min, on Chambéry's 4.5° slope about 1,100 ft/min. ICAO's noise abatement guidance, for its part, says approach procedures should not require an approach above the ILS, VASI or PAR final approach angle.

Where a runway has both an ILS and a PAPI, the two paths need not coincide. The ILS glide path crosses the threshold at its reference datum height, which ICAO sets at 15 m (50 ft), and the aircraft's glide slope antenna follows it; the MEHT is a height of the pilot's eye. On the ILS or LOC RWY 14R at Boeing Field, the chart gives a glide slope threshold crossing height of 39 ft and a visual glide slope indicator TCH of 55 ft, and warns that the two are not coincident (see instrument landing system). On FAA non-precision approaches a chart note likewise flags a VGSI not coincident with the vertical descent angle, and the visual descent point is harmonized with the VGSI angle.

Temperature can separate the two paths as well. The RNP RWY 14L chart at Toulouse warns that below −14 °C an uncompensated Baro-VNAV, flying below its designed path in the cold air, may show four red lights on the PAPI (see cold-weather altimetry).

Errors and illusions

The slope indicator is the remedy for the landing illusions. A black-hole approach over unlit terrain or water, a narrower runway than usual, and an upsloping runway or rising terrain before it all make the pilot feel higher than the aircraft is, and the instinctive descent produces a low, flat approach; a downsloping or wider runway invites a high one (see visual illusions). None of them can distort the PAPI's angle, which is why the AIM recommends an electronic glide slope or VASI when available.

The lights mislead only when used outside their design:

• Before alignment or beyond the protected area. Descent on the lights should wait until the aircraft is aligned. Ajaccio's runway 20 PAPI does not clear Punta Sant' Eliseo (4,170 ft): its protection ends 12 km from the displaced threshold, and its use beyond is prohibited.
• Offset installations. The runway 35 PAPI at Cannes is offset 5° east of the centerline.
• A large eye-to-wheel height at a PAPI not set for the type, as at Lyon.
• The wrong count. An APAPI read as a PAPI, or a single light taken for another.
• A different path. A non-coincident ILS or a cold-temperature Baro-VNAV error.

On a visual or circling approach the slope indicator may be the only vertical guidance left, and following it to the flare is part of a stabilized approach.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.