PAPI / VASI Approach Trainer
Height and distance on final give the PAPI and VASI indications and their relationship to the flight path.
Loading the tool…
Move the airplane on final - drag it in the side view, or set the distance and height - and watch the lights. Every light unit shines white above its aiming angle and red below it, so the picture depends only on the angle from your eyes down to the lights. A four-light PAPI shows four white when high (more than 3.5° on a 3° PAPI), three white slightly high, two and two on the glide path, one white slightly low and four red when low (less than 2.5°). A two-bar VASI shows red over white on the glide path - the far bar red, the near bar white; a three-bar VASI gives a light airplane the lower glide path (white, red, red) and a high-cockpit airplane the upper one (white, white, red).
The side view shows why. The dashed lines are the aiming angles of the units; the green line is the glide path; the purple mark is the threshold crossing height, which fixes how far down the runway the PAPI stands. The coral line is the obstacle clearance surface the installation is sited for - one degree below the lowest on-course signal. The AIM promises that clearance only within 10° of the extended centreline and 3.4 NM of the threshold for a PAPI (4 NM for a VASI), and the descent on the lights should begin only when aligned with the runway.
Fly the approach starts you 3 NM out, above or below the glide path: set your descent rate - about 360 fpm at 65 knots with no wind, less into a headwind - and keep the on-path picture to the threshold. The score is the share of the time from 2.5 NM with the on-path picture. Read the lights shows six pictures from the cockpit, without the side view, for you to name.
Assumption and limits - a schematic installation on a flat, level runway, never a real airport and never for navigation: the light units at the runway elevation, aimed at the FAA standard angles (PAPI: AC 150/5340-30J Tables 7-2 and 7-3; VASI: Order JO 6850.2C Appendix E), the PAPI placed by the AC's formula from the threshold crossing height you set, the VASI bars where you set them. The PAPI's red-white transition is drawn 3 minutes of arc wide (the AC 150/5345-28H maximum at the beam centre), the VASI's a quarter degree (the Order's "approximately"), above each bar's aiming angle as the Order's figures draw it. The cockpit view is zoomed (16° wide) and the lights are drawn larger than they look. Lateral offsets do not change the picture here; real units have a limited horizontal coverage and may be baffled. The picture belongs to your eyes: the wheels are lower. Visual ranges are the AIM's (miles taken as statute miles). The practice flight is a model: a constant groundspeed (65 KIAS with flaps 30° from the C172S airspeed calibration, standard day at sea level, less the headwind) and the descent rate you set, reached with a 2-second lag (a model value). The real glide path angle, threshold crossing height and restrictions of an airport are in the Chart Supplement and NOTAMs.
Sight angle to a unit = atan(height of the eyes / (distance before the threshold + the unit's distance past it)); a unit is white above its aiming angle and red below. PAPI (AC 150/5340-30J "Design and Installation Details for Airport Visual Aids", 12 Feb 2018, active, ch. 7 PAPI): aiming relative to the glide path, unit nearest the runway first, +30′, +10′, −10′, −30′ (Table 7-2; +35′ / +15′ / −15′ / −35′ for height group 4 aircraft on a runway with ILS), two-light L-881 +15′ / −15′ (Table 7-3); distance from the threshold = TCH × cot(lowest on-course signal), the third unit (or the farthest of two) - the AC's own example: TCH 45 ft, 2°50′ -> 909.26 ft, reproduced; threshold crossing heights by height group (Table 7-1: 40 ft for general aviation); the obstacle clearance surface from 300 ft in front of the PAPI at the lowest on-course angle less 1°, 10° either side, 4 NM (7.5.4.4, Figure A-81). AC 150/5345-28H (29 Jul 2019, active): the light beam split white over red, the transition within 3 minutes of arc at the beam centre, the centre of the beam being the transition. VASI (FAA Order JO 6850.2C "Visual Guidance Lighting Systems", 30 Aug 2022, active, Appendix E; it replaced JO 6850.2B, whose Appendix E gives the same values): two-bar aimed 3° (upwind) and 2.5° (downwind; 2.75° for obstacle clearance), effective glide path from the reference point midway between the bars at the upwind bar's angle (Figure E-3: A = B = C + 1/2°), three-bar 3.25 / 3.0 / 2.75°, the red-white transition of about a quarter degree, downwind bar 125-800 ft from the threshold, bars 700 ft apart (+300 / −200), threshold crossing at least 25 ft. AIM (effective 9 July 2026) 2-1-2 a, b and FIG 2-1-2, 2-1-3, 2-1-5: the pictures and their names, visual ranges, obstacle clearance within ±10° to 4 NM (VASI) and 3.4 NM (PAPI) from the threshold, descent only when aligned with the runway.
Checked: every AIM FIG 2-1-5 angle falls inside the AC's band for its picture (3.5 and 2.5° are the outer aiming angles; 3.2 and 2.8° lie between them and ±10′); the reading of the VASI transition gives the AIM's on-path picture on the Order's effective glide path at every distance (the opposite reading does not). The AIM and the AC differ on the PAPI clearance distance - 3.4 NM from the threshold against 4 NM from the surface's origin; the page gives the AIM's to the pilot and draws the AC's surface. The four-light picture is the same calculation as the Visual Descent Point Challenge's; that tool takes the line of sight as the line to the threshold crossing point, which gives the same angle on the glide path and differs slightly close to the runway (both readings shown). Descent rate for the path = groundspeed × the path's gradient / 60, the IFR Trainer's descent calculation (identical results checked); the groundspeed from the C172S airspeed calibration (POH Figure 5-1, flaps 30°) and the standard atmosphere.
- Aeronautical Information Manual (effective 9 July 2026), para. 2-1-2 a (VASI: bars, glide paths normally 3°, three-bar upper path 1/4 degree higher, red / white principle, visible 3-5 miles by day and 20 or more at night, obstruction clearance within ±10° and to 4 NM, descent only when aligned) and b (PAPI: two or four units, visible about 5 miles by day and 20 at night, obstruction clearance within ±10° and to 3.4 NM), FIG 2-1-2 (2-bar VASI), FIG 2-1-3 (3-bar VASI), FIG 2-1-5 (PAPI)
- FAA Advisory Circular 150/5340-30J, Design and Installation Details for Airport Visual Aids (12 February 2018, active; errata of 17 November 2020), chapter 7 PAPI: 7.4.4 configuration, 7.5.4.3 siting without an ILS (the distance from the threshold = TCH × cotangent of the lowest on-course signal, and its worked example), 7.5.4.4 obstacle clearance surface, 7.5.4.5 threshold crossing height, Table 7-1 threshold crossing heights by height group, Table 7-2 aiming of the L-880 (4 box) PAPI, Table 7-3 aiming of the L-881 (2 box) PAPI, Figures A-81 and A-82
- FAA Advisory Circular 150/5345-28H, Precision Approach Path Indicator (PAPI) Systems (29 July 2019, active): 1.1 types L-880 and L-881, 3.2.1 photometric requirements (white over red, the transition within 3 minutes of arc at the beam centre), 3.2.4.1 vertical adjustment (the centre of the beam is the transition band)
- FAA Order JO 6850.2C, Visual Guidance Lighting Systems (30 August 2022; it replaced JO 6850.2B, whose values it keeps), Appendix E Visual Approach Slope Indicator (VASI) System Configurations and Installation: configurations and Table E-1 (three-bar visual cues), the quarter-degree transition, obstruction clearance, visual glide path angles and threshold crossing heights, bar locations, aiming of two-bar and three-bar VASI, Figures E-3 and E-6 (aiming and obstruction clearance diagrams)
- Cessna Model 172S Information Manual 172SIM (POH/AFM Revision 5, 19 July 2004, P/N 172SPHUS05) - normal approach speeds with flaps 30° (p. 4-5) and Figure 5-1 Airspeed Calibration (p. 5-9) for the practice flight; aircraft data, Model 172S only
- Cross-tool: the four-light PAPI picture is the same calculation as the Visual Descent Point Challenge (identical results checked); the descent rate for a glide path is the IFR Trainer's descent calculation
- Model choices of this tool (stated on the page): schematic installation on a flat runway, units at the runway elevation, a zoomed 16° cockpit view with the lights drawn enlarged, the transition bands, statute miles for the AIM's visual ranges, the practice flight at a constant groundspeed with a 2-second descent-rate lag, scoring from 2.5 NM