PAPI Glide-Path Geometry | Precision AGL Academy

PAPI Installation & Glide-Path Geometry

Standard selection, unit configuration & spacing, lateral/longitudinal placement, PAPI elevation, glide-path angle & threshold crossing height (forward & reverse), individual unit aiming angles & indication-sector boundaries, check-stick verification, obstacle-clearance screening, unit elevation/foundation slope, and existing-installation inspection.

About this calculator

PAPI system, PAPI unit, aiming angle and glide-path angle are related but distinct parameters — this calculator keeps them separate throughout. A geometric 3° calculation alone does not establish complete PAPI compliance; the exact standard-specific dimensional database and obstacle limitation-surface criteria must be verified against the applicable ICAO/FAA/DGCA requirements, approved aerodrome design, and actual site survey.

Standard & Runway Basics

Governing Standard

The applicable design values (spacing, aiming angle set, tolerances) are project/standard-entered below — not hard-coded to one authority.

Runway ID

PAPI Runway End

Runway Heading (record only)

Runway Width (m · used for runway-edge distance)

Approach Category (record only)

Threshold Elevation (E_TH)

Display & Precision (all inputs are in metres)

PAPI System Configuration

PAPI system (the complete installation), PAPI unit (one optical light box) and PAPI box/housing (the physical mount) are tracked separately — the unit count below drives the array geometry, aiming-angle table and unit-elevation table together.

Number of Units (N)

Unit Spacing S_PAPI (centre-to-centre)

PAPI Side

Recorded in the report. Lateral offsets are entered as positive distances from the runway centreline on whichever side the bar is installed; ICAO normally places the bar on the left side of the runway.

Check-Stick Method Used?

RUNWAY EDGE (landing direction ↑) ══════════════════════════════════ ← inner-unit distance → ● ← S → ● ← S → ● ← S → ● P1 P2 P3 P4 (nearest runway, (farthest, highest setting) lowest setting) The bar lies square to the runway centreline at distance D_PT from the threshold. ICAO labels the units A to D from the farthest, so A = P4 and D = P1.

PAPI Location & Elevation

PAPI Distance from Threshold (D_PT)

Longitudinal placement — measured along the runway from the threshold to the PAPI reference line.

PAPI Elevation (E_PAPI)

Lateral Offset Required (D_CL−PAPI)

Lateral Offset Measured

Lateral Tolerance (±)

Enter D_CL−PAPI as a positive distance from the runway centreline to the reference point the governing standard uses (for example the inner edge of the nearest unit). Runway-edge distance = D_CL−PAPI − runway width / 2 and is shown under Lateral Location.

Glide-Path Angle & Threshold Crossing Height

Aiming angle (optical setting of one unit) and glide-path angle (the resulting visual approach path) are related but not identical — the field below is the nominal design/resulting glide-path angle used for the geometry engine, not an individual unit's aiming angle.

Nominal Approach / Glide-Path Angle (α)

3° is the normal default only — not hard-coded. Overwrite with the project/standard-defined value.

Calculation Mode

Required Threshold Crossing Height (H_{TCH, target})

Output: theoretical D_PT needed to achieve this TCH at the nominal glide angle.

Individual Unit Aiming Angles & Indication Sectors

This does not equate "3 white = exactly X°" — it models the transition-sector boundaries generated by the individual optical aiming angles you enter, per the governing specification. P1 is the unit nearest the runway and carries the highest setting; the defaults follow the common pattern θ+30′, +10′, −10′, −30′ for four units and θ±15′ for two (edit to the approved design).

Unit	Aiming Angle θ (°)	Δθ to next unit	Red/white transition angle (°)

Indication model (standard 4-unit PAPI): 4 white = well above the glide path · 3 white/1 red = slightly above · 2 white/2 red = on the glide path · 1 white/3 red = slightly below · 4 red = well below (plain-language labels; ICAO Annex 14 words these as further above, above, on, below and further below). Each unit changes from white to red at its own aiming (setting) angle θ_i, so the transition angle column equals the entered setting angle. As the aircraft descends, the unit with the highest setting angle (P1, nearest the runway) turns red first and the lowest-setting unit (the farthest) turns red last.

Check-Stick Verification (advanced module)

Check-Stick Distance from PAPI (D_CS)

Check-Stick Target Height (H_expected, override)

Measured Height (H_measured)

Tolerance (± height)

Check-Stick Result

—

Enter distance and measured height.

Obstacle Clearance — Preliminary Geometric Screening

Obstacle Distance from Threshold

Measured along the approach, before the threshold (positive). The screening adds D_PT to reach the distance from the PAPI.

Obstacle Elevation

OPS Slope Offset Below Reference Angle (°)

Obstacle protection surface slope = reference angle − this offset. ICAO Annex 14 sets the surface 0.57° (about 34′) below the setting of the lowest unit (unit A, the unit farthest from the runway) for a four-unit PAPI and 0.9° (54′) below unit A for an APAPI. The default offset follows the number of units until you edit it. Verify against the edition you design to, or enter the project value.

OPS Reference Angle

Using α instead of the lowest unit setting makes the surface steeper, which is the less conservative choice. Use it only when the governing standard defines the surface that way, and enter that standard's offset above.

Obstacle Horizontal Offset (informational)

Unit-by-Unit Elevation & Foundation Slope

A PAPI installation cannot be evaluated using horizontal distance alone — enter each unit's individual elevation if the foundation is not perfectly level.

Unit	Elevation	ΔE to next unit

Foundation Slope Correction Tolerance (%)

Method & Reference

How This Calculator Works

PAPI system, PAPI unit, aiming angle, glide-path angle and threshold crossing height are calculated as distinct, related parameters — never collapsed into a single "3° angle" assumption.

Array width

W_array = (N − 1) × S_PAPI

Centre-to-centre distance from the first to the last unit — not the physical housing width.

Basic glide-path equation

H = D × tan(α)

Example: D = 300 m, α = 3° → H ≈ 15.72 m, the geometric height difference of a 3° line over 300 m.

PAPI-to-threshold geometry

gradient = tan−¹(ΔE / D_PT)

ΔE = E_PAPI − E_TH. This is the ground gradient between the two reference points. It is informational only: it is not the glide angle or the optical aiming angle and is not compared against the nominal angle.

Forward / reverse TCH

Forward: H_TCH = D_PT × tan(α) + ΔE Reverse: D = (H_TCH − ΔE) / tan(α)

Mode A calculates TCH from position & elevation; Mode B derives the theoretical PAPI position/elevation requirement from a target TCH — a design-analysis tool in both directions. Siting standards fix the PAPI distance from the lowest on-course angle and a minimum eye height over threshold (for ICAO, the setting of unit B minus 2′); this tool works from the nominal angle α, so treat the reverse result as a first estimate and confirm it against the siting table in the applicable standard.

Check-stick geometry

H_GP = D_CS × tan(α) α_measured = tan−¹(ΔH / D)

Expected vs. measured height difference (ΔH) is checked against the applicable tolerance for a PASS/FAIL result, plus a measured glide-path angle for comparison against the required angle.

Standard-specific database

Standard	Spacing / Angle Set	Tolerances
ICAO Annex 14	Project-entered	Project-entered
FAA AC 150/5340-30J & AC 150/5345-28	Project-entered	Project-entered
DGCA / Indian implementation	Project-entered	Project-entered

Standard-specific figures are not hard-coded. The values pre-filled on this page (9 m spacing, 300 m from the threshold, 3° angle, ±0.5 m lateral tolerance, ±0.03 m check-stick tolerance, 0.5 % slope tolerance, 0.57° OPS offset) are editable placeholders, not standard figures, and the exact dimensional database and obstacle limitation criteria are not incorporated in this build.

Terminology kept separate

PAPI system (complete installation), PAPI unit (one optical light box), PAPI box/housing (physical mount), aiming angle (unit optical setting), glide-path angle (resulting approach path) and threshold crossing height (visual glide-path height above threshold) are never treated as interchangeable.

What this calculator does not do

It does not maintain the exact ICAO/FAA/DGCA PAPI dimensional database, does not perform a complete obstacle-limitation-surface / approach-surface compliance determination, and does not treat a passed geometric 3° check as full PAPI compliance. Geometry, TCH, aiming-angle framework, check-stick, preliminary obstacle screening, unit elevation and foundation slope are fully calculated; exact figure-level acceptance must be confirmed against the approved design and site survey.

PAPI Installation and Glide-Path Geometry Calculator: Frequently Asked Questions

Quick answers on PAPI placement, threshold crossing height, aiming angles and the geometric checks.

What does the PAPI Installation and Glide-Path Geometry Calculator do?

It calculates PAPI array geometry, lateral and longitudinal location, threshold crossing height (TCH), unit-by-unit aiming angles and indication sectors, check-stick verification, a preliminary obstacle screening, and unit elevation and foundation slope. It also records an existing installation and produces a PDF report.

Does a passed geometric 3° check mean the PAPI is compliant?

No. The calculator states that a geometric 3° calculation alone does not establish complete PAPI compliance. PAPI geometry is safety-critical, and the results shall be verified against the applicable ICAO, FAA or DGCA requirements, the approved aerodrome design and actual site survey data. The calculator does not perform a complete obstacle-limitation-surface or approach-surface compliance determination.

How is the threshold crossing height calculated?

In forward mode, HTCH = DPT × tan(α) + ΔE, where DPT is the PAPI distance from the threshold, α the nominal glide-path angle and ΔE = EPAPI − ETH. In reverse mode, D = (HTCH − ΔE) ÷ tan(α) gives the theoretical position needed for a target TCH. Siting standards fix the PAPI distance from the lowest on-course angle and a minimum eye height over the threshold, so treat the reverse result as a first estimate and confirm it against the governing standard. In forward mode the page also shows the eye height over the threshold at the lowest on-course angle (unit B − 2′ for a four-unit PAPI, unit A − 2′ for an APAPI) for comparison with the minimum eye height over threshold (MEHT) in ICAO Annex 14.

What is the difference between the aiming angle and the glide-path angle?

The aiming angle is the optical setting of one unit. The glide-path angle is the resulting visual approach path. They are related but not identical, and the calculator keeps the PAPI system, unit, box or housing, aiming angle, glide-path angle and threshold crossing height as separate terms. The default pattern follows θ + 30′, +10′, −10′ and −30′ for four units and θ ± 15′ for two, and you should edit it to the approved design.

How does the check-stick verification work?

The expected height on the glide path is HGP = DCS × tan(α) at the check-stick distance. You enter the measured height, and the difference ΔH is compared with the tolerance for a PASS or FAIL result. The calculator also gives a measured glide-path angle, αmeasured = tan⁻¹(ΔH ÷ D), to compare with the required angle.

How does the obstacle screening work?

It is a preliminary geometric screening of an obstacle against an obstacle protection slope set below the unit settings. ICAO Annex 14 sets the surface 0.57° (about 34′) below the setting of the lowest unit (unit A, the unit farthest from the runway) for a four-unit PAPI and 0.9° (54′) below unit A for an APAPI. The default offset follows the number of units until you edit it. It does not model the surface origin distance, width or splay, so it is not a complete compliance determination.

Are the spacing, distance and tolerance values hard-coded to a standard?

No. The standard-specific figures are project-entered. The values pre-filled on the page (9 m spacing, 300 m from the threshold, 3° angle, ±0.5 m lateral tolerance, ±0.03 m check-stick tolerance, 0.5 % slope tolerance and 0.57° OPS offset) are editable placeholders, not standard figures. The exact dimensional database and obstacle limitation criteria are not incorporated.

Why enter the elevation of each PAPI unit?

A PAPI installation cannot be evaluated using horizontal distance alone, so each unit's individual elevation is entered if the foundation is not perfectly level. The calculator then reports the elevation difference between units and the foundation slope against your tolerance. The existing-installation inspection records measured units for reference only, and measured aiming angles are checked in the Survey and Aiming Verification tool.

Important Notice, Disclaimer & Limitation of Liability

Please read before relying on any content on this page or on any result or record produced by it.

Specific to this page: Applies to the PAPI Installation & Glide-Path Geometry (calculator), including its PDF reports.

Calculated results. Results, tables, charts, status indicators (such as “OK”, “within limit” or “pass”/“fail”), recommended or selected values and the example presets on this page are first-pass planning and training estimates calculated in your browser from the values you enter, using simplified formulas, generic tables and typical values. They do not take account of every factor of a real installation, may contain errors or rounding differences, and are not a design, specification, selection, sizing, procurement or acceptance decision. A competent person must check them against the approved design, the manufacturer’s data and the applicable standard before any use. “Recommended” means an indicative estimate, not an endorsement of any rating, product or supplier.

PAPI and visual glide slope aids. PAPI geometry, setting angles, unit separation, aiming and indication values are generic estimates for learning and planning. They are not a flight-inspection, survey, commissioning or aiming certificate and do not replace the survey, flight check and approvals required by the aerodrome operator, the air navigation service provider and the regulator. Indication tables are not operational information or instructions for pilots, controllers or vehicle drivers. Do not adjust, aim, energise or return to service any visual glide slope aid on the strength of this tool.

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