PAPI Unit Separation Geometry | Precision AGL Academy

PAPI Unit Separation & Lateral Geometry

Unit spacing, inner-unit distance from the runway edge, bar length, stagger and skew — designed layout, set-out coordinates and an as-built survey check, unit by unit.

About this calculator

Enter the design values for the runway and PAPI bar, then (optionally) the surveyed position of each unit as chainage and offset or as grid easting/northing. The tool compares every unit with its design position, checks centre-to-centre spacing and uniformity, the inner-unit distance from the runway edge, longitudinal alignment of the bar and the housing clear gap, and draws the plan. Unit 1 (P1) is always the unit nearest the runway.

Quick start

Standard, Units & Display

Governing Standard

Length Unit

Decimals

Changing the unit converts every length already entered.

Runway & PAPI Bar (Design)

Runway ID

PAPI Runway End

Runway Width (W)

PAPI Side

Number of Units (N)

Side is viewed looking along the landing direction. Offsets are always measured from the runway centreline toward the PAPI side.

Design Unit Spacing (S), centre-to-centre

Inner Unit to Runway Edge, design (d_edge)

Bar Distance from Threshold, design (D_PT)

Used here as the design chainage of the bar. Setting D_PT itself from glide-path geometry is done in Calculator #12.

Tolerances & Unit Size

Tolerances are the ± values you enter from the governing standard or project specification. A blank tolerance switches that check off, but the verdict cannot show PASS while the spacing or inner-unit-to-edge tolerance is blank. Spacing and edge-distance values are pre-filled for ICAO/DGCA as an editable starting point only.

Spacing Tolerance (±)

Applied to each centre-to-centre gap. The spread between the largest and smallest gap is reported for information.

Inner Unit to Edge Tolerance (±)

Unit Position Tolerance vs Design (radial)

Project construction tolerance — placeholder, replace with the specified value.

Stagger Tolerance

Housing Width

Stagger = spread of unit chainages along the runway (a bar square to the runway has none). Housing width is the lateral footprint of one unit, used for the clear gap. The default stagger of 0.30 m is the spread allowed when every unit face lies within about ±150 mm (6 in) of a line square to the runway, a figure used in FAA-based inspection checklists and by Transport Canada; ICAO/DGCA figures and project specifications may differ, so enter the specified value. The housing width is a placeholder.

As-Built Survey (optional)

Survey Data Format

Deviation Magnification on Plan

Survey deviations are centimetres on a plan that covers tens of metres, so measured markers are drawn at design position + deviation × this factor. Numbers in the tables are never magnified.

Unit	Chainage from threshold	Offset from centreline (toward PAPI side)	Chainage	Offset from CL	Δ Chainage	Δ Offset	Radial dev.	Status

Report Details

Project Name (for report)

PAPI ID (for report)

Result

Spacing Between Units

Gap	Design	Measured c-t-c	Lateral component	Δ vs design	Limit	Status

Bar Checks

Check	Value	Requirement	Status

Design Layout & Set-Out

Unit	Along bar from P1	Offset from edge	Offset from CL	Chainage	Easting	Northing

Plan View

Design position Surveyed position (deviation magnified)

PAPI Unit Separation & Lateral Geometry · Rev. 1.1 · 26 Oct 2026

Engineering reference tool for Precision AGL Academy — not a substitute for an approved PAPI design drawing, type-test certification or a licensed survey. See the Legal Notice.

Method & Reference

How This Calculator Works

The layout is built from four design numbers — runway width, inner-unit distance from the edge, unit spacing and chainage — and every survey point is reduced to the same runway frame before it is compared.

Design positions

Unit k (k = 1 is the inner unit) sits on the PAPI side of the centreline at:

Offset_k = W/2 + d_edge + (k − 1) × S

at chainage D_PT for every unit. Bar length = (N − 1) × S. Overall lateral extent adds one housing width.

Grid to runway frame

With threshold (E₀, N₀) and landing-direction bearing θ:

chainage = ΔE·sinθ + ΔN·cosθ

across = ΔE·cosθ − ΔN·sinθ

‘across’ is positive to the right of the landing direction; the tool flips its sign for a left-hand bar so offset is always positive toward the PAPI side. Set-out coordinates use the inverse of the same relations.

Bar checks

Centre-to-centre spacing is the straight-line distance between consecutive units; the lateral component is shown beside it. Uniformity is largest gap minus smallest gap, reported for information because each gap is already checked against its own tolerance. Stagger is the spread of chainage across the bar, and skew is the angle of the line from P1 to the last unit away from square to the runway centreline. Clear gap is the smallest spacing less the housing width.

Starting values offered

Standard	Unit spacing	Inner unit to edge
ICAO Annex 14 Vol. I	9 m ± 1 m	15 m ± 1 m (PAPI); 10 m (APAPI), confirm tolerance
DGCA / Indian implementation	As ICAO (confirm CAR)	As ICAO (confirm CAR)
FAA AC 150/5340-30 (current edition)	Project-entered	Project-entered
Airport-specific / project	Project-entered	Project-entered

These are typical published figures offered as editable starting values. Confirm them, and the unit-position, stagger and housing figures, against the current edition of the governing standard and the approved design before relying on any pass result.

Terminology kept separate

PAPI unit (one optical light box), PAPI bar (the complete row of units), unit spacing (centre-to-centre), inner-unit distance (runway edge to the centre of P1, as used here; some standards measure to the inner edge of the housing, which differs by half the housing width, so check the definition in yours) and offset from centreline are not interchangeable. P1 is the unit nearest the runway; ICAO labels the units A to D from the farthest, so A = P4 on a four-unit bar. Separation and lateral position are checked here; aiming angles, glide-path angle and threshold crossing height are in Calculator #12.

What this calculator does not do

It does not check mounting height, frangibility, aiming angle or horizontal beam alignment, obstacle limitation surfaces, or the second bar on the opposite side of the runway. A PASS here means the surveyed layout matches the tolerances you entered, not that the installation is compliant.

PAPI Unit Separation and Lateral Geometry Calculator: Frequently Asked Questions

Quick answers on how PAPI bar layout, spacing and survey deviations are checked.

What does the PAPI Unit Separation and Lateral Geometry Calculator do?

It builds the design layout of a PAPI bar from the runway width, inner-unit distance from the edge, unit spacing and chainage, and compares an as-built survey against it. It reports spacing between units, offsets, stagger, skew and clear gap, and gives design set-out coordinates, a plan view, a CSV export and a PDF report.

How are the design unit positions calculated?

Unit k, where k = 1 is the inner unit, sits on the PAPI side of the centreline at Offsetk = W ÷ 2 + dedge + (k − 1) × S, at chainage DPT for every unit. The bar length is (N − 1) × S, and the overall lateral extent adds one housing width.

What spacing and inner-unit distance values does it start with?

For ICAO Annex 14 Vol. I the starting values are 9 m ± 1 m unit spacing and 15 m ± 1 m inner unit to runway edge for a PAPI, or 10 m for an APAPI, with the tolerance to be confirmed. DGCA is shown as the same as ICAO, to be confirmed against the CAR. For FAA and airport-specific bases the values are project-entered. They are editable starting values, so confirm them against the current edition of the governing standard and the approved design.

How are survey coordinates converted to chainage and offset?

With the threshold at (E0, N0) and the landing-direction bearing θ, chainage = ΔE·sinθ + ΔN·cosθ and across = ΔE·cosθ − ΔN·sinθ. Across is positive to the right of the landing direction, and the tool flips its sign for a left-hand bar so the offset is always positive toward the PAPI side. You can also enter survey data directly as chainage and offset.

What bar checks does it make?

Centre-to-centre spacing is the straight-line distance between consecutive units, checked against its tolerance. Uniformity is the largest gap minus the smallest gap and is reported for information. Stagger is the spread of chainage across the bar, skew is the angle of the line from P1 to the last unit away from square to the centreline, and the clear gap is the smallest spacing less the housing width. A blank tolerance switches that check off. The verdict cannot show PASS while the spacing or inner-unit-to-edge tolerance is blank.

Why is the deviation magnification on the plan view?

Survey deviations are centimetres on a plan that covers tens of metres, so measured markers are drawn at the design position plus the deviation times the factor you choose (×1, ×10, ×25, ×50 or ×100). The numbers in the tables are never magnified.

Does a PASS here mean the PAPI is compliant?

No. A PASS means the surveyed layout matches the tolerances you entered, not that the installation is compliant. The calculator does not check mounting height, frangibility, aiming angle or horizontal beam alignment, obstacle limitation surfaces, or the second bar on the opposite side of the runway. Aiming angles, glide-path angle and threshold crossing height are in Calculator #12.

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 Unit Separation & Lateral Geometry (calculator), including its CSV downloads, print views.

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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Notice v2.9 · 27 October 2026 · Full terms: Legal & Disclaimer