Every number in this app’s library traces to something. This document says what, and — just as importantly — how strongly each number was verified during the build.
Read §1 before using any of these values in a publication.
Three levels are used throughout the data files:
| Level | Meaning |
|---|---|
| verified | Retrieved from the cited source during this build and checked figure-for-figure. |
| medium | Well-established published engineering practice, reproduced from the standard literature, but not opened against a primary document during this build. Check before publication use. |
| low | Representative of the vehicle type; not a specific model, not a cited figure. Adequate for illustrating a configuration, not for quoting. |
What was verified during this build:
aircraft.xml) — per-wheel
coordinates, gross weights and tire pressures. Read directly during the v1.9
build for the gear configurations (§5.7) and to retire three tandem-spacing
assumptions (§5.6). Used as an independent cross-check before that (§5.3).What was NOT verified during this build:
assumedFields, and flagged in the
app. See §5.7 — and note that the six pure Figure 2 patterns are nominal
throughout.W = 500[LN/(N−1) + 12N + 36].test/run.mjs, which independently confirms that a two-axle group at 4.0 ft
is allowed exactly 34 000 lb — the arithmetic origin of the statutory tandem
limit.src/core/tires.js compute aircraft tire dimensions arithmetically; and the
inch-nominal truck tire dimension table.Derived, and therefore reproducible:
DECISIONS.md §D7.Typical practice, confidence medium — check before quoting:
Low confidence — representative only:
Computed exactly from the designation — no table, no uncertainty:
445/50R22.5, 295/75R22.5,
LT245/75R16, …). Overall diameter follows arithmetically:
OD = rim + 2 × section × aspect.H44.5x16.5-21, 52x21.0R22, 1400x530R23, …).
The three-part designation encodes all three dimensions directly.Table lookup, in src/data/tires.json:
| Size | Confidence | Note |
|---|---|---|
11R22.5 |
high | 279 mm section / 1054 mm OD. The reference truck tire of the flexible-pavement literature; dimensions agree across the major medium-truck data books. |
11R24.5, 12R22.5, 10.00R20, 11.00R20, 9.00R20 |
medium | Reproduced from the standard truck-tire literature. Not checked against a TRA yearbook during this build. |
A size that is not in the table is reported by the app as unknown and
refuses to produce geometry. It is never guessed. Adding one requires a
source and a confidence field, and the test suite fails without them.
Static loaded radius. Two models are implemented and the choice is exposed:
radiusRatio (default): SLR = ratio × free radius, 0.97 truck / 0.965
aircraft. These defaults come from the build specification and are
user-adjustable. Confidence: medium — they are reasonable engineering
values, not a cited standard.sectionDeflection: SLR = free radius − deflection × section height, with
0.32 for aircraft, following the Tire and Rim Association convention that
aircraft tires are rated at a nominal 32 % deflection.The aircraft library shipped in v1.2 with four Boeing aircraft spanning gear codes D, 2D and 3D: 737-800, 757-200, 767-400ER and 777-300ER. v1.6 adds the three wing-plus-body aircraft — 747-400, 747-8 (both 2D/2D2) and A380-800 (2D/3D2) — which had been deferred since the first build. Seven aircraft, six gear codes.
Those three are sourced by a different and better method than the original four, described in 5.5. Read that section before comparing numbers between them: on the original four the outer width is an input and the track is derived; on the three new ones the track is published and the outer width is a cross-check.
An earlier attempt was abandoned because the sources could not be reached
(the FAA database returned 403 to the fetch tool, and the ACAP PDFs exceeded
its size limit). Both were tooling limits, not access limits: the FAA site
serves the spreadsheet normally to a browser user-agent, and the PDFs download
fine with curl. Every number below was retrieved and read directly.
This table describes the original four aircraft. The three wing-plus-body aircraft take their geometry from the manufacturer footprint figures instead — see 5.5.
| Quantity | Source |
|---|---|
Gear designation (Main_Gear_Config) |
FAA Aircraft Characteristics Database |
| Wheelbase, nose to main gear | FAA Aircraft Characteristics Database |
| Main gear outer width | FAA Aircraft Characteristics Database |
| MTOW | FAA Aircraft Characteristics Database |
| Maximum design taxi weight | Manufacturer ACAP, section 7.2 |
| Tire size and tire pressure, nose and main | Manufacturer ACAP, section 7.2 |
| Percent gross weight on the whole main gear (95 %) | FAA AC 150/5320-6G, G.1.3 |
ACAP editions used: 737 D6-58325-7 Rev C (Oct 2025), 757 D6-58327 Rev H (Dec 2024), 767 D6-58328 Rev K (Dec 2024), 777 D6-58329-2 Rev G (Dec 2024).
The FAA field is not the centerline tread. Its own data dictionary defines
Main_Gear_Width_ft as “Distance between outer tires in the main landing
gear.” Treating it as the track would push every main wheel outboard by half
a dual spacing plus half a tire — for a 777 that is nearly a meter per side,
and the figure would look entirely reasonable while being wrong.
So the track is derived, never assumed:
track = outerWidth − (wheelsAcross − 1) × dualSpacing − sectionWidth
Section width comes exactly from the three-part tire designation.
Nothing in that derivation uses the manufacturers’ separately published tread figures, so agreement between them is real corroboration rather than circularity. With the dual spacings recorded in the data files:
| Aircraft | Derived track | Manufacturer published tread | Difference |
|---|---|---|---|
| 737-800 | 5727 mm | 5715 mm (18 ft 9 in) | +12 mm |
| 757-200 | 7302 mm | 7315 mm (24 ft 0 in) | −13 mm |
| 767-400ER | 9302 mm | 9296 mm (30 ft 6 in) | +6 mm |
| 777-300ER | 10 963 mm | 10 973 mm (36 ft 0 in) | −10 mm |
All four agree to within about 13 mm, on quantities of 6 to 11 meters — which
is roughly what quoting a tread to the nearest inch can account for, and no
more. test/run.mjs asserts both the derivation and this cross-check, at a
15 mm tolerance.
The 767 row previously read 9322 mm, 26 mm — twice any other residual, and the only one this rounding argument could not explain. It was recorded as “agreeing to within a few centimeters” and left alone.
It was not rounding. Its dual spacing was 1143 mm, taken as a round 45 in because no consulted document stated it. FAARFIELD 2.1.1 stores the 767-400 ER main gear wheel coordinates explicitly as X ±22.900 in, i.e. 45.800 in (1163 mm). Because the track is derived from the dual spacing, that error propagated straight into the geometry. Correcting it drops the residual from 26 mm to 6 mm and removes the outlier.
What actually moved is worth stating precisely, because it shows the derivation behaving as designed: the outboard tire of each dual pair did not move at all. The FAA outer width is the authoritative datum and is held, so a corrected dual spacing is absorbed inside it — the inboard tire moves 20 mm inboard and the strut centerline 10 mm, while the outer tire edge stays put. This is the same property 5.4 relies on when it says the outer width is preserved whatever dual spacing you enter.
Two things are worth taking from this beyond the number:
On the original four aircraft, two quantities are not constrained by any source consulted:
Every aircraft unit lists these in assumedFields, the schema fails
validation if that array is missing, and the app shows an amber notice naming
them whenever an aircraft is loaded. Set them from FAARFIELD before using the
output for pavement work — and note that changing a dual spacing re-derives the
track, so the authoritative outer width is preserved whatever you enter.
Neither assumption applies to the 747-8 or the A380-800: their footprint
figures publish every spacing, and both declare assumedFields: []. The
747-400 declares exactly one, NLG.tire.rimDiameter, because Boeing states
its nose tire as 49X17 — a two-part Type VII designation that omits the rim.
The overall diameter and section width come from the two published numbers and
do not depend on that assumption.
The 747-400, 747-8 and A380-800 were left out of v1.2 through v1.5 with this reason recorded: their wing-plus-body layouts need the longitudinal and transverse offsets of the body gear relative to the wing gear, “a single outer width closes a two-strut layout; it cannot close a four-bogie one.”
That was correct, and it was a data problem rather than a modeling one.
The offsets are not in the FAA database, and they are not in FAARFIELD either
— FAARFIELD analyzes one gear at a time and stores the wing gear and the body
gear as separate entries (B747-400 and B747-400 Belly), so it carries the
bogie geometry but not the distance between the two.
They are stated plainly in the manufacturers’ own airport planning documents, which is where they came from:
| Aircraft | Document | Figure |
|---|---|---|
| 747-400 | Boeing ACAP D6-58326-1 Rev E (Sep 2023) | §7.2.1 landing gear footprint |
| 747-8 | Boeing ACAP D6-58326-3 Rev C (Aug 2023) | §7.2 landing gear footprint |
| A380-800 | Airbus AC A380, issue Nov 01/24 | §7-2-0 footprint, sheets 1 and 2 |
The sourcing is inverted relative to 5.2. These figures publish the track, both gear positions and every spacing directly, so nothing is derived from an outer width. The outer width instead becomes an independent check:
| Aircraft | track + dual + section | Published outer width | Difference |
|---|---|---|---|
| 747-400 | 10 998 + 1118 + 482.6 = 12 598.6 mm | 41 ft 4 in = 12 598.4 mm (Boeing) | 0.2 mm |
| 747-8 | 10 998 + 1189 + 533.4 = 12 720.4 mm | 41 ft 9 in = 12 725.4 mm (Boeing) | 5 mm |
| A380-800 | 12 456 + 1350 + 530 = 14 336 mm | 47.05 ft = 14 341 mm (FAA) | 5 mm |
The residuals are the figures’ own rounding — Boeing draws to the nearest inch, the FAA tabulates to 0.01 ft.
Independent corroboration from FAARFIELD. The FAA’s FAARFIELD 2.1.1 aircraft library stores explicit per-wheel coordinates. It agrees with the manufacturer figures on every bogie dimension:
| Aircraft | FAARFIELD wheel coordinates | Manufacturer figure |
|---|---|---|
| 747-400 | X ±22 in, Y 58 in | 44 in dual, 58 in tandem |
| 747-8 | X ±23.4 in, Y 56.5 in | 46.8 in dual, 56.5 in tandem |
| A380 wing | X ±674.37 mm, Y 1699.26 mm | 1350 mm dual, 1700 mm tandem |
| A380 body | X ±764.54 / ±774.70 mm, Y 0 / 1699.26 / 3398.52 mm | 1530 / 1550 / 1530 mm dual, 1700 mm tandem |
Two wholly independent sources agreeing to the millimeter, including the
A380 body bogie’s 20 mm wider middle axle — which is why dualSpacingByRow
exists rather than the spacing being averaged. FAARFIELD’s per-strut
MgPercent also reproduces the load split from the other direction: 0.2375 × 4
struts = 95 % for both 747s, and 0.19 × 2 wing + 0.285 × 2 body = 95 % for the
A380, with both giving an equal 4.75 % per tire.
The FAA’s tabulated wheelbase is not defined consistently for these aircraft. For the 747-8 it equals the centroid of the four bogies (97.3 ft against a computed 29 655 mm, agreeing to 2 mm). For the A380 it equals the nose-to-body-gear dimension (104.6 ft = 31 881 mm exactly). For the 747-400 it matches neither, nor their midpoint — it gives 87.9 ft where Boeing’s figure gives 84.0 ft, the commonly published 747-400 wheelbase. This library uses the manufacturer figures and treats the FAA field as a cross-check only.
The FAA’s MTOW for the 747-400 is the -400ER figure (910 000 lb), which exceeds the maximum taxi weight in Boeing’s own table for the -400. The manufacturer table is used instead: MTOW 875 000 lb, MDTW 877 000 lb, from the same column of ACAP §2.1.1.
Nothing from the original build spec. Additional weight variants (the A380
alone has fifteen) are not separate entries because the geometry is identical
across them — only the weights differ, so a variant changes tire loads and
nothing else. A380-800 WV000 is the one carried, and its variant is stated on
every weight.
The 757-200, 767-400ER and 777-300ER shipped from v1.2 with
MLG.tandemSpacing declared in assumedFields: no consulted source
constrained it, so a plausible round number in inches was chosen and said to be
chosen. The FAARFIELD 2.1.1 aircraft library does constrain it.
| Aircraft | Assumed | FAARFIELD | Error |
|---|---|---|---|
| 757-200 | 1143 mm (45 in) | 1143 mm (Y ±571.5) | none |
| 767-400ER | 1422 mm (56 in) | 1371.6 mm (Y ±685.8) | 50.4 mm too long |
| 777-300ER | 1448 mm (57 in) | 1463.04 mm (Y −1463.04 / 0 / +1463.04) | 15.0 mm too short |
All three are now sourced and out of assumedFields. Only NLG.dualSpacing
remains assumed on the two-strut aircraft; FAARFIELD models the main gear only,
because the nose gear carries too little load to matter to thickness design.
Why a wrong value survived seven releases. A tandem spread is symmetric about the bogie center, so changing it moves both axle lines equally and leaves the wheelbase (a centroid), the track and the outer width completely untouched. Every derivation check in the suite passed with the wrong number in place. This is the argument for declaring assumptions in the data rather than trusting tests to find them: the tests could not have.
Sixteen configurations in aircraft/faa-5300-7.json. Every one is
schematic — kind: "schematic", flagged in the app — and the split between
what is measured and what is nominal is not the same as it is for the real
aircraft above.
Verified, and cited per gear. The wheel geometry — track, dual spacing,
tandem spacing, uneven bogie offsets, wing-to-body offset — of the ten
configurations whose representative aircraft appears in the FAARFIELD 2.1.1
library: S (F-15C), 2S (C-130), 2T (C-17A), 2D/D1 (DC10-30/40),
2D/2D1 (A340-600 WV000), 5D (An-124), 7D (An-225), C5 (C-5),
D2 (B-52), Q2 (IL-76T). Gross weights and tire pressures come from the same
entries. This is the library already used in §5.3 to corroborate the 747 and
A380, read directly during this build.
Nominal, and declared in assumedFields on every unit:
No outer width is stated on any of them. On the real aircraft the FAA’s
published outer width is the datum and the track derives from it (§5.2). Here
the track is measured and the outer width would depend on the nominal tire, so
stating one would present a placeholder as a datum. mainGearOuterWidth is
null throughout and a test asserts it.
Low — the six pure patterns. T, Q, 2Q, 3S, 3T, 3Q have no
aircraft behind them, or none whose geometry any consulted source publishes
(Q’s representative, the HS-121 Trident, left service in 1985). They are
drawn to one nominal scale — 49x19.0-22 tires, 1400 mm lateral pitch, 1450 mm
longitudinal, strut centers 1800 mm outboard of the bogie half-width, wheelbase
20 000 mm — so that Figure 2’s twelve cells stay comparable, which is the
point of that figure. Not one of those numbers describes an aircraft.
Two idealizations, both declared in the data. The C-17’s real bogie has its two rows offset laterally from each other by 38.1 mm, and one wheel in each row sits 292.1 mm out of line with the other two; both are squared up here and both are in the FAARFIELD coordinates if needed. Loads on all sixteen use the equal per-tire split that 95 % on the main gear implies, which is not how FAARFIELD apportions the wing/body aircraft (DC-10 78/17, A340-600 72/23); the geometry is sourced, the per-tire loads are nominal, and the unit notes say so.
Order 5300.7 itself — the naming convention, Table 1’s pressure codes,
Table 3’s eighteen rows with their wheel counts, nose gear types, typical
aircraft and the historic FAA/USAF/Navy concordance, and Figures 2–20 — was
read in full during this build and is transcribed in src/core/gearcode.js.
The test suite reproduces all eighteen published wheel counts from the names
alone.
Every axle carries a source string and every load a basis string, both
shown in the properties panel when you select an axle. Export unit.json
alongside any figure and the citations travel with it.
The test suite (npm test, 175 checks as of v1.9) fails the build if any axle, gear, load,
GVW, MTOW, tire pressure or multi-axle group spacing lacks provenance, and
includes a negative control that confirms the validator actually rejects a
missing source rather than passing vacuously.