# Cargo Compartment Sizing > Use when you must size a freight cargo compartment and its cargo door around standard unit load devices: look up public ULD envelope dimensions from the module catalog, check whether a ULD cross-section passes through a cargo door with the rotated orientation tried, pick the largest-volume ULD that fits a door, lay out ULD positions with a deterministic 2D strip layout and aisle allowance, compute the required cargo volume from payload mass and density, and report the adequacy verdict. Produces the ULD count, the volume utilization, the unused length and width, the door opening width and height with clearance, and the volume adequacy verdict. Trigger: cargo door opening, unit load device, ULD layout, freight hold. - Skill: `ashfordeou/cargo-compartment-sizing` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/cargo-compartment-sizing` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/cargo-compartment-sizing/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Docs & Writing - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/cargo-compartment-sizing --- # Cargo Compartment Sizing (vehicle-design/sizing/cargo-compartment-sizing) Use when the task is sizing a freight cargo compartment and its cargo door around standard unit load devices (ULDs) for a transport aeroplane: the ULD envelope catalog lookup, the door cross-section fit check with rotation, the largest ULD that fits a door, the 2D strip layout of ULD positions in the hold, the required cargo volume from the payload mass and density, and the end-to-end volume adequacy verdict. This leaf implements the layout in pure Python, stdlib only. It pairs with sizing/fuselage-sizing (passenger cabin layout and its underfloor volume budget at the whole-cabin scale) and sizing/fire-protection-sizing (which consumes the compartment volume sized here). It does not cover passenger baggage budgeting, compartment detection-zone rules, ULD restraint and loading system mechanics, or structural sizing of the shell around the door cutout. ## Domain quick reference - ULD catalog convention: a ULD is tabulated as (length_along, width_across, height) in meters, length along the fuselage axis and width across the hold. The catalog paraphrases standard public IATA container and pallet envelope data with 1 in = 0.0254 m exactly, so a 60.4 in wide unit carries width 1.53416 m; pallets carry the nominal 64 in net build height (1.6256 m) used for volume accounting. - Container volume: the envelope product of the tabulated dimensions. The corner cutouts of contoured containers mean the net usable volume sits below the envelope product; this leaf discloses that approximation and applies no correction constant. - Door fit: a ULD cross-section (width_across, height) fits an opening (W, H) when width_across <= W and height <= H, or with the rotated cross-section when height <= W and width_across <= H. - Largest ULD for the door: the catalog entry with the largest envelope volume whose cross-section fits, visited sorted by catalog key so a volume tie resolves to the earliest key. - Strip layout: n_along = floor(length / uld_length) ULDs in a row; n_across = floor(width / (uld_width + aisle_allowance)) rows across the hold, floored to 1 when the width alone admits one row; positions = n_along * n_across. The aisle allowance is 0.10 m and only separates two-abreast rows. - Utilization: utilized_volume = positions * uld envelope volume over the compartment volume length * width * usable height. The unused length and width remain after the laid-out rows. - Required cargo volume: V_req = payload_mass / payload_density. - Door opening geometry: required width = uld_width + 2 * 0.05 m side margin, required height = uld_height + 0.05 m top margin. The corner radii are the distances from the fuselage centerline axis to the opening corners at the sill height (negative for lower-lobe doors); the opening is inside the fuselage when both corner radii are within the fuselage radius. - Adequacy verdict: needed_ulds = ceil(V_req / per-ULD volume); the layout is volume-adequate when the utilized volume meets or exceeds the required volume. - FAR-25 (14 CFR Part 25) frames the transport-category context for cargo compartment design; the geometric relations above are common conceptual sizing practice, summary-only. ## Workflow 1. Fix the hold envelope and payload: compartment length, width and usable height, and the payload mass and density. 2. Choose the candidate ULD from the module catalog (ULD_CATALOG in cargo_compartment_sizing_logic.py). 3. Check the ULD cross-section through the cargo door with uld_fits_door, with the rotated orientation tried. 4. Pick the largest ULD that fits the door with max_uld_for_door. 5. Lay out ULD positions in the compartment with compartment_uld_layout, the strip layout with the aisle allowance; rework the envelope if positions fall short. 6. Compute the required cargo volume with cargo_volume_required from the payload mass and density. 7. Size the cargo door opening with door_opening_geometry: side and top margins, and the corner radii checked within the fuselage. 8. Assemble the adequacy verdict with layout_summary and iterate: lengthen the hold or take a denser load until volume_adequate is True. ## Worked example Narrowbody lower-lobe freight hold: usable length 12.0 m, width 2.2 m, usable height 1.70 m; payload 4000 kg at 120 kg/m3; cargo door 1.80 m wide x 1.68 m high. Module outputs: - LD3-46 envelope volume 3.895769 m3, LD1 5.827817 m3, LD9 11.536493 m3. - compartment_uld_layout(12.0, 2.2, 1.70, "LD3-46"): 7 positions in 1 row of 7, utilized 27.270382 m3 against a 44.880000 m3 compartment, volume utilization 0.607629, unused length 1.065300 m, unused width 0.665840 m. - cargo_volume_required(4000, 120) = 33.333333 m3, so 9 LD3-46 containers are needed (ceil). - Door checks at 1.80 x 1.68: LD3-46 fits in both orientations, LD6 and P6P-96x125 do not; max_uld_for_door returns LD1 at 5.827817 m3 as the largest catalog ULD whose cross-section passes the opening. - door_opening_geometry("LD3-46", sill -0.75 m, fuselage radius 1.975 m): required door width 1.634160 m, height 1.675600 m, top corner radius 1.234648 m, bottom corner radius 1.109108 m, both within the fuselage. - layout_summary verdict: volume_adequate False, utilized 27.270382 m3 against 33.333333 m3 required, shortfall 6.062952 m3, needed_ulds 9, door_fits True. The payload needs 9 LD3-46 containers against 7 positions, so the hold must lengthen or take a denser load. ## Verification - Confirm compartment_uld_layout(12.0, 2.2, 1.70, "LD3-46") returns 7 positions with volume utilization 0.607629, unused length 1.065300 m and unused width 0.665840 m. - Confirm cargo_volume_required(4000, 120) = 33.333333 m3. - Confirm LD3-46 passes the 1.80 x 1.68 m door in both orientations, LD6 and P6P-96x125 do not, and max_uld_for_door(1.80, 1.68) returns LD1 at 5.827817 m3. - Confirm the door geometry anchors: width 1.634160 m, height 1.675600 m, corner radii 1.234648 m and 1.109108 m, within the 1.975 m fuselage at a -0.75 m sill. - Confirm the verdict: volume_adequate False with shortfall 6.062952 m3, needed_ulds 9, door_fits True. - Confirm the identities: doubling an exact-multiple compartment length doubles per_row and positions; a ULD that fits in one orientation also passes the swapped check on a large enough opening; doubling the payload density halves the required volume. - Confirm ValueError rejection of non-physical inputs: non-positive compartment dimensions, an unknown uld_id, a ULD taller than the usable height, non-positive payload density, negative mass, and a non-positive fuselage radius. - Run the contract test offline: python3 scripts/test_cargo_compartment_sizing.py (30 tests, deterministic). ## Related leaves - vehicle-design/sizing/fuselage-sizing: passenger cabin layout and the whole-cabin underfloor volume budget that bounds this hold. - vehicle-design/sizing/fire-protection-sizing: consumes the compartment volume sized here when it fixes the protected zone and agent concentration. - vehicle-design/sizing/weight-estimation: payload mass feeding the cargo volume requirement. - vehicle-design/structures-integration/fuselage-skin-stringer: structural sizing of the fuselage shell around the door cutout. ## Pitfalls - Sizing the door on the ULD height alone: the required door height adds the top margin and the width adds both side margins, so the opening exceeds the ULD cross-section. - Forgetting the rotated orientation: a ULD that fails the straight cross-section check can still pass the door turned on its side, so both orientations must be tried before declaring a no-fit. - Charging the aisle allowance to a single row: the 0.10 m aisle only separates two-abreast rows, so one row keeps the full unused width. - Treating the envelope volume as usable volume: contoured container corner cutouts push the net usable volume below the envelope product, so the layout check is optimistic by a disclosed approximation. - Comparing required volume against per-position volume: needed_ulds is the ceiling of the ratio, and positions must meet it before the hold is volume-adequate. - Passing zero or negative inputs; the module raises ValueError instead of returning a nonsense dimension. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_cargo_compartment_sizing.py The test covers the worked-example layout contract (7 positions, volume utilization 0.607629, unused length and width anchors), the door cross-section checks with both orientations, the largest-ULD-for-door pick, the strip layout identities (doubling length doubles positions, two-abreast rows with the aisle allowance), the required volume from the payload, the door opening geometry anchors and corner-radius hypotenuse relation, the adequacy verdict (volume_adequate, shortfall, needed_ulds, door_fits) and ValueError rejection of every non-physical input listed in Verification. ## Compliance - Standards referenced, not reproduced: FAR-25 is US government work (public domain); the ULD catalog is paraphrased standard public envelope data and the layout relations are common conceptual sizing methodology, summary-only per standards-map.yaml. - compliance: STANDARDS-REF, gated: false.