# Environmental Control Sizing > Use when you must size the environmental control system of a transport aircraft: compute the cabin ventilation fresh air flow from the occupant count and per-occupant rate, roll up the cabin heat load from occupants, solar, equipment and skin with a design margin, derive the pack cooling airflow from the heat load and the supply air temperature rise, take the pack airflow as the governing maximum of fresh and cooling flow, and build the cabin pressurization schedule that holds the design cabin pressure altitude until the design differential pressure binds, then clamps at constant differential as the cabin altitude rises. Produces fresh air flow, cabin heat load, pack airflow, cabin differential at cruise and cabin altitude under both regimes. Trigger: environmental control system sizing, ECS, cabin ventilation, fresh air flow, pack cooling airflow, cabin heat load, pressurization schedule, cabin pressure altitude, differential pressure limit. - Skill: `ashfordeou/environmental-control-sizing` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/environmental-control-sizing` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/environmental-control-sizing/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/environmental-control-sizing --- # Environmental Control System Sizing (vehicle-design/sizing/environmental-control-sizing) Use when the task is sizing the environmental control system (ECS) of a transport aircraft: the cabin ventilation fresh air flow from the occupant count, the cabin heat load rollup (occupants, solar, equipment, skin) with a design margin, the pack cooling airflow from the heat load and the pack supply temperature rise, the pack airflow verdict as the governing maximum of the fresh and cooling flows, and the cabin pressurization schedule that holds the design cabin pressure altitude until the design differential pressure binds and then flies constant differential with the cabin altitude allowed to rise. This leaf implements the sizing model in pure Python, stdlib only, with FAR 25 as the reference regulatory context. It pairs with vehicle-design/sizing/ice-protection-sizing (the sibling aircraft- subsystem sizing leaf; boundary: surface anti-icing bleed heating vs cabin conditioning) and with cross-cutting/units-atmos/isa-atmosphere, whose public two-layer atmosphere relation this leaf embeds internally as private helpers only. ## Domain quick reference - Fresh air flow: m_fresh = N * rate_per_occupant (default 0.25 kg/min per occupant, the 0.55 lb/min practice), converted to kg/s by /60. - Cabin heat load: Q_occ = N * q_occ; Q_total = Q_occ + Q_solar + Q_equipment + Q_skin; Q_design = margin * Q_total (default margin 1.1). - Pack cooling flow: m_cool = Q_design / (cp * dT_supply), with default cp 1.005 kJ/(kg K) and default supply temperature rise 20 K. Pack flow is the governing maximum: m_pack = max(m_fresh, m_cool), and cooling_dominates records which side rules. - Pressurization: p_amb = p_ISA(h_cruise); p_cab = p_ISA(h_cabin design). Differential dP = p_cab - p_amb. While dP <= dP_max the schedule holds the cabin altitude at the design value; when the required differential would exceed dP_max the clamp binds, p_cab = p_amb + dP_max * PSI, and the cabin altitude rises per the inverse ISA (defaults: 8000 ft design cabin altitude, 8.9 psi design differential). - Internal two-layer ISA (private helpers, not a deliverable): p = P0 (1 - L h / T0)^(G/(L R)) in the troposphere to 11 km, then an isothermal stratosphere at 216.65 K with scale height R T_strat / G. The public atmosphere leaf is cross-cutting/units-atmos/isa-atmosphere. - Units: SI (Pa, kg/s, K, kW, m) with ft and psi accepted at the pressurization and altitude interfaces; PSI = 6894.757 Pa, FT = 0.3048 m. - FAR 25 frames the cabin conditioning and pressurization context; the relations above are standard engineering methodology, summary-only. ## Workflow 1. Fix the occupant count and the per-occupant ventilation rate, and get the fresh air flow with fresh_air_flow (returns flow_kgmin and flow_kgs). 2. Roll up the heat sources with cabin_heat_load: occupant heat at q_occupant_kw each, plus solar, equipment and skin heat, closed by the design margin to give design_heat_kw. 3. Size the pack with pack_airflow on the design heat, cp and supply temperature rise; pass the fresh flow so the verdict reflects the governing maximum of ventilation and cooling (cooling_dominates). 4. Build the pressurization schedule with pressurization_schedule at the cruise altitude, design cabin altitude and design differential: read differential_limited to know whether the clamp binds, and cabin_altitude_ft for the resulting cabin altitude. 5. For a one-call rollup run ecs_summary with every input to get the combined dict of all outputs. 6. Confirm the deterministic checks with the contract test scripts/test_environmental_control_sizing.py. ## Worked example Reference transport: 189 occupants at 0.25 kg/min each, 0.12 kW/occupant, solar 15 kW, equipment 12 kW, skin 8 kW, margin 1.1, cp 1.005 kJ/(kg K), supply temperature rise 20 K, design cabin altitude 8000 ft, design differential 8.9 psi, cruise 39000 ft then 50000 ft. - fresh_air_flow(189): flow_kgmin = 47.25 kg/min, flow_kgs = 0.7875 kg/s. - cabin_heat_load(189, 0.12, 15, 12, 8): occupant_heat_kw = 22.68 kW, total_heat_kw = 57.68 kW, design_heat_kw = 63.448 kW. - pack_airflow(63.448): cooling_flow_kgs = 3.156617 kg/s (63.448 / (1.005 * 20)), pack_flow_kgs = 3.156617 kg/s, cooling_dominates True. - pressurization_schedule(39000): p_amb = 19.677 kPa, p_cab = 75.262 kPa (the 8000 ft cabin), differential = 8.0619 psi, below the 8.9 psi limit so the schedule HOLDS: cabin_altitude_ft = 8000.0, differential_limited False. - pressurization_schedule(50000): p_amb = 11.597 kPa; holding 8000 ft would need 9.234 psi, above the limit, so the clamp binds: differential_limited True, p_cab = 72.960 kPa (ambient + 8.9 psi), cabin_altitude_ft = 8809.9 ft, above the 8000 ft design value in the differential-limited regime. ## Pitfalls - Sizing the pack on the fresh-air flow alone: the pack airflow is the governing MAXIMUM of the fresh ventilation flow and the cooling flow (3.1566 kg/s cooling dominates 0.7875 kg/s fresh in the worked example); picking the smaller flow starves the cabin heat load. - Forgetting the design margin in the heat rollup: the design heat is margin * total (default 1.1), so an undiscounted total heat under-sizes the pack by the margin ratio. - Reading the differential at cruise as the limit: at 39000 ft the 8.0619 psi differential holds the 8000 ft cabin below the 8.9 psi clamp; only above the altitude where the clamp binds does the schedule leave the design cabin altitude (8809.9 ft at 50000 ft). - Feeding a margin at or below 1: the margin must exceed 1 (the design heat carries reserve); a margin of 1.0 or less raises ValueError. - Confusing which flow cools and which ventilates: fresh air (occupants at 0.25 kg/min each) is a ventilation requirement, while the pack cooling flow derives from the heat load and the supply temperature rise; the two answer different questions. - Mixing pressure units at the interfaces: pressurization accepts ft and psi with internal conversions (PSI = 6894.757 Pa), so feeding Pa or m into those interfaces silently mis-sizes the schedule. ## Verification - fresh_air_flow(189) returns 47.25 kg/min and 0.7875 kg/s; flow is linear in the occupant count. - cabin_heat_load rolls the four sources into total_heat_kw and design_heat_kw = margin * total exactly; margin 1.5 scales the design heat by 1.5/1.1. - pack_airflow(63.448) returns cooling_flow_kgs 3.156617 kg/s to 1e-6; with fresh_flow_kgs = 5.0 the pack flow equals the fresh flow and cooling_dominates is False. - At 39000 ft the schedule holds 8000 ft exactly (differential 8.0619 psi within 1e-4); at 50000 ft the clamp binds, cabin pressure equals ambient plus 8.9 * PSI within 1e-3 Pa and the cabin altitude lands in (8800, 8820) ft. - The internal ISA is continuous at the tropopause and round-trips _h_isa_from_p(_p_isa(h)) == h to 1e-6 m in both regions. - ValueError rejects: occupants <= 0, rate <= 0, margin <= 1, any negative heat input, design_heat_kw <= 0, cp <= 0, dT_supply_k <= 0, fresh_flow_kgs < 0, cruise_alt_ft < 0, cabin_alt_design_ft < 0 and dP_max_psi <= 0. - Run the contract test offline: python3 scripts/test_environmental_control_sizing.py (33 tests, deterministic). ## Related leaves - vehicle-design/sizing/ice-protection-sizing: sibling aircraft- subsystem sizing leaf; boundary is surface anti-icing bleed heating versus cabin conditioning. - vehicle-design/sizing/battery-sizing: the electric power subsystem sizing leaf for the power offtake context around the ECS loads. - cross-cutting/units-atmos/isa-atmosphere: the public atmosphere leaf whose two-layer relation this leaf embeds internally. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_environmental_control_sizing.py The test covers the worked-example magnitudes (47.25 kg/min fresh flow, 63.448 kW design heat, 3.156617 kg/s pack cooling flow), the pressurization hold at 39000 ft with differential 8.0619 psi and the clamp at 50000 ft with cabin altitude 8809.9 ft, the regime boundary crossing monotonicity, linearity in occupants, the margin scaling ratio, internal ISA tropopause continuity and pressure round-trip in both regions, ecs_summary rollup, run-to-run determinism and ValueError rejection of every non-physical input. ## Compliance - Standards referenced, not reproduced: FAR 25 frames the cabin conditioning and pressurization context; the sizing relations above are standard engineering methodology, summary-only per standards-map. - compliance: STANDARDS-REF, gated: false.