# Avionics Bay Cooling Sizing > Use when you must size the cooling airflow for the avionics and equipment bay: rolling up the bay heat load from the LRU heat dissipations, sizing the bay cooling mass flow from the allowable cooling air temperature rise so the bay exhaust temperature stays at or below the LRU inlet air temperature limit, converting the mass flow to the volumetric flow and to cubic feet per minute, and checking each LRU case temperature against its case limit from the dissipated power and the case to air conductance. Produces the bay heat load, the required cooling mass flow and CFM, the exhaust temperature, the per LRU case temperature verdicts and the bay cooling PASS or FAIL verdict that gate the bay ventilation layout. Trigger: avionics bay cooling sizing, equipment bay cooling airflow, LRU heat dissipation, LRU case temperature check, cooling air temperature rise, case to air conductance. - Skill: `ashfordeou/avionics-bay-cooling-sizing` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/avionics-bay-cooling-sizing` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/avionics-bay-cooling-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/avionics-bay-cooling-sizing --- # Avionics Bay Cooling Sizing (vehicle-design/sizing/avionics-bay-cooling-sizing) Use when the task is sizing the cooling airflow for the avionics and equipment bay of an aircraft: the LRU heat dissipations must be carried away by a cooling air flow whose exhaust stays at or below the LRU inlet-air temperature limit, and each LRU case must stay within its case temperature limit. This leaf implements the standard bay cooling model in pure Python, stdlib only: heat load rollup, cooling mass flow from the allowable temperature rise, volumetric flow and CFM conversion, and the per-LRU case temperature check. It pairs with vehicle-design/sizing/environmental-control-sizing (the cabin conditioning leaf, which stops at the conditioning pack flow and never sizes bay airflow or LRU case temperatures) and with avionics/do160/environmental-qualification (the LRU temperature test specification side, which does not size airflow). ## Domain quick reference - Bay heat load rollup: Q_total = sum of the LRU heat dissipations P_i, each LRU power in W. The bay is a collection of line-replaceable units (LRUs) inside the avionics/equipment bay. - Cooling mass flow: m_dot = Q / (cp * (T_exhaust_limit - T_supply)), with cp = 1005 J/(kg K) for dry air. The exhaust temperature is sized to equal the LRU inlet-air temperature limit, giving the maximum cooling benefit without exceeding the LRU inlet limit. - Volumetric flow: V_dot = m_dot / rho with rho = 1.2 kg/m3 near sea level; CFM = V_dot * 2118.88 (ft3/min per m3/s). - LRU case temperature: T_case = T_inlet_air + P / (h A), where h A is the case-to-air conductance in W/K (default 12 W/K shared across the bay). A zero-power LRU sits exactly at the inlet air temperature. - Case verdict: PASS when T_case <= T_limit, else FAIL; margin is T_limit - T_case (positive for PASS, negative for FAIL). - Bay verdict: FAIL when any LRU case exceeds its limit or the mass flow is zero with positive heat, else PASS. - Units are W, deg C, kg/s, m3/s, ft3/min (CFM), K. - FAR 25.1309 frames the equipment installation context; the relations above are standard engineering methodology, summary-only. ## Workflow 1. Collect the LRU heat dissipations in W and roll up the bay heat load with bay_heat_load. 2. Fix the bay cooling supply temperature and the LRU inlet-air temperature limit, then size the cooling mass flow with cooling_mass_flow; the returned exhaust temperature equals the limit by construction. 3. Convert the mass flow to the volumetric flow and CFM with volumetric_flow. 4. For each LRU, compute the case temperature from the dissipated power and the case-to-air conductance with lru_case_temperature, then grade it against the LRU case limit with case_verdict. 5. Run the whole bay in one call with bay_cooling_summary to get the heat load, flows, exhaust temperature, per-LRU case temperatures, case verdicts and the bay PASS/FAIL verdict that gates the bay ventilation layout. 6. Confirm the deterministic checks with the contract test scripts/test_avionics_bay_cooling_sizing.py. ## Worked example Reference bay with six LRUs dissipating 400, 350, 300, 550, 450 and 450 W (2500 W total), supply air at 25 C, LRU inlet-air limit 55 C, air density 1.2 kg/m3, case-to-air conductance 12 W/K for every LRU, and case limits [60, 60, 60, 65, 60, 60] C. - Bay heat load: bay_heat_load returns total_w 2500.0 W. - Cooling mass flow: cooling_mass_flow(2500.0, 25.0, 55.0) returns 0.08292 kg/s (2500 / (1005 * 30)); exhaust_temp_c 55.0 C. - Volumetric flow: volumetric_flow returns 0.06910 m3/s and 146.4 CFM. - 300 W LRU: lru_case_temperature gives rise 300 / 12 = 25.0 K and case_temp_c 25 + 25 = 50.0 C, versus the 60 C limit a PASS with margin 10.0 K. - 550 W LRU: rise 550 / 12 = 45.83 K, case_temp_c 70.83 C, versus the 65 C limit a FAIL with margin -5.83 K; this is the undersized heat path the check exists to catch. - Bay summary: bay_cooling_summary over the six LRUs reports the 2500.0 W load, 0.08292 kg/s, 146.4 CFM, exhaust 55.0 C, case temps {0: 58.33, 1: 54.17, 2: 50.0, 3: 70.83, 4: 62.5, 5: 62.5} C with verdicts PASS, PASS, PASS, FAIL, FAIL, FAIL and a bay verdict FAIL, so the bay layout must be revised (heat path or limits) before release. Re-running the summary with the 550 W LRU reduced to 350 W gives an all-PASS set and a bay verdict PASS. ## Pitfalls - Checking the airflow but not the individual LRU cases: the bay verdict FAILs when any LRU case exceeds its limit even if the mass flow is adequate - in the worked bay three LRUs fail their case limits (550 W gives 70.83 C against 65 C) while the flow itself closes the 30 K rise. - Sizing the flow on a temperature difference that is not positive: cooling_mass_flow requires the LRU inlet-air limit above the supply temperature; a limit at or below supply raises ValueError because there is no cooling potential. - Reading the exhaust temperature as a margin: the sized exhaust EQUALS the LRU inlet limit by construction (55.0 C in the worked example); it is the per-LRU case temperatures that carry margin. - Sharing one conductance blindly: the case temperature uses the case-to-air conductance h A (12 W/K default) per LRU, and doubling the conductance halves the case rise; a high-dissipation LRU with poor local heat path fails while the average looks fine. - Treating this leaf as the cabin conditioning pack: the bay airflow and LRU case checks are this leaf's claim; environmental-control- sizing stops at the conditioning pack flow, and DO-160 environmental-qualification covers the LRU test side. - Feeding non-physical bay inputs: an empty LRU list, negative power, density or conductance at or below zero, and a case limit below absolute zero all raise ValueError. ## Verification - Confirm bay_heat_load([400, 350, 300, 550, 450, 450]) returns total 2500.0 W. - Confirm cooling_mass_flow(2500.0, 25.0, 55.0) returns 0.08292 kg/s (inside the independently verified 0.08 to 0.09 kg/s bound) and that zero heat returns a 0.0 kg/s flow. - Confirm volumetric_flow converts the reference flow to 0.06910 m3/s and 146.4 CFM, and that 1 kg/s at 1.2 kg/m3 gives 0.8333 m3/s and 1765.7 CFM. - Confirm the identities: doubling the allowable temperature rise at fixed heat halves the mass flow, doubling the heat doubles the flow, doubling the case-to-air conductance halves the case rise, and a zero-power LRU sits at the inlet air temperature. - Confirm the 300 W LRU case is 50.0 C (PASS, margin 10.0 K) and the 550 W LRU case is 70.83 C (FAIL, margin -5.83 K), and that the bay verdict is FAIL when any LRU fails and PASS only when every LRU passes. - Confirm every non-physical input raises ValueError: empty LRU list, negative power, temperature difference not positive (limit at or below supply), density at or below zero, conductance at or below zero, and a case limit below absolute zero. - Run the contract test offline: python3 scripts/test_avionics_bay_cooling_sizing.py (32 tests, deterministic). ## Related leaves - vehicle-design/sizing/environmental-control-sizing: the cabin conditioning leaf; its equipment heat line sits inside the cabin load and it stops at the air conditioning pack, it does not size bay airflow or LRU case temperatures. - vehicle-design/sizing/ice-protection-sizing: surface thermal ice protection for the aerodynamic surfaces, a different thermal sizing problem. - avionics/do160/environmental-qualification: the test specification side of LRU temperature qualification, not airflow sizing. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_avionics_bay_cooling_sizing.py The test covers the reference bay sizing contract (2500 W heat load, 0.08292 kg/s mass flow, 146.4 CFM, 55.0 C exhaust, 300 W LRU case 50.0 C PASS, 550 W LRU case 70.83 C FAIL and the FAIL bay verdict), the flow and case scaling identities, the volumetric conversion references, the convenience dict keys, determinism and the ValueError rejection of every non-physical input. ## Compliance - Standards referenced, not reproduced: FAR 25.1309 is the equipment installation context (case of an avionics bay in an aircraft); the bay cooling relations above are standard engineering methodology, summary-only per standards-map.yaml. - compliance: STANDARDS-REF, gated: false.