This section presents methods and examples of computing the steady-state heating and cooling loads of aircraft compartments. In a steady-state process the flows of heat throughout the system are stabilized and thus do not change with time. In an aircraft compartment, several elements compose the steady-state air conditioning load. Transfer of heat occurs between these sources and sinks by the combined processes of convection, radiation, and conduction in the following manner:

1. 1 Convection between the boundary layer and the outer airplane skin.
2. 2 Radiation between the external skin and the external environment.
3. 3 Solar radiation through transparent areas directly on flight personnel and equipment and on the cabin interior surfaces.
4. 4 Conduction through the cabin walls and structural members.
5. 5 Convection between the interior cabin surface and the cabin air.
6. 6 Convection between cabin air and flight personnel or equipment.
7. 7 Convection and radiation from internal sources of heat such as electrical equipment.

The subsequent paragraphs discuss methods of determining each of the heat transfer rates listed above, as well as the physiological considerations involved in the selection of proper cabin conditions that are to be maintained.