Definition
The total amount of heat, measured in British Thermal Units (BTU) per hour, that an air conditioning or environmental control system must remove from an enclosed space — such as an aircraft cabin — to maintain a desired internal temperature. Heat load includes heat entering from outside (solar radiation, hot ambient air, friction at high speeds), heat generated inside (occupants, electronics, lighting), and heat conducted through the airframe structure.
Plain English
The total amount of heat the cooling system has to get rid of to keep the cabin at a comfortable temperature. The hotter the day, the more passengers on board, and the more equipment running, the bigger the heat load.
Context Anchor
Used in discussions of cabin cooling, equipment cooling, ventilation, and temperature-control systems.
Derivation
Load comes from Old English hlad, meaning a burden or something carried. The cooling system has to 'carry away' heat — so the heat load is the burden of heat the system must handle.
Why Pilots Care
Unmanaged heat load can cause overheating of engines, avionics, or cabin systems, leading to reduced performance or component failure.
Intuition Check
Heat load does not mean physical weight or cargo. Here, load means the amount of heat a system has to handle.
Example Sentence 1
Sitting on the ramp at noon in Phoenix with six passengers boarded, the heat load on the air conditioning system was more than it could handle until they got airborne.
Example Sentence 2
High-speed flight reduced the engine heat load by improving ram-air cooling.