Definition
A unit of heat energy. One BTU is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. In aviation, BTU ratings are used to describe the heat output of fuel-fired cabin heaters and the energy content of aviation fuels.
Plain English
A way of measuring how much heat something produces. The bigger the BTU number, the more heat.
Context Anchor
Seen in discussions of fuel-fired aircraft heaters, where heater output or heat production may be described in BTUs.
Derivation
From British, because the unit was developed in the UK, and thermal, from the Greek therme meaning heat. The name simply tells you it is a British-origin measure of heat energy.
Why Pilots Care
The BTU rating tells a pilot how effectively a fuel-fired heater will warm the cabin in cold conditions.
Grounding Statement
Picture warming a small glass of water just one degree on a stove — that tiny amount of heat is roughly one BTU. A cabin heater producing 10,000 BTU per hour is putting out a lot of warmth.
Intuition Check
A BTU is not a temperature reading. It measures an amount of heat, not how hot the air is at one moment.
Example Sentence 1
The aircraft's combustion heater is rated at 35,000 BTU per hour, which is enough to keep the cabin comfortable on a cold winter flight.
Example Sentence 2
Before flight in winter, confirm the heater’s BTU output meets the expected demand.