Definition
A chemical or physical reaction that releases heat to its surroundings as it takes place.
Plain English
A reaction that gives off heat while it happens. The materials involved get warmer, and so does the area around them.
Context Anchor
Seen in aircraft maintenance and systems discussions involving batteries, fire, combustion, and chemical reactions.
Derivation
From the Greek 'exo' meaning 'outside' and 'therme' meaning 'heat.' Literally 'heat going out' — the reaction sends heat outward into its surroundings.
Why Pilots Care
Several aircraft systems involve exothermic reactions that can become hazards if uncontrolled — a shorted battery, a fuel fire, or an overheating chemical oxygen generator can all release significant heat. Recognising the term helps a pilot understand why certain malfunctions produce heat and smoke rather than simply failing quietly.
Analogy
A hand warmer is a simple example: once the chemical reaction starts, it gives off heat you can feel. The heat is produced by the action itself.
Grounding Statement
Think of a hand warmer packet: once activated, the chemicals inside react and the packet gets warm. That warmth is exothermic action — heat being released by the reaction itself.
Intuition Check
Exothermic does not mean heat is being added from outside. It means the action itself is giving off heat.
Example Sentence 1
The instructor explained that combustion in the engine cylinders is an exothermic action, which is why the cylinders need cooling fins.
Example Sentence 2
Maintenance procedures require monitoring for exothermic action when mixing certain aircraft adhesives.