Definition
A self-accelerating failure condition in which rising temperature inside a component causes a chemical or electrical reaction that produces still more heat, which raises the temperature further, and so on until the component fails — often violently. Most commonly associated with rechargeable batteries (especially lithium-ion), where an internal fault causes the cell to overheat, vent flammable gases, and ignite or explode.
Plain English
A runaway heating cycle: something gets hot, the heat makes it react in a way that creates more heat, and the cycle feeds itself until the part burns, melts, or catches fire.
Context Anchor
Seen in aircraft electrical system, battery charging, and maintenance discussions, especially when talking about battery overheating.
Derivation
From 'thermal' (relating to heat, from Greek thermē, meaning heat) and 'runaway' (a process that has escaped control). Together: heat that has gotten away from any ability to stop it.
Why Pilots Care
If undetected it can destroy the battery and start a fire inside the aircraft, requiring immediate isolation of the battery and emergency procedures.
Analogy
It is like a small campfire that gets hotter, and the extra heat helps it burn even faster. Unless something interrupts the cycle, the problem grows on its own.
Grounding Statement
Picture a battery that is already warm while charging, then begins getting hotter because the charging itself is making the heat problem worse.
Intuition Check
Thermal runaway does not mean a part is merely hot. It means the heat is feeding a cycle that makes the part keep getting hotter.
Example Sentence 1
After the passenger reported smoke coming from her tablet, the crew followed the lithium battery procedure to contain the thermal runaway before it could spread.
Example Sentence 2
The pilot shut down the battery master after smelling fumes that indicated possible thermal runaway in the main aircraft battery.