Definition
The self-ignition of a material without an external flame or spark, caused by internal heat buildup from a slow chemical reaction — typically oxidation — that cannot dissipate fast enough to the surroundings. Common in oily rags, certain fuels, and some chemicals stored in poorly ventilated conditions.
Plain English
Something catches fire on its own. No match, no spark — just a slow chemical reaction that builds up heat inside the material until it ignites.
Context Anchor
Seen in aircraft maintenance when handling oily rags, paint materials, solvents, and other shop items that can heat up if stored or thrown away improperly.
Derivation
From Latin sponte meaning 'of one's own accord,' and combustion from comburere meaning 'to burn up.' Together: burning that starts on its own. The 'on its own' part is the key — no outside ignition source is needed.
Why Pilots Care
Improper storage of oily waste in hangars or shops can lead to hidden fires that damage aircraft or endanger personnel.
Grounding Statement
A pile of oil-soaked rags left in a closed container can trap heat until the rags get hot enough to ignite.
Intuition Check
Spontaneous combustion does not mean fire appears for no reason. It means the heat source is built up inside the material instead of coming from an outside flame or spark.
Example Sentence 1
Used shop rags soaked in linseed oil must be stored in a sealed metal container to prevent spontaneous combustion.
Example Sentence 2
During the post-maintenance inspection the crew checked the hangar for any risk of spontaneous combustion from cleaning materials.