Definition
The deliberate leaning of an aircraft engine's fuel-air mixture during certain high-power operations, particularly in some turbocharged or fuel-injected reciprocating engines, to reduce an excessively rich mixture toward a more efficient ratio. Derichment systems automatically reduce fuel flow under specific conditions to improve combustion and prevent fouling or power loss caused by an over-rich mixture.
Plain English
Cutting back the amount of fuel mixed with the air going into the engine when there is too much fuel for good burning. The engine runs better when the fuel and air are in the right balance, so the system trims the fuel back.
Context Anchor
Seen in engine, carburetor, fuel-injection, and mixture-control discussions.
Derivation
Built from 'de-' meaning 'remove or reduce' and 'enrichment,' which means adding extra fuel to the mixture. Derichment is therefore the opposite action: taking fuel out of an over-rich mixture. Knowing it is simply the reverse of enrichment makes the term easy to place.
Why Pilots Care
Maintains proper engine temperatures, fuel economy, and performance once high-power demands end, reducing risk of spark-plug fouling or excessive fuel burn.
Intuition Check
Derichment does not mean shutting off fuel. It means reducing the fuel portion of the fuel-air mixture.
Example Sentence 1
The turbocharged engine's derichment system reduced fuel flow once manifold pressure stabilized in the climb.
Example Sentence 2
The pilot monitored fuel flow during climb to confirm that derichment occurred at the scheduled altitude.