Definition
A mixture control setting in which the fuel-to-air ratio delivered to the engine is at its highest fuel proportion. The mixture control knob or lever is pushed fully forward (in), commanding the carburetor or fuel injection system to meter the maximum fuel quantity for the given airflow. This setting is standard for engine start, takeoff, climb, and operations at lower altitudes where the air is dense.
Plain English
The mixture control is pushed all the way in, so the engine is getting plenty of fuel for the air it is taking in. This is the normal setting for starting the engine and flying at lower altitudes.
Context Anchor
Seen in engine-start checklists and hand-propping procedures, where the pilot verifies the cockpit controls before the propeller is pulled through by hand.
Derivation
‘Rich’ comes from the Old English ‘rice,’ meaning abundant or plentiful. In a fuel-air mixture, ‘rich’ means plenty of fuel — a generous amount relative to the air. The opposite is ‘lean,’ meaning sparse on fuel.
Why Pilots Care
A rich mixture supplies engine cooling and prevents detonation at high power settings such as takeoff, yet must be leaned in cruise to avoid wasted fuel and fouled plugs.
Analogy
It is like adding more syrup to a drink: the drink is still liquid, but the mix has more of one ingredient. A rich engine mixture has more fuel in the fuel-air mix.
Intuition Check
“Rich” does not mean expensive, fancy, or automatically better here. It means the engine’s fuel-air mix contains a higher amount of fuel; too much fuel can also cause starting or running problems.
Example Sentence 1
Before hand propping the airplane, the pilot confirmed the mixture was RICH and the throttle was cracked.
Example Sentence 2
After start the pilot may lean slightly for taxi but returns the mixture to rich before takeoff.