Definition
The proportion of fuel to air, by weight, in the mixture entering an engine's combustion chambers. It is expressed as a ratio such as 1:12, meaning one part fuel to twelve parts air.
Plain English
How much fuel is mixed with the air going into the engine, compared by weight. A small change in this proportion makes a big difference in how the engine runs.
Context Anchor
Seen in engine operation, mixture control, leaning, and discussions of engine power and temperature.
Why Pilots Care
Correct fuel-air ratio prevents engine damage, maintains power, and avoids inefficient or rough operation.
Analogy
It is like a recipe: too much or too little of one ingredient changes the result. For an engine, the ingredients are fuel and air.
Intuition Check
Do not read fuel-air ratio as the amount of fuel in the tanks compared with the air around the airplane. It means the proportion of fuel and air in the mixture the engine is actually burning.
Example Sentence 1
As the airplane climbed, the pilot leaned the mixture to maintain a proper fuel-air ratio in the thinner air.
Example Sentence 2
An overly rich fuel-air ratio caused the engine to run rough on the ground.