Definition
The proportion of fuel to air, by weight, in the mixture being delivered to the engine's cylinders for combustion. A typical reciprocating aircraft engine runs on a fuel-to-air ratio of roughly 1:12 to 1:15 by weight, depending on power setting and operating conditions.
Plain English
How much fuel is mixed with the air going into the engine. Too much fuel for the air is called rich; too little fuel for the air is called lean. The pilot adjusts this with the mixture control.
Context Anchor
Seen in engine operation and heat management discussions, especially when adjusting the mixture during climb, cruise, or descent.
Derivation
Ratio comes from Latin words meaning a reckoning or calculation. That helps here because a fuel-to-air ratio is not just an amount of fuel by itself; it is a comparison between two amounts: fuel and air.
Why Pilots Care
A correct ratio prevents overheating, detonation, and power loss while protecting engine life.
Intuition Check
Do not read fuel-to-air ratio as fuel flow only. It is the amount of fuel compared with the amount of air available to burn it.
Example Sentence 1
During climb, the pilot kept the fuel-to-air ratio on the rich side to help cool the cylinders.
Example Sentence 2
A richer fuel-to-air ratio during climb helps keep cylinder temperatures within limits.