Definition
A ratio expresses the relationship between two quantities of the same kind, showing how many times one value contains or is contained within the other. It is commonly written as A:B (read 'A to B'), as a fraction A/B, or as a decimal. In aviation maintenance, ratios describe proportional relationships such as gear ratios, compression ratios, fuel-to-air mixtures, and mechanical advantage.
Plain English
A way of comparing two amounts to show how they relate in size. For example, a 2:1 ratio means the first amount is twice as big as the second.
Context Anchor
Seen in aircraft maintenance and performance descriptions when comparing related amounts, such as fuel to air, one gear speed to another, or one aircraft dimension to another.
Derivation
From the Latin ratio, meaning 'reckoning' or 'calculation.' The original sense was the act of working out a relationship between numbers — which is exactly what a ratio still does today.
Why Pilots Care
Many engine and system specifications are given as ratios. Misreading a compression ratio or fuel/air mixture ratio can lead to incorrect maintenance decisions or engine performance problems.
Analogy
A recipe can use a 2:1 ratio, such as two cups of water for every one cup of concentrate. The ratio tells you the relationship between the amounts, not just the total amount in the container.
Intuition Check
Do not read ratio as just a difference between two numbers. A difference subtracts; a ratio compares how much one amount is in relation to another.
Example Sentence 1
The engine has a compression ratio of 8.5:1, meaning the air in the cylinder is squeezed into a space 8.5 times smaller than its starting volume.
Example Sentence 2
The gear ratio determines how many times the engine crankshaft turns for each rotation of the propeller.