Definition
A scale on an instrument or chart in which the spacing between graduations is not uniform. Equal numerical changes are represented by unequal physical distances along the scale, so the same value covers more space in some parts of the scale than in others.
Plain English
A scale where the marks are spaced unevenly. Some sections are stretched out and others are squeezed together, even though the numbers themselves go up in regular steps.
Context Anchor
Seen when reading aircraft instruments or gauges whose markings are closer together in some ranges and farther apart in others.
Derivation
From Latin 'linearis' (relating to a line) with the prefix 'non-' (not). A linear scale has marks evenly spaced along a line; a nonlinear scale does not. Knowing this makes it clear that 'nonlinear' here describes the spacing of the marks, not the shape of the instrument.
Why Pilots Care
Allows accurate reading of instruments where the physical response is not uniform across the full operating range.
Analogy
A normal ruler has evenly spaced marks. A nonlinear scale is more like a ruler where some inch marks are close together and others are farther apart, so you cannot judge every part of it the same way.
Grounding Statement
On a nonlinear scale, the spacing itself is part of how the instrument communicates the value.
Intuition Check
Nonlinear does not necessarily mean the scale is curved or irregular-looking. It means equal value changes are not shown with equal visual spacing.
Example Sentence 1
The lower end of the airspeed indicator uses a nonlinear scale, so the marks are bunched together near the stall speed and spread out at higher speeds.
Example Sentence 2
Engine temperature gauges often use a nonlinear scale to give finer detail in the normal operating range.