Definition
A mechanical analog calculator consisting of sliding logarithmic scales used to perform multiplication, division, and other mathematical operations. In aviation, a circular slide rule (commonly called an E6B flight computer) is used to solve flight planning problems such as time, speed, distance, fuel burn, wind correction, density altitude, and true airspeed.
Plain English
A hand-held tool with sliding scales that lets you do math by lining up numbers instead of using a calculator. Pilots use a circular version to work out flight planning problems like fuel, time, and wind.
Context Anchor
Seen in ground-school flight planning, manual flight computers, and backup planning when electronic calculators or apps are not being used.
Derivation
Named for the way it works: a 'rule' (a measuring device with marked scales) that you 'slide' to align numbers. The original straight slide rules used logarithmic scales so that sliding distances corresponded to multiplying or dividing values.
Why Pilots Care
Even in the GPS era, examiners and many training programs still expect pilots to compute wind correction, fuel burn, and time/distance problems on a manual flight computer. Knowing how to use one is often required for the private pilot knowledge test.
Analogy
It is like a ruler with movable number lines. Instead of measuring length, you move the scales until the numbers line up and then read the answer from another mark.
Intuition Check
Do not read “rule” here as a regulation or instruction. In “slide rule,” “rule” means a measuring scale, and “slide” means the scale moves.
Example Sentence 1
Before GPS units were common, pilots used a slide rule to calculate ground speed and estimated time en route.
Example Sentence 2
The E6B flight computer is essentially a specialized slide rule designed for aviation problems.