Definition
A handheld mechanical computer in the form of two concentric rotating discs marked with logarithmic scales, used by pilots to perform multiplication, division, and proportion-based calculations such as time-speed-distance, fuel burn, true airspeed, density altitude, and wind correction. The aviation version is commonly known as the E6B flight computer.
Plain English
A round, hand-spun calculator made of two discs that turn against each other. Pilots line up numbers on the scales to quickly work out things like how long a flight will take, how much fuel they'll burn, or what their true airspeed is — all without batteries.
Context Anchor
Seen in pilot training, written test preparation, and flight planning when using a manual flight computer instead of an electronic calculator or app.
Derivation
From 'slide rule' — a calculating tool invented in the 1600s using sliding logarithmic scales — made 'circular' by wrapping the scales around a disc instead of a straight bar. The circular form lets the scales loop continuously, so calculations don't run off the end.
Why Pilots Care
Provides reliable manual computation backup when batteries fail or GPS is unavailable, supporting accurate navigation and fuel decisions.
Analogy
It is like a non-electronic calculator made from rotating dials instead of buttons and a screen.
Intuition Check
Do not think of it as a rule about flying in circles. “Circular” describes the shape of the calculating tool, and “slide rule” means a mechanical calculator with number scales.
Example Sentence 1
Before the cross-country flight, she used her circular slide rule to calculate true airspeed from the indicated airspeed, altitude, and outside air temperature.
Example Sentence 2
During cross-country planning she used the circular slide rule to convert fuel burn rate into total gallons required for the route.