Definition
A simplified, approximate value or quick mental calculation used by pilots to estimate something close enough for practical decision-making, without performing the full precise computation.
Plain English
A handy shortcut number that gets you close to the right answer fast, even though it isn't exact.
Context Anchor
Seen in aeronautical decision-making discussions when pilots or instructors use simple estimates to think through risk, time, distance, workload, or personal limits.
Derivation
The phrase comes from an old English expression where craftsmen used the width of the thumb as a rough measuring tool. It came to mean any quick, practical estimate based on experience rather than precise measurement. In aviation, it carries the same idea: close enough to be useful, not exact.
Why Pilots Care
Allows safe, timely decisions when exact figures are unavailable or time pressure exists.
Intuition Check
Do not read “rule” here as an official aviation rule. A rule-of-thumb figure is a rough guide, not a regulation and not a precise calculation.
Example Sentence 1
As a rule-of-thumb figure, the instructor used three times the altitude to lose in thousands of feet to estimate the descent distance in nautical miles.
Example Sentence 2
During the preflight briefing the instructor shared a rule-of-thumb figure for crosswind component limits on short runways.