Definition
A practical, approximate guideline used in flying to make quick estimates or decisions without precise calculation. It is a memorized shortcut that gives a useful answer in most normal situations, but it is not a substitute for exact figures from the Pilot's Operating Handbook, performance charts, or published procedures.
Plain English
A handy mental shortcut pilots use to get a quick, close-enough answer when there is no time or need for exact math.
Context Anchor
Seen in flight training discussions when the handbook gives a quick way to estimate something, such as a turn setup, instead of requiring a detailed calculation.
Derivation
An old English expression based on using the thumb as a rough measuring tool — close enough for everyday work, but not precise. The phrase carries that same idea into aviation: a quick approximation, not an exact measurement.
Why Pilots Care
Allows rapid cockpit decisions when exact calculations would take too long or distract from flying.
Analogy
Like pinching salt between your fingers instead of measuring it with a scale.
Intuition Check
Do not read “rule” here as a legal requirement or exact procedure. Here, “rule of thumb” means a practical estimate that helps you act sensibly without doing a full calculation.
Example Sentence 1
A common rule of thumb for a standard-rate turn is to use a bank angle equal to about 15 percent of your true airspeed in knots.
Example Sentence 2
When planning a visual traffic pattern, pilots often apply a rule of thumb to estimate the turn radius needed to align with the runway.