True Airspeed (rule of thumb)

Indicated airspeed is what the cockpit instruments show, but at higher altitudes the air is thinner, so the airplane must move faster through the air to produce the same aerodynamic effects. This rule of thumb estimates that increase by adding about 2% of IAS for every 1,000 ft of altitude, giving a quick mental conversion from IAS to TAS. It is only an approximation, but it is useful because the density of air generally decreases with altitude in a way that makes true speed rise steadily above indicated speed.

Use this for quick flight-planning estimates when you need a rough TAS from IAS and altitude without a flight computer or detailed performance tables. It is most helpful in basic navigation, cross-country planning, and sanity-checking airspeed at moderate altitudes.

• Using altitude in feet directly instead of dividing by 1,000 first
• Treating the result as exact instead of an estimate
• Using this rule at very high altitudes or in unusual temperature conditions without checking more accurate data