Definition
A navigation formula stating that distance traveled (D) equals groundspeed (GS) multiplied by time (T). Used to calculate how far an aircraft will travel, or has traveled, when groundspeed and elapsed time are known. Groundspeed must be expressed in the same time unit as T (for example, knots with hours yields nautical miles).
Plain English
If you know how fast you are moving across the ground and how long you have been moving, you can work out how far you have gone by multiplying the two together.
Context Anchor
Used in cross-country planning, navigation log calculations, and in-flight estimates of how far the airplane will travel before the next checkpoint.
Derivation
D, GS, and T are simply the first letters of Distance, Groundspeed, and Time. The formula is the standard physics relationship between speed, time, and distance, applied here using groundspeed (speed over the ground) rather than airspeed, because navigation is concerned with progress across the earth's surface, not movement through the air.
Why Pilots Care
Helps determine fuel burn, time en route, and whether a planned leg is feasible given available daylight or reserves.
Analogy
Exactly like calculating how many miles you will drive at 60 miles per hour for two hours.
Grounding Statement
If the airplane is moving across the ground at a steady speed, the distance covered grows with each minute that passes.
Intuition Check
Do not use indicated airspeed for GS in this formula. GS means speed across the ground, so wind can make it different from the speed shown on the airspeed indicator.
Example Sentence 1
With a groundspeed of 120 knots and a flight time of 30 minutes (0.5 hours), the pilot calculated D = 120 X 0.5 = 60 nautical miles.
Example Sentence 2
Before departure the pilot used D = GS × T to confirm the 180-nautical-mile leg would take about one and a half hours at the expected ground speed.