IAS, TAS and Groundspeed Explained for Pilots

Three speeds, three questions

Indicated airspeed (IAS), true airspeed (TAS), and groundspeed (GS) describe different aspects of aircraft movement. Confusing them can lead to poor planning, especially when wind and altitude change.

Indicated airspeed

IAS is the speed indicated by the aircraft's airspeed system. It is directly connected to the aerodynamic pressures used by the aircraft's instruments. Many aircraft operating limitations and performance references are expressed in IAS.

True airspeed

TAS is the aircraft's speed through the surrounding air after accounting for effects such as air density. As altitude increases, the relationship between IAS and TAS changes. Pilots therefore use the appropriate performance information or calculation method for the aircraft and conditions.

Groundspeed

Groundspeed is the aircraft's speed over the ground. Wind changes it: a headwind reduces groundspeed while a tailwind increases it. This is why a route can take different amounts of time in opposite directions even when the aircraft's airspeed is similar.

Simple planning example

If a route is 120 nautical miles and the planned groundspeed is 120 knots, the basic still-air time calculation is one hour. If a headwind reduces groundspeed to 100 knots, the same distance takes about 1.2 hours. The fuel consequence must then be considered using the aircraft's actual fuel-flow data.

The practical lesson

Use the speed appropriate to the question you are answering. Aircraft limitations and aerodynamic performance often use IAS; performance calculations may require TAS; time and distance over the Earth depend on groundspeed.

AIRVENOR takeaway

When flight planning, always state which speed you are using. A single number called “speed” is not enough.