To start off we first need to know what is True Airspeed? Well, it is pretty simple it is the airspeed that your aircraft is moving through air. Now that we know that we can move onto the next portion.
Why are they different? Well the aircraft speed indicator measures pressure not speed. Your airspeed indicator will read very accurate at sea level with perfect conditions but if you add weather along with the aircraft climbing it starts reading inaccurate speeds. That is because the airspeed indicator shows a slower speed due to density increasing based on the altitude and air changes. That is the reason why you cannot directly measure true airspeed.
Now that we know why we can’t accurately measure true airspeed we need to know why does it matter? Well put simple if you use indicated airspeed to calculate fuel burn, time to destination etc you will be way off and that is why true airspeed is so important. Did you know that cruise performance charts use True Airspeed?
6-14-3 — As the altitude of aircraft increases, so will the difference between IAS and GS. Below are some very “ball-park” figures which can be used for reference. With the aircraft flying at
– 250kts IAS with no wind
– 3000ft ~ 260kts GS (+10)
– 6000ft ~ 270kts GS (+20)
– 9000ft ~ 290kts GS (+40)
– 12000ft ~ 300kts GS (+50)
There we can see a increase in speed from the set 250 knots True Airspeed as we climb through altitudes.