Effective True Airspeed

T-45C TS INav-08

Fuel, Weather, and Alternate Airfield Planning

Eftective True Airspeed

That portion of the wind triangle on the computer above or below the midline represents the headwind or

tailwind component respectively that has to be added or subtracted to/from the TAS to solve for ground

speed.

Headwind

component

Now, as we are adding or subtracting this component from the TAS we in essence have to swing the

length of the TAS line to the CUS/GS to subtract to get ground speed.

Difference

between

TAS and ETAS

When the drift angle is less than 10 degrees, you get an error of less than 1/2 of 1% (.005) but with a drift

angle greater than 10 degrees the error climbs to 1 1/2% or greater so we have to compensate for it in

some way. We use the “Cosign” of the drift angle. The designers of the Air Navigation Computer have

provided a handy “Cosign” scale just to the left of the TAS index. To find the “Effective True Airspeed” find

the drift angle, to the left of the TAS index, on the black cosign scale and directly above read the “Effective

True Airspeed”. Use this ETAS with headwind or tailwind to get the effective ground speed for that

particular leg. The drift angle correction needs only to be used if the drift angle is 10 degrees or greater.

Groundspeed

There are two types of winds associated with flying operations, TRUE and MAGNETIC. The surface

winds you receive from Approach/Departure Control and Airport Traffic Control Towers are MAGNETIC

winds, which coincide with the magnetic direction of runways. The en route winds you receive from the

forecaster are TRUE winds, and are taken from the teletype Winds Aloft Forecast (FD).

(2-02) Original

Page 8-14