T-45A UJPT & E2-C2 INAV08

Fuel, Weather, and Alternate Airfield Planning

You will note that the ground and air vectors have a common point of origin and the angle between the

two, from TH/TAS to CUS/GS is the DRIFT ANGLE. The angle from CUS/GS to TH/TAS is CRAB

ANGLE.

As a side note: a course (CUS) is predicted path over the ground.

TRACK is your actual path over the ground.

Drift angle is important because it represents the angle at which the

aircraft will drift away from the desired course if you don't put any

correction into the wind.

The crab angle is the correction that must be used into the wind to

maintain a desired course. Crab angle corrects for drift angle.

Due to the limited size of the navigation computer, only a portion of the wind triangle is plotted on the wind

side as a graphic display. The rest is solved trigonometrically. Without going much into the higher

mathematics of the computer, what we are setting up is top part of the wind triangle on the grid face.

Crosswind component

By definition, the sine of an acute angle in the right triangle is equal to the side opposite the angle divided

by the hypotenuse (side opposite the right angle). Therefore:

SINE of the Drift Angle = Crosswind Component

True Air Speed

If we rearrange the equation:

True Air Speed =

Crosswind Component

Sine of the Drift Angle

The computer puts the crosswind component on the outside scale of the wind side, and the Sine, which

you read as an angle, on the white middle scale which gives you the drift angle.

(6-99) Original

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