PREDICT FIRST
You plan a track of 090°T at a true airspeed of 120 kt. The wind is from 045°T at 40 kt. What true heading does the wind triangle give?
Two movements, added together
The aircraft moves through the air at its true airspeed (TAS). The air moves over the ground with the wind. Where you end up after an hour is the two added together.
Actually, these two motions are independent. It makes no difference whether the mass of air through which the aircraft is flying is moving or is stationary.
A wind is named for the direction it blows from, but it carries you the other way.
When the wind is blowing from the north at 25 knots, it simply means that air is moving southward over the Earth’s surface at the rate of 25 NM in 1 hour.
A wind blowing straight along your track adds its full speed, or takes it away.
If the mass of air is moving eastward at 20 knots, the airspeed of the aircraft is not affected, but the progress of the aircraft over the ground is 120 plus 20 or a GS of 140 knots.
Heading, track and drift
The direction in which the aircraft is pointing as it flies is called heading. Its actual path over the ground, which is a combination of the motion of the aircraft and the motion of the air, is called track. The angle between the heading and the track is called drift angle.
Point the nose straight along your planned track with a wind from the left, and the air carries you to the right of it. After an hour you are as far off track as the air has moved across it.
Assuming no correction is made for wind effect, if an aircraft is heading eastward at 120 knots and the air mass moving southward at 20 knots, the aircraft at the end of 1 hour is almost 120 miles east of its point of departure because of its progress through the air. It is 20 miles south because of the motion of the air.
To stay on the planned track, point the nose into the wind by just enough to cancel the drift. That angle is the wind correction angle.
In other words, if the wind is from the left, the correction is made by pointing the aircraft to the left a certain number of degrees, therefore correcting for wind drift. This is the wind correction angle (WCA) and is expressed in terms of degrees right or left of the TC.
Drawing the triangle
The FAA handbook draws the triangle with three lines. Each is as long as its speed for one hour:
- The wind arrow, from the start point, pointing the way the wind blows.
- The airspeed line, from the end of the wind arrow, as long as your TAS, swung until it meets the planned track. Its direction is your heading.
- The line from the start point to where they meet. Its direction is your track, and its length is your groundspeed.
Next, align the ruler with E and the dot at 45°, and draw the wind arrow from E, not toward 045°, but downwind in the direction the wind is blowing making it 40 units long to correspond with the wind velocity of 40 knots.
All three lines use one scale, so a knot is the same length on each of them.
The units used may be of any convenient scale or value (such as ¼ inch = 10 knots), but once selected, the same scale must be used for each of the linear movements involved.