Any vehicle traveling on the ground moves in the direction it is steered or headed to, and is relatively unaffected by wind thanks to friction between the vehicle and the ground. After all, the wind might be moving, but the ground, the medium by which the vehicle is traveling, is not. Aircraft in flight however, seldom travel in exactly the direction they are headed in because of the wind effect.
Any free object in the air moves downwind with the speed of the wind itself. This holds true for aircraft of all shapes and sizes, from commercial airliners to hot-air balloons. If an aircraft is flying in a 20-knot wind, the body of the air in which it is flying moves 20 nautical miles in one hour. Therefore, the aircraft also moves twenty nautical miles downwind in one hour. This movement is in addition to the forward movement of the aircraft through the body of air. The path of an aircraft is ultimately determined by two factors: the motion of the aircraft through the airmass, and the motion of the airmass across the earth’s surface.
The effect of wind on the aircraft causes it to follow a different path over the ground than it does through the airmass. The path over the ground is split into two parts: the track, and the true course. The true course represents the aircraft’s intended path over the earth’s surface, while the track is the actual path the aircraft has flown. In other words, the true course is where the aircraft is intended to go, and the track is where the aircraft has actually gone. The lateral displacement of the aircraft by wind is called drift, the angle of difference between the aircraft’s true heading, and the aircraft’s track.
With a given wind, the drift changes on each heading. A change of heading also affects the distance flown over the earth’s surface for a given time, known as ground-speed. A tailwind, for example, boosts ground speed, while a headwind lowers it.
When flying, the pilot or navigator must correct for wind affecting the aircraft’s course. If the pilot is trying to fly in a straight line from point A to B, but wind coming in from the left pushes the aircraft off-course, the pilot must constantly angle their aircraft into the wind to compensate for the drift.
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