When
navigating through a moving
medium such as
air or
water one must take the motion of the medium into account when
setting course for a
given location.
An
aircraft travels in the
direction in which its
nose points. If one were to determine the direction to a target and then fly in that direction in still air, one would reach the target. If there were a
wind blowing over a large area from a direction other than directly ahead of or behind the aircraft then the aircraft would
drift sideways in proportion to the
cross component of the wind. If one were to continuously
readjust one's
heading to point toward the target one would still drift, and thus fly a curving course longer than the line from start to finish, wasting
time and
fuel.
To arrive at one's intended destination efficiently one would point one's nose at some
angle into the wind from the line between the start location and the target such that the
vector sum of one's motion and the wind would have a
resultant coincident with the line. This angle is the
crab angle.