The wind is a great problem in navigation. Imagine that pilot sets the plane to fly eastwards (90°). If the wind blows from the north, it will blow the plane to the south. And the aircraft will fly to south-east (135°). So, to fly exactly to the east, pilot must set the plane to north-east. This was just an example. In reality, wind can blow from any direction and at any speed. So pilots must also watch the wind and know how to compensate the effect of the wind on the aircraft.