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When are the angle of attack and pitch angle equal?
The angle of attack for an aircraft is the angle the chord of an airfoil makes with the freestream velocity (or oncoming air flow). The pitch angle is the angle between the chord of an airfoil and a level surface (i.e. the ground). In a plane or a helicopter, the angle of attack and the pitch angle are only equal if the air is approaching the aircraft horizontally to the Earth's surface.