High Quality Content by WIKIPEDIA articles! An aircraft stabilizer may provide longitudinal (or pitch) stability, or directional (or yaw) stability. A longitudinal stabilizer is a surface that provides forces that tend to keep an aircraft flying level and with unchanging "pitch" angles relative to the airstream; the nose of aircraft is prevented from pitching up or down. A directional stabilizer tends to keep the aircraft flying straight ahead with unchanging direction relative to the airstream (yaw stabilization). Static vertical stabilizers are small wing surfaces placed behind the center of mass of an aircraft, either as part of the tail empennage or outboard on aft-swept wings. A static horizontal stabilizer may be placed either behind the center of mass, as a tailplane, or forward of it as a canard foreplane (although not all foreplanes are used for this purpose).