Another role of a horizontal stabilizer is to provide longitudinal static stability. Stability can be defined only when the vehicle is in trim;[5] it refers to the tendency of the aircraft to return to the trimmed condition if it is disturbed.[6] This maintains a constant aircraft attitude, with unchanging pitch angle relative to the airstream, without active input from the pilot. Ensuring static stability of an aircraft with a conventional wing requires that the aircraft center of gravity be ahead of the center of pressure, so a stabilizer positioned at the rear of the aircraft will produce lift in the downwards direction.