§25.173 Static longitudinal stability

(See AMC 25.173) Under the conditions specified in CS 25.175, the characteristics of the elevator control forces (including friction) must be as follows: (a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or VFC/MFC, whichever is appropriate, or lower than the minimum speed for steady unstalled flight. (b) The airspeed must return to within 10 % of the original trim speed for the climb, approach and landing conditions specified in CS 25.175 (a), (c) and (d), and must return to within 7·5 % of the original trim speed for the cruising condition specified in CS 25.175 (b), when the control force is slowly released from any speed within the range specified in sub-paragraph (a) of this paragraph. (c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 4 N (1 pound) for each 11,2 km/h (6 kt). (See AMC 25.173(c)) (d) Within the free return speed range specified in sub-paragraph (b) of this paragraph, it is permissible for the aeroplane, without control forces, to stabilise on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.