14 CFR 23.345 - High lift devices.
(a) If flaps or similar high lift devices are to be used for takeoff, approach or landing, the airplane, with the flaps fully extended at VF, is assumed to be subjected to symmetrical maneuvers and gusts within the range determined by -
(1) Maneuvering, to a positive limit load factor of 2.0; and
(2) Positive and negative gust of 25 feet per second acting normal to the flight path in level flight.
(b) VF must be assumed to be not less than 1.4 VS or 1.8 VSF, whichever is greater, where -
(1) VS is the computed stalling speed with flaps retracted at the design weight; and
(2) VSF is the computed stalling speed with flaps fully extended at the design weight.
(3) If an automatic flap load limiting device is used, the airplane may be designed for the critical combinations of airspeed and flap position allowed by that device.
(d) The flaps, their operating mechanism, and their supporting structures, must be designed to withstand the conditions prescribed in paragraph (a) of this section. In addition, with the flaps fully extended at VF, the following conditions, taken separately, must be accounted for:
(1) A head-on gust having a velocity of 25 feet per second (EAS), combined with propeller slipstream corresponding to 75 percent of maximum continuous power; and