14 CFR § 121.289 - Landing gear: Aural warning device.
(a) Except for airplanes that comply with the requirements of § 25.729 of this chapter on or after January 6, 1992, each airplane must have a landing gear aural warning device that functions continuously under the following conditions:
(1) For airplanes with an established approach wing-flap position, whenever the wing flaps are extended beyond the maximum certificated approach climb configuration position in the Airplane Flight Manual and the landing gear is not fully extended and locked.
(2) For airplanes without an established approach climb wing-flap position, whenever the wing flaps are extended beyond the position at which landing gear extension is normally performed and the landing gear is not fully extended and locked.
(b) The warning system required by paragraph (a) of this section -
(1) May not have a manual shutoff;
(2) Must be in addition to the throttle-actuated device installed under the type certification airworthiness requirements; and
(3) May utilize any part of the throttle-actuated system including the aural warning device.
(c) The flap position sensing unit may be installed at any suitable place in the airplane.