a retro fitted stall warning sender in the wing leading edge from ACI

Most aircraft are equipped with a Stall Warning system of some type. On some older aircraft, a small moveable flap located on the leading edge of the wing engages an electrical switch in the cabin, which activates a stall warning buzzer or horn. On some later aircraft, the system is pneumatic, with a small slot in the leading edge of the wing. Both types sense approach to stall, and sounds a device in the cabin, warning of impending stall.