Abstract: A backup locking system for a thrust reverser door comprises a catch pivotally mounted by a pivot pin to a fixed structure of the thrust reverser. A hook with a self-locking profile is provided at the free end of said catch for cooperation with a locking interface on the door in the event of failure of the normal locking systems. At its other end remote from said hook, the catch has a heel ending in a part-cylindrical surface which is concentric with said pivot pin. The catch is urged against the locking interface by a piston rod which presses against a front face of said heel under the force of a spring acting on a piston to which said piston rod is attached. When the rod is retracted, a double lever positively moves the catch toward a minimum opening angle, at which point the door can be opened. An unlocking spring causes the full opening of the catch to maximum open position when an opening lever is released.