A blow back prevention device for an aircraft flap, comprising: a first ball-ramp plate engaged with an output coupling, the output coupling being adapted to receive a rotatable output shaft, and rotatable therewith about a longitudinal rotation axis, the first ball-ramp plate having a first ball-ramp surface and an opposed first braking surface; a second ball-ramp plate having a second ball-ramp surface and an opposed second braking surface, and being disposed adjacent to the first ball-ramp plate such that the first and second ball-ramp surfaces, are opposed to each other, and include a ball disposed therebetween, and such that each of the first and second braking surfaces, is opposed to a respective first and second stationary braking structure,; a spring biasing the first ball-ramp plate toward the second ball-ramp plate, to space the first braking surface apart from the first stationary braking structure; and an input shaft opposed to the output coupling and extending along the rotation axis through the first and second ball-ramp plates, in a lost window arrangement therewith, the lost window arrangement being configured such that an absolute value of a rotational torque applied to the output coupling by blow back of the flap, greater than a rotational torque applied to the input shaft, causes the ball to urge apart the first and second ball-ramp surfaces,, and overcome the biasing of the spring, such that the first and second braking surfaces, are urged against the respective first and second stationary structures, to cease rotation of the output coupling due to the rotational torque applied thereto by blow back of the flap. a first ball-ramp plate engaged with an output coupling, the output coupling being adapted to receive a rotatable output shaft, and rotatable therewith about a longitudinal rotation axis, the first ball-ramp plate having a first ball-ramp surface and an opposed first braking surface; a second ball-ramp plate having a second ball-ramp surface and an opposed second braking surface, and being disposed adjacent to the first ball-ramp plate such that the first and second ball-ramp surfaces, are opposed to each other, and include a ball disposed therebetween, and such that each of the first and second braking surfaces, is opposed to a respective first and second stationary braking structure,; a spring biasing the first ball-ramp plate toward the second ball-ramp plate, to space the first braking surface apart from the first stationary braking structure; and an input shaft opposed to the output coupling and extending along the rotation axis through the first and second ball-ramp plates, in a lost window arrangement therewith, the lost window arrangement being configured such that an absolute value of a rotational torque applied to the output coupling by blow back of the flap, greater than a rotational torque applied to the input shaft, causes the ball to urge apart the first and second ball-ramp surfaces,, and overcome the biasing of the spring, such that the first and second braking surfaces, are urged against the respective first and second stationary structures, to cease rotation of the output coupling due to the rotational torque applied thereto by blow back of the flap.