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. <EOS>