A door operator for selectively opening and closing a side hinge door. The door operator may include a motor with a drive shaft, an operator arm assembly, a door position sensor, and a controller. The operator arm assembly may include an output shaft operatively coupled to the drive shaft, an operator arm, and a clutch assembly mounted to the output shaft. The door position sensor, which may be electro-magnetic, signals the controller when the door is not closed. If the door has been manually pushed open, the controller signals the motor to rotate the drive shaft in the closing direction. In automatic mode, the output shaft and the operator arm are operatively engaged, but manual force to open the door overcomes static friction between the operator arm and friction discs in the clutch assembly, operatively disengaging the operator arm and output shaft until the manual force is removed.