What is the correct procedure if a light aircraft (under1000kg) elevator/aileron/direction stop working? Is it alright to use the ballistic parachute(if the aircraft is equipped with one)? If not, then in what situation ballistic parachute is used?
If a major flight control surface fails in flight, you'll be lucky if the ballistic parachute can save you. An aileron, for instance, that goes "hard over", will put even the most docile training aircraft into a tight spiral or a roll that will quickly become a spin (because the airplane doesn't have enough speed for a complete roll and will stall). A stuck elevator could result in anything from a stall to a steep dive.
In this situation, immediate deployment of a ballistic parachute might prevent fatalities; waiting will not. Delay will allow airspeed to build, G forces on the pilot to increase (presuming a spiral or spin), and potentially the wings come off due to flutter or G forces after passing Vne -- not to mention likely exceeding the parachute's maximum deployment safety margin.
Fortunately, this kind of failure is exceedingly rare, absent the kind of damage that comes from air combat, ground impact, or extreme weather phenomena.
This one is dependent upon the control surface failure and recommended procedures from the manufacture. If the aircraft is equipped with a ballistic parachute system, the system may be activated if the pilot feels that the aircraft has departed from controlled flight with no ability on their part to regain control and recover. Typically aircraft equipped with ballistic parachutes will employ the system if a forced landing, departure from controlled flight, or other serious airborne emergencies occur.
Elevator failures have been addressed using elevator trim to control pitch and power settings. This can give the flight crew enough time to clear in emergency and maneuver the aircraft to the nearest available airport for landing using the technique stated above to control pitch. Aileron failures, depending on how the system fails, can sometimes be countered with roll inputs using a good aileron, if one still exists or using rudder to slip the airplane causing a shallow roll in the direction of the rudder input. Of all control surface failures, a rudder failure is probably the least dangerous, as the airplane still may be maneuvered using aileron and elevator inputs albeit with coordinated flight slightly compromised. on a multi engine aircraft, yaw control may be affected by means of differential thrust using the throttles. It would be advisable in the event of a rudder failure to land the airplane at an airport with runways offering the lowest possible crosswind component on approach.
Large aircraft utilizing digital flight control computers and hydraulically actuated control surfaces may be able to detect compromised flight controls and re-route commands to existing functioning flight controls to achieve a desired response from a flight crew command.
If the aircraft is equipped with a ballistic parachute, and flight and straight and level attitude is still possible, even with the control failure, the decision to use the chute may be up to a risk management decision: Is it advisable to simply deploy the chute and ride it down, or attempt a landing with compromised flight controls. These decisions are largely open-ended problems, and often come down to evaluating how compromised the control of the aircraft is, the pilots proficiency at handling such an emergency, the environment, and other resources available to the crew. SRM/CRM is never a closed ended problem.