What is the purpose exactly of a ‘Control Check’? I recently watched an Air Crash Investigation episode where Qantas flight 32 lost an engine shortly after takeoff. It was revealed that a stud pipe in a Rolls-Royce engine was made incorrectly, causing the pipe to snap and burst through the engine and the aircraft's wing, severing major hydraulic systems, and causing the ECAM to display pages and pages of errors.  

On the final approach, the captain announced he wished to perform a ‘control check’.  He said (and this is approximate): 


  When the aircraft is damaged, you have to certify it is safe to land before you try to land.  


I assume this is so that the captain can make the necessary preparations if he feels his aircraft is not safe to land. On flight 32 this was done by moving the stick left and right to simulate lining up with the runway. 



Perhaps a better question is back to the post's title. 

What are the purposes of in-flight Control Checks?

In my example of Qantas Flight 32, the stick was moved left and right on final approach to simulate lining up with the runway. This was only small left and right control deflections.  The flight deck had only minutes until landing.  What was the captain expecting to see, or not to see?  Can the captain after that brief manoeuvre be aware of his aircraft enough to make a decision? 

Does this mean then that a Control Check only comes, or should only be done, if you are worried that the aircraft cannot land?  Can it be done at any other time?  In other words: is an in-flight control check only done for landing? 



SOURCES:

Wikipedia 
Flight 32: 
https://en.m.wikipedia.org/wiki/Qantas_Flight_32
Technically speaking, a control check is part of any pre-flight. 

In the case of Qantas 32, one engine had exploded, punching holes in the wing and setting off a host of warnings, so there was a reasonable expectation that some of the controls might also have been damaged. 

What the pilot did was put the plane through some basic maneuvers to see how the controls responded in their now potentially damaged form. And found that roll rate had been affected, something he factored in on the subsequent landing. He also found that pitch control had not been affected, which meant he could make a reasonably conventional approach to the runway.  

Better to find out how much the controls had been affected when there is time and altitude to recover from an unexpected response, than to find out on final approach, where the margin for recovery is very slim to none at all. 

During flight, this is not normal. It is prudent if an inflight incident may have damaged some of the controls, so that the pilot knows the level of controllability they have to work with.