If all states are measured by appropriate sensors and interfaced to the flight control computer then allthe states may be fed back and there is no restriction on the choice of K. Sometimes however only someof the states are available for feedback. This can be because there are no sensors deployed to measure thestate or for other reasons such as because the only available positions for the sensors are away from thecg (centre of gravity) position and so do not give a true reading or because the sensors have significant dynamics (time constants etc) of their own.

How can we choose K so that eigenvalues (and poles) of the controlled system are suficiently stable, well damped and with appropriate natural frequencies? This is a key problem in flight control!

The representation of feedback is shown in figure (a). As regards the stability ofthe system the C and D matrix and the command input u_c take no part and can be neglected. State feedback can accordingly also be thought of as the problem shown in figure (b) with C = I , D = 0 and u_c = 0.