Flight Control Surfaces Repair

To prevent severe vibration or flutter of flight control surfaces during flight, design balance limitations must be observed when carrying out flight control surfaces repair.

Retaining proper balance and rigidity of flight control surfaces is critical. The effect of repair or weight change is greater on lighter surfaces than on older heavier designs. Therefore, flight control surface repair has to be done in such a way that weight distribution is not affected and flutter avoided. Flight testing is the usual method to ensure flutter is not a problem. Flutter is a flight hazard.

Too many coatings of paint, trapped water or ice under the paint, loose or vibrating trim tabs can all develop into flutter. Primary control surfaces are susceptible to wear, deformation, and fatigue failures because of the buffeting of airflow over the tabs. Trailing-edge play in the tab may increase, creating an unsafe flutter condition. Only a minor amount of tab wear can be tolerated. Care must be taken during repair to prevent stress concentration points or susceptibility to fatigue.

In order to design an effective repair to an aircraft of sheet metal construction, the stresses that act on the structure must be taken fully into the equation. Aircraft structures must be designed to accept all stresses imposed, whether by flight or ground loads, without deformation. Any repair must accept the stresses, carry them across the repair, and then transfer them back into the original structure. Changes in cross-sectional areas of aircraft structures subject to loading results in stress concentration. That, in turn, can induce fatigue cracking or eventual failure. Damage to the surface of a highly-stressed piece of metal, for example, will cause a concentration of stress at the point of damage. When rows of rivets are used to secure a lap joint, the transfer of stress at each row of rivets must be achieved without rivet shear.