Gust lock on a rudder.

A gust lock on an aircraft is a mechanism that locks control surfaces in place preventing random movement and possible damage of the surface from wind while parked. Gust locks may be internal or external.

References

(1971) "Gust lock", The Illustrated Encyclopedia of Aviation and Space 6. A. F. E. Press, pp. 1043. LCCN 68-14013. The Library of Congress Control Number or LCCN is a serially based system of numbering cataloging records in the Library of Congress in the United