The manual fly section of a multi-section telescopic crane boom is extended and retracted through the cooperative use of an extensible and retractable plunger on one of the boom extension cylinders and a manual locking pin insertable through registering openings of the fly section and outer mid-section of the telescopic boom. The latching plunger cooperates with a notched latch bar attached to the interior of the manual fly section and the plunger is actuated manually from the exterior of the boom by a rotary screw mechanism.