Method and apparatus for extending a fly section of a telescopic crane boom which fly section does not have its own power unit to effect extension or retraction. The boom includes a base section, an inner powered section telescoping within the base section, an outer powered section telescoping within the inner section, and a non-self-powered fly section telescoping within the outer section. To extend the fly section, a load pin connecting the fly section to the outer section is released using a hydraulic cylinder mounted to the base section. The load pin is locked in the released position by a locking device. The outer section is then extended, taking the fly section with it. Upon full extension of the outer section, the inner end of the fly section is latched to the inner section or base section by latch pins actuated by a hydraulic cylinder. The outer section is then retracted, exposing the fly section and unlocking the load pin from its released position. The load pin then simply slides along the bottom of the fly section until, upon full retraction, the load pin snaps into a hole at the inner end of the fly section to prevent relative motion of the fly section and the outer section. The fly section is then unlatched from the inner section or base section. At this point the outer section and inner section may be independently operated, with the fly section in fully extended position. The fly section is then retracted by basically reversing the procedure. A circuit is also provided so as to prevent both the load pin and the latch pins from being disengaged simultaneously unless the fly section is fully retracted within the outer section.