He served as mission control capsule communicator (CAPCOM) on Apollo 11 during the launch and during the first lunar moonwalk (EVA) by Neil Armstrong and Buzz Aldrin before joining the astronaut support crew for the Apollo 14 mission, on which he doubled as a CAPCOM. Thereafter, McCandless was reassigned to the Skylab program, where he received his first crew assignment as backup pilot for the space station's first crewed mission alongside backup commander Rusty Schweickart and backup science pilot Story Musgrave. Following this assignment, he again served as a CAPCOM on Skylab 3 and Skylab 4. Notably, McCandless was a co-investigator on the M-509 astronaut maneuvering unit experiment that was flown on Skylab; this eventually led to his collaboration on the development of the Manned Maneuvering Unit (MMU) used during Space Shuttle EVAs. Although he was classified as a Shuttle pilot until 1983, McCandless ultimately chose to work on the MMU as a mission specialist due to the prestige of the program (which ensured a flight assignment) and his lack of test pilot experience.