The loan runs up to and including Saturday 31 October. During that period Crawley have six league games, starting with an away trip to Plymouth Argyle this Saturday.
Hancox has struggled to get Blues first team game time over the past year because of the form of Jonathan Grounds at left-back.
Hancox made just one appearance last season which came in the FA Cup third round win at Blyth Spartans in January and he has figured just once so far this season, when he played in the Capital One Cup second round victory against Gillingham in August.
Crawley, who are managed by former Blues player Mark Yates, are currently struggling in League Two in 21st place with nine points from their opening ten games.
They collected their first win in six games last weekend with a 3-0 success away at Newport County.