CASA Executive Manager Standards Division, Peter Boyd, will leave the regulator on 2 October.
Boyd has been with CASA since since 1999, filling several roles until taking over Standards Division in 2005.
Senior Manager Operations Strategy, Roger Weeks, will temporarily fill the position as head of Standards Division starting 6 October.
CASA Director of Aviation Safety, Mark Skidmore, is believed to be considering the best way to fill Boyd's role permanently, which may hint at some form of restructure.
Since John McCormick's contract as DAS expired last year, Principal Medical Officer Pooshan Navathe, Deputy Director Terry Farquharson, Executive Manager Permissions Peter Fereday and now Boyd have all left CASA.