10.21 is a bug-fix release: small incremental improvements and bug fixes from 10.20, including:
- Updated Cessna with new, improved art files and flight model tuning
- Fixed landing gear rendering in Plane-Maker
- Crash fix for Mac when running the CRJ plugin or resizing the window
The complete change notes can be found at http://wiki.x-plane.com/Beta#X-Plane_10.21_Release_Candidate_2.