This site is being deprecated.
Please see the official X‑Plane Support page for help.
Are you perhaps using a third party plane? There is a known problem with El Capitan and third party planes that use SASL. See this article for a work around, and this article for information on the forthcoming fix.
If this is not your issue, please provide additional details: Is it when you use a certain plane? Start from a certain location or fly the same route? We need as much information as possible to try to reproduce the crash.
This site is no longer being actively maintained.