Try flying over these areas without holding S.
It has been noted by some users that there is at least one invisible wall which will either cause a full aircraft crash (straight to black screen - you crashed message) or flip you up several thousand feet. - depending on your damage model settings I guess.
I have reproduced this by flying up the river from coast at several different altitudes. Have crashed into an invisible wall every time.
It may not be the “holding S” which is causing the issue.
However, if it is…then you may have it double mapped to something else. AutoPilot engage?