Review the original item:
The terrified passengers aboard the 747 gripped their armrests as the jet
bucked in a bad stretch of turbulence after
a ride like this one, everyone knew that five minutes on Disney's Space
Mountain would feel like relaxing in a rocking chair.
You chose the correct way to fix the error:
The terrified passengers aboard the 747 gripped their armrests as the jet
bucked in a bad stretch of turbulence. After
a ride like this one, everyone knew that five minutes on Disney's Space
Mountain would feel like relaxing in a rocking chair.
The terrified passengers aboard the 747 gripped their armrests is a main clause. After a ride like this one, everyone knew is the second main clause.
You are always correct when you use a period between the two main clauses that create the fused sentence.