Well, that didn’t take long, did it? In issues #99-100, Claremont and Cockrum bring back The Sentinels, and the movement by humans to eliminate mutants from the face of the Earth. It’s also worth noting that Claremont already was writing the book as basically one long story–breaks between issues and “traditional” arcs were discernible, but the team seemed to grow and change with every issue.
These issues also contained the team’s first boat crash (technically, a boat being destroyed by a Sentinel while the X-Men were in it), above, in issue #98, and their first space shuttle crash in #99 (and their first trip into space, although not even as far as the moon), in #99.
The crash trope is strong in this one!
Oh, and one other little thing happened, after the X-Men’s shuttle got pelted with cosmic rays…
That’s right. The rays changed her name, her power-set and her clothes. Uncanny X-Men #101 was the first appearance of Phoenix.