Message from a pilot ... or what 17-, 22-, 36- and 48-Across each is?

SKYWRITING

Think of how pilots might spell out words where everyone on the ground can look up and read them.

This clue plays on two meanings of "message from a pilot." Literally, it refers to skywriting, where pilots write words in the sky with smoke. The second part hints that the answers at 17-, 22-, 36-, and 48-Across are also examples of "skywriting"—meaning they’re words or phrases written in the sky (or, in crossword terms, entered in the grid’s "sky" or top section). The pun ties the aviation term to the puzzle’s structure.