If you are in a location north of Lake Buena Vista, you could fly/drive/walk down there.
If you are south of the lake you could fly/drive/walk up there.
"Down" is just another preposition specifying the (relative) direction in which the flight goes. It would also be correct to say "Catch a charter plane, fly to Lake Buena Vista" but adding the preposition "down" gives the reader (or listener) a clue as to where the writer (or speaker) is located - probably somewhere north of Lake Buena Vista.
You can't leave out the prepostion "to", though. So it would be wrong to say "Catch a charter plane, fly down Lake Buena Vista".
Hi bhaisahab and Shenfeng. [STRIKE]t[/STRIKE]Thanks ... [STRIKE]l[/STRIKE]Let's see if I got it. [STRIKE]m[/STRIKE]Maybe 'Fly down' means southward to the lake?
If you were on a high plateau, and the lake were in a valley, you could fly down northwards to it.
Please try to construct and punctuate sentences correctly. If you have any questions about my corrections, just ask. Also, please quote complete sentences in your questions. It helps if we have as much context as possible.