As difficult as it is to prove a negative, the Airfacts Journal seems to make a convincing case of identifying the actual author.
The site’s writer has been dubious about the quote for many years and carried out much reading of Leonardo da Vinci’s notebooks and recorded writings trying to find the line, despairing that it may be in some largely unpublished collection of private letters or similar. Eventually his efforts, and those of others, were rewarded:
Well, finally the mystery has been solved, thanks to several patient detectives connected via the internet, using the vast search tools of Google Books and other electronic storehouses. People reached out to book authors and asked where ideas and narrative came from, while others sat through dreadful old copies of films. It all came together in the nerdy discussion page of the Wikiquote entry for Leonardo de Vinci, and now we can reveal the author was… drum roll please…
John Hermes Secondari. An American TV writer. In 1965.
The article goes on to describe Secondari’s career and a particular project he was involved in, as television series called ‘the Saga of Western Man’:
One of the episodes of The Saga of Western Man was “I, Leonardo Da Vinci.” The credits state it was written, produced and narrated by John H. Secondari, with “the voice of Leonardo da Vinci” played by Fredric March. Its copyright date is 1965 and it was released in 1966 by American Broadcasting. At 16 minutes and 21 seconds into the second reel, after the off-screen Leonardo narration urges people to build his flying machines, claiming after any crash “the hurts will be slight,” over gentle visuals of a wheat field panning up into a clear blue sky, the voice says:
And once you have tasted flight you will walk the earth with your eyes turned skyward, for there you have been, and there you would return.