Mike Relja, a retiree in his late 70s, received an unexpected job offer from NASA about a month ago.

The assignment: Help put a former NASA-operated Lockheed SR-71A Blackbird back in the air after a nearly 27-year hiatus.

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Relja, an ex-U.S. Air Force and NASA test engineer for the SR-71A, could not believe his ears. David Ash, a former U.S. Navy and Joby test pilot who was hired in June as a special projects director for NASA, asked Relja to provide his expertise for a secret effort to return a long-retired and neglected Blackbird to flight.

“I told the guy, ‘You know, I’m too old to be doing this,’” Relja recalls to Aviation Week. “‘I wish you well. I’d like to see you make it. It would be interesting.…