SEATTLE — On Christmas Eve 2016, Karlene Petitt, an international long-haul pilot for Delta Air Lines, received a devastating letter that threatened to end her career. She had been grounded since March pending an evaluation by a company-assigned doctor. The letter informed her of his diagnosis: She was mentally unfit for duty and would not be permitted to fly again. Petitt had then been flying commercial jets for 35 years. She’d raised three children, earned a doctorate and two master’s degrees and wrote a series of books, all while performing perfectly as a pilot. In early November of the pre…