These are the pilots that were repeatedly seen: Second Officer Don Repo (most frequently around the food-heating ovens in the lower deck galley); and Captain Bob Loft, who appeared as a life-like flesh-and-blood person sitting in First Class a few times.
And the whole story sounds more credible because passengers and crew who saw the apparitions were well-balanced professionals with a sound mind, and who actually refused to talk about what they experienced, lest they were sent to a psychiatrist and/or lost their jobs.
Even though Eastern Air Lines officially dismissed the notion of any paranormal activity on their planes, they did authorize the cannibalized parts to be removed after over a year of sightings, and apparently this action stopped the ghostly appearances.
Sounds very intriguing in itself, but the book documenting these sightings and paranormal experiences, namely The Ghost of Flight 401 by John G. Fuller, intrigued me for other reasons as well.
You can read my review for the book, The Ghost of Flight 401 at amazon and Goodreads.