The airship captain's hat is a unique, black sea captain's hat with the Brotherhood of Steel logo, found only on Lancer Captain Kells. It does not provide any protection, but does offer boosts to both Charisma and Endurance, though the Endurance benefit is not as high as its non-unique variants.
As with most characters, Kells will, over time, respawn his clothing if pick-pocketed, so the player character can obtain an infinite number of airship captain's hats. The hat can also be obtained by killing Kells while aboard the Prydwen. This hat will become unobtainable if the player character destroys the Prydwen.
Brotherhood of Steel soldiers have unique ambient dialogue when the Sole Survivor has this hat equipped. Quotes include: "Nothing to report as of yet sir," and "It is an honor to be in your presence El Capitan."