its less of a barrel roll and more of a combine maneuver using roll and pitch. the plane obviously isnt certified to do this as a transport category but what the pilot does by pitching while rolling with enough energy at the initiation of the maneuver is maintain "one G" all the way through the roll by pitching up then down so theoretically the plane always has one G on it, if its done properly, thereby not stressing anything beyond certified limits its no different than if the plane were flyings straight and level....
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