An inspirational videotape to boost sagging morale among employees of Eastern Airlines was mailed this weekend to 3,600 pilots by the Air Line Pilots Assn. The union expressed concern about the "negative atmosphere and deteriorating loyalty and pride" created by extensive layoffs and cutbacks, and it asked pilots to pass the tapes to other Eastern employees. The videotape features Charles Plumb, a former U.S. Navy pilot who spent nearly six years as a prisoner of war in Vietnam. He uses that experience as the inspiration for his taped address. "The pilot group feels Plumb's story gives important support to flight crews and ground personnel" the union said.