Apple is adding a new CEO to its ranks and continuing a long-running tradition of internal promotion. The iPhone maker on Monday announced that CEO Tim Cook will step down as chief executive in September and named senior vice president of hardware engineering John Ternus as his successor. Cook will serve as executive chairman. He has the "mind of an engineer, the soul of an innovator, and the heart to lead with integrity and with honor," Cook wrote in a press release announcing Ternus as CEO.