Boeing’s largest factory is in “panic mode”, according to workers and union officials, with managers accused of hounding staff to keep quiet over quality concerns.The US plane maker has been grappling with a safety crisis sparked by a cabin panel blowout during a flight in January, and intense scrutiny of its production line as regulators launched a string of investigations.Its site at Everett, Washington – hailed as the world’s biggest manufacturing building – is at the heart of Boeing’s operat...