A Cathay Pacific Boeing 777-300, registration B-KPB performing flight CX-253 from Hong Kong (China) to London Heathrow,EN (UK) with 270 passengers, was enroute at 9800 meters (FL321) about 20nm east of Lanzhou (China) when the crew decided to return to Hong Kong due to a cracked windshield at the captain's side. The aircraft descended to 6900 meters (FL226) and landed safely back in Hong Kong about 5.5 hours after departure. A replacement Boeing 777-300 registration B-KPO is estimated to reach London with a delay of 10 hours.