Washington (AFP) – Airlines struggling to staff their planes cancelled hundreds of US flights Saturday at the start of a long and almost certainly messy holiday travel weekend. As of mid-afternoon, with Americans gearing up for July 4 Independence Day celebrations, more than 600 flights within, into, or out of the United States had been cancelled, and more than 3,300 were delayed, according to flight tracking service flightaware.com. The numbers on Friday were grim as well, with 587 US flights scrapped among a global total of 3,061 cancellations, the site said. Sunday was also looking problema…