I've never had a problem getting A seats on CX, including the CX 831. For that flight, I was on a waitlist for approximately one week, but it cleared mon ths before the flight. As LAN, my advice is to always check codeshares. I have always been able to get an A seat on the SCL-North America flights by booking it under the AA codeshare number. I am pretty sure IB codeshares with LA on the MAD-SCl flight and I know QF codeshares on the SYD-SCL flight.