UAL Corp., parent of United Airlines, the world's No. 2 airline, filed for bankruptcy protection Monday, succumbing to continued losses and staggering debt payments it could no longer afford to make. UAL said current and future tickets on United flights will be honored, Red Carpet Clubs will continue to operate, and frequent-flier points will continue to be awarded and redeemed by United and partner airlines. The carrier says it has arranged for about $1.5 billion in loans, known as debtor-in-possession financing, needed to fund operations during a court supervised reorganization.