Two Bombardier regional jets experienced failures of their left main landing gear in separate incidents this week, and federal officials are investigating. On Monday, an Air Wisconsin CRJ-200 landed in Philadelphia without its left main landing gear, causing minor damage. There were no passengers on the plane and no injuries were reported. A United Express CRJ-700 operated by Mesa Airlines experienced a collapse of its left main gear after landing in Chicago. No injuries were reported among the 28 passengers and four crew.