The US transportation department said on Tuesday it was opening an investigation into Delta Air Lines after the carrier canceled more than 5,000 flights since Friday as it struggles to recover from a global cyber outage that snarled airlines worldwide.While other carriers have been able to resume normal operations, Delta has continued to cancel hundreds of flights daily of a crew scheduling system.Since Friday Delta has been cancelling 30% or more of its flights daily through Monday, axing 444 flights on Tu ...