Nearly two years after passengers screamed while sparks flew down a runway during the landing of an Alaska Airlines flight, the cause has been revealed.A final report by the National Transportation Safety Board (NTSB) released on Tuesday points to "incorrect" maintenance work. The flight, which departed from Seattle, Washington, made a hard touchdown in Santa Ana, Calif., during Tropical Storm Hilary after the left main landing gear collapsed on touchdown.Video recorded by a passenger captured the plane slamming into the ground at high speed. Sparks were seen flying as the plane appeared to drag its left wing along the tarmac.ALASKA AIRLINES