PHOENIX — An American Airlines flight headed to Chicago was forced to return to Phoenix Sky Harbor International Airport after the pilot reported light smoke in the cabin. An airport spokeswoman says the pilot of Flight 1172 landed safely and returned to the gate Sunday morning. The MD-80 was carrying 140 passengers and five crewmembers. Airline spokesman Jim Faulkner says mechanics believe the smoke came from a small fluid leak in an auxiliary power unit.