What happened
The aircraft arrived at its gate in Chicago, where crew members detected fumes and smoke in Zone C of the cabin. Simultaneously, the main cabin lighting failed and emergency lighting activated. Maintenance personnel discovered that the number two Auxiliary Power Unit (APU) had auto-shutdown due to a malfunction. The APU was placarded as inoperative, and the aircraft was returned to service.
Shortly after takeoff on the return flight, smoke became noticeable inside the cabin. The crew elected to return to the airport and land. Upon landing, the smoke had dissipated, and fire department personnel reported no indications of smoke or fire. Suspecting only an air conditioning problem, the crew continued taxiing toward the gate.
As they began taxiing, a flight attendant reported that the smoke was increasing. The captain stopped the aircraft and directed an evacuation. A crosswind prevented proper deployment of the slides on the right side of the aircraft. During the evacuation, an elderly passenger using slide L-4 on the left side was seriously injured by others who came down the slide after him. Additionally, five passengers received minor injuries during the evacuation.
The investigation
An investigation revealed that the smoke was caused by oil from a failed APU bearing which leaked into the air conditioning system.