What happened
Following pushback from the gate, both engines of the aircraft were started, and pneumatic power was transitioned from the auxiliary power unit to the engines. Shortly after this transition, smoke became visible within both the cockpit and the cabin. In response to the smoke, the flightcrew donned smoke masks, and the captain ordered an emergency evacuation.
During the evacuation process, 2 passengers sustained serious injuries, while 32 passengers received minor injuries.
The investigation
The investigation determined that the source of the smoke was a failure of a hydraulic check valve. This failure allowed hydraulic fluid to enter the engine pneumatic duct. Additionally, investigators noted that the 3R door evacuation slide failed to deploy during the event. Further examination revealed that the door had not fully opened; however, when the door was manually pushed during the investigation, it opened completely, allowing the slide to deploy and inflate.