Alaska Airlines Flight 528

Airline personnel reported that the airplane pushed back from the gate with the auxiliary power unit running. As the push back sequence was completed, two deicing vehicles began deicing operations prior to the flight crew configuring the airplane for deicing. Shortly after, the cabin and flight crew noticed fumes within the cabin and cockpit area. The flight crew instructed the deice crew to discontinue deicing operations and the flight crew performed the smoke removal checklist as the deicing operation stopped. The flight crew started both engines and taxied back to gate N14 where the passengers and flight crew exited the airplane via the jet way. The captain reported that he did not clear the ground deicing crew to start their deicing operations. The driver of the primary deicing vehicle reported that he informed the flight crew of the fluid types, freeze points, and concentrations. He added that there was a lot of "radio chatter" and the bucket operator began deicing operations "after we received no objections" from the flight crew. Company deicing procedures state, in part, "prior to fluid application, Ground Deicing Crew member shall confirm with Flight Crew that the aircraft is configured for deicing." The flight crew gate deicing checklist states that the APU and engine bleed air switches are to be turned off prior to deicing. At the time, the APU and engine bleed air switches were still in the ON position when deicing fluid was sprayed on the airplane. This allowed the fluid to be introduced into the air supply lines leading to the cabin and cockpit.

Verbatim NTSB analysis and probable cause from the NTSB dataset