It was not immediately clear what happened. The man was found unconscious inside a hangar.
""He was found laying down near his airplane. My understanding is he was not conscious or able to talk with first EMTs on scene and then he was transported after that ," said Bryan Howard, fire and police chief at Boeing Field.
The plane was not running at the time.
King County sheriff's spokesperson Cindi West said the man likely hit his head on a plane propeller, but spokesman for the airport said it may have been a prop start that injured the man.
The man in his 60s was taken to Harborview Medical Center in critical condition.