Investigators are still looking at the causes of both those incidents, but a preliminary report from the National Transportation Safety Board found Boeing left the bolts off the 10-week-old Alaska Air jet needed to keep the door plug in place. And Boeing suggested the Latam incident may have been caused by an incident in the cockpit and not anything to do with the plane's controls. The age of the aircraft in the United incidents suggest that the cause could lie with United personnel, rather than Boeing's well documented quality issues.