Ummm...
The reason that the terrorists were able to commandeer the jetliners on 911 has nothing to do with inadequate airline cockpit doors.
The reason is that--before 912--the standard procedures for dealing with hijackers was to cooperate with them, do what they asked, and try to calm everyone down over periods of hours. The belief was that the hijackers probably did not want to die but were somewhat mentally unbalanced, hence confrontation could trigger a disaster but cooperation had a very good chance of getting everyone out of the situation alive.
Needless to say, this is no longer standard procedure. But Nader's claim is a strong sign that, to put it politely, he is vastly underbriefed on airline security issues.
Brad DeLong