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Clearly, the only efficient solution would be one marshall per flight,
disguised in cabin crew (male or female), trained for action, discreetly carrying a weapon, seated
in FD each time one pilote has to get out.
Clearly you should stick to writing/ journalism. If a pilot can go rogue, why can't a marshall? Now you have twice as many people that have to be sane.....ie you've halved your odds. Brilliant.
Bringing an extra human to the flight deck makes no sense whatsoever, it double your chances of a takeover. The pilot can still crash the aircraft anytime they want, but now someone else also has that opportunity.....someone who hasn't been psychologically screened no less. It may make the odd person feel safer, but that just means they are a bit dim.