For example, if he or she (let's just go with he, like you did) said it to his passengers, it could be a firable offense. I'd have to hear the context. If I ran an airline, I would want to serve both Lefties and Righties, so I wouldn't want my employees making political statements. But, I might just reprimand him, rather than fire him.
If he said it over the airways to other pilots, I don't think that is, by itself, a firable offense. There would have to be more. I get the impression that pilots joke around all the time when not being the face of the company to its customers.
Do you agree? Or, do you want him fired no matter what because he disagrees with you politically? that is the typical approach of the Left: make sure people can never make a living again.