Was everyone allowed to carry guns on the plane, or was it just easier to smuggle guns onto planes? The truth is that it's not the libertarian position to simply allow everyone to carry guns onto planes. The libertarian position is that it should be up to private airlines to decide what they allow onto their planes, and what kind of security they have. I think a reasonable form of security would be to have armed guards on flights, but I'm not opposed to allowing passengers to carry either. It's just that it should be up to the airlines themselves.
It has been a while since I researched this subject, but I do know airlines voluntarily started their own screening in 1970.
While that did decrease the number of hijackings, it did not eliminate them. I think the number dropped to 10 hijackings in 1972. So that works out to one hijacking every five weeks
after airlines began their own screening. Does that sound satisfactory to you?
The most famous hijacker of all during that period was D.B. Cooper, in 1971.