Another reason for boarding from the left has nothing whatsoever to do with aircraft. During the Great War (WW1) there was a huge increase in aircraft, the majority being military. The pilots for these machines tended to come from higher echelons of society and approached aeroplanes in much the same way as they did horses - with which most pilots would have been familiar. Horses are always (in the military at least) mounted from the left, a hangover from having to wear cumbersome swords on the riders left side. If you look at old film footage of the day, you will see that aircraft are always mounted from the left, this was long before airports existed.