Why do we always board an aircraft from the left?

actually, don't always.
not aircraft entered left. small ga aircraft such cherokees have door on right side.

rick29148 wrong. small ga aircraft such cessnas have room 2 exit doors.

cessnas have doors on both sides. beechcraft bonanza a-36 , cessna 206 stationair has double clamshell doors on right while piper seneca , piper cheyenne have double gullwing doors on left.

larger turbo prop , jet aircraft, enter , exit door on left. captain occupies left seat in cockpit. has been tradition long time countries. if passengers allowed enter , deplane left side, captain keep watch on process. large aircraft had propeller engines had shut down during passenger ingress/egress. easier captain watch approaching passengers , shut down engine(s) on side of aircraft.

pre-war dc-3 had right-hand passenger door.

cargo doors on right side prevent cross on mishaps @ gate. it's easier have aircraft's cargo deplane in same orientation.

however, regional airliners such older swearingen metroliner turbo props have cargo doors on left.

why mount horse left?

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