For one thing, having a designated side for boarding makes sense in terms of airport infrastructure. You most commonly board and deplane via jet bridges, and even though jet bridges are movable to sync up with different types of aircraft, they’re always designed to be attached to the left side of a plane. With so many gates at any given airport, things could get a bit confusing if planes could line up to the right or left of the jet bridge.
But why the left, specifically? It mostly comes down to safety. “Many aircraft operations take place on the right-hand side of the aircraft — fueling, loading and unloading bags, catering, etc.,” says former pilot Dan Bubb, a professor at the University of Nevada, Las Vegas. “It would be very dangerous to board passengers on that side of the aircraft while there are vehicles and other equipment moving.”
This is especially true if you’re deplaning via a staircase rather than a jet bridge — in that case, you definitely want to keep passengers and crew away from any potentially hazardous activities.
There’s also the consideration of jetway alignment. Captains always sit on the left side of the aircraft, and because jetways are also on that side of the aircraft, “they enable captains to better align planes with the jetways and avoid having the left wings collide with the jetways,” says Bubb. “If the jetways were on the right side of the aircraft, it would be harder for the captains to see and align the aircraft with the jetways.”
Of course, that’s less of an issue these days with marshallers — the ground crew with the illuminated wands — serving as a visual aid for captains pulling in to park
And the practice of boarding a vessel from the left side actually predates airplanes entirely. Have you heard the terms port and starboard? They’re nautical terms used to delineate the left and right sides of a boat, respectively — and the etymology of the term “port side,” has something to do with boarding on the left.
According to the National Oceanic and Atmospheric Administration (NOAA), the term starboard originates from the Old English words stéor (steer) and bord (the side of a boat). In days past, sailors used steering oars to control boats, and since most people are right-handed, the oars were placed on the right side of the boat. That meant that when a boat came up to port, its left side would be next to the dock. As such, the left side of a boat became known as the loading side, or “larboard,” since passenger and cargo loading would happen on that side. But since starboard and larboard sound quite similar, sailors changed the name of the left side to “port” to avoid any miscommunication. In an interesting twist, we can now board boats on either side of the vessel, but on airplanes, we’ve stuck to the left side.
At the end of the day, it wasn’t a single factor that led to the custom of boarding from the left side of a plane, but rather a confluence of multiple factors. Ultimately, it boils down to logistics and safety. Jet bridges are always designed to match up with the left side of aircraft, and by loading passengers from the left side of planes, the right side is available for other potentially dangerous activities, from cargo loading to refueling.