The right side is where ground staff handle checked baggage, so passengers must embark and disembark from the front and rear left doors.
Why passengers board airplanes through the left door
Most airplanes carrying passengers today have 8 doors evenly distributed on both sides. However, if you frequently fly on airplanes, you'll notice that passengers must board and disembark from the left door, whether it's via jet bridge or stairs. The right doors are always sealed, and this is not arbitrary; there is a reason for it all.

Not only commercial airplanes but also most types of transport and combat planes... passengers must board from the left door. Explaining this, pilot Andrew Stagg says it is related to long-standing maritime industry conventions. According to the ship's structure, the helm must be on the left side, so passengers must embark and disembark on the left when the ship docks.
Similarly, the right side of the airplane has enough doors like the left side but is only used in emergency situations. Normally, this side is where ground staff arrange checked baggage, so passengers must embark and disembark from the front and rear left doors, whether boarding through the jet bridge or using stairs.
In the past, some airplanes allowed passengers to enter and exit through the right door. However, a former US Air Force pilot shared that this posed challenges for the captain, who is always seated on the left side of the cockpit, needing to observe outside and align the aircraft door with the connecting tube from inside the airport.
Furthermore, regulations also touch on several aviation safety issues. Airplanes need to refuel on the right side. Therefore, separating ground activities such as arranging luggage, fueling, etc., from passenger boarding helps minimize risks and saves time. If you board from the right side, it's usually in smaller aircraft like family planes or helicopters, where passengers wait for the pilot to board first before following.
According to Vi Yến/Vnexpress
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Reference: Travel guide Mytour
MytourSeptember 8, 2020