This is a jukugo word that uses the on'yomi readings of the kanji. You didn't learn this reading for 乗 yet, so here's a mnemonic to help you:
You know one person who always has a hard time boarding a plane, and it's Jourm (じょう), the simple farmhand. Jourm is what you'd call a very big boy. He doesn't fit in the tube thing you normally walk through when boarding a plane, so he has to climb up the side. It's a whole thing.
Please watch your feet while boarding.