Learn Spanish (Latin America): Mi familia viaja a Perú en avión.

In Spanish, when you talk about the mode of transportation you use to go somewhere, you use the preposition en. While it literally means "in" or "on", it translates to "by" in this context. Notice that you also drop the word "a" or "the", so you just say en avión rather than en un avión.

Why is the verb viaja singular when a family is made up of multiple people?

Even though a family has many members, the word familia is a singular noun because it refers to one single group. Therefore, it uses the singular "he/she/it" verb ending, making it viaja and not the plural viajan.

Why is it a Perú and not en Perú if we just used en for the plane?

The preposition a means "to" and shows movement towards a destination. You use it here because the family is traveling to Peru. The preposition en is used for the mode of transportation (en avión) or to say where something is already located. If you said viaja en Perú, it would mean they are already in Peru and just flying around inside the country.

Why does Perú have an accent mark on the letter 'u'?

Words in Spanish that end in a vowel normally place the spoken emphasis on the second-to-last syllable. However, the word Perú is pronounced with the strong emphasis on the final syllable. The accent mark on the 'u' tells you to break the normal pronunciation rule and stress that last sound.