Until now we've learned two rules for making a Spanish noun plural:

If the noun ends in a vowel, add -s
If the noun ends in a consonant, add -es

The second rule has some exceptions, which are summed up below.

If the noun ends in a z, drop the z and add -ces

el lápiz
the pencil

los lápices
the pencils

If the noun ends in an s or an x, add -es only if the stress is on the last syllable (which is always the case in monosyllabic words). Otherwise, add nothing.

el análisis the analysis	los análisis the analyses
el país the country	los países the countries
el fax the fax	los faxes the faxes
el torax the thorax	los torax the thoraxes

And finally, if the noun has an accent mark on the last syllable, we remove the accent mark if that doesn't influence its pronunciation.

el avión
the plane

los aviones
the planes

This is because the stress is on the second last syllable by default, so here aviones retains the stress on on even though we drop the accent mark. Note that in países we didn't drop the accent mark, since this would have changed the pronuncation, because without the accent mark país would have been pronounced as one syllable instead of two.

Usages of el avión