The unstressed ending -eo produces a sound that can oscillate, depending on the case, between [ɔw] ~ [ju] ~ [ew].

For reasons of consistency it is considered appropriate to retain it, as many authors do, for words that originally had it and still form their plural in -ei. From angeo [ˈaŋdʒɔw] “angel” one will have angei [ˈaŋdʒej], from umeo [ˈymˑju] “humid” and “humble” one will have umei [ˈymˑej] ~ [ˈymˑi], etc.

As also in Italian (consider the form aereoplano, incorrect), words relating to the concept of “airplane” can undergo various phonetic alterations. In writing, and in texts of neutral register, it is therefore advisable to use the “pure” form aeroplan (to be pronounced in speech as preferred: [aeruˈplaŋ], [ɛːruˈplaŋ]…), and to reserve forms like arioplan, rioplan for more faithful representations of spoken usage, for texts of familiar register, etc.

The same considerations also apply to other words with the prefix aero-, such as aeropòrto.

As also in Italian (consider the form aereoplano, incorrect), words relating to the concept of “airplane” can undergo various phonetic alterations. In writing, and in texts of neutral register, it is therefore advisable to use the “pure” form aeroplan (to be pronounced in speech as preferred: [aeruˈplaŋ], [ɛːruˈplaŋ]…), and to reserve forms like arioplan, rioplan for more faithful representations of spoken usage, for texts of familiar register, etc.

The same considerations also apply to other words with the prefix aero-, such as aeropòrto.