In flights that cross Schengen borders but remain in the EU Customs Union, passport checks are required but customs checks are not. However, Wikipedia says that an international airport is an airport with immigration and customs facilities. I have made such flights that had passport checks but no customs checks. However, are airports classified as international (by IATA and ICAO according to Wikipedia) still required to have the ability to do customs checks, even if they do not regularly carry out any?
This may be relevant in cases where a country were to leave the EU Customs Union.