"The airplanes were not in Uist."
Translation:Cha robh na plèanaichean ann an Uibhist.
na is also the genitive singular form for feminine nouns, eg. na Gàidhlig of the Scottish Gaelic language, na pàirce of the park, na h-Èireann of Ireland, na h-Alba of Scotland (since Ireland and Scotland have the article in genitive).
But besides that you are right, an, am, a’ is used in all other cases in singular, na in plural. :)