Whether it's "a" or "an" is not about the letters, it's about the sound. When saying letters, in English, "U" is pronounced "yoo". The Y sound, being a conson

Whether it's "a" or "an" is not about the letters, it's about the sound. When saying letters, in English, "U" is pronounced "yoo". The Y sound, being a consonant, will mean the correct indefinite article is "a". Contrasted with German, I know they just pronounce it like "oo" with no y-sound preceding it. If you pronounce the letter "U" like that, then using "an" would be correct. a/an being about the sound and not the letter is also why "an hour" is correct, despite H being a consonant, because the H is silent. That all being said, you'll find people argue over whether "an historical event" or "a historical event" is correct.