I think it sounds wrong because the letter “U” is pronounced “yew”, and y is not a vowel. You would never say ‘an yurt’ or ‘an yogurt’. If it was prounced “oo”, then “an oo F O” would sound correct.
But as it is, it just sounds wrong. And whether technically correct or not, I reject it!
It’s correct when written because acronyms are representing the words they abriviate. In this case an unidentified object. If you where to speak the acronym than just “a” is correct because you are saying the letter “U” not the word “unidentified”. Does that make sense? Different rules for written and spoken.
I think it sounds wrong because the letter “U” is pronounced “yew”, and y is not a vowel. You would never say ‘an yurt’ or ‘an yogurt’. If it was prounced “oo”, then “an oo F O” would sound correct.
But as it is, it just sounds wrong. And whether technically correct or not, I reject it!
It’s correct when written because acronyms are representing the words they abriviate. In this case an unidentified object. If you where to speak the acronym than just “a” is correct because you are saying the letter “U” not the word “unidentified”. Does that make sense? Different rules for written and spoken.
All common English-language standards and style guides will tell you the written article should be based on the verbal pronunciation of the acronym.
E.g., https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Usage/faq0167.html