It's not really about whether the first letter is a vowel or a consonant. It's all about the sound of the first letter. Does it sound like a vowel or a consonant.
An hour
A house
A united team
An unidentified team
The issue with historical is that some pronounce the h and some don't. Here's the current guidance as we see it:
When pronouncing the words historic and historical, the accent falls on the second syllable, and many pronounce them as starting with a vowel. For those people, it is appropriate to use an before historic and historical. Therefore, you have a choice depending on what sounds best for you.
There is a lot of leniency on this issue. If you're still unsure, opt for a historical and a historic as these remain preferable – especially in formal writing.
