It seems natural to me to always put an before any word beginning with a vowel. It is also natural to drop the initial h from many words, and add an before them, the biggest culprits being: hotel, hospital, half, hand and horrible. While I do sometimes say, 'a hospital' I would then always aspirate the h, it seems to be a very consistent rule. My partner is downstairs having coffee with a friend and I've noticed that they seem to drop the h from half ( for example) when coupling it with another word that has a silent h, like hour, 'an [h]our an [h]aff', but when I asked him to repeat those words he said, 'an hour and a half', weird huh?
I've just tried to say, 'a alpha, a eight' and it is very unnatural, my mouth stumbles over the words.