The usual rule about indefinite articles goes something like this:
If a word starts with a consonant, use “a” before it;
if a word starts with a vowel, use “an” before it.
Examples: an apple, a banana
However, this rule has some exceptions in the case of acronyms and the letter H, which is sometimes pronounced with some wind behind it, but sometimes it is silent.
In the case of acronyms, if the initials are pronounced like a word, like NASA, you would use “a” rather than “an.” Also, if the letters are spelled out and the first one is pronounced as if it had a consonant at the beginning like “Pee” or “Bee” or “Tee,” use “a” before it.
Examples: a NASA engineer, a BP executive
However, if the letters of the abbreviation are spelled out and the first letter is pronounced as if it had a vowel at the beginning like “Em” or “En” or “Ess,” then use the word “an” before it.
Examples: an MMS employee, an SPE paper
Similarly, words beginning with the letter H that sound like they start with a windy consonant take “a” before them, whereas those where the H is silent take “an” before them.
Windy Examples: a historically important event, a hysterectomy
Silent Examples: an hourly employee, an honorary degree
Now, some British folks might pronounce “historical” without any wind, in which case they can use “an” and get away with it. Same thing with the old Exxon folks who still live in Humble, Texas, where they have an Humble resident or two.
I saw the same error several times today that cracked me up. There was a valve that was supposed to “relive” pressure, rather than “relieve” pressure. This is something that spell checker will not catch. I think most of us would prefer the latter.