Using A or An with Initialisms

-->A brilliant graduate student recently pointed out to me that root cause analysis is abbreviated with the initialism RCA.  If we write about RCA as a type of analysis, we treat RCA as a noncount noun like weather, advice, or flour.  Example: Managers use RCA when they work to improve quality or safety. 

When we use RCA that way, as a noncount noun, we write it without any article at all (no a, an, or the). 

However, if we refer to a particular case in which people performed a root cause analysis, we would add an article, possibly "an RCA," as in "they performed an RCA." 

Why an RCA?  We don’t write an road!  Doesn’t the article an precede a vowel, as in an apple?

The rule is that, to choose a or an, we treat an initialism like RCA by its phonetics. So RCA would be, in effect, "ARE SEE AY," which begins with a vowel sound.

Correct:  She is earning an MBA.  An SEC report showed X. 

The “Y” sound is treated as a consonant: 

A U.S. visit, A European council

So is the "H" sound—at least in U. S. English:  A historic site.