The use of "an" before a word beginning with an aspirated h, such as hierarchy, is not wrong, but it is not common practice nowadays. It was formerly more common and some still regard it as more appropriate in careful style, but they are certainly in the minority, and most now see it is an archaic curiosity. It is more often found with "historical" than with other words ("an historical account").

I think the following note from the Oxford Dictionary online expresses it well:

"Is it ‘a historical document’ or ‘an historical document’? ‘A hotel’ or ‘an hotel’? There is still some divergence of opinion over which form of the indefinite article should be used before words that begin with h- and have an unstressed first syllable. In the 18th and 19th centuries people often did not pronounce the initial h for these words, and so an was commonly used . Today the h is pronounced, and so it is logical to use a rather than an. However , the indefinite article an is still encountered before the h in both British and American English, particularly with historical: in the Oxford English Corpus around a quarter of examples of historical are preceded with an rather than a"
http://oxforddictionaries.com/definition/an#DWS-m-en_gb-msdi...

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Note added at 13 mins (2011-07-26 12:43:33 GMT)
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Note, by the way (as I forgot to mention) that "an" is possible before a word like "historical", in which the first syllable is not stressed, and this is also true of "hierarchical", but with "hierarchy", in which the first syllable is stressed, it should be "a hierarchy" in any case.

I presume we are talking about written language here. Those who drop their hs will naturally use "an" in speech without thinking about it.

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Note added at 58 mins (2011-07-26 13:28:01 GMT)
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I think we're in danger of getting confused unless we distinguish firmly between spoken and written usage. People who say "an hotel" or "an hierarchical distinction" are very rare nowadays, and likely to get funny looks. It still depends a bit on the context; a few old-fashioned academics still do it in academic contexts, but even there it has almost died out (though it was still fairly common when I was an undergraduate, and I'm not THAT old). Up to the mid-twentieth century, however, as Jack says, it was common, and those whose didn't do it in formal situations were regarded as somewhat uneducated.

In written usage, however, the question is not so clearcut. Again, an before a word beginning with an h is fairly rare and getting rarer all the time, but there are certainly people who would say "a hierarchical distinction" and write "an hierarachical distinction".

Let me say firmly that I don't do this and I don't recommend it should be done. There is no rational justification for it; it's a hangover from an earlier era. My comments are intended to be descriptive, not normative. But it would not be difficult to show that there are still people who follow this practice. I say forget it, and use "a".