You probably know that ə is called schwa and that this vowel is very common in unstressed syllables. However, there are certain circumstances where one might expect a ə, but where it does not occur, at least for a lot of General British English speakers. Look at the following words:
What have these words got in common? Well, they are all stressed on the second syllable. The first syllable is unstressed and has a vowel which is spelt with the letter <a>. This vowel is pronounced æ and not ə. But notice that words like banana magician sadistic also fit this description, but for these the first vowel is ə! What’s the difference? The answer is that in the first group of words the vowel in the first syllable is followed by two consonant sounds, whereas there is only one following consonant in the second group. Here are some more words where æ is used rather than ə in an initial unstressed syllable:
cantankerous Dalmatian damnation malpractice pantechnicon
Notice that the above does not apply to words where the first syllable is a form of the ad- prefix, so adventure affliction applause etc all have ə in the first syllable.