Definite article. el ala [duplicate]

Question

I was wondering if "el ala" belonged to those group of nouns that start with an "a", where the definite article changes from LA to EL.

I also read that the indefinite article "UN" instead of "UNA" can be used before these nouns. I was wondering if this is an optional choice of a strict requirement.

Is "una ala" completely wrong or are both "un ala" and "una ala" correct?

Thank you.