The "Friend Zone" is a lot more complicated than people make it out to be, because, well, people are complex, and romance is doubly so. (Yeah, that sounded a little douchey to me too, but let's keep going.) Actually, this whole idea of the friend zone, created by "Nice Guys" is really stupid. If you were actually a nice guy, you wouldn't think this crap. You give actual, genuinely nice guys a bad name.
So, yeah, obviously, if you have a friend you find physically attractive, you're obviously going to have feelings for her. This isn't actually a bad thing, but you have to keep in mind that people sometimes want something different from their significant others than they want from their friends. Besides that, you have to try to understand exactly how you think of her. It could very well be that you are only pursuing her because, for lack of a better word, she's conveniant. She's pretty, and you get along. But she might not really be right for you.
About the whole, "once you're in the friend zone, you can't get out" bullshit, this isn't true at all. My friend told me that her best relationships developed from year-long friendships. She doesn't like to become romantically involved with people she just met, she actually likes to spend some time with them first, and get comfortable with them.
A GIRL, who you have a higher chance of romance with the longer you've known her, and the more you and her get along! Crazy, right? Is this the Twilight Zone!?
It really all depends about what you're looking for in a relationship, but if you really are falling in love with a friend, you really should do something about it. Ask her out, see what happens. It's much better than just bottling it up inside. But you can't use friendship as a crutch. So what if you've helped her, so what if you've been there for her? That does not mean she owes you. It definitely doesn't mean that you're right for her. If she doesn't think it's going to work, don't act like an ass. Take some time, sort yourself out, and move on.
So yeah, I find the idea of the "Friend Zone" utterly ridiculous. It's only the friend zone if you don't have the courage to say what you really mean.
And hiding the truth just because you feel it would be better to not shake things up will definitely come back to bite you in the long run. Besides the fact that you'll probably feel tortured every time you see her.