If the viva came down to luck, I’d be worried if mine was on Friday 13th! But it’s not about luck.
- You’re not lucky if your thesis passes with minor corrections.
- You’re not lucky if your examiners say nice things.
- You’re not lucky if you feel good about your viva.
Your PhD comes down to effort, actions and talent: the things you do over a long period of time put you in a good position for the viva.
You can be fortunate, but that’s different. Fortunate is something good happening as a result of effort. You can be fortunate throughout your PhD as a result of the questions you ask, the risks you take and the good work you do. You’ll likely be fortunate in the viva, because of all the work that you’ve done, and the talent you bring with you.
It’s not likely that you’ll be unlucky (or lucky).