Travelocity has a pretty good tool for finding published fares. It's located under: FLIGHTS|Fare Listings (look at the bottom).

To a destination like SIN, however, you will need to search ONE WAY and specify UA, because the travelocity engine will only return the first X number of published fares it finds, which means that you often don't see Z fares in markets with many fares to choose from (ie. SIN).

Also, you can just call reservations and they can tell you if a particular fare is published in a particular market. I don't recall seeing consistent Z fares for SIN. Note also, D fares to certain places (BKK, ICN come to mind) are as cheap as Z fares. You should ask reservations for a "Discounted Business Class Fare".

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