Possible to get Singapore Airlines (SQ) flight on 016 stock?
 

I'm trying to booking flights with SQ 21/22 SIN-EWR in the itinerary through united.com but the booking engine just won't show any direct SIN-EWR or vv. flights. Have to do this over the phone (agents will see the flights). Does anyone know why?

UA would prefer to sell you a seat on their own flights (through SFO/LAX) or joint venture flights (through EWR) than sell a competitor's flight.

Intentional? Accidental? With UA IT you never really know.

Taking a look at SQ SIN-EWR D fare and the fare rules say:

It really has very little to do with UA. It's SQ's plating restrictions. You'll have to buy a full C fare to ticket this through UA. That's going to be awfully expensive.

UA could show the flights at full-fare. They elect not to do so, presumably for competitive reasons in this case. In other, more complicated cases, I'd think search engine incompetence, but when flights aren't showing up on a simple one-way/round-trip search (as opposed to part of a complicated multi-city itinerary), it's because UA has chosen not to display them.

Perhaps, but my experience is there are very limited circumstances where UA will show *A flights that aren't codeshared, and usually, they will only show *A flights that as the UA #. Not always, but usually. I've definitely seen limited amounts of LH flights show up as native LH flight numbers, even when they do have a codeshare number, but again, usually will only show as the UA codeshare and not native. Sometimes I've called to get an agent to book a AC-UA connection, for example, with the AC number vs. the UA codeshare it shows on the web. And obviously, connections on non-partner carriers - CX, JetStar, etc. are booked as their native flight numbers.

There used to be a workaround on flights where award space was available, as before checkout, there used to be a place where it would suggest to buy in cash instead if you wanted. That went away a couple of years ago, maybe, when UA started showing the bundles just before checkout.

Tried this recently and failed. Even with a feeder flight on UA at the beginning of the itinerary. Tried web, phone, and TA.

RE-launched SQ direct flight from EWR
 

SQ generally has plating restrictions and few codeshares with UA, so you may be waiting for a long time.

"Never" would be my guess.