Is this a joke? A330 is the 5th most operated aircraft family for TPAC operations (excluding HI/AK since they're not really "trans") over the last decade.

Bloomberg: United May Amend $12.4 Billion Airbus Deal to Take Smaller Jets
 

United May Amend $12.4 Billion Airbus Deal to Take Smaller Jets

I can't say I'm surprised, especially with the recent move to defer 61 737s and convert the remaining four to 738s to be delivered next year. This new management team seems to be reviewing several of the fleet decisions made prior to last Spring. When United first announced the order of the 777-300ER, I was scratching my head thinking why on earth would they do this considering the 35 A350-1000s starting to arrive in October 2018. Evidently, it appears United may end up pulling a Delta style move by ordering last off the line 777-300ERs, at a major discount by Boeing, to replace 747s one for one, while defering/altering the A350 order. I do think United will end up ordering the rumoured additional 11 777-300ERs, bringing the total of 777-300ERs to 25, to finish off replacing the 747s. The economics of the higher capacity 777-300ER as compared to the ultra fuel efficient A350-1000 is not as great as one would think.

While cancelling the order is very unlikely due to cancellation costs, its very unlikely United will end up with 35 A350-1000s. The article mentions United looking at A350-900s and the A330 NEOs as potential conversions. The A350-900 would do quite well in United's fleet, especially the 280 tonne A350-900 on thinner SFO-Asia routes, like SFO-SIN and replace the remaining 777-200ERs on Asia routes, like IAD-NRT/PEK. The 787-9s can be used to open more new "long and thin" routes. (SQ uses the 268 tonne A350-900 on SFO-SIN but the 12 tonne higher MTOW A350-900 will have an extra 500 nm of range.) United looking at the A330 NEO is probably the likely 767-300ER replacement. While the 767-300ERs have been given a life extension, they will likely start going in 2021, when they start to line up against D checks. The A330 NEO is the most cost efficient replacement for these aircraft, as the 787-10 is a bit too large for a 767-300ER replacement, and the 787-10 is likely to be a 772/747 hybrid replacement on Transatlantic routes.

The article concludes with United considering the A321 NEO, along with a rumoured 737-10 MAX, as a future narrowbody aircraft. These aircraft are likely to replace the RR powered 757s. The PS 757s have yet to have fuel tank modifications, which have a hard deadline of December 26, 2018, to be completed. So, the PS 757s are likely done in 2018.

how many aircraft families fly TPAC? 787, 777, 747, A380, some A350, perhaps a few A340? 5th place merits a participation award i guess. removing YVR/SEA from consideration (not UA hubs), i would think the percentage of TPAC ops flown by the A330 is even less.

i don't have access to any neat databases, but i set filters on flightradar24 for A332/3 flying to/from both SFO and LAX. as of the time of this post, there are no TPAC flights in the air to/from either airport.

i'll rephrase: the longer stage lengths of UA's TPAC routes compared to DL's shorter TATL routes make a stronger case for the economics of the 787 compared to those of the A330neo.

With different engines, and possibly further improvements, it's imaginable that the A330neo could have different economics to the A330. Given there are none flying yet, it's not exactly fair to bash the lack of flights conducted by them.

Moreover, the reported range of an A330neo isn't far off that of a 787. In fact the 800neo is squarely between 787-8 and 787-9, the 900neo can go further than a 787-10 (but both of those significantly less than the 800neo/-8/-9).

Still, I'm no expert on aviation so I don't really know how relevant the A330neo really could be for TPAC.

I was mistaken, it's fourth. With a threshold of >4 flight/day over the last decade, there are seven TPAC aircraft families: B777, B747, A340, A330, B767, B787, A380

to potentially make the waters murkier, there's talk on a.net (apparently from an internal UA pilot messageboard) that the A350 does not have crew rest area(s) that satisfy the requirements of the pilots' and/or FAs' contract with UA - and that it's not a simple fix.

i'm sure someone else here has better information as to (a) the specifics of the issue, and (b) what affect, if any, this may have on the A350 order itself.

I've seen mentions of the contract requiring 2 comfortable (reclining, or premium?) seats in either crew rest, or the best cabin, in addition to bunks (for those flights that have 4 cockpit crew). The A350 seems to have only one seat in the overhead pilot rest area, at least on the diagrams I've seen - could that be the issue?

The issue is with the pilot rest facilities supposedly doesn't meet their requirements. FA crew rest seems to be okay.