mherdeg

There are 2 things going on here. One is easy to understand. The other is infuriating.

(1) Understanding how fares work

In order to buy a ticket, you need to have a fare cover that ticket. In order to get a fare, you need availability. In order to get availability, you need city pairs and dates.

OK so your city pairs are
6/25 EWR-LHR
6/30 LHR-EWR

The cheapest fares for this date is KHNC29S. Here's an Expertflyer fare search that shows most fares in this market:



(btw, notice that there is not a T fare filed -- we'll get to that later)

That fare has the following traits - 90 day advance purchase, $310 round trip before taxes and fuel surcharges, requires a Saturday night minimum stay.

Specifically the min stay rules for the fare say:
(You can see this by clicking 'fare rules' just before booking a K fare ticket or by using matrix.itasoftware.com or ExpertFlyer to view this fare's rules.)

If you do a search for an EWR-LHR round trip from June 25 to June 30, you will see that both
(1) the day flight EWR-LHR which departs Saturday morning and arrives Saturday night
and also
(2) the night flight EWR-LHR which departs Saturday night and arrives Sunday morning

both have K inventory. However, the only published K fare requires you to depart no sooner than the first Sunday after arrival on your turnaround point - i.e. July 3, since you arrive on Sunday 26 June.

One possible expensive journey you can book on 6/25 EWR-LHR (redeye) returning 6/30 LHR-EWR fares out with fare basis SHW26NCE, which says
By way of contrast the cheap journey on 6/25 EWR-LHR (day flight) returning 6/30 LHR-EWR fares out with fare basis KHNC29S which, again, says
I hope it's clear why — if your return portion is going to be midweek — an outbound flight that departs Saturday and arrives Saturday night is eligible for the cheaper K fare; while at the same time an outbound flight that departs Saturday and arrives Sunday is not eligible for the cheaper K fare but is eligible for the more expensive S fare.

Now, what about that "T" you were seeing which was cheaper than the "S" I quote above? Here things get interesting.

You'll notice that the one-way fare price shown for the morning flight on June 25 is a $400 K fare whereas the one-way fare price shown for the evening flight on June 25 is a $600 T fare. Where does that T fare and price come from? Why aren't you automatically getting the next cheapest fare without a min stay requirement, an S fare? Here is the other thing to understand (which is a little infuriating).

(2) United shows prices and fare classes per sector -- but mucks around with them depending on which return you choose.

If you select the cheapest economy $600 T fare EWR-LHR , on the next screen you will see that you can book a return portion of either a $1400 S fare LHR-EWR or a $1100 T fare LHR-DUB-EWR.

Suppose you pick the $1400 S fare LHR-EWR. Look *very closely* at the next screen and you'll see that you now have an S fare EWR-LHR and an S fare LHR-EWR. After you picked the T fare on EWR-LHR, based on the return you chose, UA upped your fare class on the outbound!! Why? Because the cheapest fare that can cover your journey on an EWR-LHR round trip is that S fare.

But if instead you pick the $1100 T fare LHR-DUB-EWR, on the next screen you'll see that you now have a T fare EWR-LHR (just like you picked originally) and you also have a T fare LHR-DUB-EWR. Click "Fare rules" and you'll see that what you are being sold is a round trip on the UA EWR-DUB fare basis TJ40NCE with a stopover in LHR.

In other words, it is so expensive to fly to LHR without a Saturday night stay that United actually knows to offer to fly you to Dublin with a stopover in London (and a very short amount of time at the fare's "destination" in Dublin!).

I find this stuff infuriating because the per-segment cost United shows you as you pick partial round trips is not at all transparent and the fares it's choosing are really hard to follow. There can be wonky situations with a multicity trip where you'll see that a particular sector costs "$0" (because it is being treated as a free stopover) but later flight choices raise the price much more than the individual flight. Um, hard to describe but it is very difficult to follow.