Question about flights with the same flight number
 

Maybe it's a dumb question.
Does the delayed prior flight affect the later flight departure (assuming they are different airplanes)?
Why they share the same flight number?

No delay. No issue. The reason for the double numbers is that there are only so many numbers. Don't think it happens very often.

Example: UA 900 (probably a CRJ 700) goes SAN-SFO, arriving around noon. UA 900(heavy) flys SFO-FRA, departing around 2pm, but is a 747. No issue.

Perhaps in a previous situation it was considering a connecting flight with a change of planes. Doesn't matter. No issue.

Same flight numbers have no up's, only the fact that one only gets miles for A-C not A-B-C :-S
In addition, SAN-FRA is technically a direct flight, although it is obviously not non-stop.

I think this changed recently.

Then I stand corrected. About time too though.

The only thing that changes is that the 2nd one to go up in the air will get a letter added to its call sign, to distinguish the two. So if, say UA 936, an Airbus 320 DEN-IAD is delayed, then the continuation IAD-ZRH on a 767 may be labelled as 936a. On Ch9 I've also heard ATC assign the first letter of the pilot's name as the distinguishing letter.

Edit: I should not that I _think_ but am not sure that the 2nd flight is the one that gets the letter added.

They share the flight number mostly for marketing reasons to make the flights come up more prominently in the display.

As to whether a late inbound on segment 1 results in a delay for segment 2 using separate aircraft: in my experience they may hold segment 2 a little bit if there are a substantial number of passengers connecting from segment 1 to 2. But if they delay is long or few passengers are affected, segment 2 takes off on time and the late inbound passengers misconnect. I'd estimate that if you care, you have a slightly better chance of the second flight being held for you on one flight number compared to another similarly timed inbound flight (if only because the same flight number tends to make people book on it). It is also possible for them to hold international departures for other arriving domestic flights, if a substantial number of people are involved and the delay is not too long (SFO has done this on flights I've taken, generally for up to 15 minutes). In the reverse direction (international to domestic), I don't think it's normal policy to hold a domestic connection.

I think they often assign the extra letter without knowing whether the two flights will be in the air at the same time. I also think I've heard them attach the letter to the domestic segment even when that is the first segment and not to the international segment.

It must be all marketing. Brings direct, single-flight-number, multi stop routes to the top of searches, makes it look like United serves more cities from your airport of choice.

I am actually really surprised that a big delay on the inbound leg of the flight number would not result in a delay of the outbound leg. That would seem to be a big benefit. Otherwise, these single-flight-number, multi-stop routes are basically at a disadvantage in every way -- they don't get "delay protection", you might get shaved on the mileage earned, and, a big one, if you aren't careful and book several legs on the same flight numnber as separate segments, your changs of getting an upgrade on any leg go down because you don't get upgraded in advance on any leg (even if there is availability) unless you can get upgraded on every single leg.

The single flight number routes are actually disadvantaged then in most respects, it would appear.

for the traveller, definitively a disadvantage in all aspects

Not a good example. A CR7 flight is always operated by UA Express. Two problems here. First, any UAX flight is a four digit flight number. Secondly, the only non-US destination this would be able to continue onto (either in the same aircraft or a different one) would be one in Canada.

A year and a half ago.