Why do airlines use the same flight # for multiple segments?
 

Many airlines use the same flight number for multiple segments. This is done for some flights, but not all.

For example UA 497 is EWR=>SFO and SFO=>SAN.

Why not simply use 2 different flight numbers?

Purely a guess, but i'd suspect it is due to systems not being programmed for more than 4 digit flight numbers, and they need more than 9999. Similar to the Y2K bug.

(I absolutely hate this too, as well as when they change the flight # of the same flight just to cheat and reset their stats).

This is correct. Too many flights and insufficient numbers. Partly caused by the rise in the number of code shares.

While the technical reason above is part of the problem there is a slightly more sinister version as well. A direct flight (single flight number, even if a connection and change of aircraft) historically would show up above a traditional connection in a GDS interface. So searching SAT-GUM would have always listed CO 7 first, even though it was a 737 to IAH and then a 777 on to Tokyo and a 764 continuing to Guam. Southwest generally uses this to its advantage still in terms of how it schedules flights while the other legacy carriers are getting better about avoiding it on domestic itineraries.

The EWR-SAN connection in this example is something of an exception, but UA also only has an 8a, 5p and 8p nonstop so it slightly fits in the schedule (though the 10a with a DEN connection is less total travel time).

There are some airlines where a multi-segment flight with the same flight number does stay on the same aircraft through all of the segments, but that is not true for all airlines. Where it is not true, there is the usual risk of missing the connection and the like.

WN 2325 is flight where the same aircraft goes from OAK->MSP->MDW->PHL. A passenger can book it on Southwest's web site, which shows it as a two stop flight with no plane change.

However, UA 497 uses different aircraft from the EWR->SFO (777-200) and SFO->SAN (737-900) segments.

I used to train Reservations Sales Agents at Disney World. We sold packages that included air. The difference between a nonstop and direct flight (aka multi-segment with same flight number) was incredibly difficult for some people to grasp. No matter how many times we covered it, the majority of people still got it wrong on quizzes.

Part of the curriculum was to mention what you highlight above. Namely, that it's possible to miss a connection on a direct flight because the airline might operate each segment on different equipment. All the trainers got together and decided to skip that part of the lesson. It just confused most people and it wasn't something the agents would ever explain to guests. (If a guest experiencing IROPs ever called, they were immediately transferred to the Air Desk.)

I once had difficulty with a ticket because the airline decided to reuse the same flight number for AAA-BBB and BBB-AAA.

Another reason is that in some parts of the world (notably ASECNA countries), overflight charges are capped at a maximum amount per flight number per day. So multiple segments with the same flight number will get charged only the capped fee rather than multiple fees for each segment.

Doing so would allow the airline to market the flight as a direct flight as opposed to one with transits... direct = more to charge

note that direct doesn't necessarily mean non stop...

Ooh, I hate it when they do that. DH and I once had an MCI-JFK-EDI "direct" that involved a change of planes at JFK as well as a change of terminal, requiring us to go through the TSA mess again. Worse, the MCI-JFK flight got diverted to CVG and when we arrived at JFK (in plenty of time for the JFK-EDI leg) I discovered that DL had trashed our seat assignments from JFK-EDI and DH had lost his aisle seat, which he needed because of a creaky back. A lounge angel changed our assignments for us.

My sinister reason: you don't get mileage credit for the actual miles flown- you get less because they count them as if you'd had a nonstop. If I were in charge of the world it would be illegal to assign the same flight number if an aircraft change is required.

People use non-stop and direct synonymously. It's almost a lost cause to try to correct them.

That is probably because most people outside the airline and travel industry habitually use "direct" synonymously with "non-stop". Having "direct" mean what it means in airline jargon is the source of this confusion, and probably leaves some passengers disappointed or angry that their "direct" flight involved a stop and a connection that they missed.

Not globally true. In West Africa, the term "one-stop" is often used to describe what in North America would be called a "non-stop" flight. The logic being that the flight only stops at its destination. I guess a "non-stop" flight would be one where you have to parachute out overhead the airport, but that term is almost never used. "Direct" is used to describe a flight on the same aircraft, even if there are interim stops or flight number changes.

True. Very true, in fact.

But, our Reservations Sales Agents took a lot of calls from travel agents. So, there really wasn't any wiggle room when it came to terminology.