https://en.wikipedia.org/wiki/Singap...ghts_21_and_22

This flight (and the upcoming QF project Sunrise flights stating in 2026-'27) flies 18-20 hours nonstop which would 'save four hours compared to a one-stop flight'.
I doubt it. I rather thing two hours: extra landing and takeoff costs 2x30 minutes plus one hour stop makes two hours. But, let it 4 hours, this is not the worst issue.
Taking 1-2 extra sets of crew, hunderd(s) tons of extra fuel makes such flights more costly than one stop flights on a similar distance. The extra fuel and extra crew costs paying passenger space, decreasing profits of the flight.
Moreover, LHR-SYD contains more stop airports close to the great circle route (SIN, BKK, DXB), unlike the SQ SIN-JFK polar route.

I think such flights are just prestige of the airlines 'we can do LHR-SYD non stop'. Or is there another reason these airlines operate such flights which cost much more money than a one stop flight.

Your ideas ?

I don't think airlines do anything for prestige if they are going to lose money in the process. Declining food service is one example of airlines forgoing costs over maintaining prestige.

I. suspect these ultra-long haul flights serve a need and leave full most days. I have not experienced a flight that has not been sold out in several years. Every time I get on a plane, I hear the announcement, "This is a full flight. If you cannot find space for your bag, please let us know so we can gate-check it."

Two more factors in favor of the non-stop:

1. Routing via an intermediate airport is inherently longer than a non-stop. How much longer depends on how close to the non-stop route the stop is, but there's always some added distance. The optimal non-stop routing for such a long flight, under a given day's wind conditions, can push an intermediate stop that is right under the great circle route several hundred miles away from the best non-stop routing. That's a big part of where the four hours number comes from.

2. Any connection runs a risk of a missed connection because of delays in the first leg, of lost luggage at the connection point if you check it, of the aircraft for the second leg having a problem or not being there because of an earlier problem, of weather at the connection point interfering with a smooth connection, etc., etc. That's a big part of why people prefer non-stops: if the plane takes off, you have a much higher likelihood of arriving at your destination more or less when you expect to. Is there anyone here who has never had a problem with a connecting flight? Case closed.

SIN-JFK does not currently appear to be going the polar route and instead across the Pacific, but still limited potential stops. JFK-SIN goes across the North Atlantic so several options on the way.

1: The time lost is close to 4 hours, when including the deviation from optimum routing
2: The aircraft does not need "hundreds of tons" extra. In fact, the total fuel consumption on a non-stop is considerably lower. The total fuel capacity of an A350-100 is in the range of around 165.000 litres, which equates to roughly 115 tons . Adding "hundreds of tons" extra is therefore physically impossible.
3: ULH flights require an additional 2 cockpit crew members, no more or less
4: Neither DXB, BKK or SIN are anywhere near the great-circle route between SYD and LHR. HKG, on the other hand, is. However, the GC route goes over Russia and that's not presently allowed

My idea? People armed with computers and flight planning software, coupled with exact information on aircraft performance, are fully able to make the necessary calculations. If they can make a business case out of it, I'm not here to argue that.

I can see the merits of a non stop if in business class but I really question it for economy. The amount of damage your body will sustain over two long hauls with a good break, stretch and walk imbetween would more than wipe out the time saved.

1: For SIN-JFK 4 hours is true, but SYD-LHR, I doubt.
2: 'hundreds' should read 'tens'. The 'hundreds' is indeed my nonsense. But less fuel consumption of a flight which carries lots of extra fuel whil the aircraft does not have this range ? Explain ?
3: And what about cabin crew ? They cannot work 20 hours, it is not allowed by law.
4: Exactly: QF and BA are not allowed to fly over Soviet airspace (unless the Ukraine war is resolved in 2026) so the shortest route usually goes close to the three airports I mentioned and HKG is indeed a better match. Many nonstops between W Europe and SE Asia go over the Middle East for the same reason.

Let's see. Landing fees. Carrying passengers that would need a visa for the stop. The myriad of minor mechanicals that occur prior to many departures. Crew scheduling with en-route stops. HVFers that will pay a premium for the nonstop front cabin seat. Lots of reasons.

1: 40 minutes descend, 45 minutes climb, deviation from optimum routing, 1-1.5 hours on the ground. It all adds up.
2: A non-stop flight between two points burns less fuel in total than a one-stop.
3: QF are running their PER-LHR with one set of cabin crew, giving them extended breaks. I suppose Australian law makes that possible
4: SYD/MEL-LHR are unlikely to be viable, or maybe even possible, if Russian airspace is unavailable

The "extra crew" is net zero or even a savings. You need two set of crews to fly LHR-DXB-SYD because the crew operating LHR-DXB gets a crew rest before maybe operating DXB-SYD a day or two later so you have to have a crew already in DXB to operate DXB-SYD (they probably flew LHR-DXB a day or two before). Flying with an augmented crew that is rotating out means either the same or maybe a couple less crew members depending on the airline, labor laws, aviation regulations, etc. Plus with the nonstop there's no added costs of the crew layover in DXB which would amount to a hotel room and per diem for each crew member for the 1-2 nights they're on crew rest in DXB.

Operating cost is only one factor. The other is margins on revenue. If I can spend $5 and get you to spend $6 or if I can spend $6 and get you to spend $10, which is the better option and yields a better margin? These ULH nonstops can command a premium especially in the forward cabin because time is money and that convenience can often be monetized.

Because taking off and accelerating to cruising speed while climbing to cruising altitude burns a LOT of fuel. A non-stop, even assuming that the potential stop-over point was directly under its optimal route (which is hardly ever the case), has already taken off, is already up there, and is already going that fast.

Why do you think cars get better mileage on a freeway (motorway, autoroute, autobahn, etc.) than they do around town? It's not because they inherently get better mileage at 75mph/120kph than they do at 40mph/70kph. If anything, the opposite is true; air drag goes up as the square of the speed. It's because they don't have to keep stopping and starting up again. The details are different for airplanes, but the economic principle is the same.