Optimum Distance for long haul routes?

[jabird]

I have mentioned this before on other threads, but never got a definitive answer:

What is the optimum distance - ie the distance at which greatest (fuel) economy is reached - for long haul flying?

Obviously, there will be variables, but consider:

Shorter flights:

Use huge amounts of fuel in the take off and landing cycle.
Pay two sets of airport handling charges when one might do.

Longer flights:

Need to carry much more fuel for use later on in the sector - does this not ultimately grow exponentially (until maximum range is reached).
May need additional crew for shift rotations mid-flight, hence seating or crew rest space is used for non-revenue purposes (not such an issue on B777s with rest space in crown).
Need to carry more food and have larger tanks for water / toilets + bins for waste.
May need to remove seats to gain range, either through all business service (SQ SIN-EWR / CX HKG - JFK?), or through having more premium seats than the route would otherwise demand - therefore reducing overall yields or giving away upgrades which would otherwise not be given.

This is obviously a major issue on the kangaroo routes - I understand that LON-PER is technically feasible in a B777, although I don't know at what configuration. Iirc LON-SYD comes within the scope of the A350, but technical specs and route viability are two different animals.

Obviously, EK, EY etc have a big cost advantage in DXB / AUH etc but I note EK also have additional stops on many of their DXB > Oz / NZ routes. Is this all about range, or just their way of serving more destinations?

[The SSK]

You are complicating things unnecessarily.

If you plot the fuel consumption of an aircraft against a range of sector distances, the curve will start extremely high (at extremely short distances) and will reduce rapidly before beginning to flatten out. However after the lowest point is reached, the curve will start to rise again slightly and will continue to rise until you get to the maximum range of that aircraft.

I have worked with such graphs, they are a tool of the planner's trade. They all look pretty much alike for different aircraft types, both short and longhaul, the only difference is in the scaling - although I think for turboprops the initial slope is a lot less steep.

It stands to reason, then, that the most fuel-efficient sector distance is where the curve is at its lowest. From memory (it's been a while...) for most ultra longhaul types this is about 5500km.

So in theory, a route like LON-DXB-SIN, which is pretty much on the great circle and the stop is close to the mid-point, will require less fuel than a nonstop LON-SIN. In practice the difference if any will be pretty insignificant.

I doubt you will ever get Europe-Australia nonstops, for reasons of crew duty limits. They would certainly be impossible under present rules and there is no great commercial incentive for the European or Australian carriers to stir up the industrial-relations nightmare that would ensue if they were ever seriously considered.

[jabird]

SSK,

Thanks for an excellent answer. You have described the graph in a way I can picture exactly what you are saying. I have not seen such graphs, only maximum range charts, thse being subject to well known variables as discussed at length on the threads for any airport with a 'relatively' short runway.

If you know of any such charts being online, I would love to see one.

Using your SIN example, do you know what percentage saving there would be with a stop in DXB? Of course, the reality is that EK also have a much lower cost base there, and for anyone based north of London, they can offer a regional departure through DXB. As for SQ's MAN-MUC-SIN, not sure of the logic in that one (MUC as LH hub yes, but MAN on same route?).

[jabird]

I stand corrected - SQ will sell me a ticket on the MAN - MUC sector, if I have the small sum of £1,035 to spare!

[Cyrano]

SSK,

Thanks for an excellent answer. You have described the graph in a way I can picture exactly what you are saying. I have not seen such graphs, only maximum range charts, thse being subject to well known variables as discussed at length on the threads for any airport with a 'relatively' short runway.

If you know of any such charts being online, I would love to see one.

Using your SIN example, do you know what percentage saving there would be with a stop in DXB? Of course, the reality is that EK also have a much lower cost base there, and for anyone based north of London, they can offer a regional departure through DXB. As for SQ's MAN-MUC-SIN, not sure of the logic in that one (MUC as LH hub yes, but MAN on same route?).

This is an interesting topic. One can leave aside the commercial specifics of individual routes or intermediate stops and wonder: what is the approximate cost saving in - for example - flying a 10,000km route with a fuel stop midway compared to non-stop? Of course there are other issues: additional crew costs, extra landing fees, etc., but if fuel prices continue to climb, this is perhaps going to become an option more widely considered in the future.

I want to say that I saw a reference a year or two ago to some research Rolls-Royce had done on this subject, but I haven't been able to find the reference again.

The big saving would come, I imagine, if you operate the one-stop service with an aircraft which is optimised for the 5,000km range rather than the 10,000km one, i.e. smaller fuel tanks, lower MTOW, less surplus engine weight to carry, etc.

[jabird]

The big saving would come, I imagine, if you operate the one-stop service with an aircraft which is optimised for the 5,000km range rather than the 10,000km one, i.e. smaller fuel tanks, lower MTOW, less surplus engine weight to carry, etc.

Or what about Neos / 737-9xxs pushed to the limits? Is 3 + 3 not at least the optimum cross section for passengers, and also a much shorter cross section, although payload is less?

From a booking point of view, there are times when two long low cost sectors might be cheaper than one long haul - I'm thinking about western Asia for this, obviously there is the risk of delay liability, but that's another topic.

[The SSK]

If you want to see comparative fuel burn figures per aircraft type for a given sector distance, you can use the Eurocontrol model used for calculations for the EU emissions trading scheme
Google 'Eurocontrol small emitters tool'
It's a fairly crude model which does not include a 'burning fuel to carry fuel' effect and hence does not show a deterioration in fuel efficiency beyond the optimum range.

But you can have fun comparing different aircraft types (caution: you need to use ICAO 4-character codes). If you choose a medium-haul sector, say London-Gulf, 5500km/3000nm, the clear winner is the A321. Outcomes are very sensitive to assumed seating density but generally the A330-200 is pretty good, as would be the 737-900. Triple 7s not bad, slightly ahead of the 744 and the A380. If you want to save the planet, avoid the A340s.