Aircraft performance questions......

[brasidas2000]

I've been playing this game off and on for about a year, and I have noticed some glaring errors in aircraft performance modelling.

Specifically, 777 and A340.

Not only is the A340-600 fuel burn less than a 777-200ER with GE90, but the leasing  cost of the Boeing is higher. Thereby making the Boeing less efficient in operating costs (which it isn't) due to fuel burn especially when most of the Rolls Royce Trent models are well known to be less fuel efficient than most of their contemporaries (on a thrust basis), much less the fact that in meatworld, the A340 is no longer produced due to it's inability to compete wtih dual engine efficeiencies, much less the A380 which is on life support, as is the 747-8I.

Based on the data published by Rolls Royce, the fuel burn published on the A340-600HGW model, the engines of an A340-600 is one fourth what Rolls Royce says the total fuel burn should be for cruise. The fuel burn of a GE90-92B equipped 777-200ER is little less than half the actual burn of the actual aircraft in cruise.

With such a huge disparity in actual numbers, (and this extends to other aircraft that have had their performance crippled/altered like Cseries - Take off at MTOW, Max Range...etc...) why does this game even reference actual aircraft? Why not abstract the planes to fantastical proportions as much as the actual data purported to represent them? Instead of boring designations like 737-800, one could simply refer to them as something mythical terms instead. Make a complete break with reality. That would at the very least represent the simulation more honestly. My two cents.

[schro]

I'm having a hard time understanding the exact fuel burn issue that you're describing. Can you point to the fuel burn reference documents that you're talking about so I can see for myself?

The other thing to keep  in mind is that  the plane acquisition costs in game are dynamic based upon the demand of the type, so that will vary from game to game depending on what sells well.

The A346 is mostly comparable to the 77E's real life fuel burn, but it's really the maintenance costs on the engines that have caused it's early demise.

The other thing is that the stated fuel numbers for planes are blended averages of the behind the scenes game data, which applies takeoff, climb and cruise capabilities and related fuel burn. So, for example, two different planes with the "same" fuel burn in a game could end up with vastly different real fuel burn when they are used on the same route.

[JumboShrimp]

Based on the data published by Rolls Royce, the fuel burn published on the A340-600HGW model, the engines of an A340-600 is one fourth what Rolls Royce says the total fuel burn should be for cruise. The fuel burn of a GE90-92B equipped 777-200ER is little less than half the actual burn of the actual aircraft in cruise.

When you say 346 is 1/4 of something, 772 is 1/2 of something, are you sure you are not comparing engine data vs. aircraft data (which mas either 2 or 4 engines)

[CarlBagot]

The other thing is that the stated fuel numbers for planes are blended averages of the behind the scenes game data, which applies takeoff, climb and cruise capabilities and related fuel burn. So, for example, two different planes with the "same" fuel burn in a game could end up with vastly different real fuel burn when they are used on the same route.

I really find that part of this game annoying, in another game you can compare aircraft types on a reference route and see the breakdown of climb/cruise/landing fuel usage to see which type would best fit your most common routes while here it is heavily abstracted as a KG/H figure  is hard to compare especially for different engine profiles (3 spool's clime advantage as an example) or for different speeds, (the A330/340 is  at M 0.82 while the 777 is at M 0.84 thus would have less time per flight and thus lower fuel burn than otherwise the numbers would indicate.

[schro]

I really find that part of this game annoying, in another game you can compare aircraft types on a reference route and see the breakdown of climb/cruise/landing fuel usage to see which type would best fit your most common routes while here it is heavily abstracted as a KG/H figure  is hard to compare especially for different engine profiles (3 spool's clime advantage as an example) or for different speeds, (the A330/340 is  at M 0.82 while the 777 is at M 0.84 thus would have less time per flight and thus lower fuel burn than otherwise the numbers would indicate.

It's all fairly logical though - once you get used to the numbers then everything tends to make sense.

For example, the fact that overall fuel burn makes more of a difference in fuel usage for longer flights than it does shorter flights. For those playing along at home, this means that the total fuel burn for a 727 vs an A320 for a 200nm flight will be quite comparable, even though with a 2000nm flight, the 727 will use twice as much as the A320.

[Sami]

while here it is heavily abstracted as a KG/H figure

The fuel burn figures in AWS are not these dumbed down kg/h figures in the database. The fuel usage calculation is probably one of the more complex and detailed systems here in the background.

The a/c performance database has fuel data (and other performance figures like speeds and climb/descent rates) that are divided into seven segments per flight (three climb segments, cruise, and three descent segments); each with individual & accurate data (but naturally the high data accuracy applies only to modern aircraft models, for older models such data is often derived from other comparable planes). Using this data the system can produce very realistic actual consumption figures for each sector. The individual model, engine, weight (etc) variations are then added on top of this basic model data (= there is one set of perf figures per fleet group and model variants are then adjusted with factors from the baseline values)

But for easier usage the system also calculates a single figure for each a/c type on the fuel usage. The figures are directly comparable for the same sized planes (i.e. A320 vs B737) since the system calculates the average fuel burn figure based on a typical mission profile for such a/c size class (for B737 that could be 3 hour sector for example).

The actual calculated fuel burn per each sector is not displayed to the players since I've felt that it's not necessary to be displayed. For planning purposes it could be displayed, if I ever build a sort of "planner" page where you could see all runway/payload restrictions, flight times etc.

[Luperco]

The actual calculated fuel burn per each sector is not displayed to the players since I've felt that it's not necessary to be displayed. For planning purposes it could be displayed, if I ever build a sort of "planner" page where you could see all runway/payload restrictions, flight times etc.

A planner like that would be a fantastic feature in this game. Not necessarily with fuel consumption for each segment.

I suggest to put the estimation of the costs in the route edit page. When creating a route, after the choose of the aircraft, the data about the estimate costs (and mayne revenue)   of the trip will be shown somewhere in the page.

[brasidas2000]

I'm having a hard time understanding the exact fuel burn issue that you're describing. Can you point to the fuel burn reference documents that you're talking about so I can see for myself?

The other thing to keep  in mind is that  the plane acquisition costs in game are dynamic based upon the demand of the type, so that will vary from game to game depending on what sells well.

The A346 is mostly comparable to the 77E's real life fuel burn, but it's really the maintenance costs on the engines that have caused it's early demise.

The other thing is that the stated fuel numbers for planes are blended averages of the behind the scenes game data, which applies takeoff, climb and cruise capabilities and related fuel burn. So, for example, two different planes with the "same" fuel burn in a game could end up with vastly different real fuel burn when they are used on the same route.

I'm sorry but I don't think anything you said is accurate.

The fuel burn rate between a 777-200 and an A340-500 isn't comparable. If it were, the A340 would still be around.

There are many sources of data available on the net. Even with that, all commercial jet turbines are optimized for cruise performance in RVSM airspace. If your cruise numbers are worse than a competitor's, you are not going to survive long in the market place regardless of your burn in taxi/take-off/climb/descent/landing.

However, I will reference a pprune source, as this information seems to concur with most publicly available data.

http://www.pprune.org/archive/index.php/t-220792.html

Specifically:

"Hard figures for the A340-500HGW and 777-200LR are as follows (the figures for the A346 and 773 are comparable):

Airbus A340-500HGW
engines: Rolls-Royce Trent 556
engine's specific fuel consumption: 0.568 lb. of fuel per lb. thrust per hour
engine's maximum thrust: 56,250
engine's cruise thrust: 11,000 lb.
typical seats: 313

Boeing 777-200LR
engines: General Electric GE90-110B1L
engine's specific fuel consumption: 0.530 lb. of fuel per lb. thrust per hour
engine's maximum thrust: 110,000 lb.
engine's cruise thrust: 19,000 lb.
typical seats: 301

As you can see, the GE90 has about 7% lower SFC (0.530 v 0.568) compared to the Trent 500. This would give the 777 a small advantage alone (the A340 has only 4% more seats than the 777), but the A340's problems are compounded by its higher total thrust requirements, due to its higher weight. Why is it heavier? Primarily because both the wing and the fuselage were pressed into service outside the limits of their original design. The fuselage for example is narrower than that of the 777, so it has to be longer; this increases weight in the same way that bicycle frames with narrow tubes are heavier than those with "fat" tubes (i.e. the walls have to be disproportionally thicker in the narrow tube for equal strength), and also because the longer fuselage must be reinforced to counter bending moments.

The wing's cross section and sweep were likewise optimised for a lighter plane travelling at slower speeds. So the A340 has higher SFC, lower speed (therefore more time in the cruise burning fuel), higher weight resulting in a higher total thrust requirement, and only slightly more seats to counteract these disadvantages. It is no mystery why Boeing sold ten times more 777s than Airbus did A340s last year. The only question that remains is whether Airbus made the right decision in massively reducing development and manufacturing costs by using an existing fuselage cross section. With hindsight it increasingly looks like that was a mistake (because of sharply rising oil prices), and that A340 sales will slow to an inevitable halt in the near future."

For the record, my numbers of SFC for an A340-500 on a per hour basis is 24992 lbs.

Consequently, the numbers I have on a 777-200LR are 20140 lbs per hour at cruise.

The SFC fuel burn ratio of an A340-500HGW/777-200LR at cruise would be ~ 1.241.

Secondly, the A340 has a slower cruise speed (0.82 vs 0.84 approx). This means it will burn more fuel longer than a 777-200LR on the same flight. As an example, for a 10 Hour cruise segment, the A340 would spend a little over 2 percent longer in cruise, each way. On a single leg, this translates to slightly more than 12 minutes longer in cruise, which is an additional 5000 lbs of fuel. Even if one has a lower aircraft gross weight at the end of cruise, and the SFC is something more like 0.5 rather than the average 0.568, this is still wasted fuel compared to the 777-200LR with it's higher average cruise speed.

The maintenance costs were also prohibitive. This is due to Rolls using a three spool engine design rather than a two spool engine design. http://www.pprune.org/tech-log/444991-american-twins-brit-triple-spool-engines-2.html

Even when Airbus and Rolls offered a deal to all carriers to help defray the costs of maintenance on RR engines and used upgrade incentives to increase fuel efficiency competitiveness like they did on Iberia's 757 fleet, it was not enough to save the A340 production line from being shut down as the 777 economics in operational efficiency and maintenance were superior on most levels. GE engines do tend to have a shorter overhaul interval, but if the engines are leased and the refit is planned, a switch-out and RTS isn't especially problematic, except the additional leasing of engine spares which all airlines do anyway.

Anyway, as I was saying in my primary point, the operational data is not reflected in the game as I originally asserted and simply means the economics of the "airline management" aspect are not being modeled to preserve the key aspects of aircraft operations and relative efficiencies which drives new aircraft development and determines just which one is better in the market place. This means during highly volatile economic disruptions like exorbitant fuel prices, there is no penalty for clinging to an inefficient platform and there is no reward for choosing a more efficient platform. This simply means the abstraction of the game from aircraft factual data is so extreme, it makes it less a game based on real world data, but instead based on whoever determined the abstracted values. If one looks at the average fuel burn values assigned to each type in this game, they will see that they are wildly inaccurate even when converted to metric.

Also as I stated before, there are other certain aircraft performance factors that are grossly inaccurate like Cseries CS100: Required runway at MTOW, range with full passenger load. It makes me wonder why even reference actual aircraft. The 777 was specifically designed to replace four engine commercial aircraft using ETOPS but it is made uncompetitive on a straight cost basis, on an operational basis, and I can't see the refit basis being all the impactful unless the difference is greater than 50%. So why use real world aircraft designations at all? I could just go play Airport Tycoon instead.

[Sami]

, it makes it less a game based on real world data, but instead based on whoever determined the abstracted values. If one looks at the average fuel burn values assigned to each type in this game, they will see that they are wildly inaccurate even when converted to metric..

Yep, if you see what I posted before, all data is based on reliable sources and proper data and not made up. So the "just go play Airport Tycoon" is rather insulting in my mind, since that means one has no idea and appreciation on how much detailed data is on the background on all these simulations.

If you have some proper reliable data (not forum posts..), and spot a clear error, then naturally the error should (and will) be corrected. But in such case please make a bug report post so it is properly tracked and made sure it is not forgotten (etc.).

[schro]

I'm sorry but I don't think anything you said is accurate.

<snip>

You can think anything you'd like to think. Your analysis is missing the point of what myself and several posters pointed out, in that the in-game presented fuel burn numbers are an average based upon a number of things that none of us except for the game administration actually know. This will become evident if you go in game and place an A340 and a B777 on the same route and compare their fuel cost that's calculated once the route actually flies (as we're not given total fuel burn for a flight). I suspect the difference will be more apparent at that point. For example, if the 777's average was calculated with an average 8 hour mission but the A340 was calculated with an average 12 hour mission, that would bring the averages closer together than they would be for a comparable stage length (due to takeoff/climb out being a larger proportion of the average for the 777).

Additionally, the pprune data that you're referencing is based upon Boeing's marketing sheets and not real-world 77L flight data. Looking back in the archives, that specific data was slapped around with a trout here - https://www.airwaysim.com/forum/index.php/topic,46666.msg257690.html#msg257690 . It is also fairly poor form to bring up differences between the 77E and A346 and then to provide data from the 77L and A345 as a way of making your original 77E vs A346 argument.

Regardless, I don't know all the answers. I play the game and enjoy it without nit picking the details. Generally things are close enough for horseshoes and hand grenades, so that works for me.

[NovemberCharlie]

Also bear in mind the 777 has in reality a superb payload range performance, cargo capacity and economics.
I am convinced the case for the 777 will improve dramatically once cargo gets implemented.