Both the 747 and the 320 are very fuel hungry. Flights longer than 10 hours in the 747 don't seem possible

Tonight I did my best to follow a real world 747-8 flight (CLX7606 / LX-VCC) from Luxembourg to Rio de Janeiro (ELLX-SBGL).

I loaded 90% fuel (all fuel tanks full, except the stabilizer tank and the reserve tanks, which are unusable at the moment, due to bugs), and I only loaded enough cargo to balance the center of gravity properly, for all phases of flight. The fuel and payload page looked like this:

That fuel should have been more than enough to reach the destination, since the 747 has a 16 hour autonomy, and even with 90% fuel it should fly over 14 hours, especially with no passengers and limited cargo. This flight was 10.5 hours long, and the map confirmed that:

So basically I should have had about 4 hours of extra fuel. In theory.

I departed 30 minutes after the real world flight, directly from the runway, in order not to waste additional fuel and time by taxiing. I replicated all the maneuvers of the real flight, I did all the altitude changes they did, and I kept the same Mach speed (.84 to .85). I began the flight climbing to FL290, 20 minutes later I climbed to FL310, 2 hours later to FL320, 6 hours later to FL340, and 30 minutes later to FL360.

I was plagued by the usual autopilot bug, that prevents it from being able to keep the altitude and speed, and makes the aircraft oscillate up and down like a roller coaster. I fought that by using the FLCH mode every time the altitude oscillations got to an amplitude larger than +/- 300 feet over/under the set altitude, which were triggering ATC to warn me about not keeping my altitude. But I doubt that autopilot bug is enough to explain the huge fuel usage.

In the end, as the fuel remaining reached about 7%, I decided to divert to Salvador. But even that was not close enough. I had to re-divert to the Arcaju airport. While doing that, on LNAV, the autopilot did a 360 turn, for no reason. When it attempted a second 360 turn, I enabled heading select towards the runway.

It was too late. Because of that 360 turn the remaining fumes in the tanks have been exhausted, the engines flamed out, and I crashed in the ocean, just 4NM from the runway. The correct decision would have probably been to divert to Recife. And if I was a real life pilot I would have probably been fired, if I survived, for not diverting much earlier.

At the time of the flameout I have flown for 9 hours and 8 minutes. I have flown just 4300NM, and the 747 has in theory an autonomy of 8000NM. Even with 90% fuel, due to the 3 unusable fuel tanks, it should easily reach at least 7000NM, especially with no passengers and limited cargo. It barely reaches 60% of that.

At this point I fail to see how you can do a flight lasting more than 10 hours in this aircraft, without cheating and adding more fuel during the flight.

Not sure exactly what caused this high fuel usage. I noticed the calculated gross weight doesn’t decrease during the flight, maybe the simulator does the performance calculations based on that, which means the fuel usage doesn’t decrease as it should, when the weight of the fuel carried decreases.

And this is just one of my many attempts to fly the 747 properly. I consistently experienced this high fuel usage issue on each of my attempts.

The gross weight calculation issues also seem to affect the aircraft pitch attitude. As you burn more fuel in the 747, you should be able to climb higher, which is desirable as the jet engines are more efficient at high altitudes, where the air is colder. Instead the aircraft pitches up too much, up to 10 degrees, if you attempt to reach high altitudes, like FL430. Even when you used most of the fuel.

So most likely that’s the mechanism that causes the high fuel usage: the calculated gross weight doesn’t decrease as fuel is used, the aircraft pitches up more than it would if the gross weight was calculated correctly, increasing drag, which reduces efficiency, increasing the fuel usage. Either that, or they didn’t model correctly the increased efficiency of the engines at low air temperatures. Or both.

And the problem doesn’t seem to be 747 specific, the A320 has a similar behavior. The previous weekend I attempted a flight in the A320 from Seattle to Ireland (KSEA to EINN). That’s a 3900NM flight, so a bit over the 3500NM autonomy of the A320. But with no cargo and passengers on board, and full fuel tanks, I should have been able to do that easily.

From what I heard from a Ryanair 737 pilot, that’s usually how they fly the newly delivered 737s from Seattle to Europe: empty, with a refueling stop in Ireland. So I tried to replicated that in the A320, which has similar autonomy to the 737.

I started from the gates at KSEA, departed, climbed directly to FL390, which probably is a bit too high for an aircraft full of fuel, but I was too lazy to do step climbing that particular day, especially since I’m not very familiar with the A320. But it seemed to have no trouble reaching that altitude. I cruised at Mach .78, the normal cruise speed.

The autopilot in the A320 was much better than the one in the 747, it had no trouble keeping the altitude and speed, and followed LNAV properly.

Long story short, I wasn’t even close to being able to reach Ireland. I considered diverting to Iceland, but even that seemed risky, although the total distance would have been just 3200NM, so well within the 3500NM theoretical range of the A320, especially with no passengers and cargo.

In the end, to be on the safe side, I decided to divert to Greenland / BGSF, so just 2500NM total flight distance. About 70% of the 3500NM autonomy. I landed there with just 4% fuel remaining, after a 6 hour flight, so it was a good decision. I would have never been able to reach Iceland. But normally the aircraft should be able to fly for at least 7 hours, especially with no passengers and cargo, so clearly the fuel usage was way too high.

So the fuel issue of the 747 seems to affect the A320 as well, and probably for the same reason: the calculated gross weight didn’t decrease at all during the flight, which was probably the cause of the high fuel usage.

After fully refueling in Greenland at BGSF, I departed for Bucharest, Romania, LROP, with the same empty aircraft, no passengers or cargo, a planned 6 hours 2700NM flight, well within the A320 range of 3500NM, especially with no passengers and cargo.

I may have wasted a bit of fuel by starting my descent about 150NM too early, but other than that, it was a flight similar to the previous leg, and I should have had plenty of fuel upon my arrival at Bucharest. Instead, this is how the fuel looked after I landed at Bucharest:

image

Only 106 KG of fuel left, so about 0.6%. About the fuel capacity of two small cars. Yes, if I was a real pilot I should have probably been fired for not diverting to refuel somewhere along the route, but I wanted to do a thorough test of the capabilities of the MSFS implementation of the A320.

For me right now the high fuel usage is the most frustrating part about flying the airliners. Especially in the 747, that has 3 unusable fuel tanks, as a bonus. The autopilot issues in the 747 are a close second, in my top issues list. The low FPS when start with long flight plans, especially in the cockpits, would be the third, but it’s something I could live with.

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Yeah the fuel is off on both. There are a couple of quick fix mods that correct it to more real world though. I got them off nexus mods.

Look at the 3rd page.

https://www.nexusmods.com/microsoftflightsimulator/mods/

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Thanks for trying to help me, but for the time being I’ll wait for an official patch.

Looking at the A320 mod, it seems to just reduce the fuel flow needed for certain levels of thrust, and I’m not convinced that’s the correct way to fix this. And the 747 mod doesn’t do anything, the config file is identical to the original one.

And if the real problem is, as I am guessing, that the gross weight that doesn’t decrease during the flight, changing these configs would just increase the efficiency of the engines uniformly, over the entire duration of the flight, but they would do nothing to model the increase in the efficiency of the aircraft when it becomes lighter, as the fuel is burned.

So with those mods you would probably end up with much better efficiency than in a real aircraft at the beginning of the flight, when the gross weight has the correct value, to compensate for the lower efficiency towards the end of the flight when the gross weight incorrectly remains the same as at the beginning of the flight.

So the efficiency of the aircraft would still be linear, instead of constantly increasing during the flight, as it should. And attempting to fly at higher altitudes, climbing in steps as you use fuel, would still reduce efficiency, instead of increasing it.

I mean, if you are desperate to fly longer, the A320 mod is a possible solution. But the problems seem more fundamental than just a few incorrect parameters. And if I was that desperate, I would have just cheated by loading more fuel during the flight.

Later edit: I also checked the idle fuel flow on a real A320 neo, from this YouTube video:

On startup, once the engine stabilized, the idle fuel flow was 280 kg/hour. Converting that to pounds, it would be about 620 pounds/hour. The value in the original configuration file is 600, so very close to the real value. But the A320 mod adjusts that to 364 pounds/hour, which is incorrect.

Then I checked the performance specifications for the LEAP-1A engine: 15–16 g/kN/s fuel usage, and 120.6 kN max thrust per engine on the A320 neo. That would mean up to 6730 kg / hour, at max thrust. The original value in the config was 10,000 pounds/hour, so about 4500 kg / hour, so in theory that value might actually need to be increased, not decreased. The configuration file from the mod changes that to 6323 pounds / hour, which would be about 2900 kg / hour. Less than half of the real value.

So, as I guessed, the root cause of the problems is probably deeper than just a few configuration files, probably a combination of bugs and inadequate modelling. And I would not be entirely surprised if the A320 neo from MSFS was not made from scratch, but is actually based an older Airbus. And that’s probably what they did for the 747-8I. They seem to have started from a 747-400, and converted it to an 8I. I mean when you load the fuel before the flight the tanks are still labeled incorrectly, like in the 400: reserve 2 and 3, instead of reserve 1 and 4.

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You are correct, all airliners need to be finished/optimized. The author of that fix did say it is not real worl values but it will give you the range of the normal aircraft. He did say it was an ugly fix. I tested the A320 using a fuel planner est from Phx to Den. Flight+reserve est about 15,900lbs I loaded 16,400. When I landed I had 5% fuel with about 1.5 hrs flt time so it is pretty good workaround at least in simulator terms.

thanks!! needed that for A320 and Cj4. please make the post recommended to everyone!! :grinning:

I posted the mod here if you want to check the difference value change just in case, also wingflex
“Fuel Consumption & Wing Flex Improvment”

Thanks. I will look into it next weekend but, again, I’m not very optimistic about good fix coming from changing those parameters.

I did more testing on Sunday in the A320 with the default settings, which further confirmed my hypothesis that the bug that keeps the displayed gross weight frozen for the entire flight is the most likely cause of the high fuel usage.

I did a 5 hour flight starting with full fuel, no cargo or passengers, weather set to the clear skies default, cruising at FL390 at a constant Mach .78.

The fuel flow remained more or less constant for the entire duration, oscillating around 3830 kg/hour. The pitch attitude also remained constant for the entire duration of the flight, about 5 degrees nose up, instead of decreasing as the fuel was being used. This is clearly not correct, both the pitch attitude and the fuel flow should have decreased while crusing.

After those 5 hours, with the aircraft having just about 15% fuel remaining, I descended from FL390 to FL290. At FL390 and Mach .78 the ground speed and total air speed were around 450 knots. After descending to FL290 I manually adjusted the airspeed until the ground speed and total air speed were again around 450 knots.

Normally, at that stage of the flight, with the aircraft very light after using most of the fuel, I would have expected an increase in fuel flow, at the same TAS, after descending from FL390 to FL290. Instead the opposite happened. The fuel flow actually decreased, by 15% from 3830 kg/hour to 3270 kg/hour after descending lower, while keeping the same 450 knots TAS.

This proves that, since the gross weight doesn’t decrease for the entire flight, you have to fly the entire flight at an altitude that is optimal for flying with full fuel tanks, even when the tanks are almost empty. So that’s the first bug.

Another bug, I started the flight with about 21,200 kg of fuel. I landed after 6 hours with 250 kg remaining in the tanks. And in the fuel used statistics it was showing I used 16,884 kg of fuel. 250 + 16,884 = 17,134 kg of fuel total. So what happened with the remaining 4000 kg of fuel? Did it leak out of the tanks? :slight_smile:

To summarize:

  • about 15% extra fuel is burned because of the the frozen gross weight bug, maybe more.
  • another 19% fuel is lost somewhere, almost like the aircraft has a hidden fuel leak.

Those two bugs alone would add up to a 33% lower autonomy of the aircraft. The aircraft should have a 3500NM autonomy, with full fuel tanks. 66% of that is 2300NM. No wonder I wasn’t able to fly more than 2700NM, even with an empty aircraft, with no passengers and cargo. No other bugs are needed to explain that.

Still, I plan to do some further testing, to see how the fuel flow varies with the gross weight and altitude, for various initial fuel loads (loading a lower amount of fuel before the flight freezes the gross weight at a lower value for the duration of that flight).

But I will have to wait for the next weekend to do more testing, as I have a real job too in addition to this unpaid MSFS beta tester job.

Thanks, you might sent your reports to Zendesk, to be sure they had the ticket.

Yes, I plan to do so tomorrow morning.

The engine cfg for the 747 has the profile as way too hungry. Thrust at sea level is about 66k instead of the 26k listed, specific fuel consumption should be about .33 instead of .5 and inlet is about 77 instead of the really low number they have. These are interpolations of the real data, though I do not claim they are completely accurate. The 747 cfg they have now is …ummm strikingly similar to the A- 320 engines. Hope this helps. Happy flying!

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Yes, that was my guess as well, that the NEO is just a CEO copy/paste, with a few changes. But still, even the 320 CEO has a 3300NM autonomy, compared to the 3500NM of the NEO. That’s only a 5% difference, not the huge difference I’m experiencing during flights. Even if they just took the A320 CEO and put a NEO label on it, it shouldn’t be this bad.

As I was saying two days ago, one bug is that at the end of the flight the fuel used + fuel remaining doesn’t match the fuel loaded at the start of the flight. About 4000kg of fuel are just missing at the end of the flight. The question is, which of the two values is incorrect: fuel used, or fuel remaining.

If the fuel used is incorrect, then the autonomy wouldn’t be affected by the bug. If the fuel remaining is incorrect, obviously the autonomy would be affected. The good thing is I made a 6 hour video of the entire flight, exactly for this purpose, to analyze in detail what happened during the flight. And these are the statistics for the cruise segment:

14:16:32 - 19458kg available + 1456kg used = 20914kg total
18:30:18 - 3362kg available + 14380kg used = 17742kg total

So 4.23 hours of flying, 16096kg used according to the available statistics, 12924kg used according to the used statistics. The indicated fuel flow was around 3830kg/hour, which would be about 16200kg over 4.23 hours.

This means that the available fuel information is probably correct, and the used fuel statistics are incorrect. So it seems that the second bug, that looked like a fuel leak, is just cosmetic, it’s not affecting the real fuel usage.

I realize I should have specified in my post I was referring to the 747. However, though I have never flown the A-320, I suspect you are experiencing some of the same issues. I think your analysis of the the missing fuel causes me to scratch my head too! I guess it’s like when they make whiskey, there is always an amount that seeps into the barrel, right? :wink: Seriously, in RL the FAA would not have yet issued airworthiness certificates for either of these aircraft, but we keep whacking away at it and sharing info. There is already something good in this sim, but it is more dependent on the community to get it right than it should be. I just hope the new folks don’t get too discouraged.

I have heard that there is a group of simmers who are going to work on the A-320 and publish their results free of charge. I don’t have any info. But if you are not a part of that group yet, it might be a good idea. And keep hammering away at Zendesk. Happy flying!

Could the gross weight staying constant despite fuel burn also trigger the high approach angles of attack?

Yes, that would make sense.

Just wanna add to it. I’m showing 50kg/min on the A320. This is at FL360 (you can guess my winds/temps/pressure) at TOC and about an hour before what would be my TOD.

I never made it to the TOD because I ran out of gas. Probably because it was burning a ridiculous amount of fuel and was still at MTOW 6 hours into the flight.

probably the best you can do for now is choose unlimited fuel. rubs me the wrong way, I know, but unless you are willing to play with the engine config, it’s probably the only solution. If you check out flightsim.com, there is a project for an a320 of “payware quality but free of charge.” don’t know because I don’t fly the a320, but it might be worth checking out.

Here’s my suggestion for you Engine CFG.

high_fuel_flow = 13600
fuel_flow_max = 13600

[JET_ENGINE]
thrust_scalar = 1.0

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