I just noticed fuel burn on the Longitude is LOWER when ice control systems on. Is this a bug? I would assume that this would burn more or the same fuel but reduce thrust as some gas is sent to the engines to heat systems. Maybe a bug where power is reduced correctly is causing fuel burn to incorrectly also be decreased?
Video showing bug: https://www.youtube.com/watch?v=CY2rIbEJNsE
I don’t see a bug here.
As soon as you turned on engine anti ice, N1 decreased from 85.9% to 81.5%, hence the lower fuel flow.
Depends on how the anti ice is working on the Longi.
If it’s always on and feed by bleed or electrics, there should be an increase of fuel burn over a long period.
If it’s just activated and on standby until ice is detected it won’t be noticeable at the first glance.
Decrease of engine thrust could be a feature of the anti ice system to reduce stress to the engine when in icing condition.
I don’t know any airplane where anti ice is continuous on. Doesn’t make sense since it does noticeable decrease performance.
The decrease in N1/thrust is simply due to the increased bleed air demand. Has nothing to do with stress reduction.
Well, then antiice is on if bleed is taken from the engine and reducing N1.
In that case, there should be an increased fuel flow compared to to the lower N1, or lets say the FF should be the same like before, but you have less N1 and therefore less thrust with the given FF.
To get the previous thrustlevel, you need to add power to the engine → increased fuel flow.
Longi should have A/T, so normally A/T should increase thrust if bleed will take some power.
From what I have been able to find…
This airplane uses a pneumatic de-icing systems. Where bleed air from the engines are sent to heat components. Based on what I am able to find about 5% of thrust when engaged.
Fuel flow should increase to compensate for the lost thrust to maintain speed and climb to compensate for the loss in thrust. In addition even if this compensation is NOT done the fuel flow would just stay the same and the thrust would be decreased by 5% due to the thrust gas being diverted to heat the wings and such.
So this is a flaw in the simulation from what I am able to see.
Joe
To confirm if this is indeed a bug, you would need to do the test again, but increase thrust to the initial setting of 85.9% with anti ice on and check the fuel flow (and IAS).
Can’t imagine that you experience a 5% N1 decrease with an identical fuel flow. Not even with the tiny Longitude engines.
This isn’t the case with e.g. the A320 and the 767 IRL.
@bitstop @Sunday748
Just checked the Learjet 60 AFM and as expected it looks like anti ice doesn’t cause an increased fuel flow.
The performance tables only mention lower thrust limits and a 0.1% lower en route climb gradient with anti ice on.
IF there would be an increased fuel flow due to the use of anti ice, the performance tables would show that, either with additional tables or with a note how much the fuel flow increases.
I did some more testing you can see here.
YouTube at this moment is still making the HD version available.
But it is a bug in the simulation… And you can’t manually increase the throttle the 5%. It jumps to max when you manually try to increase it. I even set my speed to be M0.7 thinking that would give me room to adjust. I all so turned off weather just to ensure no additional variables.
If you go to max thrust and the you turn on ice protection the system isn’t reducing n1 or changing FF at all.
Apparently a very inconsistent behaviour and partially indeed a bug.
I never understood why Asobo with zero experience in flight sims opted to simulate very complex planes like the TBM and the Longitude. They even failed with simulating the pretty basic DA-20!
Asobo is not doing the modeling anymore. Microsoft has someone else doing that now. It was the group that did the update GPS systems.
Maybe @Bishop398 from WT can comment on this.
Might also be better to move this to the “bugs” section so it gets visibility there.
The behavior here is almost entirely accurate and fairly thoroughly simulated.
You can see in the attached videos that the engines spool up for a second when turning A/I on; in the Longitude the system spools the engines up a bit before opening the bleed valves to the A/I system in order to counteract the impending N1 drop due to bleed air load. So the resulting overall drop is usually very small or unnoticeable. We do simulate all of that, the engine spool time, the bleed air valve delay, the N1 drop (from the increased spool up), and the slight thrust efficiency loss.
We don’t have hard numbers for the FF change but our understanding is that it’s basically trivial on the C700. The reason for this is that the compressor is running the same speed with the same mixture ratio, just that some HP air is diverted and so resulting thrust output is lower.
What does seem potentially a bug here is if the A/T is not compensating, but given where the video shows the throttle it may have already been at the N1 limit for that FADEC mode. The “jump” there seen in the mode N1s is correct as those are the detents. The range is not continuous.
Something we can double-check, although the OAT shown here is far too low to be needing A/I. You wouldn’t be running it at cruise unless you were at a much much lower cruise altitude.
EDIT: Forgot to add, for the initial video posted, that is also not a bug. The FADEC N1 limits with A/I on are different than with them off, so the large drop being seen in those conditions is just due to the lower N1 limit at the current alt/temp.
I have a friend who flies a business jet for his job. He said in no situation has he seen FF decrease when turning on ice protection. He said it normally would increase as the AP adjusted for the loss in thrust. But he said if you assumed that they didn’t model the AP adjustment the FF would just stay the same and you would see power loss.
@Bishop398 thanx for the detailed explanations, but this doesn’t match @bitstop s observation: If you go to max thrust and the you turn on ice protection the system isn’t reducing n1 or changing FF at all.
@bitstop To avoid any condusion, automatic thrust control/adjustment is an autothrottle/autothrust, not a AP function.
The bizjets your friend are flying are apparently working the same way like most other jets and e.g. the A320 and the 767.
But without knowing which jets your friend flies, his info doesn’t really help.
It does match the observations. As I said, the C700 will first spool the engine up (add N1) before extra bleed load is introduced. Then when bleed load is introduced, the drop from this spooled up state is basically back to where the N1 started. Since the N1 is the same as before the process started, the FF is the same, and the diverted bleed air only results in a slight thrust reduction.
This is all simulated.
I see, very impressive 
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