Below 2000lbs fuel + negative G's = flameout?
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Now before the experienced pilots in here tell me to RTFM let it be known that I did. I know that a flameout can be caused by pulling negative G’s when the fuel tanks are almost empty (TO-BMS1F-16CM-1 page 123):
Please note, as the reservoir tanks start to empty the possibility of negative Gs starving the engine of fuel increases significantly. Avoid negative G loads with less than 1000lbs. of fuel remaining.
The manual says this happens when the fuel remaining is below 1000lbs, two RSVR tanks each with 480lbs = 960lbs, so I assume they mean 1000lbs specifically contained in the RSVR tanks and not total fuel quantity right?
So to understand the fuel system a bit better I ran some tests in the TR_BMS_05_AAR aerial refueling training mission and found out that the RSVR tanks start emptying once the total fuel QTY dips below 2000lbs and immediately afterwards you can force a flameout with 100% certainty (this picture shows 1966lbs total remaining and Fuel QTY SEL shows that the RSVR tanks are both still at about 99% full).
That doesn’t make any sense to me though. Shouldn’t the RSVR tanks be the last tanks to be emptied since the diagram on page 121 shows that they’re the last tanks that lead to the engine? Also why can I cause flameouts repeatedly if the RSVR tanks dip even the slightest bit below 480lbs? Is this intended behaviour?
To see for yourself what I’m talking about:
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Start training mission TR_BMS_05_AAR for aerial refueling
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Jettison the payload
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Switch to CAT I
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Engage AB and deplete fuel until <2000lbs
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Pull negative G’s
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Instant flameout
I’m not sure if this feature was implemented with 4.34 or not but I’ve ran into three seperate occasions of this happening in the past two days of testing and never before so it’s new to me. Maybe I’m dealing with a sudden increase of realism but it would be cool if someone could explain to me what’s happening
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Engine feed norm?
Same happens when engine feed is off (4.33) imho
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F-16D model right? …
Look at Fuel knob selector. Try the same on the “Norm” position. Does it also happens?
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Yep, Engine Feed is by default on Norm when you start that mission and was so too when I caused a flameout near the end of a 2 hour campaign mission.
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F-16D model right? …
Look at Fuel knob selector. Try the same on the “Norm” position. Does it also happens?
Same thing happened on Fuel Knob Sel on Norm, just tested it again.
EDIT: Fuel Selector on Norm. Yes it’s the D-Model but it also happened to me in a F-16CM-50, although that might’ve been well below 2000lbs.
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… ok … will try to repro when time permit.
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This is because you are failing to understand the system in full and you are giving a scenario not even covered under what you referenced. That quote is talking about just normal engine running not full AB operation. Even with normal conditions the system cannot maintain full fuel flow in full AB forever. There are two ways that the engine is fed through the res tanks. One is bolted to the floor and thus negative Gs impact or remove it’s ability to feed the engine. You doing that while asking for max fuel flow of course you are going to flame out.
This situation is also not one you are going to be duplicating in real life scenarios. The goal of the BMS team is to make a simulation that behaves as close to real as they can. This is just making it more realistic to how the real deal would behave.
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Engine feed norm?
Same happens when engine feed is off (4.33) imho
Another failure to understand how the res tanks feed. If you go to engine feed off and go inverted or negative G you are removing most, if not all, of the systems ability to feed the engines. That you should be able to duplicate at any fuel load and would be the normal response to an abnormal input.
Or from BMS1F-16CM-1 Page 123 in reference to the engine feed knob.
“With the knob in OFF none of the pumps work and fuel transfer occurs only via gravity and siphoning action. During heavy manoeuvring or negative G the engine may flame out.”
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IIRC, AB depletes fuel from the reserve tanks faster than the pumps can feed it from the rest of the fuel system. Flameout can occur if the reserve tanks are not near full : with neg Gs, fuel will flow away from the feeder and flameout occurs, as you feed air instead of fuel.
Check again in your scenario, but put the fuel quantity selector switch (near the HSI) to RSVR. If the Aft and Fwd needles are not near 480, that’s your explanation
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Chirschn posted a link to screenshot on post#5:
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Well, the fuel QTY switch is on NORM and not RSVR, so I cant really tell if my theory is correct
I cant tell if those 500 fwd lbs and 1500 aft lbs are in the wings or in the reserve tanks
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Well, the fuel QTY switch is on NORM and not RSVR, so I cant really tell if my theory is correct
I cant tell if those 500 fwd lbs and 1500 aft lbs are in the wings or in the reserve tanks
Indeed
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This is because you are failing to understand the system in full and you are giving a scenario not even covered under what you referenced. That quote is talking about just normal engine running not full AB operation. Even with normal conditions the system cannot maintain full fuel flow in full AB forever. There are two ways that the engine is fed through the res tanks. One is bolted to the floor and thus negative Gs impact or remove it’s ability to feed the engine. You doing that while asking for max fuel flow of course you are going to flame out.
This situation is also not one you are going to be duplicating in real life scenarios. The goal of the BMS team is to make a simulation that behaves as close to real as they can. This is just making it more realistic to how the real deal would behave.
The AB doesn’t matter only the G’s do. I just tested it again. Fuel Flow Rate at idle 800 and I managed to cause a flameout again at just below 2000lbs. I only wrote about the AB to remove the excess fuel faster to get below the 2000 mark. I’ll do more testing tomorrow but in the meantime maybe you can answer these two questions: are the RSVR tanks supposed to be the last tanks to be drained or not? What’s the significance of the 2000 lbs mark?
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Well, the fuel QTY switch is on NORM and not RSVR, so I cant really tell if my theory is correct
I cant tell if those 500 fwd lbs and 1500 aft lbs are in the wings or in the reserve tanks
Cruz
Check my last commit , I had issues with AI and flame out though I checked that fueflowrates are correct and RSVR should never empty …
So considering the report of the OP I suggest to have a deeper look
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Well, the fuel QTY switch is on NORM and not RSVR, so I cant really tell if my theory is correct
I cant tell if those 500 fwd lbs and 1500 aft lbs are in the wings or in the reserve tanks
I’m not sure I follow? The fuel qty selector is on Norm in the picture above. The first picture I posted showed another screenshot with the RSVR tanks both at 500lbs. You can load up the AAR mission for more detail.
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Cruz
Check my last commit , I had issues with AI and flame out though I checked that fueflowrates are correct and RSVR should never empty …
So considering the report of the OP I suggest to have a deeper look
I can have a deeper look indeed
I’m not sure I follow? The fuel qty selector is on Norm in the picture above. The first picture I posted showed another screenshot with the RSVR tanks both at 500lbs. You can load up the AAR mission for more detail.
There are 2 fuel switches : the ENG FEED switc, on the left panel. And the FUEL QTY switch near the HSI that you showed on the picture above.
ENG FEED should stay in NORM to ensure fuel feeds from both the aft and fwd reserve tanks.
FUEL QTY has no impact on the fuel system, only on the fuel quantity display. I asked for you to put it on RSVR so we can check what the reserve tanks level actually is, since its the only one that matters there I didnt find your pic with it on RSVR and both needles at 480 lbs. I just tested, and I can trigger a flameout with both reserves at around 480 lbs, which should not be possible indeed So a deeper look will be had
EDIT : ok, found your pic, and I have the same.
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I can have a deeper look indeed
There are 2 fuel switches : the ENG FEED switc, on the left panel. And the FUEL QTY switch near the HSI that you showed on the picture above.
ENG FEED should stay in NORM to ensure fuel feeds from both the aft and fwd reserve tanks.
FUEL QTY has no impact on the fuel system, only on the fuel quantity display. I asked for you to put it on RSVR so we can check what the reserve tanks level actually is, since its the only one that matters there I didnt find your pic with it on RSVR and both needles at 480 lbs. I just tested, and I can trigger a flameout with both reserves at around 480 lbs, which should not be possible indeed So a deeper look will be had
EDIT : ok, found your pic, and I have the same.
Oh okay! Sorry if I caused any confusion
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So I checked. This is a peculiarity of the D model, I believe
Essentially, in normal conditions, the engine is fed equally from the FWD and AFT circuit. However, the D model has around 1000lbs less in the FWD circuit compared to the AFT circuit, because of that pesky backseat.
The result is that in a low fuel situation (like around 2000 lbs), the FWD reserve is not full anymore, when the AFT full reserve is still full and has 1000 lbs more in the main AFT tank to back it up.
As a result, in neg Gs, what happens in the code now is that the FWD tanks being not full and with neg Gs, the FWD pump ingests air (because fuel is displaced to the top of the tank) and that causes a flameout.
So the code works as intended, but to be extra sure, I’m asking a specialist to see if our modeling of the situation when only one of the 2 pumps ingests air is correct.
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The AB doesn’t matter only the G’s do. I just tested it again. Fuel Flow Rate at idle 800 and I managed to cause a flameout again at just below 2000lbs. I only wrote about the AB to remove the excess fuel faster to get below the 2000 mark.
OK just a language issue then as the way it was written sounded like you were doing this test at 2k fuel in AB which is just a little crazy. As long as the engine feed knob is in normal this shouldn’t be a thing. The two fuel pumps in the reservoir tanks will easily keep the engine running at 800 PPH regardless of inverted or negative G…
I’ll do more testing tomorrow but in the meantime maybe you can answer these two questions: are the RSVR tanks supposed to be the last tanks to be drained or not? What’s the significance of the 2000 lbs mark?
Yes the reservoir tanks are the last to empty. L3crusader covers the significance of 2k fuel is and this would be BMS only. The real deal won’t do that at your stated 800 PPH if the engine feed knob is in normal. If it is in the off position it would certainly flame out but that wouldn’t be limited to 2k fuel but at any fuel quantity remaining.