Triggering fuel emergency , tank percentage reduction

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alan CXA651
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Joined: 15 Mar 2016, 08:23

Triggering fuel emergency , tank percentage reduction

Post by alan CXA651 »

Hi Scot and team.
I did not see on the list of corrections in the latest connie update , about the 10% fuel in any tank being reduced as was discussed some where in the connie thread as at 10% in tanks , with careful management allows you to fly for almost 2 hrs before they run dry.
regards alan. 8)
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P*Funk
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Joined: 18 May 2017, 06:34

Re: Triggering fuel emergency , tank percentage reduction

Post by P*Funk »

One thing is for sure, the percentage should be different for each tank to ensure it accounts for how some of the tanks are smaller than others. Percentage of one tank is therefore a misleading measure of the total remaining.

In fact I don't even understand why a single tank reaching below 10% is an emergency when you can easily cross feed fuel. Shouldn't an emergency be 10% of total fuel?

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CAPFlyer
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Re: Triggering fuel emergency , tank percentage reduction

Post by CAPFlyer »

There's been no update since Scott discussed changing the logic, but the change also discussed was also more to change the alert to only be a "the crew knows" thing and not an "everybody knows" thing, so you wouldn't get the same ding.

As for the amount, 10% of the biggest tanks only gives you about 45 minutes of normal cruise, so it's right on the margin for being a "fuel emergency". I don't believe fuel leaks are simulated within Accusim directly (I will gladly stand corrected), but as you can crossfeed, the point is you shouldn't get any tank below 10% by itself, the others should be pretty close as well. The reason for it being "first tank" triggering the issue is simply because you don't want it to be "all tanks" since then people will drain the 3 other tanks and leave 1 at 10% just to keep from getting gigged, but putting the airplane in a dangerous configuration in the meantime.

Also, 10% of total fuel is ~20 minutes of flying at normal cruise. If it was to be "total fuel", then the amount would be closer to 20%.
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sloppysmusic
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Re: Triggering fuel emergency , tank percentage reduction

Post by sloppysmusic »

The 10% seems fair to me. Begs me to ask 2 related questions though:

1) Does career punish you for carrying too MUCH fuel? ie not being efficient and costing the company money re slower/heavier/higher burn costs (not related to good power management during flight)?

2) A 'random' emergency (not triggered by one's own incompetence) usually means diverting an overweight plane to a nearby strip. Obviously the safe thing to do would be a fuel dump before descent. I see the dump 'stops' at a very low limit of 30 gals in the outboard tanks and 70 inboard (Somewhat below 10%..). (Manual states "any emergency dumping" leaves you this low)
a) Wouldn't a fuel dump be required at cruise altitude before descent legally, meaning this dangerously low value would almost guarantee an inevitable glider-like landing?
b) After forcing you to declare an emergency (eg throwing a sick passenger at you) would career punish you for dumping fuel below the 10% value in any tanks?

Thanks and apologize in advance if I have missed something blatantly obvious so far.

flapman
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Re: Triggering fuel emergency , tank percentage reduction

Post by flapman »

CAPFlyer wrote:As for the amount, 10% of the biggest tanks only gives you about 45 minutes of normal cruise
CAPFlyer wrote:Also, 10% of total fuel is ~20 minutes of flying at normal cruise
Hello again CAPFlyer,

I disagree that these numbers are appropriate for the L-49. I don't think you're considering how large these fuel tanks are. The fuel and payload manager (and the aircraft.cfg) show tanks 1 & 4 have 1550 gals, and tanks 2 & 3 have 820 gals. For a power setting, lets use FL220 off the chart (2160 RPM, 120 BMEP) and 353 total gal/hr.

10% x 1550gal = 155gal / 353gal/hr = .439 hrs.... or 26 minutes of gas.

But remember, this is all four engines burning off a single large fuel tank, which only contains 10% of it's fuel capacity. All other fuel tanks are at 0%. This is bone dry.

For users that are having this problem (I'm not, I landed with wwwwwayyyyyy too much gas :lol: ) I can describe the scenario which is causing a problem. Proper fuel system operation would have the quantity (in gallons, not percent) balanced early in the flight, with the remainder of the time spent on "tank-to-engine." Thus, when the "first tank" reaches 10% fuel remaining, all other tanks should have almost the same quantity of fuel. 10% of 1550 gal is 155 gal, and 10% of 820 gal is 82 gal, thus we know the larger tanks will reach 10% first.

When the 10% low fuel warning is issued, a properly managed aircraft will have 155 x 4 = 620 gallons on board (tanks 1 &4 at 10%, tanks 2 & 3 at 18.9%)

620gal / 353gal/hr = 1.756 hrs or 1:45 of fuel remaining until exhaustion.

If we go off your assertion that, "10% of total fuel is ~20 minutes of flying at normal cruise," then the math works as follows:

4740gal x 10% = 474gal / 353gal/hr = 1.34 hrs or 1:20 of fuel remaining. It's an improvement, but not really.

Current U.S. airline regulations use the standard "fly to most distant alternate, then 45 minutes" IFR reserve requirements for all domestic operations. Including propliners. Of course we live in a different regulatory environment than the late 1940's when our Connie entered service, but it's a good standard to work with.. and probably close to the margins they used anyways.

If I use the :45 standard, here are the fuel tank percentages which would equal standard domestic IFR fuel regulations in the Constellation.

4.27% fuel in tanks 1 & 4, which would be 8.07% in tanks 2 & 3. This leaves a little over 66 gal in each tank.

And really, 66 gallons isn't an emergency situation. If you're over the alternate with this little amount of gas on board, you still have the full IFR reserve. In the airline world, we generally don't declare an emergency until we reach :30 fuel endurance. Expecting to land with less than :45 justifies a "minimum fuel" advisory at most.

If we go off the :30 standard.

2.85% fuel in tanks 1 & 4, which would be 5.38% in tanks 2 & 3. This leaves a little over 44 gal in each tank.
And this is using 353 total gallons burned per hour, almost the highest charted fuel burn.

P*Funk
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Re: Triggering fuel emergency , tank percentage reduction

Post by P*Funk »

That's some solid math and I definitely wish that there'd be an update to make it more in line with that than the rather excessively conservative emergency with 13% fuel remaining that we currently face if we actually operate the aircraft by the standards in the manual.

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