Hornet internal fuel capacity changing?

Over the last couple weeks playing I've noticed something a little odd. when spawning into a hornet I've noticed the internal fuel level fluctuates. keeping in mind that the percentage bar is always at 100% and the engines are just coming to life so it hasn't been all been burnt.

The number appears to fluctuate by about 250lbs. So is the hornet modeled to include everything from expansion at different temperatures and fuel densities? because you're blowing my mind if that's the case. And if so I'm curious to know if it is something modeled on other aircraft.

What environmental conditions would allow for the highest fuel capacity and which would be the lowest?

Thanks everyone!

So fuel capacity is measured in lbs, the weight of jet fuel changes based on atmosphere. So on different maps or with different weather conditions, a full fuel load while having the same volume, will have different weights.

So fuel capacity is measured in lbs, the weight of jet fuel changes based on atmosphere. So on different maps or with different weather conditions, a full fuel load while having the same volume, will have different weights.

If we are being pedantic the weight doesn't change, the volume of space taken up by a lb of fuel changes. So you can fit less fuel into the aircraft.

One way of testing this I guess would be start a mission at -20C and one at +30C and see what difference in fuel load you get. Then you could work out the expansion coefficient over 50C of change and see if that matches the difference in full fuel weight. Be interesting to see if its modelled!

The mass of an object is not affected by weather conditions, the density is. Fuel can expand and contract due to temperature fluctuations but it’s weight remains constant.

EDIT: sniped.

:gun_sniper::thumbup: Sorry hehe

Indeed, good correction! Density changes not mass.

In the screenshot that wags posted a week or so ago showcasing the jet carrying harms, jdams, and whatnot. he also had a screenshot showing the cockpit and mfds. in that picture, if you look at the fuel quantity it says over 11,000lbs. commonly I read 10,800 something at full internal.

In real life, the indicated fuel quantity varies a little bit (usually within couple of hundred pounds or so IIRC). I'd guess it is probably a combination of factors that is in play here, including the inaccuracy of the fuel gauges, variations in fuel density and when exactly the shutoff valves cut the refueling flow when the tanks fill up.

IIRC, pounds per gallon differs about three tenths of a pound from the high end to the low end in your normal temperature ranges. There were charts in the maintenance manuals for fueling and high/lo cal fuel quantity calibrations to account for temp. That was for JP-8, so not sure how much it varies with other gas types.

Mine usually has 10700 at 100%, but as soon as I'm weight off wheels with bags, this starts to go up until it hits 11000. I've never seen that number right from the beginning though.

Fuel capacity is volume. To good approximation the fuel tank is a constant volume. Fuel quantity is mass. To very, very good approximation fuel material is constant.

DCS (and most real operator) handles fuel as a massed material which makes sense so it can be treated as conserved. There are other reasons like mass is a better measure of total energy and for weight calculations but those are unimportant for the current topic.

The amount of volume a mass of a grade of fuel occupies depends on its properties, mostly temperature. This is one reason why fuel tanks are not filled completely as any thermal expansion would result in a spill.

And lastly there is the question of how does an F/A-18 measure its internal fuel. I'm not expert but it's generally a calculated network of capacitance and possibly some float sensors.

We definetly fill the tanks completely full, and that’s necessary to do a high/low cal for the fuel probes in the tanks.

The tanks have P/V valves, and will vent and pee. Very common in summe. External bags are more suseptable as they don’t have the bladders that the internals do.

Do the totalizator values oscillate like first post describes ever? It might be an artifact of mid-air spawn.

I think remembering an effect of the indicated fuel quantity changing a couple of hundred pounds within few moments after the IFEI is powered up. After that it remains fairly stable, perhaps changing within a few counts, on a stationary aircraft.

Other airplanes show slight fluctuations in indicated fuel quantities as well, and in many cases it tends to vary some depending on airplane's attitude, so it is nothing unique to Hornet.

Do the totalizator values oscillate like first post describes ever? It might be an artifact of mid-air spawn.

I can’t speak for the -18 directly, but in the -15 world we have what’s called an FQI rollback that will happen during engine shutdown. The total total(that’s what we call it) will commonly roll back a few hundred pounds as the jet powers down. It doesn’t happen when shutting off external power hence why we either have to get a fuel reading before shutdown or apply external power for the FQI to be accurate. With the -18 being digital, not sure if the same thing applies to them. For us, that could either be a product of the FQI itself or the signal that comes from the signal conditioner (box that takes all the inpits from the fuel probes and controls the FQI).

Of course this is all trivial information which doesnt mean mich to anyone here. :)

Fuel density changes with temperature. Ballpark for jet fuel / JP is about 6.7 lb/gal on a standard day (15 deg C), give or take. So, on a hot summer day you are going to fill the tanks but show slightly less than a full bag on the gauges. On a cold winter day, you can fill the tanks and actually over-fuel the jet slightly in terms of weight, although you haven't magically made the tanks bigger- the fuel itself is just denser, is all.