Ballast Fuel

[3-2-1-Now 126]

Hi,

 

I'm trying to find out how to compute ballast fuel, and under what CoG configs it is required.

 

If I load the aircraft with a forward CoG, according to the LM, at all fuel loads the CoG remains in limits, so this is OK.... I'm guessing the issue arises when everything is unloaded? In the sim, unless we start empty, the aircraft can never be unloaded and thus it is not an issue. Am I in the ball-park? The result being we don't actually need ballast (excluding the case where CoG goes out of limits due to fuel burn)?

 

Best regards,

Robin.

[PHXMD-11 86]

I'm trying to find out how to compute ballast fuel, and under what CoG configs it is required.

 

I think ballast fuel is really only used when the aircraft is very light or empty. When you need ballast fuel, I think (could be wrong on this) you add the same amount of ballast fuel to meet the amount of block fuel. So, if you need 32,000 pounds of fuel you carry an additional 32,000 pounds of fuel for a total of 64,000 pounds.

 

In the sim, unless we start empty, the aircraft can never be unloaded and thus it is not an issue.

 

The MD-11 can be unloaded via the load manager in between flights without having to shut down FSX.

 

The result being we don't actually need ballast (excluding the case where CoG goes out of limits due to fuel burn)?

 

As far as I know, that is correct.

[Dick 47]

Ballast (fuel) is used to bring the cog of the MD-11 in zerofuelweight conditions back into cog limits. Ballast fuel is unusable during the flight and therefore is not really counted as fuel. ( therefore the distincton between useable fuel and unuseable fuel The MD-11 is critical to bring into the zfw limits. When empty or light the tailengine shifts the cog very much aft and in this case ballast fuel in the centertank needs to be loaded. By design the fuselage part before the wing is much larger then the part aft of the wing. Therefore when fully loaded there is considerable more weight before the wing compared to aft of the wing. In this case the MD-11 will have a very forward CG and ballast fuel may be requried in the tailtank to bring the cg back into the required range.

 

You cannot really talk about cog as such, it is less confusing to talk about either the cog in zerofuelweight condition (zerofuel cog) and the cog of the aircraft in takeoff condition, iow loaded and with all the useable fuel.

 

The reasoning that we only need ballast fuel in case the cog goes out of limits due to fuel burn is incorrect. Once the Md-11 is in limits in zfw condition, fuel (or the use of fuel) will never bring you out of limits. (mind you.. were are talking about the MD-11 here, this statement is not valid for all aircraft types as for the MD-11 the cog consequences of fuel burn have been already incoperated in the zfw cog limits)

 

As to your initial question how to compute the ballast fuel: you will need to calculate the cog of the aircraft in zfw condition. If the computed value is out of limits you need to add ballast ( which could be anything like e.g. sandbags as used in the past, LD3 containers filled with concrete or e.g. ballast fuel. If the zfw cog is aft of the limits the ballast will need to be loaded before the wing, in the opposite case the ballast will need to be positioned aft of the wings.

 

As not to confuse yourself guys do not see ballast fuel as fuel but see it as dead weight that is required to bring the aircraft to a required cog value.

 

Let me know if not clear

b rdgs / Dick

[3-2-1-Now 126]

@Bank_Angle: seems that function is not available in FS9.

 

@Dick: I appreciate the difference between useable and ballast fuel. If you play with the LM, you will find that if the ZFWCG is in limits, it is extremely difficult to find a configuration that causes fuel burn to put the CoG out of limits.

 

Let us take the case of an empty MD-11. Between ~28.5 klbs and ~100.9 klbs of fuel, the TOCG is aft of the aft limit (out of limits). How do we compute ballast fuel for this case? The maximum CoG is reached at ~35.0 klbs of fuel, with an aft CoG position of 37.5% MAC (limit 31.1% MAC).

 

Not knowing the moment arm of any of the tanks, how can we compute ballast fuel for this case?

 

PMDG know as the LM and FMS compute CoG position, so why is there no auto-computation of ballast fuel based on desired ballast tank? We have the option of Aux, Center or Tail, that is entered as A, 2 or T in the FMS, in 'H' configuration. I could sit and compute it for the worst-case scenario (max aft limit in empty configuration), then try and derive tables based on different ZFWCG, but that is a hell of a lot of work when the answer is already available.

 

I understand PMDG probably can't release perf data (I know they have said this is an issue before), but not offering any means of doing a computation for a function that is apparently completely modelled seems a bit strange.

 

Best regards,

Robin.

[Dick 47]

Robin;

 

My knowledge is based on real life MD-11 weight&balance knowledge and not so much on the way PMDG has modelled the concept of ballast fuel in the simulation.

 

I appreciate the difference between useable and ballast fuel. If you play with the LM, you will find that if the ZFWCG is in limits, it is extremely difficult to find a configuration that causes fuel burn to put the CoG out of limits.

Indeed this is correct. The MD-11 comes with a zfw envelope that has extra restrictions when carrying higher fuel loads resulting in a situation where once inside the zfw limits, fuel will never bring you out of any envelope. Not sure if this correctly simulated here.

 

In real life we did have the arms and based on the ballast fuel weights and the corresponding moment could be calculated which would be "translated" in a CG.

 

When using ballast fuel you are not allowed to use the H distribution system. I do not see this restriction being modelled. Actually the H distribution or "7.5 to 1" as we called it, was an option that was not bought by all MD-11 operators. It is mainly useful for freighters but less for the pax operations.

Any tank on the aircraft can only contain either ballast or useable fuel. Iow if you would carry ballast fuel in the upperaux you would loose that tank for the useable fuel I do not see this modelled in the simulation. The same is valid for any ballast fuel in the tailtank.

 

It is also very unlikely that you would want to carry ballast fuel in tank 2, Fuel in this tank would not change the COG ( based on the location of the tank). Ballast fuel would be carried in the Upper aux in case of being tailheavy and in the tailtank in case of being noseheavy.

 

I am also surprised seeing the tocg being mentioned and even to see as pilot input. In our operation the ToCG was calculated by the aircraft and only a required input if the aircraft buildin weight&balance system was inop.

 

Like you did I can only conclude that we have no real means of computing the required ballastfuel in the simulation, we are left with swopping some lowerdeck positions around or offloading freight, which in real life would also be most of times the solution. On top of that I see some other restrictions and regulations not being modelled rendering whatever means of calculating we would have inaccurate.

 

any questions, let me know - b rdgs Dick

[3-2-1-Now 126]

Interesting............. it appeared the fuel controller managed the fuel so it considered a tank "dry" when it reached the ballast fuel limit. It wasn't obvious that the tank was completely unusable (quite a different scenario!).

 

I never got my head around the fuel system last time I read about it (a while ago) so I'll dig back into the sim to see what it does. I suspect however that you're correct in that if you tell it that it has the 'H' option, it is always in operation.

 

An hour later...

 

Checking the manuals, it states (and note - a contradiction exists):

 

* Aux tank can be either usable only, or ballast and usable

* Tank 2 can only have only usable or ballast, with ballast limited to 25,000 lbs

* Tail tank can only be ballast only

* Only one tank can be assigned for ballast

* When tail tank contains ballast, two additional switches must be operated on the maintenance panel (I guess CBs for the tail tank pumps?)

 

The contradiction then appears:

 

* The fuel must be all ballast or all usable.

 

...which is inline with what you were saying regarding the being either ballast or usable.

 

Aside from the two switches for the tail tank, it seems it is modelled?

 

Best regards,

Robin.

[Dick 47]

Hi Robin, need to see if later tonight I have time to look into the simulation a little more.

 

as to your message : "Aux tank can be either usable only, or ballast and usable" is technically spoken not incorrect. The Aux fuel tank consists of two parts, the upper and the lower. The upper aux can contain either ballast or useable and the lower aux can only contain useable fuel. There could be a scenario where the upper contains ballast while the lower aux contains useable fuel rendering the afore statement not incorrect. Although in the airlinebizz you do not want phrase things like this I guess.

As to consider a tank "dry".. even 1 kg of ballast fuel in the tail or upper aux would make you "loose" this tank and your max fuel upliftcapacity shall be lowered by the capacity of the tank involved. ( which if I remember correctly was 5 tons for the tail and 18?or so tons for the upper aux .

 

best regards Dick

[Dick 47]

Need to correct myself on the upper aux tank issue, I think I was wrong in stating that ballast and useable could not be mixed in the upper auxtank. For the tailtank this is true but not for the Aux.

There is lot modelled. It was good to see that the H fuel mode indeed comes with the higher Tow weight and a different tocg compared to the nonH fuel mode which is correct as the fuel is loaded in a different sequence. What I noticed is that the H mode is applied from the LM but once you are in the aircraft you can select ballastfuel while in real life, when carrying ballast fuel, the H mode cannot be used.

 

Coming back to the original question from Robin as to how to calculate the correct amount of ballast in case of an out of trim aircraft...: although having done this for years in real life, in the simulation I have not been able to work this out.

b rdgs Dick

[3-2-1-Now 126]

Hi Dick,

 

OK - if if I intend to load ballast fuel I should de-select the 'H' mode in the LM to get correct function (though this will apparently reduce MTOW as well)?

 

Regarding ballast fuel itself - I guess an amount of ballast that results in CoG remaining in limits at the critical fuel load (max aft CoG position) would be sufficient?

 

I might spend some time creating some figures...

 

Best regards,

Robin.

[Dick 47]

Robin,

Yes, at least in real life the H mode should be deselected when you carry ballastfuel. That will give you indeed a lower certified tow. If you are interested I can explain the difference between the H mode and the non H mode.

Not sure what you mean with your last sentence, when an md-11 is out of trim it normally is noseheavy and you would require ballast fuel in the tail. Tailheavy out of trim aircraft would in real life only happen in empty freighters. Out of trim would mostly happen on cargo aircraft when all or most positions are filled and when all pallets have more or less the same weight. E.g when you have a full load of e.g flowers or any other single commodity load. If you have mixed cargo you would position the heavy pallets in the back and the lighter ones up front to compensate for the fact that you have many more positions in front of the center of lift then pallets aft of the center of lift . You could see it as a seasaw where you have a whole lot more children at one side compared to the other side.

In real life fuel generally would not bring you out of trim. In the H mode fuel is nicely divided between center and tailtank balancing out the cog effect of the fuel. In the non H mode the tail fills last meaning that you need to have a blockfuel over 111 tons, that being the case you will have a restricted zfw meaning that your aircaft has empty positions which woud allow you to trim the aircraft with your payload.

Brgds Dick

 

[3-2-1-Now 126]

If you are interested I can explain the difference between the H mode and the non H mode.

Yes please!

 

Not sure what you mean with your last sentence, when an md-11 is out of trim it normally is noseheavy

Hmm - I find this condition very difficult to create. If I empty the aircraft, it is tail heavy. If I add two containers as far forward as possible in the lower cargo hold, it remains in limit.

 

The only time I can get a forward out of trim is if I configure the freighter for a forward loading. Passenger seems near impossible to get an out of limits forward CoG.

 

I hope I'm making sense.

 

Best regards,

Robin.

[Dick 47]

Robin,

Away from my fs computer for the coming weeks so nothing I can varify in the simulation

 

As to fuel.

Originally the md-11 came to us with only one way of fuelling. It was called basic ground fuel schedule and meant that fuel tanks were filled in sequence, first 1/2 and 3 ( being the wing tanks), after that the aux tank would be filled and if still fuel needed to be uplifted the tail tank was the last to be filled. If no fuel went into the tail the ac ended up nosehavy.

Later MdD offered (against payment) an option for another way to fuel the aircraft, the basic ground fuel schedule ratio 7,5 to 1. After having filled the wings the remainder of the fueluplift would be split and for each 7,5 tons of fuel in the aux, 1 ton would go into the tail tank. The ratio comes from the fact that the aux contains 7.5 times the fuel that the tailtank can hold. In terms of cog the forward cog movement because of the fuel in the aux tank would be cancelled out by the aft cog movement because of the fuel in the tail. That is why you will see a different tocog for the same payload/fueloplift.

Both do carry the word ground fuel schedule in their name because in flight everthing changes again. Once climb power is selected just after t/o, the aircraft will transfer as much fuel as possible/available to the tail until the optimum cog is obtained. Depending on the actual fuelload/useage the system will keep trying to achieve the optimim cog by playing the fuel in the tail. Before landing fuelquantities in each tank will be put back according to the applicable fuelschedule.

Before able to fly, all aircraft need to have their center of lift and center of gravity close to each other, if the load is such that this cannot be achieved the aircraft will need to be trimmed with the horizontal stablizer. Every degree that the horizontal stabilizer is out of the neutral position will cost extra fuel and the md-11 uses a lot of extra fuel when trimming is required.

Once ballast fuel was used in the tail, the 7,5 to 1 cannot be used anymore as useable and non useable fuel cannot mixed. Also the fuel management system that improves the cog in flight cannot be used. In real life when planning to have ballastfuel in the tail we were slapped straightaway with a 3 0/0 extra fuel penalty due to the expected extra required trimming.

 

As to your other comments, indeed pax aircraft are in normal operation always in trim. When the md-11 is tailheavy this is indeed normally easy to fix and should only occur in empty freighter ferry flights. This would effectively mean that a Freighter Cannot fly empty unless something is done with ballast. We were not even allowed to park an md-11 empty during a layover.

When the freighter has all positions filled and the pallets all have more or less the same weigt it is quite possible that the aircraft will be too noseheavy to fly. Now, when you add ballastfuel in the tail you will loose your higher t/o weights and you will be surprised with the 3 percent fuel penalty and you will need to go back to the drawingboard.....

Brgds Dick

[3-2-1-Now 126]

Very informative! Thanks!

 

I can understand not parking empty! :lol: OOPS!

 

 

Best regards,

Robin.