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MOFF

CG runs out of range on longhauls....MD11

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Hi,on my first two longhaul flights with the MD11, I was not always in the cockpit and had to recognize that the CG runs out of range on both flights. In my brain is nailed: Sit down in the cockpit and do nothing else than normal ops, she will do all for you. I guess I make a mistake in thinking of fuel handling, but I have no glue, what I have to do to avoid CG OUT OF RANGE...Help needed, please...BestSteffen

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Steffen,The FUEL system in automatic mode will do all the necessary transfers to keep the CG at a certain target within range (this is called tail fuel management). However there are conditions where due to a bad initial fuel planning the FMS has no way to correct the situation. Using the load manager, the PMDG menu or the MCDU REFUELING page in FMS mode will distribute the fuel in the tanks in a standard way that ensures that the CG will be OK during the flight, as long as your payload is distributed so that in conjunction with the distribution of fuel the CG during take-off is within the acceptable range.If you load fuel by placing it at the various tanks by yourself (MCDU REFUELING page AUTO or MANUAL options) you better know what you are doing otherwise you can well end up in a CG OUT OF RANGE situation. For example if you take off with a rather aft CG and an almost empty tail tank, there is nowhere from where the fuel can be moved forward to reduce the CG.Additionally, there is the option to define some ballast fuel to deal with some situations like a short flight with an empty aircraft.In general, if you use the Load Manager, and ensure that the resulting ZFWCG and TOCG are well within limits, you will have no problems.

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Can I just clarify "well within limits"?I've had this "CG OUT OF RANGE" issue a couple of times, despite perfectly within-range CG at T/O.In the example I'm thinking of, I have 1/3 pax, and the Load manager at low fuel shows permissible (black) CG. As I push the fuel up, CG goes Red... and eventually back to black. I can understand this - you can add some fuel and be out of balance, more to get back in, and too much, so to speak.Am I right in thinking that "well within" means I should probably add more fuel than *just* gets me back into the black zone? I think I've literally added just enough to turn my CG figures from red to black; 20 mins into my flight I get an error, I'm pretty sure the 2 are related.And finally - is anything going to be published so we can do the maths on this and work it out? Or is that going to be caught up by Boeing's performance data embargo?

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Hi Michael, thx for joining in. I filled the tanks using the PMDG menu in FSX. What I read this will be the exact way instead of Loadmanager, the TOCG and ZFWCG was always in normmal ranges, but the TOCG was always a bit high, at 31.1 or so.I will have a deeper look at it. But some data about how much I will need in the tail tank for having a longhaul flight to not get the CG out of range will be very nice.As far as I understood, there is no chance inflight to pump some fuel from the main tank back to the tail, right ?BestSteffen

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So, now I am totally confused...I used the Loadmanager, FSX and MD11 loaded. I filled the fuel up to 77000 tons. Sent it to FSX, ZFW is 159,9 tons.Tank 1: 18350Tank 2: 29050Tank 3: 18350AUX: 11350Tail: 0From the loadmanager:ZFWCG: 30.0TOCG: 28.5I thought that we need fuel in the Tail tank... Why is there nothing. Further help needed, can

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Well, if the tail serves a similar purpose as in an Airbus (that is - Mach trim) then it will only be filled in flight by transfer pumps to prevent tuck-under... After takeoff the tank would be filled from the wing and center tanks in order to shift the CG further to the back, which explains why you need about 4.4 trim units on takeoff, and just 1.x during cruise.Niklas

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Next fishing in the dark sea: Filled up the Tanks using the FMC option AUTO. Putted 5000 in tail and splitted the rest in the others, 37000 KGS totally.After tankering the system starts to pump the tail empty down to 100KGS, now 20 mins in flight the CG grows from 31.1 slowly to 33.8 and I will be not surprised if it will go up tro OUT OF RANGE.So sorry guys, that wonderfull to read all your hints, it seems not to help here... I would like to understand this better.YES. I did a RTFM...:-)Steffen

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Hmm, I'm pretty sure the tail shouldn't be loaded at all during refueling - otherwise you have a tail heavy, unstable airplane for takeoff... So the fuel system works fine until there (just not the refuelling function) - Very strange. To my own embarrasment I haven't checked my cruise CG - well, I haven't had any problems with it either. As far as I know the tail trim system is automatic, and there isn't a way to control it manually(since thats a major chance to screw up). Where does your CG wander during cruise, further to the back? That would be just what the mach trim intends, not only to prevent nose-down above the critical mach number, but also to increase the specific range of the airplane. When you had trouble with the CG in cruise, was it too far rear or too far in the front?Cheers,Niklas

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Hi Niclas,thanks for the try to help me out. I am enroute now and the CG OUT OF RANGE warning occurs after appr. 1 hr of flight 34.1 yet after 01:11hrs.The stab trim is on 1.2. The CG was going slowly more up started at 31.1 and now as I said 34.1, and will go further up...:-(Thy again.Steffen

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Nicolas.. it is not true that the tail should be empty of fuel during fueling or on the ground. The aircraft is certified to have fuel in the tail both on the ground and during take-off.In my real world job I did a weight&balance training training on the md-11 and the following are some highlight. I do not know (yet) what has been modelled on the PMDG so the following is only valid on RW md-11's1- based on the basic ground fuel schedule you first fill the wingtanks ( 1-2-3) as far as I remember 66.4 tons will go in there. If more fuel is needed that will go in the centre tank - upto 111.1 tons. If still fuel is required 5 tons can be loaded in the tailtank. Fuel in the centre tank will bring the aircaft so much forward in terms of CG that the aircraft will not to be too tailheavy when you load fuel in the tail.Fuel burn in flight is the other way rond, first tail fuel is used and after that centre and wingtanks are emptied.2- Once in flight if the CG is not at the optimium (around 31.5% CG) the tailfuel management system will take fuel from the centre/wings tank and pump it to the tail to improve cg. Depending on payload weighdistribution on the plane, during the last stages of the flight (when the centre tank is empty) there may be less need for this. 3- there is an alternative way of sequencing the fuel( the 7.5 to 1 ground fuel schedule). This is an extra modification/option that not all airlines bought. I doubt if PMDG has modelled this.4- remember that because of the tailengine an empty md-11 will be very tailheavy when empty and a freighter will even be out of CG range when empty. Some ballast(fuel) will be required to bring the ac back in the allowable cg range. As there is a lot more payload area in front of the wing (the pivot in terms of CG) a loaded aircraft could end up very noseheavy when weights are distributed equally.Pax aircraft do already have more equipment weight (seats/galleys etc) in front of the pivotpoint to make them slighly more noseheavy when empty so these birds can fly empty b rdgs / Dick

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Steffen,This is quite complicated to explain here. You can find more information in the SYSTEMS manual FUEL section (look at page FUEL.10.16 for CG management).A couple of points:- Fuel is loaded to the tanks in a predefined way. Tail tank will get some fuel only if the total amount of fuel you load is above a certain level. - The 7.5:1 ground scheduling option that Dick mentions in point 3, is modeled (H fuel distribution). It will apply when you are close to the maximum TOGW. - The cargo model has a different empty weight CG than the passenger (because of no furniture, galleys etc.)- In a situation of a low payload and a short trip that requires low fuel (e.g. a ferry flight) you should consider adding some ballast fuel to keep your CG within limits. This is modeled and you can define it from the FUEL INIT page.

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I understand totally what you say Michael, and I am very thankfull about your engagement but if I fill up the tanks with the weights described above using the Loadmanager. The settings and the flight lenght is a typical flight not a ferry or a very heavy one, a normal flight in my point of view. The behaviour as also described occurs without any doing by myself. I say it again: I understand the fuel management, also after reading the book. BUT IT HAPPENS ! So to explane the theory ever and ever again make not the real sense to me if in praxis I have the problem with the pax version, sorry.Since I am a cargo fan, my interest in pax flights is not the biggest, so I will use the cargo

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Happened both to me and a friend in fligth as well as a severe imbalance between left and right tank (1 and 3) I had to correct myself.I do not understand why because I used the FMS refueling based on correct values ... with remaining fuelThe plane is 1/3 of charge, on long haul flight and got imbalance in the middle of the atlantic as well as the CG OUT OF LIMIT alert requesting manual actions.Is that normal and what can be done ?

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In the load manager, the TOCG value will peak at 35,000 lbs of fuel (16,000 kg). If your plane will be less than half-full, I recommend setting the fuel to 35k lbs and moving cargo around to get the TOCG within a good range before setting your final fuel load. The TOCG at 35k of fuel will be your max CG during flight. If you will be flying so empty that this CG is out of range even with all the cargo up front, then you will need some ballast fuel. - jason

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