MD11 CG out of range problem, the reason?

[Guest Urs Wildermuth]

Hi folks, on my initial long haul flight ZRH-NRT I got a CG out of range warning about half ways into the flight. The QRH sais it has to do with the fuel distribution and I tried to rectify it but to no avial in flight. However, after the flight I started to analyze the problem and found the reason why this happens, it's actually pretty simple and has nothing directly to do with the fuel. As I heard from Markus today that others have had this problem, I thought I'll let you know what I found out. The problem is with the balance limits at Zero Fuel Weight. If you check the load planner, you will notice that with the airplane full to the limit, you end up with a ZFWCG of 30-34. I assume here that this refers to the MAC% at ZFW. If so, that is way too far back. In real life, the ZFW CG is limited to about 25-26% MAC. If the plane is that tailheavy, as fuel burns off the CG at gross weight shifts backwards. The fuel system will try to counter that by emptying the tail fuel but runs out of options once that is done. So I think the aft CG limit at ZFW, which is indicated as 34 (I translate that as 34% MAC) is wrong in the load planner and should be adjusted to the said values to prevent the airplane being out of trim. As an additional info, the MD11 also has a fuel burn increase if CG is too far forward. Rechecking my data, the recommended MACZFW is around 25%. So if you aim for that when you load your plane, you'll be just fine. Hope that helps, if there are questions feel free to contact me. Best regardsUrs WildermuthLSZHhttp://www.danur.com

[Guest lorenz88]

Probably your aircraf was bad balanced. I mean...if you get a CG too close to the aft limit even at takeoff and you have a fight that burns-off fuel and fuel burn-off produces a CG change it is not always said that your CG will not go into forbidden areas (then you get CG ALERT advise).My suggestion is to load the aircraft with CG not too close to the aft limit. In this way you are sure not to get the CG ALERT.Maybe someone else has a better and more precise explanation...perhaps it was only a case since even I had this alert but only in one flight against the more or less 20 that I have done with the MD11...Best RegardsLorenzo Chiovaro

[Guest jbos]

Due to the tail engine, a lightly-loaded MD-11 is capable of becoming tail heavy during flight as the fuel burns off.When loading your plane, I recommend checking your TOCG in the load manager at both your actual fuel load and at 35,000 lbs (16,000 kgs) of fuel. With the fuel schedule, 35,000 lbs is when your CG will be most aft. After this, the wingtip tanks will drain, moving the CG forward again. If your CG is too high at 35,000 lbs of fuel, then move some cargo towards the nose to keep the plane balanced. If absolutely necessary, you can also use ballast in the aux tank to keep the CG forward, but this should only be required if you have no cargo to place in the nose (e.g. if you are ferrying an empty plane).Unfortunately, the 1/3 Pax random autoload does not compensate for this and almost always produces a too-aft loading. Sometimes even the ZFWCG is out of range. The 2/3 autoload will load too far aft about 10% of the time.

[mfrantz 45]

Urs,Nice to hear from you. I couldn't attend the Hergiswil weekend this time as I did last year, hope you had a nice time. You are right about the ZFWCG limits displayed in the Load Manager. In contrast to the TOCG limits that come out of tables that depend on GW, the ZFWCG limits we set are constant and correspond to the min/max values allowed to be entered on the WEIGHT INIT page of the MCDU (LSK 6R). Actually there is a small mistake there, the max entry limit should be 32.0 instead of 34.0. Your observation: "If you check the load planner, you will notice that with the airplane full to the limit, you end up with a ZFWCG of 30-34"is the other way round: an empty aircraft will give a ZFWCG >30.0 (this is the DOW CG). A fully loaded aircraft will give a more fwd ZFWCG.Anyway, I see two issues here that I will try to address in the next update:a. In order to assist users to properly distribute payloads we need to provide more useful limiting of the ZFWCG. I have found some operational limits for the ZFWCG that are a function of the ZFW weight, and I plan to implement them into the Load Manager. b. The 1/3, 2/3, FULL and random buttons on the Load Manager currently do a random distribution of payload that ends up to the required total payload. This can lead to a ZFWCG out of normal ranges (especially with the 1/3 button). In the next update I will change the logic so that payload is distributed in a way that ensures that ZFWCG remains within the (new) limits.

[Guest Urs Wildermuth]

Hi Michael, >Nice to hear from you. I couldn't attend the Hergiswil weekend>this time as I did last year, hope you had a nice time. I did indeed. It was a great satisfaction to demonstrate to one of the more extreme anti FSX guys that the MD11 will fly on a laptop with a 2 gig centrino and a 7900/512 cards. Apart, loads of good plane talk with Markus and others, what would a MD11 guy like me wish for. >>You are right about the ZFWCG limits displayed in the Load>Manager. >In contrast to the TOCG limits that come out of tables that>depend on GW, the ZFWCG limits we set are constant and>correspond to the min/max values allowed to be entered on the>WEIGHT INIT page of the MCDU (LSK 6R). Actually there is a>small mistake there, the max entry limit should be 32.0>instead of 34.0. Ok, if this is MAC% that 34 is WAY out of aft limit. The envelope I have in my manual shows what I indicated above, the maximum in operational flights is around that. Markus has the original references, so he can give you the figures you need in case you need the original, I am out of the country from tomorrow until Dec 1. >>Your observation: >"If you check the load planner, you will notice that with the>airplane full to the limit, you end up with a ZFWCG of 30-34">is the other way round: an empty aircraft will give a ZFWCG>>30.0 (this is the DOW CG). A fully loaded aircraft will give>a more fwd ZFWCG.Yes, an empty MD11 will usually need ballast. The loaded aircraft FULL as per your loadplanner gave me somewhere around 34. I do recall that for balance reasons it was often not possible to put all the ULD's possible or to put the heavy ones in front in order to keep the CG in check. Mind, my first jet, the Caravelle, was absolutely incapable of flying without load. We had to put loads of sandbags in the forward hold in order to be in limits :)>>Anyway, I see two issues here that I will try to address in>the next update:>>a. In order to assist users to properly distribute payloads we>need to provide more useful limiting of the ZFWCG. I have>found some operational limits for the ZFWCG that are a>function of the ZFW weight, and I plan to implement them into>the Load Manager. >Yes, the aft limit for the ZFW needs to be moved radically forward, at around 26-27 % MAC. >b. The 1/3, 2/3, FULL and random buttons on the Load Manager>currently do a random distribution of payload that ends up to>the required total payload. This can lead to a ZFWCG out of>normal ranges (especially with the 1/3 button). In the next>update I will change the logic so that payload is distributed>in a way that ensures that ZFWCG remains within the (new)>limits. >It was the "Full" one that got me a CG of aft of 30%. Actually, there are so called standard load procedures, which you could adapt for the planner, which will tell you what to put where under which conditions. Unfortunately, your configuration is not what I know (I was more used to the 3 class version where the fancy seats and toys would already do wonders for the LIDOW. When we planned the MD11, we were telling the load control to plan the MACZFW for 25%. That is the one where the least payload penalty occurrs. Basically, if you manage to get your load distributed such, then you can't go wrong. Maybe that is a hint for how to get the distribution going on your random loads. I have now saved a distribution which gives me that, with about 177 tons of ZFW, that is a pretty normal load for the '11, so I am sorted, now that I know what to look for. Anyhow, if you need anything, please holler, Markus has got my e-mail. Best regardsUrs

[Guest Urs Wildermuth]

Hi Lorenzo, >Probably your aircraf was bad balanced. I mean...if you get a>CG too close to the aft limit even at takeoff and you have a>fight that burns-off fuel and fuel burn-off produces a CG>change it is not always said that your CG will not go into>forbidden areas (then you get CG ALERT advise).>Yea, of course it was, but the point is that the load planner lets you do that without the CGZFW gets red. It's been 7 years since my last load plan for an '11, so I forgot to check the limits as per the AOM so I ended up in the situation I was in. Won't happen again :)>My suggestion is to load the aircraft with CG not too close to>the aft limit. In this way you are sure not to get the CG>ALERT.>Maybe someone else has a better and more precise>explanation...perhaps it was only a case since even I had this>alert but only in one flight against the more or less 20 that>I have done with the MD11...>>Best Regards>Lorenzo ChiovaroWell, for once, I know what was going on so that is why I decided to tell you guys. If you keep your CGZFW at around 25, you'll be fine and so will the plane. Best regardsUrs

[Guest Urs Wildermuth]

Hi jbos, I should have said in my initial post I reckon but I used to be a load controller in a previous "life" before my time as a flight dispatcher so yes, I am well aware of this. Only with the amazing absence of bugs on this plane, I did rely on the load manager to scream at me too much. Once I opened my books somewhere over Sibiria, my expression was probably alike the one of Wyle E Coyote once the Roadrunner has him again falling from great hights :)>When loading your plane, I recommend checking your TOCG in the>load manager at both your actual fuel load and at 35,000 lbs>(16,000 kgs) of fuel. With the fuel schedule, 35,000 lbs is>when your CG will be most aft. After this, the wingtip tanks>will drain, moving the CG forward again. If your CG is too>high at 35,000 lbs of fuel, then move some cargo towards the>nose to keep the plane balanced. If absolutely necessary, you>can also use ballast in the aux tank to keep the CG forward,>but this should only be required if you have no cargo to place>in the nose (e.g. if you are ferrying an empty plane).It's the other way around. The ZFWCG needs to be within limits and your TOCG is assured to be unless the guy fuelling it has had a bad fuel-day. With standard fuel distribution, the whole purpose of the ZFWCG is that throughout the flight, you will never get out of aft CG during the fuel envelope. Apart, the MD11 is notorious for rising it's nose above load planners who don't watch what they are doing while loading it. Several of them have sat on their tails as a consequence when they were being loaded. Embarrassing. Thankfully I never managed that but then again, my first "love" of a plane was the SE210, where you were out of aft trim almost all the time so checking after tail heavy planes is almost second nature. I once got trapped in the flight deck after a towing maneuver because the stick that kept it from falling on it's behind fell off during the tow. Could not walk back to the tail to release the airstair (which would have solved the problem) so I had to start up the APU and call the guys back to find the stick and put it back. :) >>Unfortunately, the 1/3 Pax random autoload does not compensate>for this and almost always produces a too-aft loading.>Sometimes even the ZFWCG is out of range. The 2/3 autoload>will load too far aft about 10% of the time.Well, yea, the lazy options... Reckon I was simply too lazy to click 300 pax seats so that is how. But I'll recall to make sure the cargo holds are set up properly. Mind, that reminds me, where is the open hold in the tail? I don't think it's modelled? (edit, yes it is, but there is no door, so I missed it. Is there a way to specify how much you wanna put in there? Apart, also the aft hold can take big ULD's and the front can take small ones, but I reckon that goes too far.

[Guest XM DUDE]

>Due to the tail engine, a lightly-loaded MD-11 is capable of>becoming tail heavy during flight as the fuel burns off.>>When loading your plane, I recommend checking your TOCG in the>load manager at both your actual fuel load and at 35,000 lbs>(16,000 kgs) of fuel. With the fuel schedule, 35,000 lbs is>when your CG will be most aft. After this, the wingtip tanks>will drain, moving the CG forward again. If your CG is too>high at 35,000 lbs of fuel, then move some cargo towards the>nose to keep the plane balanced. If absolutely necessary, you>can also use ballast in the aux tank to keep the CG forward,>but this should only be required if you have no cargo to place>in the nose (e.g. if you are ferrying an empty plane).>>Unfortunately, the 1/3 Pax random autoload does not compensate>for this and almost always produces a too-aft loading.>Sometimes even the ZFWCG is out of range. The 2/3 autoload>will load too far aft about 10% of the time.http://www.precisionmanuals.com/images/forum/pmdg_trijet.jpgIn the real world the MD-11 needs Balast in the forward cargo bey to safely fly, just a little side note.Michael p.

[Dick 47]

thank you all for your contributions here..Would like to add one detail however.It is mentioned various times that the empty MD-11 does not fly without ballast fuel in the centretank. This is true only for the freighter version. Like Urs i worked with RW MD-11's and the pax aircraft that we had flew empty without ballast fuel.One aspect that also is intresting in understanding the MD-11 is the fuel distribution systen. What I have seen the PMDG behaves differently compared to the RW MD-11's I worked with. In RW there are 2 ways of fueling the aircraft;A- basic ground fuel schedule. First tanks 1-2-3 are filled. If more fuel needed the centre tank will be filled and lastly the tailtankB- the 7.5 to 1 ground fuel schedule. Here you will also fill the 1-2-3 first. After that the centre and the tail tank are filled ate the same time in the ratio 7.5 to 1. That is to say 1 ton in the tail for every 7.5 tons that go into the centretank.By doing this the forward CG effect of the centretank fuel will be cancelled out by the fuel that goes into the tailtank resulting in much less CG shifts in flight because of fuelburn. I have not been able to recreate this fuelbehaviour on the PMDG.. ( or did I miss something ? ) This option was developed after the introduction of the MD-11 and is an option available. As far as I know not all airlines did modify the aircraft.In real life when despatching MD-11 we would never bother about TOCG or LACG. Douglas delivered the aircraft with a ZFW enveloppe that was designed in such a way that once you were inside the CG limits at ZF weight you would always be and stay in limits in flight as well. This ojective was achived by inserting the extra lines into the ZF envelope. (6 extra restictions lines for the freighter and 7 for the paxaircraft.) have a nice day ! Dick

[mfrantz 45]

>In RW there are 2 ways of fueling the aircraft;>A- basic ground fuel schedule. First tanks 1-2-3 are filled.>If more fuel needed the centre tank will be filled and lastly>the tailtank>B- the 7.5 to 1 ground fuel schedule. Here you will also fill>the 1-2-3 first. After that the centre and the tail tank are>filled ate the same time in the ratio 7.5 to 1. That is to say>1 ton in the tail for every 7.5 tons that go into the>centretank.Dick,The B option (called "H" fuel distribution for "Heavy" as opposed to "L" for "Light" in option A) allows for a higher TOGW of 630,500 LBS instead of the 625,500 LBS of the A option. The H fuel distribution is modeled in our MD-11, but will be applied only when the requested fuel will result to a TOGW > 625,500 LBS. Otherwise the L option is assumed.

[mfrantz 45]

Urs, Thanks a lot for your feedback. I do have the charts that give the ZFWCG operational limits and I will implement them in the Load Manager (including the "lazy" options) for the next update.

[Guest Urs Wildermuth]

Hi Dick, >>In real life when despatching MD-11 we would never bother about TOCG or LACG. Douglas delivered the aircraft with a ZFW enveloppe that was designed in such a way that once you were inside the CG limits at ZF weight you would always be and stay in limits in flight as well. This ojective was achived by inserting the extra lines into the ZF envelope. (6 extra restictions lines for the freighter and 7 for the paxaircraft.)Exactly. So the ZFW CG is the vital bit, get that right and forget the fuel effect, it takes care of itself. I guess the extra lines you quote are the ones around 26/27% MAC for the aft limit. Ours could fly without ballast fuel I believe, but usually they took some anyhow. IIRR the DOCG was just about at the aft limit.

[Dick 47]

>>In RW there are 2 ways of fueling the aircraft;>>A- basic ground fuel schedule. First tanks 1-2-3 are filled.>>If more fuel needed the centre tank will be filled and>lastly>>the tailtank>>B- the 7.5 to 1 ground fuel schedule. Here you will also>fill>>the 1-2-3 first. After that the centre and the tail tank are>>filled ate the same time in the ratio 7.5 to 1. That is to>say>>1 ton in the tail for every 7.5 tons that go into the>>centretank.>>Dick,>>The B option (called "H" fuel distribution for "Heavy" as>opposed to "L" for "Light" in option A) allows for a higher>TOGW of 630,500 LBS instead of the 625,500 LBS of the A>option. >The H fuel distribution is modeled in our MD-11, but will be>applied only when the requested fuel will result to a TOGW >>625,500 LBS. Otherwise the L option is assumed. >Hi Michaelthx for your reply here. Yes I did read that with the PMDG we have the H and L ways of fuelling with the corresponding difference in TOGW.With our RW MD-11's it was different. At all times we had a choice between the basic ground or the 7,5 : 1 fueling schedules and indeed the 7.5 to 1 would give you a higher max TOW. We would have 286.00 or 629.200 versus the 630.500 that you have. The fuelschedule did not depend on the ttl fuel / weight of the aircraft. The 7,5 to 1 would give you a far less resticted envelope so we would always use this, apart from some specific MEL etc cases were we would downgrade the aircraft to basic ground fuel schedule. On the loadplan there was a special box to tick which envelope the dispacther/loadmaster had used to inform the crew.b rdgs Dick=

[Dick 47]

>Hi Dick, >>>>In real life when despatching MD-11 we would never bother>about TOCG or LACG. Douglas delivered the aircraft with a ZFW>enveloppe that was designed in such a way that once you were>inside the CG limits at ZF weight you would always be and stay>in limits in flight as well. This ojective was achived by>inserting the extra lines into the ZF envelope. (6 extra>restictions lines for the freighter and 7 for the>paxaircraft.)>>Exactly. So the ZFW CG is the vital bit, get that right and>forget the fuel effect, it takes care of itself. I guess the>extra lines you quote are the ones around 26/27% MAC for the>aft limit. >>Ours could fly without ballast fuel I believe, but usually>they took some anyhow. IIRR the DOCG was just about at the aft>limit. >>Hi Danur;ok.. the extra lines I quoted were 3 extra forward limitations and 3 (or 4 for the pax aircraft) aft limitationsThe forward ones would restrict the ZFMCG when there was a lot fuel in the centre tank to prevent a situation where we would be inside the ZFM envelope and where this centretank fuel would bring the aircraft (forward)out of the TOW envelope. In essence Douglas combined the two envelopes. Aft we would also 3 extra lines that would be valid when there was a lot of fuel in the tail tank to avoid a situation where we would be inside the zf envelope and the tailfuel bringing the aircraft out of the (aft)TOW envelope. They were not so much CG related as well weight related. The applicability of the lines depended on the weight of aircraft.( which is essence means the fuel OB as that would be the restricting factor.I brought this up as it would be nice if PMDG could simulate fuel behaviour the same way.. then we would have a situation that once ok on ZF CG in the loadmanager we would always be ok and no further hassle.As to the DOCG; yes in the freighters that was also aft of the limits. Actually the freighters were so tailheavy that we were not even allowed to park them empty ( especially on these sloping ramps to drain the rainwater) Our aircraft actually had a tipover alarm with a weightsensor in the nose gear.. when not enough weight on the nosegear (due to the aircraft being empty or incorrect (off)loading sequence the tipover alarm would activate and you would have your head chopped of !!b rdgs / Dick

[Guitarman 0]

Hello Michael,In the meantime, I like to ask you if it's possible that the loadmanager can be modified so that we can input how many pax there are instead of 1/2, 2/3, full or random ? Is it also possible for the B-747 loadmanager because it's the same situation ?To the RW MD-11 pilots (Urs and Dick, sorry if I forget someone),Thank you so much for your information. If it's possible, can you PM me ? I like to have contact with you and will send you in return my e-mail address.Many thanks in advance,PeterBelgiumFlightsimulator is not a simulation, it's a way of life ....