# What are the limits to lateral Centre of Gravity imbalances?

I've never seen lateral CG limits prescribed for aircraft (though I have very imited experience in this realm). Presumably, aircraft are designed to keep the center of balance as close as possible to center of lift, with any imbalances are far more likely to occur fore to aft than side to side.

But, for instance, what if fuel only drained from one wing during flight? How much of an imbalance would this create?

Are there on-board systems in general aviation to prevent this?

What are the effects of a lateral imbalance on handling, and how could pilots compensate, if possible?

Edit 1: Please do not restrict your answers to fuel loads. Is there a percentage-based (e.g. greater than X % aircraft total wingspan) or positional-based (e.g. lateral CG moved outside of the fuselage) DNE from an engineering standpoint?

Edit 2:

Also seeking design input, considering experimental and purpose-built aircraft such as White Knight Two exist.

I also note that despite the larger cargo volume of the Airbus Beluga, it cannot carry near as much weight as the smaller-volume C-5 Galaxy. Is this a benefit of the C-5's design in placing the wings above the fuselage or is there such a thing as vertical CG limits?

• On the Beluga -- what you're likely seeing is a structural weight limit -- vertical CG isn't an issue on any aircraft I know of – UnrecognizedFallingObject Jan 23 '15 at 2:10
• I assume that you are only interested in fixed wing? You might consider editing the question to make this explicit. – Simon Jan 23 '15 at 11:35
• nope, all answers welcome. speaking of rotary wing (especially the cargo variety), i'm curious how sling-loads play into the equation. – Erich Jan 23 '15 at 11:49
• Helicopters have quite narrow lateral cg limits, and they are different for left and right. If you want to know more, ask a new question. The comments are not suitable for answers. – Peter Kämpf Jan 23 '15 at 13:58
• i see my original question was too broad. i'll continue prodding in a separate question. – Erich Jan 25 '15 at 22:53

Lateral c.g. limits are prescribed for large aircraft both for max fuel imbalance and for what is being carried in the fuselage. It's basically impossible to exceed the max lateral imbalance in a passenger aircraft. Exceeding the max in wide body cargo aircraft can be done and is checked for, but it takes an unusual load to exceed the max.

The 747-400, for example, has a max lateral imbalance of 10,000,000 inch-pounds. If you load 207,205 lbs on one side only of the main deck of a 747-400 freighter using size code M pallets, you will exceed the maximum. To see this happen (if you're using Firefox or Chrome--IE won't work) go to 747.terryliittschwager.com, dismiss the flash screen, and then click on test load #6 near the bottom of the page. That will trigger exceeding the lateral imbalance limit. Scroll down to the Lateral Load Imbalance graphic to see the max lateral loading envelope and the point that shows the load exceeding it.

Here are some approximate weights for a 747-400 freighter:

basic operating weight 365,000 lbs

max fuel 360,000 lbs

max cargo 245,000 lbs given the 610,000 max zero fuel limitation and a 365,000 bow

Note that you cannot carry max fuel and max cargo. A common 747-400 max takeoff weight is 875,000 lbs.

• great site! i see where your lateral imbalance example exceeds the envelope. what units are the x-axis values in? – Erich Jan 23 '15 at 5:10
• @erich Millions of inch pounds. Eventually I'll get the axes properly labeled (if I ever finish the project). The axis labeling will a bit tedious since if the user makes the weight unit kilograms, that axis labeling will have to become inch-kilograms, which is a real bastardized unit in my opinion, but that's how Boeing shows it for kilograms. – Terry Jan 23 '15 at 7:31
• Millions of inch pounds, well, that's ridiculous. On the other hand, we should be probably happy that it's not a dozen^5 inch-pounds, right? /sarcasm – yo' Jan 23 '15 at 9:38
• @yo' The U.S. Metric Board was established and funded in 1975 to convert the U.S. to the metric system. One of the many sins of the Reagan administration was bowing to conservative pressure to defund and then abolish that board in 1982. That was a really bad move, so obviously bad that I won't bother saying "in my opinion". – Terry Jan 23 '15 at 20:03
• @erich To the best of my knowledge, no U.S. aircraft manufacturer uses the SI system as a standard or publishes W&B info in metric as the standard. My guess would be that all other countries do. What Boeing does (or at least did up until I retired in 1999) is design and publish in the US measurement system. However, in the Boeing manuals I have, there is a kilogram page following each pound page for the convenience of SI users. This produces the oddity of inch-kilograms for moments. Also, for each 'gallon' page there is a following 'liter' page. – Terry Jan 23 '15 at 20:13

Putting cargo in the right place to maintain lateral CG position is easy, there is little place other than near the center to put it.

It's a different story with the fuel in the wing tanks, a wing with more fuel is heavier and will tend to drop down and requires the pilot to correct for that using the ailerons. This will introduce drag which will increase fuel consumption and lower the speed.

The limit described in the manual is usually in weight of fuel imbalance. And where the limit is depends from plane to plane and relies on the inherent roll stability and the roll control of the ailerons.

• what percentage of total weight is fuel, typically? understand that likely this varies by aircraft, so some sample upper and lower limits to that range would be helpful to the calculation. – Erich Jan 23 '15 at 1:14
• Jet A is roughly 0.8 kg per litre, and a B747 holds something like 185,000 litres. That makes the fuel around 150,000 kg, or something like 30-35% of the total weight. A Cessna 172s's fuel is something like 150kg (210 litres of avgas at 0.7kg/litre), making it around 14% of the total weight. Both situations assume full fuel load at maximum takeoff weight – Jon Story Jan 23 '15 at 1:29

Lateral trim is indeed important. Although there are no "limits" prescribed for most light GA aircraft you can get a feel for how an imbalance can affect flight in some of them.
This is actually a fun exercise in a Cherokee:

• Fill one tank to the tab (~16 Gallons, 96 pounds of fuel)
If you're solo this should be the right tank.
If you take an instructor the tank on the side of the lighter person.
• Fill the other tank completely (~25 gallons, 150 pounds of fuel).
If you're solo fill the left tank.
If you take an instructor fill the tank on the side of the heavier person.
• Start, run-up, take off, and climb out on the tank you filled to the tabs.
• Fly around for 30-45 minutes to burn off some fuel.
With typical fuel burns you should wind up with 6-7 gallons, or around 40 pounds of fuel left in the low tank. That's roughly a 100lb imbalance 3 feet out on the wing.
• Turn into any wind, slow to around 80 knots, bring the ailerons to neutral, and wait a few seconds.

In most cases if the aircraft is rigged properly there will be a slight roll toward the full tank (heavy side). You will find you need to maintain some light aileron pressure in order to keep the wings level.

Remember to switch to the full tank after completing this little demonstration.
You don't want the engine to quit on your way home - that would be embarrassing!

A similar demonstration can be arranged with a Cessna 172, or really any aircraft that has a fuel selector which allows you to burn fuel from one wing exclusively on demand, but you should consult the POH and/or a qualified flight instructor about the particulars of the aircraft's fuel system before you go and try this on your own. I happen to know it's relatively safe in a PA-28, but I can't speak for the fuel systems on other aircraft.

The net effect of poor lateral trim is that the pilot (or autopilot) needs to maintain some control input to counteract the aircraft's tendency to roll toward the "heavy" wing.
Large aircraft (and "fancy" light aircraft like the Beechcraft Bonanza) have aileron trim to help out with this, but for aircraft without that particular luxury the pilot needs to hold the yoke/stick to maintain the desired control input.

As others have mentioned, larger aircraft often have valves and pumps to transfer fuel between wing and fuselage tanks to help achieve proper lateral trim.
The best most General Aviation aircraft can usually offer in this regard is a fuel selector that lets you burn from the left or right tanks: You burn fuel from the heavy wing until it's not heavy anymore.

Non-Fuel loads can be managed in a similar way: If they're liquid provide several storage tanks and pump them around as needed (there are gliders that do this with water ballast, though I believe that's primarily for pitch trim). If they're solid (or a liquid load in fixed containers), load them so they're well balanced, and secure them so they don't shift in flight.

The weight imbalance is serious. There is even a fuel balance valve in most jets and a warning for fuel imbalance. However, this valve played a significant role in the accident of Air Transat Flight 236.

As well, in some small passanger aircrafts, you might be weighted prior to boarding and seated so that the plane is balanced.

• (I'm not a professional here. If a better answer appears, I may delete this one since it's not really detailed.) – yo' Jan 23 '15 at 0:23
• You don't need to delete it, unless it's wrong or irrelevant: if a better answer comes in it will just get more votes :) your post answers the question so is perfectly acceptable on the site, even if lacking a little in derail or scope – Jon Story Jan 23 '15 at 1:30
• @JonStory Well, if you think :) (btw, I've got about 40k rep so I do know how SE works ;) ) – yo' Jan 23 '15 at 2:00
• Hey, you could have just made one REALLY good answer that got 200 rep a day for like 200 days :p – Jon Story Jan 23 '15 at 8:14