I have a question. This came to mind after recently flying on a commuter plane where the pilot requested a few passengers move aft until after takeoff. How is the CG determined on a commercial airliner, where the number of people vary and the size of the people vary also along with the size and weight of the luggage / cargo. With the advent of supersizing fast food meals, I would think the variety of the weight range of passengers would have an effect on the CG. I know with the r/c models, 1/4" can make a huge difference. Just thought I would toss this out, to satisfy my curiosity. Thanks

Depends on the size of the plane. Commuter planes are more sensitive than wide-bodies to passenger placement.
Although a passenger's movement from front to rear can be detected in a wide-body, as most of those fly fully automatically, only the autopilot knows.

I used to fly on P'3 Orions when I was in the Navy. Our flight engineer would calculate "Weight and Balance" as part of his preflight.

The engineer's calculations would start with the dry weight of the plane (about 65,000 lbs). Then he would add the weight of fuel, about 200# per passanger, then other weights for additional items (luggage, torpedos, etc.). He even had a specialized 'whiz wheel' (slide rule) to help figure this out. About the only thing that would change were people moving around (taken care of with the trim tabs and autopilot) and the burned-off fuel. Oh yeah, dropping a torpedo would make a difference too, but the trims helped there too.

I imagine that it's pretty similar for airlines, large and small.

Just my input.

Bob

We had a Tristar doing demo's in South America. At Lima. Peru one day...(high and hot), one takeoff almost didn't take off!
The various speeds, V1; can't stop now, Vr; rotate, V2; liftoff were computed for about 50 people.
When the pilot looked back into the cabin, he saw close to 200 people! Marketing had invited everyone in the terminal to take the flight!
Something like a Shorts 330 with that no-chord wing... It must be a bear when the weight shifts!

Like the others said it IS an issue. It's still an aircraft after all.

I suspect the ticket agents seated everyone from front to rear and didn't take note that the plane wasn't full for some reason.

With even a group of 20 or so the normal size distributtion of the people will all average out. The only time this would be tossed in the flusher is when you need to transport the entire Japanese Sumo wrestling team in first class at the front of the plane and they all insist on sitting together....

Normally a foward balance as your commuter had would be considered "safe" but I wonder if there were issues with the rotate speed involved. Being that far forward the Vr would rise as the elevator authourity would need to be more than normal and that would lengthen the takeoff roll. Perhaps to excess for that runway? Not being an airline pilot I'm just guessing of course but I do know that the power on those types of aircraft isn't like our overpowered models. They need to worry about stuff like that.

Heard an interesting tale related on Saturday morning on one of my favourite radio programmes.

The show host had been given a trip to Scott Base and flew down in a C130. On arrival, the passengers were asked to remain seated; and again; thumping and banging on the cargo door; still came the call "Please remain seated".

They did; for four hours while they unstuck the rear door which had frozen up. They had to get the first two cargons out before the passengers could disembark. Otherwise the Herky would have sat on her tail.

Plastic:

The CG of an airliner is determined by using a loads form. On this form the weight and moment arm of the plane, passengers, cargo, fuel, etc. are all added up. The position of the resulting weight/moment index is then plotted on a diagram of the CG envelope. Standard passenger weights are used and vary depending on the time of year (heavier in the winter). Standard weights are also used for luggage. Bulk cargo has to be weighted individually. Standard weights for unussual loads such as a football team charter are of course diferent to those of regular passengers.

The whole process of preparing this form by hand takes all of 5 minutes. For larger airplanes, the process is often computerized and performed by a loadmaster at the station, although doing it by hand is still not that big of a deal.

Airliners have very wide CG ranges to accomodate all kinds of loads. Also, if one section of the airplane has more passengers than another, say coach is full, but business and first are empty, the plane can be balanced by appropriately distributing the cargo in their compartments to compensate. It is not uncommon to have to move people around in a small airliner.

cirrus

I recall reading about a large turbine aircraft way back when that crashed on take off. The cargo pallet containers were not secured properly and during the take off roll they all rolled back drastically changing the cg and crashing.

I have heard some Mooney pilots say that when trimmed for a climb if they shift their head back and forth they feel it in the plane controls. Glad my Cherokee is not that sensitive.

I was a loadmaster on c 130's a long long time ago! The cg becomes more critical as the weight approaches the max takeoff weight. The permissable cg range becomes less probebly because of the limitations of the control surfaces to move that weight. also.it is not unusual to take off at a weight that is too high to land at because the stresses of landing are higher than takeoff so you have to dump fuel b 4 landing, not a happy thing to have to do if your having a problem in the first place!
On large full size planes we use simplified moments;the true number of moments divided by 1000 so 50 people that weigh 20 lbs more than you thought don't really have that much effect.
Fuel and cargo have the most effect .you cannot move the fuel and there are weight limitations
on the deck that varies thru the cargo compartment.
As the weight goes up due mostly to fuel the cg moves foward so I suspect that commuter plane wanted people to move to the rear until he had burned off some fuel.. On that small a plane the weight of the people had more effect. I am a beginner in rc so I apologise for the lecture. Also,my info is dated(I'm old). I'm sure there is more accurate info available. Just trying to help in an area I have some limited knowledge. CAZ

Caz, that is pretty much as I "heard" it. The flight Christchurt - McMurdo is something like 11 hours with a go/no go at 5 hours out of CHCH for weather. As this was also one of the early flights for the season (third or fourth I believe) they would also have been heavy on cargo as well. The item reported a "luggage limit" of 20 lbs per person. For a female reporter that would be just the makeup kit (???? ) A photographer who was on the flight took his cameras and the clothes he stood in...jokes about sleeping out with the dogs were made.

I can imagine that the fuel burnoff would create some problems at the other end, especially if they optimised CG range for the mid flight area. Is this how it is done for long flights?

Can't resist this one!! [sm=biggrin.gif]

I developed the weight and balance system for our DC9/MD80 series fleet, and we do indeed use FAA approved standard weights for passengers and baggage. (those weights were increased in May due to the recent Beech 1900 crash where the airplane was apparently overloaded, and possibly loaded out of CG).

On the MD80, baggage is normally placed in the aft most bin, then the mid and then if necessary, the forward bin. This is based on experience that the CG will move forward as more pasengers are added (see below), and loading weight aft first will assist in keeping the CG within limits. If the airplane is very lightly loaded (bad news for an airlilne) with less than 40 -50 passengers, we split the bags between the mid and aft bins.

Once the passengers, bags and fuel are loaded, the actual pre-departure calculation can be made either on paper by the flight crew, or by an approved computerized system.

On the DC9/MD80 series, the cabin is divided into two parts - forward and aft, with the dividing line around row 17, roughly mid cabin. The passenger service agents assign seats according to a specific plan, more or less evenly, to keep the airplane within limits. Due to its length, the MD80 starts out with an empty CG very near its aft limit, and as passengers are added, the CG moves forward, so that with all 165 seats full, the CG is very near the forward limit.

The inflight movement of passengers and flight attendants with 200 pound galley carts makes a significant difference in CG and trim, (and surely focuses your concentration if the autopilot is inoperative) and must be accounted for by reducing the manufacturer's forward and aft CG limits accordingly. The result is that the "company" CG limits are more restrictive than those established by the manufacturer.

One other item that can affect an airplane's sensitivity to CG is the galley and seat layout. Since the MD80 CG tends to move forward with passenger loading, if the airplane has an aft galley (which is included as part of its empty CG condition), occupancy of the forward cabin is difficult to balance with aft cabin passengers, causing these airplanes to be very forward CG sensitive. Occasionally with this configuration, we do have to move passengers to remain within our CG limits... On the MD83, we also have auxiliary fuel tanks forward and aft of the wing, into which we can load ballast fuel if the flight is relatively short and we don't otherwise need the fuel.

Generally speaking, on the MD80 series it is easier to move passengers than baggage, since with normal passenger loads, the CG will be going forward, and the aft bin will already be full, leaving no room for additinoal bags to move the CG aft.

HTH!

Jim

probligo

For our MD80 fleet, the enroute fuel burn is not an issue, even at our max endurance of 5+ hours, since the fuel tanks lie on or near the CG and our procedures require it to be burned so it will not disturb the CG.

On 747s and some other large airplanes (the XB-70 and Concorde, before it stopped flying), fuel is stored at locations far away from the CG ( in the horizontal stabilizer for long distance flights on the 747-400, for instance), so flight crew procedures are developed to ensure the fuel is burned so as to keep the CG in limits.

As I recall, the Concorde and XB-70 also moved fuel to retain the optimum CG as a function of Mach number, but that is the extreme limit of my understanding....

HTH

Jim

I am an airline pilot, currently a Captain on Boeing 767 for British Airways, but I have flown most of the fleet over the years, (not Concorde) but including a 44 seat (the HS 748) and a 64 seat twinTurbo prop, the British Aerospace ATP.
As others have said, the take off weights and CG positions are mostly computed by the Dispatcher (loadmaster in the military) using a computer, but when I flew the twin prop commuter type aircraft we often had to do it ourselves.

Using a prepared form you fill in the weights of that individual aircraft, and its empty CG.
Then add the crew and pantry weights.
The loaders and check-in staff tell you the number of passengers, the number of bags, and where the passengers are seated. The seating is usually approximate - the cabin is divided into 4 sections and you get the number in each section.
Using standard weights (say 78 kg for a male, 68 for a female) you work out the effect of the passengers on the CG, then tell the loaders where to put the bags to optimise the CG (usually towards the aft end of the available range).

Sometimes with half a load of commuters (who don't carry heavy luggage) you have to move them around to get the CG right. The 748 and ATP were very nose heavy when empty. We had to put half a ton in the aft cargo compartment for an empty positioning flight. So with half a load you had to tell check-in to leave the front quarter of the cabin empty. The door is at the front so what do the lazy passengers do? They sit near the front anyway and we have to move them back to their assigned seats. And they have to stay there for the whole flight. If the aircraft is nose heavy on landing it takes quite a heave to get the nose up in the flare.
The ATP had a straight wing so fuel use did not significantly affect the CG, but on faster swept wing aircraft the CG has to remain within limits for the whole flight.

I hope that answers the question. CG is critical, even on airliners. Passengers normally tend to spread out, but a check-in plan helps distribute them evenly. Some aircraft are naturally nose heavy so you fill them from the back, and move people aft if necessary.

Probligo, I made the flight from Christchurch to McMurdo a couple of times, but not in C130s. I allways had the pleasure to be in a C141. The first trip I made in August 1998 was a turn around or as we called them "boomarang" flight. We flew all the way to McMurdo only to abort the landing due to crosswind. Five hours down and five hours back! A C130 could make it in nine to eleven hours depending on wind, a C141 or a C5 took five hours. Now some flights are made in C17s and I've heard that they are faster. I was allowed to carry 70 pounds of baggage and that was for a six month stay. I was lucky to have some friends in the RNZAF SQ3 that would invite us over to Scott base for drinks on Sunday afternoons, I put away alot of Lion Red and CD with that group.

Geez, and I complain about my 24 hr layovers............

cirrus

Is the 737 not as sensitive as some other planes? The fuse looks shorter than some others. This thread just got me to thinking of the times I've flown Southwest. They don't do assigned seats, and I've seen half-empty planes with everyone sitting right up front.

A guy in my club works at the new Air&Space museam out at Dullas. He was talking about the Concorde's CG the other day. Apparently, the one they are putting on display will have a large pile of sandbags inside, since if you totally defuel a Concorde, it sits on it's tail. He also said that at supersonic speeds, the control surfaces don't move, all control is though CG shifting by pumping fuel fore-aft, and side-to-side. I don't have any direct knowledge of this though, but it sounds possible. (Aparently the Concorde doesn't really do much more than fly in a very straight line at 50k feet at supersonic speeds.) Can anyone who acutally worked with one verify this?

Rotorwrench,

You lucky B***** getting down to the ice. It has always been a dream of mine. I think that the Herky would be more fun, but a whole lot less comfortable than the C141.

I can still dream...[sm=sleeping.gif]

The 737's shorter fuselage makes it easier to keep in balance, but am unsure if it can remain within limits with half a pax load all sitting in front [&:]... When developing a weight and balance program, one of the initial assumptions is that people normally spread out randomly*, and will not "clump up" in a group, so if a half load was on the airplane, human nature would have them dispersed somewhat

MD80 series airplanes are "twitchier" on their CG limts due to the long fus... The price of doing 2x3 seating instead of 3x3 as done by Boeing in the 737s.

Best,

Jim

* FYI, when developing a weight and balance program, the seats are "loaded" mathematically using the window seats forward to aft, then adding the aisle seats, then the center seats. The process is repeated by "loading" the window seats aft to forward, then the aisle seats, then the center seats. The most restrictive condition is used to set the operator' CG limits.

Kirk:

Fuel was pumped around in Concorde to keep the CG within limits with the added benefit of reducing trim drag. But aircraft control in cruise was through the control surfaces like in any airplane. Moving fuel to a tail tank to reduce trim drag is common in long range airliners. Because of Concorde's wing/fuel tank design, there was a lot of CG shift with fuel burn necesitating a lot of fuel pumping around the system.

cirrus