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Skisandy
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Center Of Gravity With Large Business Class?

Thu Nov 28, 2013 8:21 pm

I just flew on a United 777 IAH-GIG and observed that about half of the plane's length was business class, the other half economy (where I was, unfortunately).

My question is: Doesn't the concentration of pax in the back affect the CG significantly? There are literally double the number of passengers, per square foot, in the far back, than there are up front and in the middle of the plane.

Sure - the business class SEATS may weigh more, but then - there are double the number of economy seats in the same square footage in back.

At times - I see that an airline moves people around before takeoff, if there are too many up front or in the back, to get the CG right.

Of course, this is not possible with the business/ economy divide. In fact - business class on that flight was half empty, and economy was totally full - all of us way back behind the wing.

This doesn't look right to me, but apparently it works. Any comments? Thanks.
 
roseflyer
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RE: Center Of Gravity With Large Business Class?

Thu Nov 28, 2013 8:32 pm

On a 777, the CG range is wide enough so that it does not matter where people are seated as much as it does on small airplanes. The regional jets and turboprops are more significantly impacted by CG and you'll see passengers being forced to move based on CG requirements. An ERJ-145 with its long narrow cabin and natural desire for people to sit up front and away from the engines often do require moving people around.

On a 777 with a large business class cabin, economy weighs more when occupied. Business class seats can be heavy, but the passenger weighs more than all but the most elaborate first class seats, so economy does push the CG aft. It's usually accounted for when load planning where the CG is of the plane. Not all 777s are the same as far as I know, but I'm not an expert.
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Cubsrule
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RE: Center Of Gravity With Large Business Class?

Thu Nov 28, 2013 9:25 pm

Quoting Roseflyer (Reply 1):
An ERJ-145 with its long narrow cabin and natural desire for people to sit up front and away from the engines often do require moving people around.

  

UA CR7s--with their large F and Y+ cabins--can also have trouble when fairly lightly loaded.
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MesaFlyGuy
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RE: Center Of Gravity With Large Business Class?

Thu Nov 28, 2013 10:43 pm

I had a trip with UA once from EWR-MYR where the plane was about 60% full. They had to move about 10 people further back, but they were nice and asked for volunteers. I would've done it had I not paid extra to be in Economy Plus.
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AR385
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RE: Center Of Gravity With Large Business Class?

Thu Nov 28, 2013 11:40 pm

Quoting Skisandy (Thread starter):
At times - I see that an airline moves people around before takeoff, if there are too many up front or in the back, to get the CG right.

I remember once on an AR 747SP MEX-LIM they did that. They stressed it would only be for takeoff. They didn´t move anybody in F though and at the time there was no C.
 
citationjet
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RE: Center Of Gravity With Large Business Class?

Mon Dec 09, 2013 12:31 am

Quoting Skisandy (Thread starter):
At times - I see that an airline moves people around before takeoff, if there are too many up front or in the back, to get the CG right.

Many years ago I was a non-rev on an AA 767-200 from ORD to BRU in First Class. During the taxi out to the runway, we were asked to move to the first row in the economy section just for takeoff to help with CG. About two minutes after takeoff the flight attendant motioned for the two of us to return back to our FC seats. I remember it being difficult to walk "uphill" to FC during the climbout.
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Starlionblue
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RE: Center Of Gravity With Large Business Class?

Mon Dec 09, 2013 12:59 am

Quoting Roseflyer (Reply 1):

On a 777, the CG range is wide enough so that it does not matter where people are seated as much as it does on small airplanes.

Just to give you an example, here is an example load sheet for a fictitious 737 Classic (the one used in the EASA ATPL exams). As you can see in the graph bottom left, the acceptable CG range at MTOW is somewhere between 8% and 26% of mean aerodynamic chord (MAC). The MAC on a 737 is something like 4 meters so this would be about 70cm. Doesn't sound like much but looking at the boxes above the graph, you can see how you can move literally 100s of kg of freight and dozens of pax back and forth, and still be well within acceptable CG range.

A 777 is a much larger aircraft so the tolerances are greater.

Still early morning here so forgive me if my back of the envelope numbers are off.

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dispatchguy
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RE: Center Of Gravity With Large Business Class?

Mon Dec 09, 2013 9:36 am

When an airline takes an aircraft - they create what is called an LOPA - Layout of Passenger Accommodations. That shows the arms and moments of the various rows, cabin arm centroids, galleys, and all the other stuff in the pax cabin, and the centroids for the cargo bins.

They then take the certified weight and balance envelope, and curtail it to their operational envelope. Lets say that Airplane X has a certified envelope from 5%-30%. They will then take the curtailments out - passengers moving from cabin to cabin, flight attendants moving around, retracting flaps and landing gear (if they change the CG at all - some do, some dont), galley carts moving around, last second bags - that curtails the operational envelope now to 8%-27%.

UALs 777 LOPA will have been adjusted when they created the new cabin layout, and their new operational curtailments would have been created and loaded into their W/B system. When they drew up the new LOPA, they'll discover how the new cabin interior will affect the CG.
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PhilBy
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RE: Center Of Gravity With Large Business Class?

Mon Dec 09, 2013 11:30 am

Quoting Roseflyer (Reply 1):
Business class seats can be heavy, but the passenger weighs more than all but the most elaborate first class seats

If we believe what we read there was an issue a few years ago with 340-600's where the excessive weight of the first class suites pushed the CG too far forwards. The weight/sqm of these exceeded the weight per sqm of the cattle class.
 
Tod
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RE: Center Of Gravity With Large Business Class?

Wed Dec 11, 2013 6:50 pm

Quoting dispatchguy (Reply 7):
When they drew up the new LOPA, they'll discover how the new cabin interior will affect the CG.

The LOPA gives a general feel for the outcome, but does not contain all the required data.

As a reconfiguration integrator, once our engineering is complete a new w/b document is created.

In addition to the factors already mentioned, the structural, mechanical systems, actually monument weights and other modifications are included in the computations.

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