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This is something I've wondered about for a long time. On really large airplanes, like a 747-400 or A380 for example, I would assume that the weight of the passengers on board is a significant to the flight characteristics of the plane. And it seems like it would be pretty easy to put all the passengers in an odd place (like all at the front or back) and as a result significantly change the flight characteristics of an aircraft.

So, assuming I'm right (tell me if I'm not), how do commercial aircraft counter this problem? Do they do their best to distribute passengers throughout the craft with seat assignments? Or do they have ballast they can move around (like fuel ballast)? Or is the weight shift not significant enough to warrant anything along these lines?

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On the EMB-145 the passenger cabin was split into 5 zones and we were given passenger totals in each zone from the flight attendant prior to leaving the gate. If balance is off (this happened quite rarely for us) you would tell the flight attendant "I need 2 passengers from zone 5 moved to zone 1" or "I need a passenger from zone 3 moved to zone 2" or some other similar instruction. The flight attendant would solicit volunteers and if no one did, they would instruct someone to move. The move was only for takeoff and once airborne they could go back to their normal seat.

After moving passengers we would re-calculate the weight and balance to verify we were in the envelope and set takeoff trim.

Similar methods are used in larger airplanes with the cabin split into zones.

The only time we would use ballast is when we had a full cabin and not enough bags in the cargo (not common). In those cases we would ask ops for ballast and they would put something in the cargo. They would try to make it useful if an outstation needed something else they would just load 50 lb bags of sand.

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What kind of weight figures are used to determine the weights? Do you assume everyone weighs around 150lbs? Or is there some sensor somewhere? I've just never been put on a scale before boarding, so I have to wonder :). –  Jay Carr Feb 10 at 15:42
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In the summer everyone weighed 170 lbs and in the winter it was 180 (or something close to these numbers). Each airline will use standard weights which either come from the FAA (higher weights) or the airline can go through some process to use their own winter/summer weights. –  casey Feb 10 at 15:44
    
So, just to check, in your experience the airline wouldn't assign seats during the ticket purchase phase in order to create a proper balance? –  Jay Carr Feb 10 at 15:45
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Considering passengers can change to empty seats at kiosks or with any agent willing to help them, I don't think there was any algorithm in the ticketing system, nor do I believe it was even aware of the passenger zones (I know it didnt know which rows had extra O2 masks, for example). On a light flight, people also move around from assigned seats. We check weight and balance on every departure and we are the final (and only) word in ensuring we are in the envelope. –  casey Feb 10 at 15:48
    
Well, that does make sense, you are the one flying the plane after all. Thanks for the info :). –  Jay Carr Feb 10 at 15:49
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A few items just to add to the background info:

  1. Large aircraft aren't just interested in being within the envelope, but also want to be close to an optimal c.g. location. For the 747-100, -200, -400 a zero fuel weight c.g. of 26.6 percent of the mean aerodynamic chord is the usual aiming point. This is done to minimize fuel burn while still staying within limits as fuel is burnt.

  2. Large aircraft may also require different operating procedures when the c.g. is within certain parts of the c.g. envelope. The one that occurs to me offhand is the requirement for using a low-gross-weight-aft-cg takeoff procedure for 747s. There's also a restricted forward c.g. limit on the 747 when an operator has elected to use increased aft c.g. loading limits.

  3. As your airspeed increases, a c.g. close to the forward or aft limits becomes less critical because the tail plane has more authority.

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I'm not sure how sensitive/flexible the widebodies are but on the smaller regional jets, the crew will use the "whizwheel" to determine CG location. The aircraft is divided up into n regions (depends on how large/long the aircraft is). If the CG is too far forward, the crew will require that a few passengers to move from the forward region to the aft regions.

I'm certain that the same thing happens on the larger jets but the crews receive weight an balance from dispatch or whomever does it for them for the purposes of review.

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So, to your last comment, some airlines are large enough that they calculate all of this out prior to boarding, while some airlines just do it on the ground before take off? –  Jay Carr Feb 10 at 15:44
    
In those cases, the pilots will send the passenger boarding totals to dispatch via ACARS and get a report back with weight and balance information. –  casey Feb 10 at 15:54
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Jay Carr, its always done before pushback. At some airlines, the pilots do it with the mechanical calculator "whizwheel" and paper, and some pilots get it from dispatch. Either way, its computed and verified prior to pushback. –  h54 Feb 10 at 16:01
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@h54 Sorry, I should have been more careful with my wording. "On the ground" meant, "before pushback" in my mind. Thanks for the clarification. –  Jay Carr Feb 10 at 16:42
    
No problem at all. Glad I could clarify. –  h54 Feb 10 at 16:43
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On an under-booked flight the passengers are spread around through seat assignments.

Emergency exits are kept clear, and if someone tries to sneak there for the extra leg room they are asked to return to their seats (at least the underage passengers are).

Source: I was on such a underbooked short haul flight for a school trip, we plus a handful of other passengers only filled a third to half of the plane. On the return flight we coincided with another school trip returning and there was a clear split between our class and the other school.

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The emergency exits don't have to be kept clear. The seats next to them however may only be occupied by persons able to open them, so they have to be able-bodied and adult. –  Jan Hudec Feb 10 at 18:59
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