GoAmtrak

I haven't seen any reports on the new Q400 thread, but how are they handling w/b issues with the F cabins? Would F pax be forced back, then allowed to move back forward after the seatbelt sign is off? What if it never comes off for the whole flight?

ASEFlyer

I've never had to wait for the seatbelt sign. Just the 10k ding.

freshairborne

The further the weight to be moved is from the center of gravity, the bigger effect it will have on the CG. I don't fly RJs, but the principles are the same. I've had to reseat pax from one end to the other on every plane I've ever flown at United.

Little-known fact, but not (yet) something the airlines consider when moving pax: an airline aircraft gets better gas mileage as the CG moves back.

Anyone being asked to move back should get a fuel-savings discount!

FAB

wdrg

Nice idea.

Interestingly, I was recently on a flight where the FAs insisted the pax all move to the front to make handing out the beverages easier.

Quokka

People shouldn't be assuming 1) that since the aircraft is past takeoff it's ok to move or 2) that FAs know squat about W&B issues.

W&B CG calculations aren't just for takeoff, they need to be within limits for the entire flight. Also realize that depending on the aircraft, its loading and use, the CG can move substantially as the flight progresses and fuel is burned. There's an NTSB report of a horrific crash where a commuter aircraft managed a takeoff but due to an out of limit CG it became uncontrollable on approach and crashed killing most on board.

rpcbind

Absolutely agrees, but if you're referring to the PC-12 in Montana, it was a bit extreme with one wing tank full, with the other empty (due to fueling w/o Prist). Definitely a W&B issue, though far beyond pax distribution.

FriendlySkies

Not sure how that would work. It's only happened to me on the all Y Q400s.

freshairborne

I am unfamiliar with the limits of any RJs, but it's not unheard of for there to be different CG limits for takeoff, cruise, and landing with some planes. True that the FAs, along with most others, would likely be unfamiliar with at least the CG (balance) issues. Most folks understand weight issues, though.

FAB

PSU Mudder

It's probably less likely to happen in a bird with an F cabin as there are fewer seats and less weight up front to begin with.

On the Qs without the F, everytime they have asked, I've volunteered to move because I could get a whole row to myself in the back. What I lost in leg room was easily made up by the increase in wingspan and personal space.

Quokka

Yes, the point is that keeping the CG within the envelope isn't just for takeoffs like some of the previous posters who change seats afterwards and the FAs that wink at them seem to think.

Even people who, very wrongly, think that CG matters for takeoffs only, should remember that any and all approaches/landing attempts could instantly turn into a takeoff. This is why most plane's CG diagrams usually sensibly show, for a given weight, the same CG limit range specified for takeoff, landing and approach.

And yes, planes can have different max takeoff and landing weights, they get lighter as they burn fuel, and some have larger permissible CG ranges at lower weights. But that doesn't mean N pax deciding to move themselves from from zone X to Y are within that range. On some CRJ types for example, while the CG range envelope is slightly wider at lower weights, the forward CG limit for takeoff/landing/approach is the same %MAC value for all weights.

No, I was thinking of a BE1900 crash that killed 18 people.

LaserSailor

You should take 3a or 4 A anyhow, especially if you have an overnight bag. You have to salmon swim to get your bag from 1A or 2A...

Hammer0425

Maybe from a major shift of cargo but certainly not from passengers moving seats. I can't think of a single scenario where passengers cannot move seats during flight that would affect the stability of the aircraft. Take off is a different story but after that point, you can move.

freshairborne

Here's an example of how the CG limits of a 757 work. First, the limits are expressed in %MAC (Mean Aerodynamic Chord-simply put, the average distance from the front of the wing to the back of the wing) instead of in linear measurement. The results are the same, though. Second, there is no envelope printed in the airplane flight manual. It is a table of weight vs %MAC figures, and the forward MAC figures vary depending on whether it's for taxi, flight, or takeoff & landing. The aft limit is only dependent on weight.

Our (UAL) load planners take the final weights and locations of the pax, cargo, and fuel and crunch the numbers, then send the CG location & elevator trim settings to us either just before pushback or more commonly during taxi. We, as pilots, do not do these calculations, we just take their numbers and make adjustments according to them.

I think pilots from some other airlines might do these calculations themselves on a worksheet of sorts, but I am not sure of this.

In my time flying before United, I did these numbers before every flight, but in the last 26, I've never done them on a United plane.

The point of all this is that the CG limits can change depending upon weight as well as phase of flight or taxi. That's how the 757 is; the 767-300 doesn't care what phase of operation it's in, just what its current weight is. Suffice it to say, though, that the range is pretty small. For a max weight 767-300, the fore and aft limitations are within 6.8 percentage points of the MAC.

What are the dimensions of the MAC on these planes? Ellefino!

FAB

No, not necessarily true. It depends upon any one or more of these: the particular limits for a given airplane, the amount of weight moved, and how far it's moved.

It's unlikely that someone not familiar with all of the limitations on a given airplane and was able to calculate them "on the fly" could decide if moving during flight is within legal parameters.

But of course, that begs the question of how someone can walk from the front to the back without causing problems.

The short answer is that the manufacturer has determined that the CG shift is not enough to cause problems in flight, where the aerodynamic control forces available are greater (due to greater airflow over the control surfaces) than they are during takeoff or landing.

So a little weight moving a little distance isn't going to be a big deal. A lot of weight, bigger. A lot of distance with a lot of weight, biggest.

Also, the relative amount of weight vs the weight of the aircraft is a factor.

This stuff is right out of Aerodynamics for Naval Aviatiors and a few thousand other "Ambien-replacement" books.

FAB

LaserSailor

Old thread alert - That crash, IIRC, was principally caused by inadequate pitch control due to incorrect maintenance, and weight/balance was a minor contributing factor,

Trying to fly a plane without pitch control, ugh.