It would "seem" that having more weight on one side of an aircraft [one extra seat per row, one extra passenger per row, some possible extra weight in the overhead bin on one side] would make an aircraft slightly unbalanced.
If an aircraft has 20 rows of extra seats on one side, those seats must weigh something...each seat weight x 20. If a 100 pound passenger is in each of those extra seats, that's 2000 extra pounds if all those seats are filled.
Perhaps this weight is negligible in the grand scheme of aircraft weight. I've noticed that on the DC-9 that the overhead bins on one side of the plane are larger than those on the other side. More weight there?
At some level I realize that these aircraft are not unbalanced, but don't know why. Can anyone provide me with the "why"? Thanks in advance for any information and insights?
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- GalaxyFlyer
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Re: Do 5-abreast setting arrangements unbalance airliners?
One word —Arm. Moment equals Weight times Arm. Not enough arm. We could put 18 pallets weighing 90,000# on one side of a C-5 and nothing on the other side, no problem, not even noticeable.
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- convair880mfan
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Re: Do 5-abreast setting arrangements unbalance airliners?
That's very interesting. I'll have to do more reading on that. Thanks GalaxyFlyer.
I guess that puts to rest the myth that when passengers move inside an airliner that the pilot has to retrim.
I guess that puts to rest the myth that when passengers move inside an airliner that the pilot has to retrim.
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- GalaxyFlyer
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Re: Do 5-abreast setting arrangements unbalance airliners?
Well, longitudinal pax movements might depending their portion of the total weight and arm. In a Citation, someone moving forward to the small galley would cause a fair bit of trim change, same person in Global might have a very small trim change, in a 747, none at all. Side to side, not noticeable in any plane that I’ve seen. I’ve dropped munitions off on side of a fighter and slightly noted it until pulling G on the dive recovery and suddenly 5 Gs just made it quite noticeable.
Re: Do 5-abreast setting arrangements unbalance airliners?
convair880mfan wrote:That's very interesting. I'll have to do more reading on that. Thanks GalaxyFlyer.
I guess that puts to rest the myth that when passengers move inside an airliner that the pilot has to retrim.
That is not a myth. As passengers move forward and aft. The aircraft retrims to account for the movement of the passengers
A flight attendant told me that every time she walked up to the forward part of the aircraft the pilots opened the cockpit door and told her hello
After a few iterations she asked how they knew she was there? They told her that they could see the autopilot trim out her movements and they could tell whether she was moving forward or aft depending on which way the trim wheel was moving.
For airplanes athwartships placement of mass is not a big deal. But for helos it does matter where the mass is athwartships-wise (side to side) because the center of gravity needs to be within limits under the rotor disc and not just forward/aft like it is for airplanes.
Re: Do 5-abreast setting arrangements unbalance airliners?
Many years ago a LOT flight became unstable and almost crashed because the pilot told the passengers to watch something interesting on the right side of the airplane.
Too many poles moved to the right-hand-side.
Sorry, I know it's old, but I couldn't resist.
Too many poles moved to the right-hand-side.
Sorry, I know it's old, but I couldn't resist.
Re: Do 5-abreast setting arrangements unbalance airliners?
GalaxyFlyer wrote:One word —Arm. Moment equals Weight times Arm. Not enough arm. We could put 18 pallets weighing 90,000# on one side of a C-5 and nothing on the other side, no problem, not even noticeable.
Bingo! And on a 5-wide aircraft, the outer 2 seats on each side are in balance with each other, so the weight difference is in a single row of seats and an aisle on the centerline, which has the absolute least amount of moment arm.
Re: Do 5-abreast setting arrangements unbalance airliners?
The one incident where the pilot moved the passengers to gain better aircraft control was Delta 1080 with the jammed elevator
He moved all of the passengers to the front of the cabin. By doing so he was able to regain some pitch authority between using the engines and stabilizer.
Before the CG change he and the FO had the control column at the forward stop and the aircraft continued to pitch up. He was able to stop the pitch by retarding the engines.
He moved all of the passengers to the front of the cabin. By doing so he was able to regain some pitch authority between using the engines and stabilizer.
Before the CG change he and the FO had the control column at the forward stop and the aircraft continued to pitch up. He was able to stop the pitch by retarding the engines.
Re: Do 5-abreast setting arrangements unbalance airliners?
Early eighties flying the Fokker F27 with no autopilot we used to quantify the cabin attendants in hundreds of feet per minutes. A petite lady maybe only a couple of hundred feet as she moved up and down the aisle. Others far more…
Re: Do 5-abreast setting arrangements unbalance airliners?
Spacepope wrote:GalaxyFlyer wrote:One word —Arm. Moment equals Weight times Arm. Not enough arm. We could put 18 pallets weighing 90,000# on one side of a C-5 and nothing on the other side, no problem, not even noticeable.
Bingo! And on a 5-wide aircraft, the outer 2 seats on each side are in balance with each other, so the weight difference is in a single row of seats and an aisle on the centerline, which has the absolute least amount of moment arm.
Not quite correct, actually it’s the aisle that’s offset
There isn’t more weight on one side of the aircraft because the seats closest to the aisle on the three seat row are actually on the aircraft centerline. The two seats to the left and right of that balance the lateral load
It’s the aisle that’s offset so any lateral imbalance will only be from the passengers that are in the aisle at that moment, that’s a negligible amount of weight
If you walk from coach to first class on a DC9 or MD80 you can see this, as the offset aisle takes a ‘jog’ to the right where you enter the front cabin and the aisle then runs in the center of the cabin
between the 2-2 seating
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