My flight just announced that it will be pretty empty, and that it is important for everyone to sit in their assigned seats to keep the weight balanced. What would happen if everyone, on a full flight, moved to one side of the plane?
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The front to back balance is a lot more important for an aircraft. It sort of hangs from, or balances on, its wings. It’s a bit like a seesaw front-to-back, and the weight on both ends must be equal or it’ll want to tip. The stabilizers (small horizontal wings on the fin) can compensate for this in some regard, but if the balance is too far off, the compensation is so severe that there’s not enough leeway left for maneuvering.
Moving from side to side will probably not do much. Except getting you yelled at by the crew for not following clear safety instructions.
I routinely fly nearly-empty ferry flights (when planes need to be moved due to maintenance, repairs, upgrades), usually 10-15 people in an A320 including crew. We get the same briefing, but really all it does is annoy the pilots slightly, because they have to re-trim the plane (turn a knob or command the flight confuser to do it). They seat us mostly over the wing (plane’s center of gravity). It could be potentially dangerous if we all moved to the rear during a cognitively difficult flight phase, e.g. turning for final on a busy airport. I wouldn’t want to add to the pilot’s workload at that moment.
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Former aircraft load-planner here.
To answer OP’s question, much of this would depend on the size of the aircraft, but shuffling weight around on a plane is always something of concern.
Moving from one side of the aircraft to the other wouldn’t have nearly as much of an effect as moving everyone from the front or to the rear.
Think about the shape of a plane as a big lever with the fulcrum running sideways through the wings also down the middle of the plane.
The plane is not very wide, so moving everyone to one side wouldn’t give you much leverage to affect the plane’s orientation.
However, the plane is much longer front-to-back, so adding all the weight at either end will have a much greater effect.
There is so much that goes on behind the scenes with aircraft operations, it’s fascinating.
And while it’s not for the faint of heart, if you want a dramatic example, read about the crash of National Air cargo flight 102 in Afghanistan.
In this instance, they were carrying a heavy load of wheeled vehicles. When taking off, the vehicles were not secured properly and slid to the back of the aircraft throwing off the center of balance to the point where the pilot could no longer control the plane.
This is a dramatic example.
If all the passengers on a 747 went to the back of the plane, the pilot would likely just compensate for this with some added trim or moving fuel.
EDIT: I’ve also dated a lot of flight attendants. Sometimes they just tell people to sit where they are so it’s less ass-pain for them with people running all over the place. And they also have to maintain (somewhat) the integrity of the fare-system for seats.
If you fly model RC planes or paper airplanes you get to learn this lesson hands on but without all the crashing and dying. You want a plane that has a *center of gravity* located very close to the *center of lift* or the plane will be hard to control.
Center of what, now?
Planes like anything have a *center of gravity* (CG or CoG) basically the average location of all weight—the force pulling the aircraft toward the earth. For example, the CG of a uniform sphere is the center. With a lollipop, CG is closer to the middle of the sweet part because it’s heavier than the stick.
Similar to the concept of CG is the *center of lift* (CL), which is the average location of all upward (aka lift) forces of the wings.
Planes are symmetrical left to right. And cargo/people are located in the middle, left to right. So what matters then is matching the CG and CL along the long axis of the aircraft.
Imagine you balance a [wood yardstick](https://www.didax.com/pub/media/catalog/product/cache/541f549daeb5bdd8bd77b8568b2d1c3a/2/1/211553.jpg), which represents the plane, on your index finger. To do that it has to be located in the middle. Your finger/arm/muscles provide the lift forces. Gravity represents gravity (:)).
Pretend the 0″ mark is the nose and the 36″ mark is the tail.
Now tape a roll of quarters at the 24″ mark. Try to lift the yardstick from the middle and what happens? The heavier end rotates down around the lift point. The plane is tail heavy and wants to pitch up. Because the CG is behind the CL.
Likewise, a roll of quarters at the 12″ mark rotates down on that end. The plane is nose heavy and wants to pitch down.
Flying RC planes I find that a slightly nose heavy plane is easier to control than a slightly tail heavy plane. A very nose heavy plane is very hard to control and I’ll likely crash and a very tail heavy plane is impossible to control and I’ll definitely crash.
One theory about what happened with Flight 93 on 9/11 is that when the passengers rushed the cockpit it unbalanced the plane, with the last audible words from the cockpit being that they needed to pull up.
Side to side, nothing at all. You can only be a step or two off center.
Front to back is a different matter. If everyone is at the front or back of the plane, the elevator control is used to raise or lower the nose to keep it level. That’s normally no problem. But there are limits. The center of gravity needs to be within those limits or it is possible for the plane to become uncontrollable when the elevator is no longer enough to keep it level.
On 1 “side”? Not much. Airplane W&B is more concerned with fore and aft, not really sides though. Helicopters do care about this though
It’s not side to side you have to worry about, it’s front to back. There’s something called weight and balance that is calculated for every flight of every aircraft with passengers, even small Cessna 172s. Airplanes need to have the weight centered around the middle (chord) of the wing because very bad things can happen if the center of gravity is too far off.
One side as in left right? Not much. One side as in front back? It could make a huge difference. Before each flight the weight of an aircraft is calculated and the weight distribution is adjusted. This decides how the cargo will be loaded, how the fuel will be loaded (there’s multiple fuel tanks on an airplane), and how the passengers are sitting is taken into account. If the passengers are not sitting where they’re supposed to, this can throw off the calculations, and the weight distribution of the aircraft. Since people, if they’re free to do so, will probably choose to mover further forward, this can make the aircraft nose heavy. Depending on just how many people we’re talking about this can create a situation where the nose of the aircraft will want to drop and the elevators at the tail (the horizontal fins that control the pitch of the aircraft) won’t be able to compensate for it, especially during landing when the aircraft is approaching slowly, and the slower it’s going, the less control authority and stability it has.
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