How Centre of Gravity (CG) is adjusted?

Question

I've gone through the following questions on Stack Exchange and other few questions too:

Why is managing CG (centre of gravity) important?

How are the limits of the center of gravity chart established?

I still do have 2 questions in mind:

1. How do they adjust the fuel quantity/weight based on the passengers' weight on the plane? (was there any situation where few fat guys have been asked to move to front or back because I never saw any airliner weighing their passengers before boarding)
2. As the fuel starts burning, the pilot will fill/empty/exchange it from wing to wing or other storage location to balance CG, Will this cause imbalance (moving out of defined tolerances) in CG based on the passengers' weight?

Answer 1

The main issue is what is referred to as the Weight and Balance of the aircraft. When an aircraft is in flight, its weight and balance needs to be within a controllable range in order to ensure the plane can both recover from a stall condition and rotate on takeoff. This is visualized by the weight and balance "envelope:

As you can see, there are limits where the aircraft is not fit for flight. We must stay inside the drawn lines in the center.

To answer your detailed questions:

1. On airlines, a "Magic" number was used to approximate all weight for passengers and baggage. This caused at least one notable accident: Air Midwest Flight 5481. As a result of this accident, policies were changed to get more accurate baggage and passenger weight.

   In GA flight, it is not uncommon for pilots to ask for passenger weights. Usually, as a rule, we add a bit to that in case people are embarrassed (10%). It is very common for pilots to assign seating on smaller aircraft with heavier people sitting in a location where the distance to the datum is smallest (near the front).

2. Weight and balance can be moved out of limits by fuel burn. In fact, a few questions on the private pilot written exam deal with this exact scenario. Most planes have tanks near the CG, so they don't experience problems with fuel burn. In planes with an aft belly tank, the CG can move forward significantly and out of limits. This would cause the aircraft to be unable to pitch up as desired. Swept wing jets that use horizontal stabilizer tanks in the tail of the aircraft have computers and pumps to move the fuel as desired to keep their CG in the desired place.

   Pilots of smaller/less traditional airplanes need to be conscious of this, and plan accordingly. Especially those with a-typical fuel tank positions. Pilots of larger commercial style aircraft are less concerned with the exact position of 1 or 2 large people or heavier bags because their aircraft has a much bigger capacity and less "sensitive" envelope. They still are affected by the physics, though, just like everyone else.

Answer 2

In the majority of airplanes fuel is stored in the wings and tanks which are positioned at or near the center of gravity at empty. This limits the forward-back moment arm of the fuel so as fuel is used up the center of gravity does not shift forward or back that much. Balancing fuel weight is therefore more of a question of lateral CG, a pilot (or automatic onboard system) can pump fuel or switch tanks to balance fuel load. The seating configuration of passengers does not have much impact on lateral CG as their moment arm is very small.

In light aircraft passenger weight and position is a big concern, however passengers are actually one of the smallest weight components on a commercial airliner compared to the plane itself, fuel and cargo. You won't see overweight people being asked to move to balance a commercial aircraft. Seating systems deal with numbers and are designed to spread the load evenly across CG. Even if all the passengers of a half-empty commercial jet sat at the back there would still be enough trimming power to keep the airplane in balance.

Answer 3

In some newer planes i.e.B-787, A-380 etc., the CG is adjusted after loading the cargo. The landing gear have solid state scales built in and in the extreme tailcone is a jackscrew with a very heavy weight attached, usually made of dense depleted uranium. This can be moved laterally +/_ several feet to adjust the CG.