The Center of Gravity of an Airplane

Center of Gravity
Photo: Getty/Yuji Sakai

Center of Gravity (CG): 

If you've ever been on a teeter totter, then you know about center of gravity. Teeter totters are fun, but when it comes to airplanes, the center of gravity is a characteristic that makes the design and loading of an airplane very important. 

NASA defines the term Center of Gravity as the average location of the weight of an object. 

In general, the center of gravity of an airplane is the point at which it would balance if it were suspended in air, or the point at which most of its mass is focused.

Its distance from the datum is found by dividing the total moment by the total weight of the airplane. The center of gravity can be thought of as the point at which all of the aircraft's mass is concentrated, or the "heaviest" part of the airplane.

In a typical light general aviation aircraft, the center of gravity is located just aft of the firewall, or just aft of where the engine is. The engine, avionics, pilot and passengers are the heaviest components on an airplane, and the location of the heaviest items on the airplane will be the approximate location of the center of gravity. On larger aircraft, the CG may vary wildly with fuel locations and loading considerations, making the loading of the aircraft imperative to a successful flight. 

Each individual aircraft is weighed carefully upon certification, and the aircraft's center of gravity and moment arm location are both determined and provided to the owner or operator.

These numbers are printed on an official weight and balance form that is kept with the aircraft's operating manual. Anytime there are any modifications to the aircraft, its structure or its systems, a new weight and balance will be calculated and a new data sheet will be created. If a new GPS unit is installed, for example, the aircraft is then re-weighed and a new center of gravity will be calculated and recorded.

 

The pilot in command, or a company's loadmaster or dispatcher should always calculate the weight and balance of an aircraft before flying to ensure that with the included payload (baggage, passengers, fuel, etc), the aircraft will remain within its maximum weight limitations and within its center of gravity limitations, per the pilot operating handbook for the specific aircraft being flown. 

A center of gravity too far forward or too far aft can cause problems for the pilot, and either condition can be dangerous. A CG that's too far forward can decrease performance. An aft CG can increase performance, but in most small aircraft, will make the airplane unstable and potentially create a situation in which the pilot does not have enough elevator control to recover from a potential stall/spin scenario.