|Once the pilot or operator is sure the aircraft does not exceed any weight limitations it is necessary to
conduct a trim analysis in order to determine the position of the CG. When loading an aircraft, it is not just
a matter of how much is loaded into the aircraft, but where you put it as well. The location of the CG is
critical to aircraft stability and elevator effectiveness. Loading an aircraft in such a way that a nose heavy
situation is likely to happen will make it less easy to handle inflight, especially when commencing take-off
or landing. If the aircraft is tail heavy, there is a risk of tip-over and will most certainly cause structural
damage to the aircraft itself. This means that improper balance of the load carried by the aircraft could
result in serious control problems and unsafe situations both on the ground as in flight. These kind of
situations should be avoided at all times by making sure that the location of the CG lies between predetermined
limits provided by the manufacturer. These CG limits are the forward and aft centre of gravity
locations within which the aircraft must be operated at a given weight. The CG limits are commonly called
the CG envelope or range. In order to find out where the actual CG is located and how the aircraft should
be trimmed in order to make it more comfortable to fly, a so called trim sheet should be filled in.
Every segment of the payload carried on the aircraft has a certain arm from a specified datum. All data
together decides whether the aircraft is either in or out of balance. The trim sheet calculates moments for
each specific payload segment like the fuel load, passengers, baggage or cargo. First, a dead load and
passenger index table is used to determine the effect of these weights on the change of CG (8). The passenger
compartment configuration is an important factor when calculating the actual CG. A configuration
that accommodates business class passengers means a less forward CG as cabin density decreases (9).
Second, the trim chart is used to determine the CG at Zero Fuel Weight (ZFW) (10). After fuel is loaded, a
fuel index correction is accomplished and plotted into the trim chart in order to determine the CG at Takeoff
Weight (TOW) (11). Should these points lie between pre-described limits, the aircraft is in balance when
loaded according to these calculations. From this point on, trim tab setting, take-off run data and landing
data can be calculated and cross checked.
From the statements above it is quite easy to understand that the CG has to be located between defined
limits in order to fly safely and legally. These limits can be expressed as a percentage of the Mean Aerodynamic
Chord (MAC). MAC is the chord of an imaginary rectangular wing that has
the same aerodynamic characteristics as the actual wing (figure 1). In effect, this imaginary wing replaces the more
complicated plan form of the actual wing for calculation purposes (figure 2) and can be used in
relation to the aircraft. It is obvious that the CG has to lie between these two limits in order to be balanced.
The concept of MAC is used by aircraft designers when they determine stability characteristics of
|Figure 1: Typical Mean Aerodynamic Chord
|Figure 2: MAC in relation to the aircraft