The center of gravity range for an aircraft is the limits within which the aircraft must balance. It is identified as a forward most limit (arm) and an aft most limit (arm). In the Type Certificate Data Sheet for the Piper Seneca airplane, shown earlier in this chapter, the range is given as follows:
CG Range: (Gear Extended)
- S/N 34-E4, 34-7250001 through 34- 7250214 (See NOTE 3):
- (+86.4″) to (+94.6″) at 4,000 lb
- (+82.0″) to (+94.6″) at 3,400 lb
- (+80.7″) to (+94.6″) at 2,780 lb
- Straight line variation between points given.
- Moment change due to gear retracting landing gear (–32 in-lb)
Because the Piper Seneca is a retractable gear airplane, the specifications identify that the range applies when the landing gear is extended, and that the airplane’s total moment will be decreased by 32 when the gear retracts. To know how much the center of gravity will change when the gear is retracted, the moment of 32 in-lb would need to be divided by the loaded weight of the airplane. For example, if the airplane weighed 3,500 lb, the center of gravity would move forward 0.009″ (32 ÷ 3500).
Based on the numbers given, up to a loaded weight of 2,780 lb, the forward CG limit is +80.7″ and the aft CG limit is +94.6″. As the loaded weight of the airplane increases to 3,400 lb and eventually to the maximum of 4,000 lb, the forward CG limit moves aft. In other words, as the loaded weight of the airplane increases, the CG range gets smaller. The range gets smaller as a result of the forward limit moving back, while the aft limit stays in the same place.
The data sheet identifies that there is a straight line variation between the points given. The points being referred to are the forward and aft center of gravity limits. From a weight of 2,780 lb to a weight of 3,400 lb, the forward limit moves from +80.7″ to +82.0″, and if plotted on a graph, that change would form a straight line. From 3,400 lb to 4,000 lb, the forward limit moves from +82 to +86.4″, again forming a straight line. Plotted on a graph, the CG limits would look like what is shown in Figure 4-19. When graphically plotted, the CG limits form what is known as the CG envelope.
In Figure 4-19, the red line represents the forward limit up to a weight of 2,780 lb. The blue and green lines represent the straight line variation that occurs for the forward limit as the weight increases up to a maximum of 4,000 lb. The yellow line represents the maximum weight for the airplane, and the purple line represents the aft limit.
Empty Weight Center of Gravity Range
For some aircraft, a center of gravity range is given for the aircraft in the empty weight condition. This practice is not very common with airplanes, but is often done for helicopters. This range would only be listed for an airplane if it was very small and had limited positions for people and fuel. If the empty weight CG of an aircraft falls within the empty weight CG limits, it is known that the loaded CG of the aircraft will be within limits if standard loading is used. This information will be listed in the Aircraft Specifications or Type Certificate Data Sheet, and if it does not apply, it will be identified as “none.”
Operating Center of Gravity Range
All aircraft will have center of gravity limits identified for the operational condition, with the aircraft loaded and ready for flight. If an aircraft can operate in more than one category, such as normal and utility, more than one set of limits might be listed. As shown earlier for the Piper Seneca airplane, the limits can change as the weight of the aircraft increases. In order to legally fly, the center of gravity for the aircraft must fall within the CG limits.