The ultimate test of whether there is a problem with an airplane’s weight and balance is when it is loaded and ready to fly. The only real importance of an airplane’s empty weight and EWCG is how it affects the loaded weight and balance of the airplane, since an airplane does not fly when it is empty. The pilot in command is responsible for the weight and balance of the loaded airplane, and he or she makes the final decision on whether the airplane is safe to fly.
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Example Loading of an Airplane
As an example of an airplane being loaded for flight, the Piper Seneca twin will be used. The TCDS for this airplane was shown earlier in this site, and its CG range and CG envelope were also shown.
The information from the TCDS that pertains to this example loading is shown in Figure 1.
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| Figure 1. Example loading information pertaining to TCDS |
For the example loading of the airplane, the following information applies:
- Airplane Serial Number: 34-7250816
- Airplane Empty Weight: 2,650 lb
- Airplane EWCG: +86.8"
For today’s flight, the following useful load items are included:
- 1 pilot at 180 lb at an arm of +85.5"
- 1 passenger at 160 lb at an arm of +118.1"
- 1 passenger at 210 lb at an arm of +118.1"
- 1 passenger at 190 lb at an arm of +118.1"
- 1 passenger at 205 lb at an arm of +155.7"
- 50 lb of baggage at an arm of +22.5"
- 100 lb of baggage at an arm of +178.7"
- 80 gal of fuel at an arm of +93.6"
To calculate the loaded weight and CG of this airplane, a four-column chart is used in Figure 2.
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| Figure 2. Center of gravity calculation for Piper Seneca |
Based on the information in the TCDS, the maximum takeoff weight of this airplane is 4,200 lb and the aft-most CG limit is +94.6". The loaded airplane in Figure 2 is 25 lb too heavy, and the CG is 1.82" too far aft. To make the airplane safe to fly, the load needs to be reduced by 25 lb and some of the load needs to be shifted forward. For example, the baggage can be reduced by 25 lb, and a full 100 lb of it can be placed in the more forward compartment. One passenger can be moved to the forward seat next to the pilot, and the aft-most passenger can then be moved forward.
With the changes made, the loaded weight is now at the maximum allowable of 4,200 lb, and the CG has moved forward 4.41". [Figure 3] The airplane is now safe to fly.
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| Figure 3. Center of gravity calculation for Piper Seneca with weights shifted |
Adverse-Loaded CG Checks
Many modern aircraft have multiple rows of seats and often more than one baggage compartment. After any repair or alteration that changes the weight and balance, the A&P mechanic or repairman must ensure that no legal condition of loading can move the CG outside of its allowable limits. To determine this, adverse-loaded CG checks must be performed and the results noted in the weight and balance revision sheet.
During a forward adverse-loaded CG check, all useful load items in front of the forward CG limit are loaded and all useful load items behind the forward CG limit are left empty. So, if there are two seats and a baggage compartment located in front of the forward CG limit, two people weighing 170 lb each are seated and the maximum allowable baggage is placed in the baggage compartment. Any seat or baggage compartment located behind the forward CG limit is left empty. If the fuel is located behind the forward CG limit, minimum fuel will be shown in the tank. Minimum fuel is calculated by dividing the engine’s METO Hp by 2.
During an aft adverse-loaded CG check, all useful load items behind the aft CG limit are loaded and all useful load items in front of the aft CG limit are left empty. Even though the pilot’s seat will be in front of the aft CG limit, the pilot’s seat cannot be left empty. If the fuel tank is located forward of the aft CG limit, minimum fuel will be shown.
Example Forward and Aft Adverse-Loaded CG Checks
Using the stick airplane in Figure 4 as an example, adverse forward and aft CG checks are calculated. Some of the data for the airplane is shown in Figure 4, such as seat, baggage, and fuel information. The CG limits are shown, with arrows pointing in the direction where maximum and minimum weights are loaded. On the forward check, any useful load item located in front of 89" is loaded, and anything behind that location is left empty. On the aft check, maximum weight is added behind 99" and minimum weight in front of that location. For either of the checks, if fuel is not located in a maximum weight location, minimum fuel must be accounted for. Notice that the front seats show a location of 82" to 88", meaning they are adjustable fore and aft. In a forward check, the pilot’s seat will be shown at 82", and in the aft check it will be at 88". Additional specifications for the airplane shown in Figure 4 are as follows:
- Airplane empty weight: 1,850 lb
- EWCG: +92.45"
- CG limits: +89" to +99"
- Maximum weight: 3,200 lb
- Fuel capacity: 45 gal at +95" (44 usable) 40 gal at +102" (39 usable)
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| Figure 4. Example airplane for extreme condition checks |
In evaluating the two extreme condition checks, the following key points should be recognized. [Figure 5]
- The total arm is the airplane CG and is found by dividing the total moment by the total weight.
- For the forward check, the only thing loaded behind the forward limit was minimum fuel.
- For the forward check, the pilot and passenger seats were shown at the forward position of 82".
- For the forward check, the CG was within limits, so the airplane could be flown this way.
- For the aft check, the only thing loaded in front of the aft limit was the pilot, at an arm of 88".
- For the aft check, the fuel tank at 102" was filled, which more than accounted for the required minimum fuel.
- For the aft check, the CG was out of limits by 0.6", so the airplane should not be flown this way.
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| Figure 5. Center of gravity extreme conditions check |