Within the following text, information concerning major repairs or major alterations does not apply to any aircraft within the light-sport category. This category of aircraft is not eligible for major repairs or alterations.
Any major alteration or repair requires the work to be done by an FAA-certificated mechanic or repairman or facility. The work must be checked for conformity to FAA-approved data and signed off by an aircraft mechanic holding an Inspection Authorization (IA) or by an authorized agent of an appropriately rated FAA-approved repair station. A Repair Station record and/or FAA form 337, Major Repair and Alteration, must be completed describing the work. (Reference Title 14 of the Code of Federal Regulations (14 CFR) part 43, appendix B, for the recording of major repair and alterations requirements.) A dated and signed revision to the weight and balance record is made and kept with the maintenance records, and the airplane’s new empty weight and empty weight arm or moment index are entered in the POH/AFM.
Weight and Balance Revision Record
Aircraft manufacturers use different formats for their weight and balance data, but Figure 7-3 is typical of a weight and balance revision record. All weight and balance records should be kept with the other aircraft records. Each revision record should be identified by the date, the aircraft make, model, and serial number. The pages should be signed by the person making the revision and his or her certificate type and number must be included.The computations for a weight and balance revision are included on a weight and balance revision form. Appropriate fore-and-aft extreme loading conditions should be investigated and the computations shown. The date those computations were made is shown in the upper right corner of Figure 7-3. When this work is superseded, a notation must be made on the new weight and balance revision form, including a statement that the new computations supersede the computations dated “MM/DD/YY.”
The weight and balance revision sheet should clearly show the revised empty weight, empty weight arm and/or moment index, and the new useful load. An example of these entries can be found at the bottom of Figure 7-3.
Weight Changes Caused by a Repair or Alteration
A typical alteration might consist of removing two pieces of radio equipment from the instrument panel and a power supply that was located in the baggage compartment behind the rear seat. In this example, these two pieces are replaced with a single lightweight, self-contained radio. At the same time, an old emergency locator transmitter (ELT) is removed from its mount near the tail, and a lighter weight unit is installed. A passenger seat is installed in the baggage compartment.
Computations Using Weight, Arm, and Moment
The first step in the weight and balance computation is to make a chart like the one in Figure 7-4, listing all of the items that are involved. The new center of gravity (CG) of 36.4 inches aft of the datum is determined by dividing the new moment by the new weight.Computations Using Weight and Moment Indexes
If the weight and balance data uses moment indexes rather than arms and moments, this same alteration can be computed using a chart such as the one in Figure 7-5. Subtract the weight and moment indexes of all the removed equipment from the empty weight and moment index of the airplane. Add the weight and moment indexes of all equipment installed to determine the total weight and the total moment index. To determine the position of the new CG in inches aft of the datum, multiply the total moment index by 100 to get the moment, and divide this by the total weight to get the new CG.
Determining the CG in Percentage of Mean Aerodynamic Chord (Percent MAC)
This procedure is the same as found in Chapter 5, Single-Engine Aircraft Weight and Balance Computations. Refer to the load conditions and CG information found in Figures 7-5, 7-6, and 7-7 to compute the CG in percent MAC:
The loaded CG is +36.4 inches aft of the datum.
The MAC is 58.0 inches long.
The leading edge mean aerodynamic chord (LEMAC) is located at station 25.98.
The CG is +36.4 – 25.98 = 10.42 inches aft of LEMAC.
Use the formula in Figure 7-8 to determine CG in MAC percentages.
The loaded CG after alteration or repair is located at 17.9 percent MAC.