Equipment Change and Aircraft Alteration

When the equipment in an aircraft is changed, such as the installation of a new radar system or ground proximity warning system, or the removal of a radio or seat, the weight and balance of an aircraft will change. An alteration performed on an aircraft, such as a cargo door being installed or a reinforcing plate being attached to the spar of a wing, will also change the weight and balance of an aircraft. Any time the equipment is changed or an alteration is performed, the new empty weight and empty weight center of gravity must be determined. This can be accomplished by placing the aircraft on scales and weighing it, or by mathematically calculating the new weight and balance. The mathematical calculation is acceptable if the exact weight and arm of all the changes are known.

Example Calculation After an Equipment Change

A small twin-engine airplane has some new equipment installed, and some of its existing equipment removed. The details of the equipment changes are as follows:

  • Airplane Empty Weight: 2,350 lb
  • Airplane Empty Weight CG: +24.7"
  • Airplane Datum: Leading edge of the wing
  • Radio Installed: 5.8 lb at an arm of –28"
  • Global Positioning System Installed: 7.3 lb at an arm of –26"
  • Emergency Locater Transmitter Installed: 2.8 lb at an arm of +105"
  • Strobe Light Removed: 1.4 lb at an arm of +75"
  • Automatic Direction Finder (ADF) removed: 3 lb at an arm of –28"
  • Seat Removed: 34 lb at an arm of +60"

To calculate the new empty weight and empty weight center of gravity, a four column chart is used. The calculation would be as shown in Figure 4-26.

In evaluating the weight and balance calculation shown in Figure 4-26, the following key points should be recognized.

  • The weight of the equipment needs to be identified with a plus or minus to signify whether it is being installed or removed.
  • The sign of the moment (plus or minus) is determined by the signs of the weight and arm.
  • The strobe and the ADF are both being removed (negative weight), but only the strobe has a negative moment. This is because the arm for the ADF is also negative, and two negatives multiplied together produce a positive result.
  • The total arm is the airplane’s center of gravity, and is found by dividing the total moment by the total weight.
  • The result of the equipment change is that the airplane’s weight was reduced by 22.5 lb and the center of gravity has moved forward 0.67".

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