Figure 3-2.  Ramp scales. Figure 3-3.  Load cell scales. 3-3 Preparation for Weighing Scale Preparation All scales for aviation use, manual or electronic, must be  protected when stored or shipped, and they must be checked  periodically for accuracy. The maximum recognized period  between calibration checks is 12 months; however, this period  may be reduced by an airworthiness authority dependent on  the conditions of use. Scales in daily use may require a shorter  interval and/or testing to determine the continued accuracy  of the unit. Scales should be returned to the manufacturer for  proper calibration and testing.  In general, weight procedures may vary with the aircraft  and types of weight equipment employed. The weighing  procedure contained in the manufacturer’s maintenance  manual should be followed for each particular aircraft. The  major considerations in preparing an aircraft for weighing  are described in the following paragraphs. Mechanical and electronic scales shall be inspected prior to  use and set to zero. This is done by adding and removing  a weight, then rechecking for zero. This process should be  repeated until a steady zero setting is obtained. The scales  should be located in the same environment in which they 