To Weigh an Aircraft

New weight and balance information is regularly required for most all aircraft; small or large. Installation or removal of equipment, after repairs and modifications, or a frequent requirement of the aircraft or operators maintenance program, requires...


Equipment selection

Scale selection should start with the weight range needed and jack point or tire size must be considered. Small to medium aircraft will use a small cell with a 1-inch or smaller jack point. The jack points must fit into the cell concave top and be retained to prevent a cell to jack point ejection or spit during jacking. Larger cells will be used for larger aircraft weight ranges and larger jack points.

The decision point for cell and cup size will be at the medium to large jet selection. Regional airline aircraft and large corporate aircraft usually require a large cell kit to be used due to their larger jack points. We often see a small 50,000-pound cell being used for larger aircraft. In many cases this may have the weight capacity but not the retention and proper fit for the jack point.

For platforms, consider the tire size and ability to chock the tire on the platform to prevent movement. Equipment used to locate the aircraft on the platform and proper ramp selection are also important points to consider. It’s tempting, but never consider using automotive-style scales or 1 percent accuracy platform units as these units do not have the accuracy or resolution required for aviation use.

Always read and understand the scale manufacturer’s manual as well as the airframe manufacturer’s manuals prior to weighing and selection of the type scale to use. Aircraft maintenance personnel must understand the process, procedure, and instructions; remember to use the right tool for the right job.

Resolution and accuracy

As a general rule scales less than 10,000-pound capacity will measure in a 1-pound count and scales over that and up to 25,000 pounds will count in a 2-pound count. Large jet 50,000-pound scales will measure in a 5-pound count and so on. Always use a scale with the proper size and count resolution. The idea is to pick the right scale size and resolution for the aircraft type and accuracy needed.

Accuracy for aviation use is touted to be one-tenth of 1 percent or three indicator divisions, whichever is higher to full span. This is a release tolerance for the unit as calibrated and released from the lab and quality scales will meet this specification.

In the field tolerance is a different story. In general the tolerance expected in use would be one-fourth of 1 percent or five divisions to span. Due to the application, jacking, level of the hangar floor, and attention of the technicians conducting the job, there will always be variables in the weighing. Aircraft weights from point to point in rotation should be within range of the target weight from the old weight and balance.

Conclusions

When selecting a scale system it is important to remember to select a scale that is the correct type, size, and has the correct weight range for the application. The aircraft manufacturer’s manual should always be consulted for the proper selection and directions. Some manufacturers do not recommend a particular scale or type. In these cases, look at the technical data provided to do the job, the calculations, formulas, and any charts that are provided. Many manufacturers will include the jack point locations and wheel center locations in their manuals, so it is up to you to decide which type of scale to use. 

 

Larry Jackson holds an A&P certificate with IA and has 31 years of experience in the maintenance and care of all types of aircraft and helicopters. His company www.aircraftscales.com is a division of Jackson Aircraft Weighing Service. For more information call (561) 281-6179.

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