Two types of scales are typically used to weigh aircraft: those that operate mechanically with balance weights or springs, and those that operate electronically with what are called load cells. The balance weight type of mechanical scale, known as a beam scale, is similar to that found in a doctor’s office, in which a bar rises up when weight is put on the scale. A sliding weight is then moved along the bar until the bar is centered between a top and bottom stop.
The sliding weight provides the capability to measure up to 50 lb, and the cup holds fixed weights that come in 50 lb equivalent units. As an example of this scale in use, let’s say the nosewheel of a small airplane is placed on the scale with an applied weight of 580 lb. To find out what the applied weight is, a technician would place 550 lb of equivalent weight in the cup, and then slide the weight on the beam out to the 30 lb point. The 580 lb applied by the nosewheel would now be balanced by the 580 lb of equivalent weight, and the end of the beam would be centered between the top and bottom stop.
A mechanical scale based on springs is like the typical bathroom scale. When weight is applied to the scale, a spring compresses, which causes a wheel that displays the weight to rotate. It would be difficult to use this type of scale to weigh anything other than a very small aircraft, because these scales typically measure only up to 300 lb. The accuracy of this type of scale is also an issue.
Electronic scales that utilize load cells come in two varieties: the platform type and the type that mounts to the top of a jack. The platform type of electronic scale sits on the ground, with the tire of the airplane sitting on top of the platform. Built into the platform is an electronic load cell, which senses the weight being applied to it and generates a corresponding electrical signal. Inside the load cell there is an electronic grid that experiences a proportional change in electrical resistance as the weight being applied to it increases. An electrical cable runs from the platform scale to a display unit, which interprets the resistance change of the load cell and equates it to a specific number of pounds. A digital readout on the display typically shows the weight. In Figure 4-12, a Piper Archer is being weighed using platform scales that incorporate electronic load cells. In this case, the platform scales are secured to the hangar floor and stay permanently in place.
In Figure 4-13, a Mooney M20 airplane is being weighed with portable electronic platform scales. Notice in the picture of the Mooney that its nose tire is deflated (close-up shown in the lower right corner of the photo). This was done to get the airplane in a level flight attitude. This type of scale is easy to transport and can be powered by household current or by a battery contained in the display unit. The display unit for these scales is shown in Figure 4-14.
The display unit for the portable scales is very simple to operate. [Figure 4-14] In the lower left corner is the power switch, and in the lower right is the switch for selecting pounds or kilograms. The red, green, and yellow knobs are potentiometers for zeroing the three scales, and next to them are the on/off switches for the scales. Before the weight of the airplane is placed on the scales, each scale switch is turned on and the potentiometer knob turned until the digital display reads zero. In Figure 4-14, the nose scale is turned on and the readout of 546 lb is for the Mooney airplane in Figure 4-13. If all three scale switches are turned on at the same time, the total weight of the airplane will be displayed.
The second type of electronic scale utilizes a load cell that attaches to the top of a jack. The top of the load cell has a concave shape that matches up with the jack pad on the aircraft, with the load cell absorbing all the weight of the aircraft at each jacking point. Each load cell has an electrical cable attached to it, which connects to the display unit that shows the weight being absorbed by each load cell. An important advantage of weighing an aircraft this way is that it allows the technician to level the aircraft. An aircraft needs to be in a flight level attitude when it is weighed. If an aircraft is sitting on floor scales, the only way to level the aircraft might be to deflate tires and landing gear struts. When an aircraft is weighed using load cells on jacks, leveling the aircraft is easy by simply adjusting the height with the jacks. Figure 4-15 shows a regional jet on jacks with the load cells in place.