Weight and Balance: A review of fundamentals

July 1, 1999

Weight and Balance

A review of fundamentals

By Kevin E. Bricker

July 1999

Weight influences more aircraft performance variables than any other single parameter. These include the typical items such as takeoff and landing characteristics, acceleration capability, maximum velocity and rate of climb. While it should be obvious that an overloaded and/or improperly balanced aircraft requires more power, has a greater fuel consumption rate, and its stability and control characteristics will be seriously affected; the effects of changes of equipment and the addition of paint don't often register as having a large impact on the performance of the aircraft.

Weighings - why, how, and when?
The aircraft basic weight and associated center of gravity must be accurately known at all times to ensure a safe aircraft operation. Non-military aircraft are weighed per specified operator's procedures or for prescribed schedules or instructions.

According to 43.13-1B, the removal or addition of equipment results in changes to the center of gravity (CG). The empty weight of the aircraft, and the permissible useful load, are affected accordingly. You must investigate the effects of these changes as the aircraft flight characteristics may be adversely affected. Information upon which to base the record of weight and balance changes to the aircraft may be obtained from the pertinent Aircraft Specifications, Type Certificate Data Sheet (TCDS), prescribed aircraft operating limitations, aircraft flight manual, aircraft weight and balance report, and maintenance manual.

Also, removal or addition of minor items of equipment such as nuts, bolts, rivets, washers, and similar standard parts of negligible weight on fixed-wing aircraft do not require a weight and balance check. (Rotorcraft in general, are more critical with respect to controlling changes in the CG position.) The procedures and instructions in that particular model's maintenance or flight manual should be followed.

Platform scales can also be used on light aircraft as with this Piper Archer II in a 3-point reaction process.

Aircraft weighing
All aircraft, regardless of type, are weighed in the longitudinal and lateral level conditions using any of the approved scale type procedures. When weighing on scales, the procedure used assumes that the lateral level condition is 0.0 degrees when the tires have been properly serviced, and the OLEO struts (main gear) evenly extended. The weighing facility floor should not exceed 0.25 inches of slope per foot. The lateral level condition is controlled by jack extension when using load cell (top-of-jack) type equipment.

As aircraft weighings are required, weighing with calibrated scales is the only sure method of obtaining an accurate basic weight and center of gravity of the aircraft.

A variety of scale types and equipment may be used for weighing to include: electronic top-of-jack load cells, portable platform scales, or stationary pit scales.

Organizations that operate, maintain, or modify aircraft, are responsible for having all weighing equipment under their jurisdiction calibrated periodically and certified by a government inspector of weights and measures, or by commercial scale officials. If the operator of such an organization does not have any scales, they have an option of hiring an outside company who specializes in weight and balance, and the calculation of the center of gravity for aircraft.

The entire aircraft weight may be considered to be concentrated at the center of gravity. Therefore, the moment of the aircraft about the datum is the weight of the aircraft times the horizontal distance between the C.G. and the datum. Example:

If the weight of this airplane is 2000 lbs. and the arm from the datum to the center of gravity is 16 inches, the moment of the aircraft about the datum is 2000 x 16 or 32,000 in. lbs.

Preparing the aircraft for weighing
To maintain consistency in determining a basic weight condition, an orderly preweighing procedure must be followed. The following is a recommended procedure to prepare an aircraft for weighing:

• Thoroughly clean the aircraft inside and out, removing accumulated dirt, grease, and moisture. Allow sufficient drying time prior to weighing.
• Assemble the required weighing equipment, including scales, jacks, leveling bars, level, tape measure, plumb bob, chalk line, etc.
• Drain the fuel in accordance with the maintenance manual instructions. The attitude for fuel draining is normally the aircraft's ground attitude; however, it may differ for your aircraft. Always consult the applicable technical document prior to defueling.
• Remove all load items such as cargo, magazines, blankets, and any other equipment that do not have a fixed position in the aircraft.
• Check all reservoirs and tanks for liquids such as drinking water, washing water, engine oil, hydraulic fluid, cooling fluids, oxygen, etc. Reservoirs and tanks should either be empty, or filled to their normal capacity prior to weighing. (Check manufacturer recommendations for procedures specific to the aircraft you are weighing. Never weigh an aircraft with partially filled reservoirs or tanks.

Move the aircraft to the area where it is to be weighed. Remember that an aircraft must be weighed in a closed hangar with all blowers of ventilating systems turned Off. The aircraft brakes will be Off to reduce the scale slide and thrust loads, which would possibly give erroneous scale readings and weighing results.


Check that the flap position, seat positions, horizontal stabilizer, etc. are in accordance with the aircraft maintenance manual.

Conduct a supervised inventory check of the equipment actually in the aircraft. Remember: A weighing without an associated equipment inventory is of no value. Correct the chart A as necessary to itemize accurately all items of fixed operating equipment that will be included in the basic weight determined by the weighing.

Correct, if needed, the historical records based upon the equipment inventory.

Aircraft leveling methods
All weight and balance computations are based on measurements taken when the aircraft is in the level position. This position is achieved when the longitudinal and lateral axis of the aircraft are in the horizontal plane. This is determined using a spirit level or (plumb bob) in conjunction with the aircraft manufacturer's built in method of determining the level condition of the aircraft.

The spirit level method is performed by placing a leveling bar squarely on the leveling lugs, which are located either inside or outside the aircraft. Then place a spirit level (or inclinometer) on the leveling bar. The aircraft is level when the bubble is centered (or the degrees indicator shows 0.0 degrees). Be sure to level the aircraft in both the longitudinal and lateral directions.

Weighing procedure (general) on mobile platforms
When weighing the aircraft on mobile electronic weighing platforms, follow the general procedure as described below. Additional procedures such as warm-up, leveling, etc. may be contained in the system's operating manual. Read and heed before proceeding with the weighing. As a general rule, perform the following:

• Ensure that all scales are within their prescribed calibration date.
• Warm up electronics for a minimum of 20 minutes, or as dictated by the system operating instructions.
• Zero the scales.
• Tow the aircraft onto the scales. Back aircraft off after pre-activating (exercising) scales; tow aircraft back on to scales.
• Level the aircraft, both horizontally and laterally in accordance with leveling instructions in the aircraft manual.
• Record weight readings and dimensional data on an applicable form. If a large discrepancy is noted, check to see where they may have occurred. If no error is found, reweigh the aircraft. (A large error is 0.2 percent of the basic weight (0.002 x basic weight), or 0.2 percent Mean Aerodynamic Chord (0.002 x MAC), or 0.2 inches for rotorcraft).
• When so specified, a second weighing shall be accomplished. Remeasurement of dimensional data is not required.
• Remove the aircraft from the scales.
• Determine the tare weight in accordance with appropriate scale instructions. Enter the tare weight readings on the weighing form.
• Finish the calculations on the appropriate weigh form and enter into the historical data.
• Stow the equipment.

The aircraft has now been successfully weighed and the center of gravity has been calculated.

3-point and 5-point reactions
Following are the calculations used in most scenarios. For purposes of weighing familiarity, we will discuss common problems and techniques using two aircraft configurations — a 3-point and 5-point reaction. The reaction you use will be dictated by the type of aircraft and gear you're dealing with. A Cessna 172 with tricycle gear, for instance, uses a 3-point reaction. A Boeing 747, with a more complex gear, uses a 5-point reaction.

Three (3) Point Reaction

The 3-point reaction Weight/CG is determined as follows:

CG = AxB

A common method for weighing aircraft utilizes jacks and load cells. Be sure the aircraft is level in both directions when using this method.

Where: A = Distance between nose and main reactions
B = Weight, as nose (or tail)
C = Basic Weight ~ (As weighed +/- corrections)
A = 150 inches
B = 220 pounds
C = 4,220 pounds (220 + 1,995 + 2,005)

This distance is then subtracted from the known main gear fuselage station to determine the CG in inches (Fuselage Station) from the aircraft's' reference datum or station 0.0.

Figure 2: Five-point reaction

Five (5) Point Reaction
The 5-point reaction Weight/CG is determined as follows:

Note: Distance C is the mean of distances A and B, ie:

A + B 2

The center of gravity calculation is determined in the same manner as the 3-point reaction, except that multiple reactions (nose and mains) require individual centerline dimension data.

Referencing center of gravity
Whenever a CG is referenced to a main wheel location it should always be corrected to relate to its reference datum, and then its moment calculated. The purpose of this correction is twofold:

First in terms of measurements, it relates the CG to the common reference datum. Second, in terms of total moment (and basic weight), it establishes the necessary mathematical datum point for subsequent calculations with regard to future operations and maintenance.

(See following illustration for calculations.)

Correction of a CG to a common reference datum (at nose or at wing leading edge) is accomplished as follows:

Figure 3: Referencing center of gravity

A = 626 inches forward of main reactions
B = 50 inches aft of main reactions
C = A + B

The CG is 676 inches aft of the reference datum.

A listing of major terms and definitions used in weight and balance is available on the next page.

Please note that information presented in this article offers a general procedure for weighing an aircraft. Always refer to the aircraft's maintenance manuals and the weighing equipment instructions for details.