Chapter 16: Aircraft Weight and Balance

Supplied B1 Module 07A textbook: PDF pages 359–392.

Weight, mass and balance limits

Weight and balance affect performance, stability and controllability. Use the aircraft's approved definitions and loading limits; terms such as empty mass, basic mass and useful load vary with the documentation. A legal total mass does not guarantee a legal centre of gravity. Evaluate both the loading envelope and the relevant operating configuration. Consistent units are essential: kilograms with metre arms produce kilogram-metre mass moments in a mass-based calculation, while force-based calculations use force units. Do not mix the two conventions.

Datum, arm and moment

The datum is the chosen reference plane for longitudinal measurements. The arm is the signed distance from the datum to a mass or force location. Moment is mass or weight multiplied by arm using the consistent convention of the calculation. Locations ahead of the datum may be negative if the documentation defines them that way. A datum is not necessarily the aircraft nose or centre of gravity. Keep the sign convention and moment-index scaling consistent throughout the calculation.

Centre-of-gravity calculation

The centre-of-gravity arm equals total moment divided by total mass or weight. List every item, calculate its signed moment, sum masses and moments, then divide. For 1000 kg at 2 m plus 200 kg at 3 m, total mass is 1200 kg and total moment is 2600 kg·m; CG is about 2.167 m. Compare this arm against the approved envelope at the corresponding mass. A mean of the individual arms is wrong unless their masses are equal.

Weighing preparation

Use the manufacturer's weighing procedure and controlled aircraft configuration. Establish required fuel and fluid conditions, installed equipment, cleanliness, level attitude and environmental limits. Calibrated scales need adequate capacity and suitable support arrangements. Prevent wind or movement from corrupting readings. Record tare for blocks, adapters or other support equipment as specified. Use jacking or wheel-scale precautions relevant to the method. Weighing records must describe what was actually on the aircraft; otherwise an accurate scale reading can produce an inaccurate empty-mass report.

Equipment changes and records

An installed item adds both mass and moment; a removed item subtracts both. Use verified item mass and location, update equipment lists and produce the required revised report. For an aircraft at 1200 kg and 2400 kg·m, removing 20 kg at 3 m gives 1180 kg and 2340 kg·m; the new CG is approximately 1.983 m. Do not subtract an item's mass without its moment. If configuration or accumulated changes are uncertain, follow the required reweighing process.

Load movement and percent MAC

Moving an item changes moment without changing total mass. Moment change equals item mass multiplied by the signed distance moved. The resulting CG shift equals this moment change divided by total aircraft mass. When CG is expressed as percent mean aerodynamic chord, percent MAC equals 100 times (CG arm minus leading-edge MAC arm) divided by MAC length. Use the same datum and units for all lengths. Check fuel-use and loading sequences if the envelope can change during operation.

Apply the knowledge

Work through these examples before starting the quiz. Explain why the other choices would not meet the requirement.