Mass and Balance in Multi-Engine Aircraft

Key Principles of Mass and Balance in Multi-Engine Aircraft

The mass and balance multi engine process involves systematically recording all masses—basic empty mass, crew, passengers, baggage, and fuel—along with their respective arm (distance from the reference datum). For multi-engine aircraft, fuel is often stored in multiple tanks (inner and outer wing, central, or even in the fin or horizontal stabiliser), and each tank's position affects the aircraft's CG as fuel is consumed.

Fuel Tanks and CG Movement

• Fuel Location Impact: Consuming fuel from tanks positioned far from the CG (like in the tail or outer wings) causes the CG to shift more than from tanks near the centre. For example, burning fuel from aft tanks moves the CG forward, while burning from forward tanks moves it aft.
• Fin or Stabiliser Tanks: These tanks can offer design advantages, such as improved trim or increased range, but also introduce risks. If too much fuel is burned from these tanks, the CG may move outside safe limits, potentially affecting stability and control.

Calculation Procedures

• Data Entry: All items (crew, payload, fuel) are entered into a manifest, with moments calculated by multiplying mass by arm. For many modern twins, all arms are positive, simplifying calculations.
• Mass States: Calculations are performed for zero-fuel mass, ramp mass, take-off mass, and landing mass, each considering the relevant fuel and load changes.
• Regulatory Requirements: Aircraft must be weighed at intervals or after significant modifications. Regulations specify when recalculation or reweighing is necessary, ensuring data remains accurate.

Extracting and Using Data

• Manufacturer Documents: Use provided documents to find fuel tank capacities, positions, and standard mass values. Deviations (e.g., optional equipment, extra tanks) must be accounted for.
• Jacking Point Calculations: For precise CG determination, reaction forces at jacking points can be used to calculate both total mass and CG location.

Practical Considerations

• Always check that the loaded aircraft remains within the CG envelope for all phases of flight, especially as fuel is consumed.
• Understand the sequence of fuel usage, as this directly affects CG movement and aircraft handling.

Common Scenarios

• Extraction of values for non-standard configurations (e.g., extra crew, optional tanks) is a frequent exam and operational requirement.
• Calculations may require conversion between fuel volume and mass, especially in US gallons to pounds or kilograms.