Determination of Centre of Gravity

Understanding Centre of Gravity (CG) Determination for ATPL Mass and Balance

The centre of gravity is the single point through which the total aircraft mass is considered to act. For ATPL Mass and Balance, you will express longitudinal CG either as an arm from the manufacturer’s datum (a fixed reference) or as a percentage of the Mean Aerodynamic Chord (MAC) from its leading edge. Correct CG determination is critical to stability, controllability, and performance. A forward CG generally increases stability but raises stall speed and requires greater elevator authority and trim, increasing drag and fuel consumption. Conversely, an aft CG reduces required tail downforce and can improve efficiency, but it also reduces longitudinal stability and may approach neutral stability, increasing the risk of stall or pitch excursions.

Determination of CG follows a standard procedure used across GA and airline operations under EASA/FAA aviation regulations and the AFM/POH or Loading Manual. Start with the Basic (or Dry Operating) Mass and its arm. Add each item (crew, passengers, baggage, fuel) by calculating moments: moment = mass × arm. Sum all masses and moments, then compute CG arm: CG arm = total moment ÷ total mass. Convert volume fuel to mass using density (e.g., 140 L × 0.714 kg/L). To express CG as %MAC: %MAC = [(CG arm − MAC leading-edge arm) ÷ MAC length] × 100. You must verify the CG lies within the forward and aft limits for the specific Take-Off Mass (TOM) and Landing Mass (LM), because allowable CG ranges vary with weight. Also consider configuration changes: for example, landing gear extension can create a pitch-up or pitch-down moment depending on the gear geometry and its location relative to the CG.

The question bank reflects common ATPL problems and procedures. You will practice shifting passengers or cargo between stations to achieve a target CG: CG shift (Δx) = (m moved ÷ W total) × Δarm. You will use arms in metres (or inches), compute large moments, and convert to %MAC. You will interpret envelope charts to find safe limits (e.g., forward 8.0% MAC to aft 26.5% MAC at a given mass), and assess operational impacts: forward-CG aircraft are very stable but require significant elevator input, while aft-CG loading reduces tailplane restoring moment and can increase stall risk. Questions also address operator responsibilities: under airline procedures, the operator defines operating items and crew weights included in Dry Operating Mass, applies standard or actual passenger weights per regulation, and ensures mass and balance compliance at dispatch and before landing.

For safe operations and exam success, apply disciplined procedures: verify CG at zero-fuel, take-off, and landing; account for fuel burn and its potential CG travel; set appropriate trim; and understand how CG affects rotation, stall speed, and go-around margins. Always rely on the AFM/POH and the company Loading Manual, and remember that even small mass shifts at long arms can meaningfully move CG.

What This Question Bank Covers

• Definitions and fundamentals: datum, station, arm, moments, CG, MAC, %MAC expression.
• Step-by-step CG calculations, fuel density conversions, and moment summations.
• Converting CG from arm to %MAC and interpreting CG envelopes versus mass.
• Effects of CG on performance and handling: stall speed, trim drag, stability (forward vs aft CG, neutral stability).
• Operational scenarios: passenger/cargo shifts, determining required mass to move CG to a limit, gear-extension pitching moments.
• Regulatory and procedural context: AFM/POH and Loading Manual use, operator responsibilities for Dry Operating Mass and standard weights.