Mass and Balance
Mass and Balance - what the exam checks
The NCAA PPL tests whether you can build a loading sheet from the datum forward and then read the answer as a flight-safety decision, not just as arithmetic on a calculator.
Core rules
- Moment (kg·m) = mass × arm, where the arm is measured from the datum, an arbitrary vertical reference plane chosen by the manufacturer.
- The empty aircraft is weighed to give the basic empty mass (BEM), which already includes unusable fuel and full oil. Adding crew, usable fuel, baggage and freight gives the take-off mass.
- Limits to respect: maximum take-off mass, maximum landing mass, maximum zero-fuel mass and the forward/aft CG limits, usually shown as a CG envelope in % MAC between LEMAC and TEMAC.
- CG position changes handling: a forward CG lengthens the take-off run, raises the stall speed and increases stick forces; an aft CG reduces static stability and makes stall and spin recovery harder.
Worked example
BEM 750 kg at 2.20 m; pilot 75 kg at 2.00 m; fuel 90 kg at 2.10 m; baggage 10 kg at 3.00 m. Moments: 1650 + 150 + 189 + 30 = 2019 kg·m. Total mass 925 kg. CG = 2019 ÷ 925 = 2.18 m. If the envelope runs 1.95–2.45 m the load is inside the CG limits, but the zero-fuel mass (925 − 90 = 835 kg) must still be compared with MZFW, and the landing mass checked against MLW after trip fuel is burned off.
Traps
- Counting fuel volume as if it were mass, or leaving fuel out of the mass column while including its moment.
- Wrong Avgas conversion: 100LL is about 0.72 kg/L (6 lb/US gal), so 100 L is roughly 72 kg.
- Checking take-off mass only, and forgetting that the landing mass is lower and the CG moves as fuel burns.
- Ignoring the aft limit after adding a rear-seat passenger or baggage.
- Assuming a load sheet removes your responsibility: the PIC must verify the calculation.