Mass and Balance
What the exam tests
Mass and balance tests mass limits, CG limits, moment calculations, %MAC and loading philosophy. Expect a numerical question with a table or simple data.
Core rules
- Maximum take-off mass, maximum landing mass, maximum zero-fuel mass; MZFM excludes usable fuel.
- CG position = total moment / total mass; moment = mass × arm.
- %MAC = (CG arm − LE arm) / MAC × 100.
- Forward CG: higher stall speed, better stability, longer take-off; aft CG: lower stability, shorter take-off, possible pitch-up.
- Floor loading and compartment limits matter as much as total mass.
Worked example
Basic mass 50,000 kg at 25% MAC. Add 5,000 kg at 10% MAC. New CG = (50,000×25 + 5,000×10) / 55,000 = (1,250,000 + 50,000) / 55,000 = 1,300,000 / 55,000 = 23.6% MAC. The CG moves forward by 1.4% MAC. Reasoning: use consistent units; %MAC can be used directly as the arm if all items are expressed in %MAC.
Traps
Using mass instead of moment; mixing metres and %MAC; forgetting zero-fuel mass; ignoring aft cargo compartment limits; assuming fuel burn does not change CG.