Mass and balance: loading, CG limits, mass limits and their effect on flight planning
Domain 6 covers mass limitations and centre of gravity: which masses are checked where, and how loading moves the CG.
Mass limitations
- MTOM (structural, often further reduced by performance), MLM, MZFM, MRWM (ramp) and OEW/DOW.
- Checks: TOM ≤ MTOM; LM (TOM − trip fuel) ≤ MLM; ZFM (DOW + payload, no fuel) ≤ MZFM; ramp mass ≤ MRWM.
- Underload = the smallest of (MTOM − TOM), (MLM − LM) and (MZFM − ZFM).
- A last minute change is allowed within the operator's procedure (commonly up to 500 kg) only if all mass limits and the CG envelope remain satisfied and no passenger reseating is required.
Centre of gravity
- %MAC = (CG − LEMAC) / MAC × 100, where the CG is total moment divided by total mass.
- Forward limit: nose-heavy, more tail downforce and trim drag, longer take-off run, higher stick forces, and rotation may be restricted.
- Aft limit: reduced static stability, lighter stick forces and greater risk of a tail strike during rotation.
- Fuel movement matters: centre-tank fuel then wing-tank fuel shifts the CG, so sequencing affects trim.
Worked example
DOW 44,600 kg, payload 12,000 kg, take-off fuel 6,100 kg; MTOM 66,000 kg, MLM 62,000 kg, MZFM 57,000 kg. ZFM = 56,600 kg ≤ 57,000 kg. TOM = 56,600 + 6,100 = 62,700 kg ≤ 66,000 kg. LM = 62,700 − 4,920 trip fuel = 57,780 kg ≤ 62,000 kg. Underload = min(3,300; 4,220; 400) = 400 kg, so MZFM is the limiting limit. With a total moment of 1,216,380 kg·m at 62,700 kg the CG is 19.40 m; with LEMAC 17.00 m and MAC 6.00 m, %MAC = 2.40/6.00 × 100 = 40%, inside a 15–42% envelope.
Traps
- Including fuel in the zero fuel mass check.
- Checking MTOM at destination instead of MLM.
- Subtracting taxi fuel instead of trip fuel when computing landing mass.
- Ignoring the CG effect of a last minute change in a forward hold.