Mass and balance with a last-minute passenger change
You are preparing a Britten-Norman Islander BN-2B for a charter from WBSB to Labuan (WBKL). Basic empty mass is 1 950 kg with ARM 2.60 m. The forward baggage compartment holds 30 kg at ARM 1.20 m, the aft baggage compartment 40 kg at ARM 4.30 m. Six passengers are seated: two at ARM 2.90 m (85 kg each), two at ARM 3.50 m (80 kg each) and two at ARM 4.10 m (75 kg each). Fuel loaded is 300 kg at ARM 3.10 m. MTOW is 2 994 kg; the forward CG limit is 2.60 m and the aft limit is 3.20 m. At the gate, one passenger weighing 110 kg replaces a 75 kg passenger in the aft row.
1. Compute the take-off mass and CG position before and after the passenger substitution, and state whether the aircraft is within limits.
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Before substitution:
- BEM: 1 950 kg × 2.60 = 5 070 kg·m
- Fwd bag: 30 × 1.20 = 36
- Aft bag: 40 × 4.30 = 172
- Pax row 1: 170 × 2.90 = 493
- Pax row 2: 160 × 3.50 = 560
- Pax row 3: 150 × 4.10 = 615
- Fuel: 300 × 3.10 = 930
Total mass = 1 950 + 30 + 40 + 170 + 160 + 150 + 300 = 2 800 kg. Total moment = 5 070 + 36 + 172 + 493 + 560 + 615 + 930 = 7 876 kg·m. CG = 7 876 / 2 800 = 2.81 m. Within the 2.60–3.20 m envelope, mass 194 kg below MTOW.
After substitution: row 3 mass becomes 110 + 75 = 185 kg, an increase of 35 kg at ARM 4.10 m. New total mass = 2 835 kg. New moment = 7 876 + (35 × 4.10) = 7 876 + 143.5 = 8 019.5 kg·m. New CG = 8 019.5 / 2 835 = 2.83 m. Still within limits; the CG has moved 0.02 m aft.
2. The passenger's actual mass was estimated at 110 kg; the standard mass is 84 kg. What operational and safety considerations arise?
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Using standard masses when actual masses are clearly higher undermines the validity of the load sheet. The 26 kg underestimate here changes the CG by only 0.02 m, but on a smaller aircraft or with a heavier substitution the error could push the CG outside the aft limit, reducing pitch stability and elevator authority. Under mass-and-balance regulation, actual masses must be used where they are known or can reasonably be established, and the commander must be satisfied with the load sheet before flight. The correct action is to re-weigh or confirm the passenger's mass, re-issue the load sheet, and verify the CG is within limits before departure, rather than relying on the standard-mass assumption.