Weight and Balance - Aft CG on a Short Runway
You are loading a light twin (MTOW 2,100 kg, MLW 2,000 kg) for a 480 NM sector. Basic empty mass is 1,420 kg with a moment arm of 2.85 m. You plan to carry two pilots (85 kg each at 2.40 m), 120 kg of baggage in the aft hold (4.10 m), and 240 kg of fuel (2.95 m). The CG envelope limits are 2.55 m forward and 3.05 m aft. The departure runway is 1,200 m, and the AFM take-off chart is based on a mid-envelope CG. The aeroplane has a published aft-CG penalty of 4 % on take-off distance and a 6 % reduction in static stability.
1. Compute the loaded CG and determine whether the loading is within the envelope.
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Compute total mass and total moment.
- Basic empty: 1,420 kg × 2.85 m = 4,047 kg·m.
- Pilots: 170 kg × 2.40 m = 408 kg·m.
- Baggage: 120 kg × 4.10 m = 492 kg·m.
- Fuel: 240 kg × 2.95 m = 708 kg·m.
Total mass = 1,420 + 170 + 120 + 240 = 1,950 kg. Total moment = 4,047 + 408 + 492 + 708 = 5,655 kg·m. CG = 5,655 / 1,950 = 2.90 m.
The CG of 2.90 m is within the envelope (2.55–3.05 m) but is 0.15 m aft of the mid-point of 2.80 m, so it is in the aft half. The mass of 1,950 kg is below MTOW 2,100 kg and below MLW 2,000 kg. The loading is legal.
However, legality is not the same as suitability: at 2.90 m the aeroplane is close to the aft limit, which has handling and performance consequences, as the next task discusses.
2. Assess the effect of this CG on take-off performance and handling, and recommend a loading change.
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An aft CG reduces the tail-down force required in pitch, which reduces trim drag and can slightly improve cruise speed and fuel burn. But it also reduces static and dynamic longitudinal stability, making the aeroplane more sensitive in pitch and more prone to over-rotation at rotation speed.
For take-off, the AFM chart is based on a mid-envelope CG. The aft-CG penalty of 4 % increases the take-off distance: 1,200 m TORA with a chart value near the limit could become unacceptable. The 6 % reduction in static stability means the rotation and initial climb are less forgiving, especially with a crosswind or a gust.
A practical remedy is to move some baggage forward or reduce the aft hold load by 40 kg, which would shift the CG forward by roughly 0.05 m to about 2.85 m, restoring margin. Alternatively, load fuel in the forward tanks if the fuel system permits. The decision should be based on the actual loading sheet, not on the assumption that a legal CG is a good CG.