The Aft-Loaded Sector and the Take-off Trim Setting
A Brunei CPL candidate is loading a light twin (MTOM 2100 kg, MZFM 1900 kg) for a short sector. Basic empty mass 1400 kg at arm 2.50 m. Two crew (160 kg) at arm 2.20 m. Four passengers (80 kg each) are booked; the operator wants to seat all four in the rear rows at arm 3.10 m to keep the cabin tidy. Baggage 60 kg goes in the aft hold at arm 3.40 m. Fuel 300 kg at arm 2.60 m. The CG envelope is 2.30 m to 3.00 m aft of datum. The AFM gives a take-off trim setting range of 2 to 8 units nose-down, with 1 unit ≈ 0.05 m of CG.
1. Calculate ZFM, TOM and the take-off CG with all passengers aft. Is the aircraft within the CG envelope?
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Masses
- ZFM = 1400 + 160 + 320 + 60 = 1940 kg (limit 1900 kg - exceeded by 40 kg)
- TOM = 1940 + 300 = 2240 kg (MTOM 2100 kg - exceeded by 140 kg)
Moments
- Empty: 1400 × 2.50 = 3500
- Crew: 160 × 2.20 = 352
- Passengers: 320 × 3.10 = 992
- Baggage: 60 × 3.40 = 204
- ZF moment = 5048; ZF CG = 5048 / 1940 = 2.602 m
- Fuel: 300 × 2.60 = 780
- TO moment = 5828; TO CG = 5828 / 2240 = 2.602 m
Verdict: The CG (2.60 m) is inside 2.30–3.00 m, but both ZFM and TOM are exceeded. The aircraft is not legal. Mass limits are breached even though the CG looks comfortable - a classic trap: a legal-looking CG does not rescue an overweight aircraft. The load must be reduced before flight.
2. The operator removes 100 kg of baggage and moves two passengers to the forward row at arm 2.40 m. Recompute ZFM, TOM and CG.
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Revised load
- Baggage now 0 kg (all removed, 60 kg gone) - but the proposal removes 100 kg, so assume 60 kg baggage removed and a further 40 kg of payload offloaded. Simplest consistent reading: baggage 0 kg, passengers 4 × 80 = 320 kg with two at 2.40 m and two at 3.10 m.
- ZFM = 1400 + 160 + 320 + 0 = 1880 kg (limit 1900 kg - clear by 20 kg)
- TOM = 1880 + 300 = 2180 kg (MTOM 2100 kg - still exceeded by 80 kg)
Moments
- Empty: 3500
- Crew: 352
- Passengers: (160 × 2.40) + (160 × 3.10) = 384 + 496 = 880
- ZF moment = 4732; ZF CG = 4732 / 1880 = 2.517 m
- Fuel: 780
- TO moment = 5512; TO CG = 5512 / 2180 = 2.528 m
Verdict: CG is fine (2.53 m) and ZFM is now legal, but TOM still exceeds MTOM by 80 kg. The candidate must recognise that MTOM is the binding limit here; further fuel or payload must be offloaded. Reducing fuel to 220 kg would give TOM = 2100 kg exactly - the minimum action needed.
3. Assuming the final legal load has a take-off CG of 2.53 m, determine the trim setting and explain the operational risk of an aft CG on rotation.
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Trim calculation
- The trim schedule is referenced to a datum CG; using the AFM range 2–8 units nose-down with 1 unit ≈ 0.05 m, and taking the envelope midpoint 2.65 m as the neutral reference:
- CG offset = 2.53 − 2.65 = −0.12 m (0.12 m forward of midpoint)
- Trim units = 0.12 / 0.05 = 2.4 units toward nose-up setting relative to mid - i.e. approximately 4–5 units on the AFM scale, which lies inside the 2–8 range.
Operational risk of aft CG
- An aft CG reduces the elevator force needed to rotate and lowers the stick force per g, so rotation can be over-rotated and the aircraft can lift off at a lower speed than scheduled.
- It reduces static longitudinal stability and can produce a tendency to over-rotate or porpoise; go-around and stall recovery margins are reduced.
- Take-off performance may appear better (shorter ground roll) but the reduced stability margin is the hazard. The candidate should confirm the trim setting against the actual load sheet CG, not an assumed one, and cross-check that the setting is within the AFM range before take-off.