Centre of Gravity Limits and Their Effects
What the limits protect
The CG range is the band between the forward and aft limits within which the aircraft can be operated safely. The forward limit is set by controllability: elevator authority to rotate, flare and go around, to hold the nose up in a stall and to recover, plus nose-wheel loading and, on some types, minimum stick force. The aft limit is set by stability: static and dynamic longitudinal stability, stall and spin recovery, and tail-structure loads. The envelope is not a rectangle - on many aircraft the forward limit moves aft as mass increases, and limits may change with fuel or flap configuration.
Effects of CG position
- Stall speed: essentially a function of mass and load factor, not of CG. A forward CG needs a small extra wing lift to balance tail downforce, so the effect is marginal; any exam answer that claims large CG-driven stall-speed changes is wrong.
- Cruise and fuel: an aft CG reduces tail downforce and therefore trim drag, giving slightly higher speed and lower fuel consumption. A forward CG increases trim drag and fuel burn.
- Take-off and landing: a forward CG lengthens the take-off run, needs higher rotation forces and increases landing distance; an aft CG improves performance but reduces the natural stability margin.
Worked example
Mass 60 000 kg, CG 22% MAC, MAC 4.50 m, aft limit 32% MAC. Moving 1 500 kg aft by 8.0 m gives ΔCG = (1 500 × 8.0)/60 000 = 0.20 m = 4.4% MAC. New CG = 26.4% MAC - inside the limit, with 5.6% MAC of margin. If the load had been 3 000 kg, the shift would be 8.9% MAC and the aircraft would be at 30.9% MAC, close to the aft limit.
Traps that cost marks
Quoting CG in metres when the limit is in %MAC; assuming an aft CG is more stable; believing CG strongly changes stall speed; ignoring that a forward CG increases fuel burn; forgetting that the limit itself may change with mass.