Aft centre of gravity, a 2.5g gust and an accelerated stall warning
A charter operator is loading a 3,000 kg MTOW twin piston aeroplane. The aft CG limit is 2.45 m aft of datum; the load sheet gives 2.49 m. The pilot says the difference is only 4 cm and signs the load sheet. The aircraft is 150 kg below MTOW. During climb at 8,000 ft in moderate turbulence, the aircraft encounters an upward gust that momentarily raises the load factor to 2.5 g. The stick force is unusually light, the nose pitches up, and the stall warning sounds at 105 KIAS. The flaps-up 1g stall speed at the actual weight is 85 KCAS. The pilot reduces power and pushes forward; the aircraft recovers. The pilot later asks whether the aft CG had anything to do with the event.
1. Calculate the stall speed at 2.5 g and explain why the stall warning sounded at 105 KIAS. What other factors could have contributed?
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Stall speed increases with the square root of load factor: Vs(n) = Vs(1g) × √n. At 2.5 g, Vs = 85 × √2.5 = 85 × 1.581 ≈ 134 KCAS. At 105 KIAS the load factor that would stall the wing is (105/85)² ≈ 1.53 g. The aircraft was therefore already close to the accelerated-stall angle of attack at 105 KIAS; the gust and the aft CG both contributed.
The aft CG reduces the tail downforce required, so the wing may be carrying slightly less load and the 1g stall speed may be marginally lower, but the reduced pitch stability and very light stick force make it easier for the gust to pitch the nose up and for the pilot to over-control. Turbulence, the upward gust and the light elevator forces all contributed to reaching the warning at an airspeed well above the 1g stall speed.
2. Explain the effect of the aft CG position on static margin, stick force, stall speed and spin recovery. Why is even a small aft exceedance significant?
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Moving the CG aft reduces the static margin, which is the distance between the CG and the neutral point. A smaller static margin reduces longitudinal static stability and reduces the stick force per unit of load factor, so the aircraft feels light and is easier to over-rotate or over-stress. The tail downforce is reduced, so the wing carries slightly less load and the 1g stall speed may be marginally lower, but the aircraft is less resistant to pitch disturbances.
An aft CG also makes spin recovery more difficult because the pitching moment that maintains the spin is stronger and the elevator authority is less effective in the recovery direction. A 4 cm exceedance is significant because the CG envelope is a certification limit, not a advisory target. Stability margins are nonlinear, and loading errors can combine with turbulence, gusts or an accelerated manoeuvre to produce a stall or loss of control.
3. What actions should have been taken before departure, and what should the pilot do if an aft CG exceedance is discovered in flight?
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Before departure, the pilot should have refused to sign the load sheet and recalculated the CG. The load should have been redistributed forward, baggage removed, fuel reduced, or passengers reseated until the CG was within the approved envelope. A 4 cm exceedance is not acceptable even at 150 kg below MTOW, because the limit applies at all weights within the envelope.
If an aft CG exceedance is discovered in flight, the pilot should move load forward if possible, avoid turbulence and abrupt manoeuvres, reduce speed to a safe margin above the stall, and land at the nearest suitable airport. Stick forces will be light, so smooth, small inputs are essential. The aircraft should not be flown into known turbulence or used for further training until the loading is corrected.