If an airplane is loaded to the rear of its CG range, it will tend to be unstable about which axis?

If an airplane is loaded to the rear of its CG range, it will tend to be unstable about which axis?

• A. Longitudinal axis
• B. Vertical axis
• C. Diagonal axis
• D. Lateral axis

Answer: (D)

Pitch stability is what’s being assessed here. When the airplane’s weight is toward the tail, the center of gravity moves aft, reducing the restoring pitch moment provided by the tail and horizontal stabilizer. A small disturbance in pitch then isn’t as quickly opposed, so the aircraft tends to continue pitching rather than return to level. That makes it unstable about the lateral axis, the axis running wingtip to wingtip around which the airplane rotates when it pitches. In other words, a rearward CG reduces longitudinal (pitch) stability, making instability more likely about the lateral axis.

Pitch stability is what’s being assessed here. When the airplane’s weight is toward the tail, the center of gravity moves aft, reducing the restoring pitch moment provided by the tail and horizontal stabilizer. A small disturbance in pitch then isn’t as quickly opposed, so the aircraft tends to continue pitching rather than return to level. That makes it unstable about the lateral axis, the axis running wingtip to wingtip around which the airplane rotates when it pitches. In other words, a rearward CG reduces longitudinal (pitch) stability, making instability more likely about the lateral axis.