What happens to stability at high altitudes?

At high altitudes, the atmosphere becomes less dense, which affects the aerodynamic characteristics of an aircraft. As altitude increases, the amount of lift generated by the wings diminishes due to the lower air density. Consequently, this reduction in lift can lead to decreased overall stability, particularly concerning the aircraft's response to disturbances.

While general stability may reduce at high altitudes, spiral stability can actually increase. Spiral stability refers to the aircraft's ability to resist entering a spiral dive, which can be enhanced as the aircraft's performance characteristics change with the thinner air. As the aircraft becomes less responsive to minor control inputs due to the reduced airflow over the control surfaces, it may become more stable in certain flight scenarios, like avoiding sharp turns or overcontrol.

Thus, the claim that stability is generally reduced while spiral stability increases captures the nuanced behavior of an aircraft's stability characteristics at high altitudes.