During cruising, an airplane is practically level. It is flying perpendicular to the direction of gravity. The aircraft is still flying following Earth’s curvature and continuously changes its orientation. However, not only its orientation changing but also the direction of gravity, having the same amount of change. As a result, a spirit level on a flight will not show us the difference in orientation of the airplane due to its motion following the curvature of the Earth.
A flat-Earther brought a level to a flight and performed an “experiment” to determine if the Earth is flat or a sphere. He observed that the level did not indicate the change of orientation if Earth is a sphere, and incorrectly concluded that the Earth is flat.
The origin of his error was his hypothesis. He thought if Earth is a sphere, then the airplane will continuously change its orientation, and a level should indicate that. In reality, the direction of gravity also changes by the same amount as the change in orientation.
References
- Explained: Why a Spirit Level on a Plane Does Not Show Curvature “Corrections” – Mick West – Metabunk