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.
‘Flat’ and ‘Level can mean approximately the same thing in everyday conversation. But in the context of science and engineering, they have different meanings. ‘Flat’ describes a planar surface. ‘Level’ means at the same height or perpendicular to the direction of Earth’s gravity.
Flat-Earthers interpret the word ‘level’ as if it always has the same meaning as ‘flat.’ However, in the scientific context, the two words have different meanings. Insisting that both words to have the same meaning is the fallacy of appeal to definition.
The fallacy of ‘appeal to definition’ is using the definition of a term to support an argument as if the term cannot have other meanings or even conflicting meanings. Flat-Earthers often use this fallacy, for example, over the word “theory.”
A communication problem can occur when a term gets misinterpreted to mean other than what was intended. A simple clarification should quickly correct the problem. The appeal to definition arises if the clarification is refused, and the person insists on using the wrong & unintended meaning of the term, and use it to support their arguments.
A higher position is farther up, away from the center of the Earth, relative to sea level. Conversely, a lower position is closer toward Earth’s center, relative to sea level. Two positions are level if they are at the same height from sea level.
Some flat-Earthers are unable to understand this. To them, ‘level’ means straight. They are wrong. In Earth sciences, height is measured from a plane of reference, usually the sea level.