What is the major cause of spatial disorientation?

Spatial disorientation primarily arises from the orientation illusion, which occurs when a pilot's perception of their position and motion in relation to the Earth's surface is distorted. This is particularly dangerous in flight, as pilots rely heavily on their sensory systems to maintain orientation. The inner ear, eyes, and proprioception all contribute to how a pilot understands their position in three-dimensional space. However, in certain conditions such as in-flight maneuvers, lack of visual reference, or over-reliance on instruments, the sensory inputs can clash and lead to misjudgment of the aircraft's attitude.

In many scenarios, specifically ones involving the absence of visual cues or when flying in clouds, the brain may misinterpret the input from the vestibular system, leading to the illusion of motion or position that does not correspond to reality. This can result in dangerous situations if the pilot reacts based on this incorrect perception.

While other factors like G-forces, visual misinterpretation, and navigation errors can contribute to challenges in flying, they are secondary in terms of causing the specific type of disorientation identified in the context of this question. Thus, the orientation illusion stands out as the major contributor to spatial disorientation.