I wrote "a leading killer" for military pilots and I mean it. During Desert Storm the US Navy lost 15 aircraft in non-combat accidents. 7 of those 15 involved SD.
From
the US Navy:
Spatial Disorientation (SD) is an aviator’s misperception of the attitude, position, or motion of his/her aircraft relative to the Earth’s surface and gravitational vertical. SD is a serious threat to flight safety and is the leading aeromedical cause of Class A mishaps in Naval aviation. SD is also a leading killer across the DoD and in civilian aviation as well.
From
the US Air Force:
7.8. Spatial Disorientation (SD). SD has proven to be a leading killer of F-16 pilots. Although SD is most common at night or in IMC, it can and has happened in day VMC. Reference AFPAM11-417 for information on the causes of SD, how to avoid it, and how to mitigate its consequences.