The most common type of pilot error in CFIT accidents is spatial disorientation,[6] a loss of situational awareness often resulting from reliance on the proprioception system for the perception of relative motion in conjunction with a loss of visual cues during dark or cloudy conditions. In these situations, pilots misunderstand their position, velocity, and/or heading during flight, which can lead to inappropriate control commands relative to the altitude of the surface of the Earth below and immediately ahead, on the course they are flying. Fatigue can cause even highly experienced professionals to make significant errors, which culminate in a CFIT accident.[7] Furthermore, pilots of tactical air fighters may experience g-force induced loss of consciousness (G-LOC), leading to CFIT accidents.