Spatial disorientation specifically refers to the lack of  orientation with regard to the position, attitude, or movement  of the airplane in space. The body uses three integrated  systems that work together to ascertain orientation and  movement in space.  All this information comes together in the brain and, most  of the time, the three streams of information agree, giving  a clear idea of where and how the body is moving. Flying  can sometimes cause these systems to supply conflicting  information to the brain, which can lead to disorientation.  During flight in visual meteorological conditions (VMC),  the eyes are the major orientation source and usually prevail  over false sensations from other sensory systems. When  these visual cues are removed, as they are in instrument  meteorological conditions (IMC), false sensations can cause  a pilot to quickly become disoriented.  The vestibular system in the inner ear allows the pilot to  sense movement and determine orientation in the surrounding  environment. In both the left and right inner ear, three  semicircular canals are positioned at approximate right angles  to each other.  [Figure 17-3]  Each canal is filled with fluid  and has a section full of fine hairs. Acceleration of the inner  Prevention is usually the best remedy for spatial disorientation.  Unless a pilot has many hours of training in instrument flight,  flight should be avoided in reduced visibility or at night when  the horizon is not visible. A pilot can reduce susceptibility  to disorienting illusions through training and awareness and  learning to rely totally on flight instruments.  Under normal flight conditions, when there is a visual  reference to the horizon and ground, the sensory system in the  inner ear helps to identify the pitch, roll, and yaw movements  of the aircraft. When visual contact with the horizon is lost,  the vestibular system becomes unreliable. Without visual  references outside the aircraft, there are many situations in  which combinations of normal motions and forces create  convincing illusions that are difficult to overcome.  The somatosensory system sends signals from the skin, joints,  and muscles to the brain that are interpreted in relation to the  Earth’s gravitational pull. These signals determine posture.  Inputs from each movement update the body’s position to the  brain on a constant basis. “Seat of the pants” flying is largely  dependent upon these signals. Used in conjunction with visual  and vestibular clues, these sensations can be fairly reliable.  However, the body cannot distinguish between acceleration  forces due to gravity and those resulting from maneuvering  the aircraft, which can lead to sensory illusions and false  impressions of an aircraft’s orientation and movement.  ear in any direction causes the tiny hairs to deflect, which  in turn stimulates nerve impulses, sending messages to the  brain. The vestibular nerve transmits the impulses from  the utricle, saccule, and semicircular canals to the brain to  interpret motion.  Figure 17-3.  The semicircular canals lie in three planes and sense motions of roll, pitch, and yaw.  • Vestibular system—organs found in the inner ear that  sense position by the way we are balanced  •  Somatosensory system—nerves in the skin, muscles,  and joints that, along with hearing, sense position  based on gravity, feeling, and sound  • Visual system—eyes, which sense position based on  what is seen  Spatial Disorientation and Illusions  17-6  YAW  ROLL  YAW  PITCH  PITCH  ROLL  The semicircular tubes are arranged  at approximately, right angles to each  other in the roll, pitch, and yaw axes.  Vestibular nerve  Endolymph fluid  Ampulla of semicircular canal  Hair cells  Cupola  Semicircular canals  Otolith organ 