Spatial Disorientation Trust Your Instruments
By Roger V. Shaw, II
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Spatial disorientation is defined as: "A state characterized by an erroneous sense of one's position and motion relative to the plane of the earth's surface. "How does this occur in an aircraft? Pilots want "shoe leather" answers to keep themselves out of trouble in the air. We will briefly discuss the orientation senses, and one of the most common illusions experienced by pilots-THE LEANS. Spatial disorientation is caused by the senses of the body misrepresenting the pilot's position in space; vision (eyes), vestibular (inner ear), proprioceptors (muscle/tendon senses... "seat of the pants"). For our limited discussion, we will discuss the two most dominant senses, which are vision and the vestibular system, and how they provide orientation to pilots. However, do not exclude a the importance of the proprioceptors or the auditory systems in attempting to understand the human body in space, as all the systems interact under varying conditions. The eyes, the most important source of information, send pictures to the brain about the aircraft's position, velocity, and attitude relative to the ground. This works great on clear days in VFR (visual flight rules) conditions with a well-defined horizon; but in poor visibility, night flying, or IFR (instrument flight rules), a pilot can experience visual illusions (runway and approach illusions). A note of caution is that even on a clear VFR day, the eyes can play tricks, since up to 90 percent of orientation is provided by visual cues. Time and space do not allow a full discussion on this very important sense that can cause serious spatial disorientation. The part of the vestibular system that we will focus on are the semicircular canals in the inner ear. These canals are filled with fluid that indicates rotation on the yaw, pitch, and roll axis. They are actually accelerometers that sense changes in velocity. |
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Problem: After 10 to 20 seconds of constant angular acceleration of the inner ear's fluid, the sensation of motion transmitted to the brain can be false. The pilot can be in a turn and not know it. In addition, there is a fixed acceleration threshold, below which the semicircular canals cannot sense any rotation at all. This threshold is approximately 2 degrees per second; if the rotation is gradual enough, the pilot won't sense any change and will develop "the leans." The leans can occur in the same or opposite direction of the motion. Both will occur when movement is below the threshold of sensitivity for the semicircular canal.
First case, a roll to the left-below the threshold-is unnoticed until the pilot looks at the attitude indicator. The pilot's semicircular canal never registered the initial turn, but the senses now produce a right wing-down sensation, which can linger for a long time. Second case, leans that occur in the same direction. The initial movement is rapid, caused by gusty conditions or clear air turbulence. When the plane is gradually returned to wings level, the pilot still thinks he/she is leaning in the direction of the initial displacement.
In all cases of spatial disorientation, the pilot must rely on the flight instruments when making control inputs - and must be patient until the false sensations dissipate.
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