Third is the vestibular sense, which is the other half of our sense of hearing. Deep within the inner ear, adjacent to the organ of hearing, or cochlea, is our organ of balance - the vestibular apparatus. It is divided into two parts - the three semicircular canals and the two otoliths.
The three semicircular canals are oriented 90 degrees to one another. They are angular accelerometers that detect pitch, roll, and yaw. Adjacent to them are two otoliths - linear accelerometers that detect motion to the front, back, left, and right. They also sense the earth's gravity.
The semicircular canals and otoliths have threshold limits, which means they cannot detect accelerations below a certain amount. If you were in a slowly rotating room with a turn rate of less than three degrees per second, you wouldn't be able to detect the motion. A rotating restaurant is an example. These restaurants take an hour to turn 360 degrees; they turn at 0.1 degree per second. Because this rate is below your reception threshold, you won't sense the room's rotation - unless you have a visual cue, such as the changing scenery out the window.
Likewise, our linear accelerators can't detect very slight changes, but they are so sensitive it's difficult to drop below this threshold. When riding an elevator, it's easy to sense its motion, even though it may be moving up or down slightly. However, if the elevator is moving up or down at a constant velocity and is not accelerating, you sense no motion at all. And because you have no visual cues of motion, the lack of acceleration fools the proprioceptive sense. Thus, you can be in a rapidly moving elevator and have absolutely no sense of motion.
In an aircraft, any one of these same senses can be dangerously fooled, but it's the vestibular sense that most often is out of step with the others.
For example, when an airplane rolls to the right very slowly, you may not sense it until the otolith notices the change in the gravity, leading you to look outside. The sight of a "tilted" horizon causes you to roll the aircraft back to wings level.
In this example, the undetected right roll creates a subthreshold, and the correcting, detected left roll creates a super threshold. In other words, your brain thinks you're in a 30-degree left bank when you are actually level because your body detected the left roll only. This creates a sensation most pilots call "the leans."
The leans is probably the most common and least dangerous form of spatial disorientation (in VFR conditions, of course). In steady turns, however, the vestibular sense can't detect a sensation of roll, but - even worse - if the turn lasts for 30 or 40 seconds, this sense may become "fatigued" and lose the turning sensation entirely.
After making a steady turn for 40 seconds you may not realize that you're turning anymore (unless you have a visual cue). Then, when you roll the wings level, your body will think it's rolling in the other direction. The result can be a tendency to want to roll right again. If you're deprived of visual input, you may go ahead and resume your right-hand turn, which usually degenerates into a descending spiral. Before the days of artificial horizons and reliable directional gyros, this form of spatial disorientation often caused a "graveyard spiral," a graphic term for a prolonged turning descent to ground impact.
Another common form of acceleration mismatch occurs during the climb after takeoff. As you climb gently and accelerate, you might sense that you're pitching nose up. Even though you're in a gentle climb, this feeling may be so strong you feel like you're climbing past the vertical. Without visual cues to correct this inaccurate sensation, you can develop a very strong tendency to push the nose down.
This form of spatial disorientation can occur easily when you take off into a dark night sky with no ground lights nearby and no other visual cues. The motion of the airplane can fool the inner ear, and if you don't ignore the inaccurate sense and rely on visual cues instead (the instrument indications), you may tend to push the nose down. Uncorrected, that could cause you to fly, wings level, into the ground about three miles from the end of the runway. This is how spatial disorientation can lead to a controlled flight into terrain - CFIT - accident.
A take off toward the ocean from a coastal airport on a hazy day can create a similar situation. Once you cross the beach, the gray sky and gray water merge to effectively "erase" the horizon. The effect can be the same as flying in clouds. Flying over the water creates another hazard because, without a horizon, pilots involuntarily try to put the sun overhead. Around 5 p.m., pilots have rolled into 80-degree banks in an attempt to keep the sun straight overhead.
The most important thing you can learn about spatial disorientation is to rely on your instruments if you don't have good visual cues. Any factor such as darkness, haze, and bad weather, that interferes with your outside visual references - can lead to spatial disorientation. Know your instruments and learn to rely on them, even when the weather is good. When two of your three spatial senses tell you "tilt," believing and flying by the instrument references you see will keep you straight, true - and safe.
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