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Spatial disorientation: cues and consequences

Donald Anders Talleur


In 2004 the Human Factors working group of the Crew Resource Management and Spatial Orientation Conference defined spatial disorientation as “a failure to sense correctly a position, motion, or attitude of the aircraft or of oneself within the fixed coordinate system provided by the surface of the earth and the gravitational vertical.” This is a fancy way of describing what happens when a pilot’s orientation becomes discombobulated.

And should a pilot fall victim to such circumstances, the outcome can be disastrous to say the least.

This month we start a twopart review that will cover the more important aspects of spatial disorientation, specific disorientation illusions and provide some tips for prevention.

While the exact figures are not certain, the best available data seem to agree that spatial disorientation (SD) accidents, while not the leading cause of accidents in general, are generally fatal. By “generally,” I mean the data show that roughly 90% of accidents attributed to SD are fatal. That’s bad news for pilots!

Even worse news is that there is not a perfect solution to the problem, and the problem is indeed us, the human in the cockpit.

That’s not to say that only the pilot is responsible and that there are no outside influences to SD, but rather that we humans are rather poorly designed for correctly sensing three-dimensional motion (six degree of freedom if you also want to consider acceleration within each dimension).

Although we do a darn good job of handling flight with threedimensional motion most of the time, situations crop up that sometimes set us up for sensory confusion.

Before directly tackling the contributors to SD, we should first consider the bodily sources for cues as to our position in space: vision, the vestibular system, auditory cues, and somatosensory cues.

When considering vision, there are both conscious and subconscious cues that are received through focal and ambient vision respectively.

The focal vision is what we are attending to directly and is in focus, while the ambient vision picks up activity outside of the narrow focal view, such as peripheral motion.

The vestibular system is wholly contained within the ears, and two organs provide information about motion the body is experiencing; the semicircular canals and the otoliths.

The semicircular canals sense angular accelerations (most often referred when talking about roll recognition).

Another set of inner ear organs, called the otoliths, facilitate recognition of linear acceleration.

The ears also gather auditory cues, but are concerned with frequency, decibels, and duration rather than motion.

For instance, a change in engine power setting is marked by both a frequency change and a change in overall loudness (decibels). In the context of transient sounds, such as a marker beacon or other short term sound, the duration of that sound sometimes provides information.

This information is most often useful in maintaining situational awareness, but clearly, a loss of that can lead to spatial disorientation.

Somatosensory cues include both proprioceptive and cutaneous tactile sensor cues. Body movement provides proprioceptive cues, specifically movement of the muscles, joints, tendons and assist vision in providing feedback for movement.

Cutaneous tactile sensors extract cues from objects the pilot is in contact with, such as the control column. While the vestibular system is sensitive to G-loading, the body’s reaction to Gs, and motion in general, is usually noticed more readily.

However, it should be understood that somatosensory pressure receptors, those that sense the position of the body through pressures on the feet, back and gluteal region, are often in agreement with cues from the vestibular system, even if those cues are not correct for the motion being experienced. This conflict can lead to a powerful sensation of motion that is incorrect. However, with ample visual cues, even these incorrect sensations may not be overwhelming enough to cause an accident.

So in all, there are lots of cues during flight that tell us things about our position in space; and that’s supposed to help keep the aircraft right side up. However, when cues are lacking or conflicting, then SD sets in. If the SD is the result of conflicting cues that we are aware of, then we say that the SD is recognized.

But, if we have a false perception of the aircraft’s orientation and are unaware of how it got to that state, we say that the SD is unrecognized. Recognized or unrecognized, if the conflict is not resolved, an accident may be imminent.

Sometimes the pilot recognizes that they are experiencing SD, but the conflicting cues are so powerful as to disrupt flying performance to the point where a timely recovery can not be completed.

So the key is to 1) recognize the SD, 2) resolve the conflict, and 3) recover the aircraft.

Recognizing the conflict occurs in two very basic ways:

1) the pilot realizes that a sequence of aircraft motions will lead to cue conflict, and 2) the pilot recognizes, by trusting their instruments, that a cue conflict has occurred during the course of recent motions. The next step is to resolve the conflict.

This is a matter of trusting the instruments and not ones’ own body. If you have experienced SD then you know that it can be hard to ignore the body’s signals, but it must be done. Naturally, some effort must be made to ascertain the validity of the instrument readings, since some sort of instrument failure may have occurred.

For this task it is important to understand how the various instrument indications are associated with each other; something that all pilots hopefully master during partial panel training.

If the instruments, or some subset of them can be trusted, then a recover is usually possible.

However, if control of the aircraft is influenced by either the errant body motion cues, and/or faulty instrumentation, then degraded flying performance can easily lead to loss of aircraft control.

What exactly generates the false motion cues in the first place is a somewhat complicated issue. These false cues can be due to subthreshold motion; motion or accelerations that are below the body’s ability to perceive them (less than about 2.5 deg/sec).

For example, if the aircraft is rolled into a coordinated turn slowly enough and to a relatively shallow angle of bank, the semicircular canals may not signal the turn at all. If this occurs and the pilot then looks back to the instrument panel, he or she will feel disoriented when realizing that the aircraft is actually in a turn. If the acceleration is slow enough, regardless of which dimension the acceleration occurs in, the pilot may not feel any change at all.

The false cues can also be due to sustained motion is a particular dimension. The “leans” is a good example of this phenomenon.

If the pilot enters a suprathreshold bank (one that is perceptible) and maintains that angle of bank long enough, the fluid in the semicircular canals will settle and the fine hairs that sensed the initial acceleration will no longer sense any acceleration at all. If there are ample visual cues, the pilot will remain properly oriented at this stage in the manoeuvre.

But without ample outside visual references, if the pilot notices the unanticipated bank and attempts to correct, during deceleration to wings-level the semicircular canals will falsely signal a turn in the opposite direction.

To compensate for this false sensation, the pilot who does not trust the instruments may unwittingly roll back into the turn they were trying to roll out of.

Another source for false cues deals with cross-coupling of signals from the semicircular canals. This can occur when the fluid of one semicircular canal is brought into motion, motion that is correct given the aircraft’s motion, and then another canal’s fluid is brought into motion by head movement. The classic scenario for cross-coupling is making a turn, and while in the turn the pilot swiftly tilts his or her head over perhaps to pick up something from the floor.

Suppose the turn was to the right while making this type of head movement. The resulting sensation of motion will be a sudden rolling in the opposite direction. If the pilot responds to the perceived motion without consulting the instruments, they may roll even further to the right, steepening the angle of bank.

Coriolis illusion is the name given to this particular SD.

The combination of subthreshold accelerations, sustained motion after suprathreshold accelerations, and cross-coupling effects can combine in ways to create some interesting and dangerous illusions. Likewise, a class of illusions called somatogravic deal with the misperception of non-vertical gravitoinertial forces.

Next month we discuss these illusions in more detail and provide some tips for resolving or altogether avoiding SD.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.?