Donald Anders Talleur
This month we continue with part two of a review of spatial disorientation (SD) with more examples of illusions that lead to SD and tips for mitigating the problems of SD during flight.
Last month I introduced the basic concepts of spatial disorientation.
Examples of more familiar spatial disorientation (SD) problems were given, namely the coriolis illusion and the leans. While coriolis force is a somatogyral illusion (illusion dealing with fluid movement within the semicircular canals), the leans are a combination of illusions due to false sensation in both the semicircular canals as well as the otoliths of the inner ear.
But before we continue talking about illusions that involve both sensory systems let’s go over a few other common illusions that deal with one at a time.
The graveyard spin is a fairly nasty somatogyral illusion involving primarily the semicircular canals. It occurs as a result of a prolonged spin in which the angular motion sensation experienced by the pilot slowly subsides.
The real problem occurs as the pilot begins to recover from the spin. As the spin rotation slows during recovery, the fluid in the semicircular canals moves, deflecting the fine hairs in the direction opposite to the initial direction of motion. At this point, the pilot easily senses that the spin has changed direction and puts in correction that effectively restores the original spin. This type of illusion is most typical after at least five turns in a stabilized spin.
While graveyard spins are concerned mainly with rotation about the vertical axis of motion, another type of illusion is concerned with the longitudinal axis. Similar to the sub-threshold bank illusion, which is also caused by motion about the longitudinal axis of the aircraft, the post roll illusion is experienced upon rollout from a banked condition.
Typically overbanking in the direction opposite the original bank during leveling of the wings leads to banking in the previous direction. This phenomenon is apparently most prevalent with rollout from high-angle of bank rolls. Once again, the semicircular canals are at fault for creating this illusion.
Another class of non-visual SD illusions are the som - atogravic illusions.
These illusions are due to sensory confusion as the result of gravity. The inner ear otolith membrane movement in re - sponse to linear acceleration, either aircraft induced or head movement induced, causes the sensation of an acceleration.
However, if head tilt and actual aircraft acceleration occur simultaneously, confusion can arise when determining the true vertical. Lack of visual cues, such as at night, and the resulting confusion over true vertical can lead to SD. Three potential illusions of this class are the false perception of pitch, false perception of attitude during turns, and the Gexcess effect.
In level flight, such as during takeoff roll or cruise, a sudden acceleration can play a nasty trick on the inner ear. Since gravity works more or less parallel to the vertical axis during these phases of flight and inertia works parallel to the flight path, a resultant force occurs that is parallel to neither axis.
The resultant force is what the pilot perceives as vertical. In the case of forward acceleration, a false sense of climb can result when adequate visual references are unavailable (the misperception is still there even with good visual cues, but those visual cues are more powerful than the inner ear cues for maintaining orientation).
During deceleration, a false sense of dive may occur. This is particularly problematic during cruise flight, in that decelerating too fast can lead the pilot to think he is in a dive, and pull back, slowing the aircraft even more!
Similar to the false perception of pitch, when the pilot is in a sustained coordinated turn the resultant gravitoinertial force is effectively what we call a “G” and is parallel to the vertical axis of the aircraft. The only problem here is that in a prolonged turn, the perception may be of level flight for reasons explained earlier.
However, should the turn become uncoordinated, the resultant G-force in the turn is no longer parallel to the vertical axis, and the pilot is set for misinterpreting attitude. In a skidding turn, the perception will be that the aircraft is turning in the opposite direction and the pilot might compensate by banking ever further into the direction of the original turn. If the aircraft is already slow and the skid continues, this situation can easily lead to a stall-spin incident.
G-excess effect, despite how it sounds, has to do with the false perception of attitude created by sustained g-loading greater the 1 G.
When the head is tilted off vertical while undergoing normal 1 G maneuvering, the pilot experiences less than 1 G of pull. As the head approaches 90 degree tilt from vertical, the pull experienced now matches the actual force due to gravity.
However, if the G-loading exceeds 1, and the head is tilted from vertical, the Gs experienced by the head will exceed 1, creating the sensation that the head is tilted farther from vertical than it actually is. This effect is particularly problematic when looking up, into the direction of turn. The perception of attitude will be confused under such circumstances and can lead to overbanking and loss of altitude.
One last illusion that general aviation pilots seem to have problems with (owing to the death rates) is the graveyard spiral. This particular illusion stems from confusion created both by the semicircular canals and the otoliths.
After the pilot has been turning for a while, a constant velocity is reached and the sensation of turning, as well as the sensation of banking can be lost. This is so because the hairs of the semicircular canal do not respond to constant angular velocity and the otoliths do not respond to constant linear velocity.
Similar to the graveyard spin, recovery attempts can actually lead the pilot to bank further in the original direction.
Any loss of altitude due to the steep bank can lead to increased back pressure, which effectively tightens the turn, hence the graveyard spiral has begun.
While these illusions, and those discussed last month are hard to eliminate, the problems that each creates for the pilot can at least be mitigated by assuring 1) good visual flying conditions during the daylight hours, 2) a good visual scan and awareness of the visible horizon,
3) a good instrument scan during the nighttime, 4) and above all else, solid situational awareness.
Inner ear disorders not withstanding, it is still possible to get into an SD situation even if everything else is done correctly.
The next steps are the critical ones. First, the pilot needs to recognize that SD has interfered with flying. Second, the instruments and/or outside environment must be used, and trusted, to assist in correcting the attitude to normal.
In deciding what to trust, keep in mind that the outside environment may have contributed to the SD (visual illusion of some sort). Sometimes the best solution to avoiding SD in flight situations where it might be likely is to let the autopilot help. An autopilot can right the aircraft while the pilot gets his head back in order. In any event, flying straight and level while minimizing head movements will speed up recovery.
If all the above fails to help, then it’s time to get outside assistance. A call to air traffic control is in order, but remember that controllers are not necessarily pilots.
Tell them what you need right off the bat (e.g. a heading out of the clouds for instance), for if you are truly experiencing SD, there is usually insufficient time to negotiate a recovery plan.
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.