Which Way Is Up?

Spatial disorientation — a primer

February 1, 2002

In the mid 1950s, AOPA and the University of Illinois joined forces to devise a curriculum that would train a noninstrument pilot to get out of weather conditions into which he had inadvertently flown. A report titled "180-Degree Turn Experiment" was published in June 1954. One of the conclusions was that "Pilots who had no previous experience with instrument flying cannot expect to survive their first experience under actual instrument conditions, except by mere chance." That's not too surprising, is it? Do you remember your first time under the hood? It wasn't easy, was it? Did you feel you were rapidly losing control of the airplane?

The FAA used some of the results from that report in its Aviation Safety Program publication Understanding Spatial Disorientation. In it, the FAA posed the question, "How long can a pilot, with or without a current instrument rating, expect to live after they experience spatial disorientation?" The FAA goes on to say that the University of Illinois found the answer to that question by having 20 student subjects fly into simulated instrument weather, and all went into graveyard spirals or roller coaster-style maneuvers. The time from entry into instrument conditions to loss of control ranged from 20 to 480 seconds, with the average being 178 seconds — that's just two seconds short of three minutes.

Did the FAA tell the whole story? Not really. The university study didn't look at pilots who had current instrument ratings, as was implied by the FAA's question. In fact, none of the 20 pilots had logged any instrument time, either actual or simulated, at all . Remember, this was back in 1954 and the requirements to get a private pilot certificate were different. Nineteen of the subjects had private certificates and one was a student pilot. Four of the pilots had just received their certificates with only 31 hours of total flight time. The high-time pilot had 1,625 hours. The average total flight time for the group was 274.6 hours.

The pilots were all given a briefing on how to make a 180-degree turn and on how to make a controlled descent. Then they went flying in a Beechcraft Bonanza. This aircraft was chosen because it was thought to be representative of the most complex light single-engine airplane normally flown by the nonprofessional, noninstrument-rated pilot. It was thought that if the techniques could be learned in this aircraft, they could be learned in any single-engine aircraft. Only two of the 20 pilots had any Bonanza time.

Amber plexiglass was used to cover the windshield and the side windows. The subjects wore nonpolarizing blue goggles to simulate instrument conditions. It was also decided to use only those instruments required for visual flight rules, plus a turn indicator. Subsequently, the attitude indicator, directional gyro, and vertical speed indicator were all covered.

So what did they have in this group? A collection of virgin instrument pilots flying an unfamiliar and not too forgiving aircraft under partial-panel conditions. Sounds like a setup for failure by today's standards. But this was 1954, and it did give insight as to what such a pilot would do when instrument meteorological conditions (IMC) were encountered for the first time. Nineteen of the pilots put the aircraft into a graveyard spiral and the other put the aircraft into a whipstall.

Those were the results for the first flight in the training program. The pilots all had five to six more instruction periods that included both discussion and flight training. What was clear on that first flight was that sooner or later they all lost control of the aircraft and they were all victims of spatial disorientation.

What is spatial disorientation? Technically, it is an erroneous sense of one's position and motion relative to the plane of the Earth's surface. Practically speaking, it's an incorrect sense of position, attitude, or motion in relation to what is actually happening in the airplane. Still a little confused? One of my students put it nicely when he said that it's just not knowing which way is really up when you're flying.

How do we correctly orient ourselves when flying? We use three systems: the visual; the vestibular (inner ear); and the proprioceptive (seat-of-the-pants sensations). Of the three systems, the visual is the most important. It is estimated that up to 90 percent of our orientation information comes in through this system. Lose that one and we're forced to use two systems upon which we normally don't rely. As we'll soon see, the vestibular system, which provides more information than the proprioceptive system, can be easily fooled. This is what leads to disasters.

The visual system is often divided into the central visual system, a small area extending across an arc of less than 2 degrees (but is the only area capable of seeing 20/20) and the peripheral visual system. The orientation information from the visual system comes mainly (about 90 percent) from the peripheral visual system. We normally rely on some sort of horizon line for this orientation. If we lose this horizon line, proper orientation becomes difficult to maintain. You can demonstrate this with a cardboard tube from a roll of paper towels. Close one eye and look through the tube with the other eye. As you're doing this, try standing on one foot. It's somewhat difficult. You've knocked out the peripheral vision (and the major portion of any horizon line), leaving only the central vision. Now, instead of using the tube, hold a clenched fist about one inch in front of the open eye. Maintaining balance on one foot is much easier. You've eliminated the central vision, but you are maintaining the peripheral vision.

When we fly in IMC we have to use our central vision to interpret the flight instruments and build a mental picture of our orientation. This isn't easy to do at first, and lots of practice is required to maintain proficiency. This is one of the reasons why obtaining an instrument rating is difficult for some pilots.

Let's take a look at the vestibular system. There are two components to this system that give orientation information: the semicircular canals and the otolith organ, sometimes called the static organ. We'll look at the otolith organ first.

The otolith organ consists of a gelatinous layer, which slides over hair cells. As the gelatinous layer slides back and forth, and also from side to side, it causes the hair cells to bend and send nerve impulses, giving the brain information about this linear motion. A problem with this system is that it can't tell the difference between gravity and G loads. If a pilot tilts his head upward the gelatinous layer slides backward, if he tilts his head downward the layer slides forward. If we could somehow get inside his head and physically move the layer forward or backward we could cause the sensation of tilting downward and upward, respectively. This is exactly what happens with G loads.

A pilot making a catapult shot off of an aircraft carrier may experience the sensation of tilting upward because the gelatinous layer rapidly slides backward as the airplane is shot forward. The unwary pilot may push the stick forward to counter this perceived climb and end up putting the airplane in the water. Don't laugh — it's happened before. Military pilots aren't the only ones susceptible to this illusion, which is called the somatogravic illusion. There have been several cases of business jet pilots succumbing to this illusion too. They were making instrument approaches in IMC, reached the missed approach point, and had to execute a missed approach. Because of the rapid acceleration when they applied full power, they experienced an illusion of an extremely nose-high attitude, pushed the yoke forward to counter the illusion, and ended up popping out of the clouds and into the ground.

The semicircular canals are three canals oriented perpendicular to each other and are pretty much aligned with what we pilots know as the roll, pitch, and yaw axes. These canals are sensitive to angular — or rotational — motion. Each canal is filled with a fluid called endolymph and at the base of each canal are small hair cells. These cells sense the motion of the endolymph.

Speaking of the roll axis canal, when a pilot is flying straight, the hair cells are not bent and the sensation is that of being vertical. When the pilot rolls the airplane into a left bank, the fluid undergoes an acceleration and the hair cells bend, giving the sensation of tilting to the left. After about 15 seconds the fluid catches up with the turn and the hairs straighten out. This gives the sensation of being vertical again. If the pilot is in visual meteorological conditions (VMC), he usually won't experience this sensation because the visual cues about the horizon line override the incorrect vestibular information. If he is in IMC, he needs to believe the flight instruments, which are still indicating the turn, and ignore the erroneous vestibular information. This can be easier said than done, especially if the pilot is not used to flying in instrument conditions. If he believes his vestibular system, he is spatially disoriented.

Suppose the pilot now rolls the airplane back to straight flight. The endolymph moves in the opposite direction as before and the hair cells are bent the other way. The vestibular system now perceives a tilt to the right. If the weather is VMC the pilot probably won't even notice the incorrect vestibular sensation. However, some pilots are sensitive to this and will attempt to right themselves by leaning in the direction that feels vertical to them, which is back to the left. These pilots are experiencing the leans and will continue to lean to the left until the endolymph comes to equilibrium again, which is about 15 seconds. If the weather is IMC the pilot may very well feel the false sensation of a right turn. This illusion is called the somatogyral illusion, and the leans are one form of it.

Our disoriented pilot in IMC may attempt to correct his false turning sensation to the right and roll the aircraft back to the left because that attitude feels straight to him. He's not relying on or believing his instruments. Meanwhile his altitude is decreasing because he hasn't been compensating for the decrease in the vertical component of lift while in the turn and is now in a spiral. He may notice the decrease in altitude and attempt to correct it by pulling back on the yoke, which is what one would do if one were truly in straight flight. However, because he is in a spiral he merely tightens the turn and may end up in an accelerated stall. This is the scenario of the graveyard spiral.

If a pilot rolls the airplane at a very slow rate, less than about two degrees per second, the vestibular system won't notice the roll at all. After about 15 seconds a pilot could wind up in a 20- to 30-degree bank and not realize it. When he does realize it, the reaction is to quickly roll the airplane back to straight flight, which causes the vestibular system to incorrectly perceive a turn to the right and — spatial disorientation, somatogyral illusion, and graveyard spiral.

Things can really get complicated if the pilot in IMC turns his head while the airplane turns, pitches, or yaws. Suppose, for example, he turns his head a full 90 degrees to the right in order to talk to the person in the right seat. His semicircular canals that correspond to roll are now aligned such that they react to any pitching motion of the airplane while the canals that correspond to pitch are aligned such that they react to any rolling motion of the airplane. If the airplane rolls to the right, the pilot feels as though the airplane is pitching downward. If, instead, the airplane pitches down, the pilot feels as though the airplane is rolling to the left. This type of mix-up in information is called the cross-coupling illusion or the Coriolis illusion.

Have you ever noticed that when doing unusual attitude recovery under the hood, the instructor has you close your eyes and put your head down? This is so you can experience the Coriolis illusion. With your head tilted 90 degrees forward, your yaw canal reacts to rolling motions, while your roll canal reacts to yawing motions. If the instructor really wants to be mean, he can have you put your head down and turned to the side. That way all three canals get erroneous information. The pitch canal reacts to yaw, the yaw canal reacts to roll, and the roll canal reacts to pitch. Now that's really confusing. The moral is that when you're in IMC don't make fast head movements, and believe your instruments!


Richard J. Hackman, O.D., is a naval aviation optometrist and aerospace physiologist. He is also a CFI, CFII, and MEI.