MAY 2008 – NO. 24
When pilots lose their sense in the air
It didn't take long after the Wright Brothers made their first tentative forays into the air, in the early 20th century, for pilots to learn that flying was an extremely hazardous activity. Crashes, injuries, and deaths in aviation's first 40 years were sometimes caused by faulty or ill-designed equipment, but a large portion were the result of pilot error: poor judgment, bad physical coordination, and, sometimes, the brain being deceived by its own senses.
In the era before the widespread use of onboard navigational instruments, most pilots had no trouble keeping their planes going where they wanted them to go — as long as they could see the horizon or physical features, natural or man-made, of the earth. They flew "by the seat of their pants," which refers to the "feel" they developed for piloting a craft in the three dimensions of space. But as soon as darkness fell, or fog or clouds obscured their view of the earth, a pilot's senses could easily become confused. Aviators would fly into a bank of clouds, for instance, and come out the other end with their wings cockeyed or, in some cases, even upside down — all the while thinking they were level.
These failures happen because our balance system was designed for one thing: two-legged travel on Earth's terra firma. We are a terrestrial species. The adage that if man were meant to fly he would have been born with wings isn't quite complete. He would also have been born with a vestibular system programmed to work flawlessly in the air. On land, gravity acts on the body as a uniform, constant force. Our sense of balance relies on it as a reference point, as do the balance systems of virtually every creature on the planet. But when we venture into the air, gravity no longer seems constant; moving through the three dimensions of space, the brain can be led to misinterpret centrifugal force — the pull caused by turning and climbing and diving — as gravitational force.
You might have experienced this phenomenon while traveling on a commercial jet. You look up, say, from reading a book to glance out a window and notice that the plane is in a sharp turn. One wing is higher than the other, and perhaps you can even look beyond the end of the lower wing to see features of the earth. Yet you don't feel as though the plane were tilted on its side. Proprioceptors in your muscles and joints sense pressure on your back and bottom, as if gravity were still pushing against you normally. Since the plane is on its side, gravity should be pushing you not into your seat but toward the earth, which at that moment might be in the direction of the person sitting next to you, or the row of seats on the other side of the aisle. But the pressure you're feeling on your posterior isn't produced by gravity; it's made by the centrifugal force generated by the plane's turning motion. It's the same force that pushes lettuce to the perimeter of a salad spinner. You are sideways to the earth, but your brain doesn't perceive the change in position because centrifugal force feels like gravity.
Each of the body's three equilibrium components — vision, proprioception, and the vestibular system — is vulnerable to illusion during flight. But the most troublesome to a pilot are tricks that can be played on the vestibular system's semicircular canals. As you may recall, the canals measure acceleration, not velocity. That's a good thing in most respects because if they measured velocity our canals would constantly be firing. They would measure such things as the rotation of Earth beneath our feet (about 1,000 mph at the equator) and the movement of Earth around the sun (about 67,000 mph). But thankfully the canals register only changes in motion, and once the acceleration slows down or stops, the canals tell your brain that the motion has ceased, which, during land-based activities, it usually has. But in the air, that may not be the case. During a long, smooth turn in an airplane, that motion may continue for a long time. After about twenty seconds, however, the little motion-sensing appendage within the canal, the cupula, fails to sense the slow acceleration and returns to the neutral position, falsely indicating to your brain that the turn has ended. According to the Civil Aerospace Medical Institute, in some planes it would be possible to perform a full 360-degree loop so gradually that, with your eyes closed, you would not be able to tell when you were upside down.
Perhaps the worst vestibular glitch, as far as pilots are concerned, is that when a turn in one direction begins to slow down, it may have the effect of causing the canals to register the change in velocity, but in the opposite direction. This error occurs because the cupula, having returned to the neutral position after the initial acceleration of the turn, misreads the deceleration as movement in the opposite direction, sending false signals to the brain. In a spiraling dive, the pilot, if he's relying only on his vestibular system, will make wildly inappropriate "corrections" to his course. These "rotation illusions," as they're called, are okay as long as a pilot has vision to corroborate and fine-tune the signals from the vestibular system. The redundancy of the equilibrium system shows its value here. Errors from the vestibular apparatus are corrected by what the eyes see.
There's a special name for all the tricks the vestibular system can play on a pilot in the air: "spatial disorientation," literally, the inability to know up from down. It took several decades of research, and what amounted to a large-scale propaganda campaign aimed at pilots, before ear deaths began to diminish. But as Kennedy's death and those of hundreds of pilots a year demonstrate, the problem hasn't disappeared.
Because the physiology of the vestibular system was not well understood in the early 20th century, the first aviation medical specialists — who today would be called flight surgeons — entertained some terrifically misguided notions about the organ's role in flying.
In 1912, prospective military pilots were given a bizarre test that purportedly measured their equilibrium. A candidate sat on a piano stool and was spun around for several minutes at high speed. "If he vomited, he was rejected," Dr. Isaac Jones wrote in 1937 in Flying Vistas: The Human Being Through the Eyes of a Flight Surgeon. "This was not so good — particularly as it was rejecting the normal! With his head wobbling around like that, it was natural and normal for him to be made sick."
As I explained in chapter 1, anyone with a normally functioning vestibular system can be made motion sick. The only ones who can't are those with disabled or diseased vestibular systems. To varying degrees among different people, when a mismatch occurs among any of the three components of balance, motion sickness ensues. In the case of someone being spun in a chair, the vestibular system, once the spinning has reached a constant speed, is signaling the brain that there's no motion, while the eyes detect movement. Doctors of that era mistakenly believed that if someone could be made sick on a piano stool it proved he had a dysfunctional vestibular system — the opposite of the truth. But besides not being a good test of the health of the vestibular system, the piano stool test wasn't even a good predictor of whether someone was prone to airsickness. Many candidates who passed the test went on to get sick at one time or another during a flight.
Several other balance tests were used by the military air corps at that time. A U.S. War Department document in 1912, cited by Harry Armstrong in Principles and Practices of Aviation Medicine, declared that the following tests for equilibrium to detect otherwise obscure diseased conditions of the internal ear should be made:
- Have the candidate stand with knees, heels and toes touching.
- Have the candidate walk forward, backward, and in a circle.
- Have the candidate hop around the room.
All these tests should be made with eyes open, and then closed; on both feet, and then on one foot; hopping forward and backward, the candidate trying to hop or walk in a straight line. Any deviation to the right or left from the straight line or from the arc of the circle should be noted. Any persistent deviation, either to the right or left, is evidence of a diseased condition of the internal ear ... These symptoms, therefore, should be regarded as cause for rejection.
While these tests may sound strange for a pilot to take, they do resemble a typical exam that a modern vestibular physical therapist like Karen Perz would use to test the dynamic balance of a patient. However, the tests don't identify the specific contribution to balance of the "internal ear," as they were designed to. A person could fail any of these tests and still have a perfectly functional vestibular system, if the other two components of balance weren't contributing as they should, or if, say, there was muscle weakness in the legs.
It was thought at the time that the vestibular system could be "trained" to improve its capabilities, and hence a pilot's skills. One method was to use a bizarre-looking device called the Ruggles Orientator, which looked like an oversized steamer trunk encircled by enormous gimbals. On top of the "trunk" was a cockpit where a person sat. The motor-driven gimbals allowed an operator or the student pilot to orient the contraption in any position a flier might find himself in, in all three axes. It was supposedly a key to "developing and training the functions of the semi-circular canals and incidentally ... to accustom [pilots] to any possible position in which they may be moved by the action of an aeroplane while in flight." What's more, by blindfolding a student while in the Orientator, "the sense of direction may be sensitized without the assistance of the visual senses. In this way the aviator when in fog or intense darkness may be instinctively conscious of his position." But this view of the importance of the vestibular system was, of course, more hypothesis than fact and was later reversed.
Vision's contribution to orientation, in pragmatic terms, was well understood by the pioneer pilots of the period. But science hadn't yet unlocked the mystery of why this should be so. Every pilot knew that if he wanted to live very long he could fly only during the day, and only in clear weather. Before about 1925, "nobody actually went into a cloud if he could possibly avoid it," Jones wrote in Flying Vistas. "In fact we always waited until the wind died down before going up — usually about 5 p.m. or about sun-up. Our pusher planes were barely able to maintain flying speed. They were unsafe except under the best weather conditions — and not safe even then! It never occurred to anyone to fly in bad weather. So at first we had no chance to study the problem of flying blind. At that time all we knew was that we must make an earnest study of the 'Ear and Aviation.'"
Flying during daylight hours in good weather wasn't much of a problem in the early days of flight, when airplanes were more of a novelty than they were useful transportation. But in the period during and just after World War I, the restriction of daytime-only flying proved vexing.
Transporting mail by air, for instance, was handicapped by this limitation. Mail pilots didn't use maps or compasses to travel cross-country. Flying only during the day, in good weather and at relatively low altitudes, they simply followed railroad tracks, noting their progress by reading the names of towns printed on water towers. The tracks were more than just a visual marker. As dusk approached, pilots were compelled to land, for safety. Then, on a prearranged schedule, they transferred the mail to a train, which would continue traveling through the night. In the early 1920s, the government decided to speed up cross-country mail service. They knew that if planes could carry the mail from coast to coast, they could gain several days on the train-plane system. So officials came up with a plan to illuminate flight paths for night travel, building beacons every few miles. As long as they could see the lights, mail pilots could fly safely at night. The first route of this kind was built between Chicago and Cheyenne, Wyoming, a distance of roughly 900 miles. These two points were strategically selected. Their location allowed planes leaving in the morning from either coast to reach them by dusk. Once there, pilots could continue flying all night along the lighted airway and reach the other end by dawn, when they could safely resume their routes to the coast. Within a decade, some 18,000 miles of airway routes had been illuminated across the United States.
The one drawback to this system was foul weather. Heavy clouds, thick fog, driving rain, or snow — these conditions would ground a pilot day or night. Something more was needed to keep the mail planes in the air every day, regardless of weather.
And something also was needed to combat the appalling number of ear deaths among military aviators. Pilots needed a way to determine their plane's position in the air without using their eyes. Isaac Jones, who coined the term "ear deaths," and his colleagues set out to thoroughly examine how the brain senses its orientation in the air. The net result of their studies was that a healthy vestibular system was indeed important for a pilot's "feel of the ship," as Jones describes it, meaning his sense of the plane's motion as it floats along in the sea of air. Contrary to the opinion of many veteran pilots, however, no amount of training or experience or machismo would allow a pilot to continue flying when he lost visual contact with the earth.
That would take technology. According to Jones, the Wright Brothers were the first to invent and use instruments that informed a pilot of his plane's position in the air. In 1912, they used a simple piece of string eight inches long that dangled in front of a pilot's head. "So long as this string pointed directly at the pilot's nose, the ship was flying without slipping or skidding," wrote Jones. Two years later, the brothers invented two other orientation instruments, a "pendulum bank-indicator," which described how a plane was turning in the air, and a rate-of-climb indicator. But Orville and Wilbur "found it difficult to get even their own student pilots to use these instruments, because of the humiliation when other aviators would say, 'The students of the Wright Brothers find it necessary to use instruments in flying.' In other words, for many years instruments were not popular. Fliers took pride in scorning them."
The ultimate solution to the riddle of "blind" flying came from technology used by ships. The marine "gyrocompass," devised by a Dutchman in 1885 and patented in the United States by the American inventor Elmer Sperry, solved a critical navigation problem on the high seas: in a steel-hulled ship, especially one fitted with electric motors, a magnetic compass wasn't reliable, requiring cumbersome shielding to be effective. The gyrocompass, as the name implies, used a massive spinning wheel, electrically operated, that interacted with the rotational force of the spinning earth to maintain a constant orientation to the north-south axis. Sperry, along with his son Lawrence, who was the youngest licensed pilot in the United States at the time and an aviation fanatic, then dreamed up a way to miniaturize the gyro components so they could be used in an aircraft — but not to determine direction.
The younger Sperry built a machine whose basic concept could have been lifted from the pages of an otology textbook. For he imagined that if three independent gyroscopes could be oriented to one another like the three semicircular canals of the inner ear, then motion along those axes could be controlled automatically, without input from a pilot. In fact, he wasn't thinking of the structure of the vestibular system when he came up with the design, but of the three types of motion a pilot can control in an airplane, pitch, roll, and yaw. Those three axes are, however, roughly the same as those of the semicircular canals. The pitch of an airplane is analogous to the motion of nodding your head yes. Roll is movement along the horizontal axis, during which one wing tilts higher or lower than the other, the same motion as when you tilt one ear toward your shoulder. And yaw is movement along the vertical axis, as when you shake your head side to side to indicate no. Lawrence Sperry called his invention a gyroscopic autopilot.
When he finally worked out the considerable problems of how to harness it to the controls of an airplane, he traveled to Paris to unveil his creation — weighing just 40 pounds and measuring 18 by 18 by 12 inches — to the world. The date was June 18, 1914, and the occasion was an air safety competition offering a $10,000 prize to the person whose invention was judged to have the best potential to improve flight safety. Appearing last among 57 competitors, Sperry, just 21 years old and having earned his pilot's license only nine months earlier, took off from the airfield in a Curtiss C-2 biplane with his French copilot and his autopilot, or as the French called it, a stabilisateur gyroscopique. He made three passes along the Seine that day, but the third was the most spectacular, calling for exquisite balance on the part of both the men and the plane itself.
On cue, Sperry and his companion wriggled out of their cockpits and gingerly stepped out onto each wing, grasping the struts for support. The plane would lurch momentarily with the shifts in weight but always righted itself quickly due to the gyros in the autopilot. The two men waved nonchalantly to the stunned spectators and judges in the reviewing stands below. Later, after Sperry was awarded the top prize, he demonstrated to French officials how his invention could even allow a plane to land and take off without human hands touching the controls.
Though the gyroscopic autopilot wasn't widely adopted by airplane manufacturers until decades later, Sperry designed other instruments that allowed pilots to fly "blind" as early as the 1920s. One was called a turn indicator, similar in function to the one designed by the Wright Brothers, which displayed the direction (left or right) and rate of a plane's turning. It was considered the essential instrument for blind flight. Invented in 1917, the Sperry turn indicator was standard equipment on most large military aircraft by the mid-1920s. There was one monumental problem, however. Pilots had no faith in it or any of the several other orientation instruments introduced later, such as the artificial horizon, which tracks a plane's pitch-and-roll movements against a gyroscopically stabilized horizon. Like students of the Wright Brothers, they preferred to believe the illusion-inducing input from their bodies' own navigational sensors: the vestibular system. Many of them paid for this conviction with their lives.
Those with the most piloting experience, according to Jones, were the ones most difficult to convince. Once again, technology, combined with some astute psychology, came to the rescue. This time it was an upgraded version of the piano stool that had been used to screen out pilot candidates during World War I. Instead of attempting to make a man so dizzy he would vomit, military flight instructors began using the spinning chair in the late 1920s to demonstrate to their students how orientation instruments could outperform the vestibular system in blind flight.
This new version of the stool was the Bárány Chair, the very instrument that the Austrian otologist Robert Bárány had designed to test subjects in his research on the vestibular system. Looking like a slimmed-down barber chair, equipped with high-quality bearings that gave it an exceptionally smooth ride, it had been used in the Air Service for several years as a teaching device. A few minutes in the chair were enough to demonstrate how the semicircular canals could send false signals to the brain. With other members of the class looking on, a student would sit blindfolded and be spun around at various speeds. After a certain amount of time, the chair would come to a stop. The instructor then asked him if he was still moving. The student invariably would answer yes — but in the opposite direction.
One day, a lieutenant colonel by the name of William Ocker brought a portable Sperry turn-and-bank indicator with him to one of the Bárány Chair demonstrations. Isaac Jones describes its voodoo:
The pilot is rotated — preferably in the presence of other pilots. He is turned to the right and then the chair is stopped. He calls out, "I am turning left, to the left." He is then turned very rapidly to the right and then slowly to the right. He may say, "I am not moving," or "I am turning to the left" — whereas all the observers assure him he is turning to the right. The pilot is then told to look into the instrument box. A flashlight in the box shows a turn-and-bank indicator and a compass. As he looks at the instruments he is again rotated. He watches the instruments while he is being rotated. He says, "I am turning right; the indicator also shows I am turning right." When the speed of the chair is slightly reduced, he will say, "The indicator shows I am turning right; my senses tell me that I have stopped," or "The indicator says that I am turning to the right; but I feel that I am turning to the left." It frequently happens that the pilot who has just had this demonstration will argue that his sensations are correct, in spite of what the instruments tell him. In that case it is helpful to have him stand by and watch someone else go through the same performance. As a rule a few such experiences in the turning-chair will convince the pilot that he can rely upon the instruments. His thought then is "Oh yes, I have that feeling of turning, but I am not actually turning." From that moment his problem is solved.
Even seasoned pilots, after going through such a demonstration, were amazed by the seemingly miraculous abilities of the gyroscopically controlled instrument. Suddenly it became clear how useful, and in many cases absolutely essential, orientation instruments could be to a pilot. The Bárány Chair exercise quickly became standard training protocol for military pilots (and is still used today by the air force). Next, pilots were given extensive training flying "under the hood." In a two-seater biplane, the student's cockpit would be sealed by a canvas cover so that he could not see out; behind him in the other cockpit was the instructor, who could take over the controls if the student faltered. The Ruggles Orientator and, later, the Link Trainer, a more advanced flight simulator, were also used for instrument training.
It still took many years for a majority of pilots to trust their planes' orientation instruments. Jones reported that some diehards would return them to the manufacturer as defective and were incredulous when technicians told them they were operating perfectly. "When you start to question the instruments, that's where it gets dangerous," says Colonel George Maillot, a retired air force pilot and pilot instructor who's had several scrapes with spatial disorientation. "You can fall into a trap, unless you're Steel Man himself, to the [input from the semicircular canals], and they start telling you things, and you start believing them, almost to the point where you feel like you're standing on your head. And then it's straight-down time."
A pilot's visceral distrust in his instruments may be a testament to the powerful, primitive need for the brain to believe what its senses tell it, and to disregard or at least downplay everything else. Perhaps that's hardwired as a survival strategy, forged in a world where we needed fast, nearly automatic responses to stimuli — such as discerning the barely perceptible breathing of an approaching predator and knowing it was time to flee. It's difficult for the higher parts of the brain to override what the lower brain perceives as accurate sensory signals of any kind.
The jet age has presented new problems for pilots. With their tremendous speed and acceleration, jets are able to create forces that can disorient pilots in ways never imagined by Sperry or Jones.
One such problem, called the G-excess illusion, occurs to the inner ear's gravity and linear force sensors, the otolith organs, which measure horizontal and vertical linear accelerations, including the force of gravity. Tiny crystals, the otoliths or otoconia, embedded in a gelatinous material, push against hair cells in response to linear movement. They also measure tilt. Imagine a business card coated with motor oil. A layer of beads is then sprinkled onto the card. The beads easily remain in place as long as you keep the card horizontal. But if you tilt it at an angle, say, 45 degrees, gravity tends to pull the beads downward, toward the earth. That's sort of what happens in the utricle, the otolith organ that senses movement in the horizontal plane, when it's tilted: the otoliths are pulled downward. Even though they don't move far because of the sticky stuff that holds them, there's enough movement to give off signals to the brain that the head is tilted. But any time the G-forces on the utricle exceed 1 G (the force of gravity on the surface of the earth), strange things can happen. A modern jet pilot experiences these types of accelerations often, and the sudden surge of power can be so enormous that, even though the trajectory of the plane is perfectly horizontal, the otoliths are pulled backward severely on their gelatinous substrate. It's as if you took that oiled business card with the beads on it and pushed it forward quickly. The beads would all tend to move backward. The brain misreads these signals from the otoliths as head tilt. The pilot thus believes he is going up when in fact he is level. His natural response? To push the stick forward to lower his trajectory. When this happens in a low-altitude flight, it can result in something aviation experts rather morbidly call the "lawn dart effect."
The most common vestibular illusion in jet flight is something called "the leans," which is also experienced by prop-plane pilots. It happens when flying in turbulent air, usually at night or "in the soup." A pilot may be concentrating on his instruments to remain level and straight, but momentarily looks out the cockpit. If the plane suddenly rolls or tilts when he's looking up, and then gradually returns to level, the semicircular canals will register the sudden tilt — if it has a force great enough to trigger them — but not the gradual return, whose force is below the detection threshold. The brain then perpetrates an illogical hoax against the pilot, insisting for several minutes that he is still "leaning" over to one side.
On his final flight after 20 year career in the air force, Lieutenant Colonel Gregory Davis was in the left seat of a two-seater T-37 jet when he came down with a bad case of the leans. He thought this was ironic because his job as chief of aerospace physiology at Sheppard Air Force Base in Texas had made him an expert on this sort of illusion. He taught not only pilots, but flight instructor trainees, and had experienced the leans many times before. "The flight was uneventful until we descended back into the weather [clouds] for the formation approach and landing," he wrote in a report for a U.S. Air Force Research Laboratory. Making several turns on the descent to the runway, watching the lead plane in his squadron to stay in formation, he wasn't able to glance at his attitude position instruments to stay oriented. "It was really amazing," he continues. "There I was, on the wing of an aircraft, and I had a really good set of the leans. I could see the sun peeking through the clouds above me, telling me my airplane should be right side up. No, that didn't cure the leans. I had [a copilot] sitting next to me to tell me what attitude we were in at any given time. No, that didn't cure the leans. I knew the somatogravic [seat-of-the-pants] sensation told me I was right side up. No, that didn't help either ... It was so bad, I was sure we flew most of the approach inverted."
Spatial disorientation continues to be a major headache for the military air services. While the overall aircraft accident rates have steadily declined over the last thirty years, the rate of mishaps caused by "spatial-d," as air force pilots call it, has remained unchanged. This intransigence is mostly due to the increased performance capabilities of modern planes, especially single-seat fighter planes like the F-16, which can generate tremendous acceleration forces that the vestibular system simply can't handle. As the performance capabilities of aircraft increase, so does the frequency of spatial disorientation. Between 1991 and 2000, according to Lieutenant Colonel Davis, spatial disorientation has cost the air force sixty lives and about $1.4 billion in aircraft. That's an average of seven fatalities and over $100 million every year.
On the civilian side, accident and fatality rates have declined steadily over the past 20 years, including accidents due to spatial disorientation. The National Transportation Safety Board listed 1,614 general aviation accidents in 2004, with 556 fatalities. Of the 312 accidents involving fatalities, about 40 percent, or 125, were estimated to be caused, at least in part, by "continuation of flight into weather for which the pilot was not qualified," according to the NTSB. Which means a pilot without an instrument rating got caught in or thought he could deal with poor visibility. In other words, spatial disorientation probably reared its head.
And when orientation is lost, the odds are stacked against a pilot's survival. In 1983, the Federal Aviation Administration reported that in a recent five-year period more than five hundred spatial disorientation accidents had occurred, with a 90 percent fatality rate.Going back a little further, a study done by the University of Illinois in 1954 was the first to investigate precisely how such accidents occur. Twenty pilots participated in the study. They ranged from 19 to 60 years old, and their flight time varied from 31 to 1,625 hours. None had any instrument training. In the experiment, each pilot was of course accompanied by an experienced pilot-observer. To simulate instrument conditions, the subject put on blue-tinted goggles, which prevented him from seeing through the orange-tinted cockpit windows of the Beech-craft Bonanza. The observer then noted how long the student pilot was able to control the aircraft, and the consequences. Flying blind, with only their vestibular system and proprioception to guide them, pilots could maintain control for just under three minutes. At that point, 19 out of the 20 pilots went into what is called a graveyard spiral. The plane begins to turn in one direction, which is inevitable under these conditions. It happens so gently at first that the pilot doesn't notice it because the rotational velocity is beneath the threshold detectable by the semicircular canals. As the turn continues, the nose of the plane angles downward, increasing the airspeed, which the pilot usually does notice. To slow down, he instinctively pulls back on the stick. But instead of decreasing the speed, as would normally happen, the turn tightens and airspeed spikes. Sensing the mounting speed, the pilot continues pulling back on the stick, worsening the situation. A diving spiral, a rapid, dizzying corkscrew descent through the air, ends with the plane breaking up either in midair or on impact with the earth.
Original art courtesy Rob Grom.
Excerpted from Balance: In Search of the Lost Sense by Scott McCredie, by permission of the author and courtesy Little, Brown and Company. Copyright © 2007 by Scott McCredie. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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