2 Spatial Disorientation: How to Have an Accident, Confidently
Pilots do not intend to damage themselves or their aircraft, yet
pilots crash. I guess that's why we call them "accidents," to
differentiate them from the stupid things we plan on doing.
To oversimplify a whole lot, we can divide accidents into simple
categories:
- the airplane is out of whack;
- the pilot is out of whack;
- somebody else whacks the pilot or his airplane;
- acts of God.
To get the theology out of the way pronto, since this is not our main
subject, let me just say that, knowing God personally, I can say that
not only do people ask Him to do a lot of things that He doesn't have
any particular interest in doing, but also that people blame Him for a
great many things He didn't do in the first place. He put us into a
livable environment, gave us motor skills and a fine analog computer,
some basic essential instincts, and a parenting system -- really,
quite enough to live prudent and safe lives if we pay attention to
details.
And to get the sociology out of the way, I can also say with some
authority that whatever I say here isn't going to affect the other guy
one little bit, because he isn't here listening, for one thing, and
neither of us can predict his behavior, for another.
And let's get the airplane thing out of the way, too. That's your
responsibility, not mine. You've purchased a proper, careful annual
inspection, you've researched the vagaries of your machine and its
fittings, you do a careful pre-flight followed by a positive control
check, and from the moment you attach the tow cable you have
contingency plans for disaster in hand.
So that leaves the pilot. Hi! Here we are! That's us!
Let me say right now that not one pilot goes into an accident
intending to have one. And inspecting accident records shows that
some of these pilots are experienced and skillful. We expect stupid,
careless, clumsy pilots to have accidents, so illogic leads us to
conclude that any pilot having an accident is stupid, careless, or
clumsy. If you've ever damaged an aircraft, you've felt this illogic
from some of your comrades, have you not?
So our aim here is to explain some ways in which a bright, careful,
skillful pilot like yourself might be surprised by an accident.
2.1 Accidents of Thinking
Frank Caron, in his Technical Soaring article on French glider
accidents (V. XXIII, No, 3, July, 1999 p. 71-5)), uses James Reason's
error model of cognitive errors leading to glider accidents. The
breakdown is in Figure One:
cognitive processes accidents fatalities injuries
------------------------------------------|-----------------------------------|
routines | 37 (16%) 1 (10%) 6 (15%) |
inaccurate representation of glider status| 55 (23%) 7 (70%) 11 (28%) |
inaccurate representation of environment | 64 (27%) 9 (23%) |
incorrect choice of procedure | 54 (23%) 2 (20%) 10 (25%) |
bad resources management | 9 ( 4%) |
1 ( 3%) wrong intention | 12 ( 5%) |
3 ( 8%) violations | 5 ( 2%) |
------------------------------------------|-----------------------------------|
Totals 235 (100) 10 (100) 40 (100)
Figure 1: Cognitive Errors Leading to Glider Accidents
Notice that the entries beginning with "in-" account for about three
quarters of accidents and injuries and 90% of fatalities. Basically
they imply, "I didn't realize exactly what was going on!" My goal
here is to show you that figuring out the situation incorrectly is
something we are prone to do, a design feature, not a bug. I don't
believe that French pilots are basically any different than we are,
even though they might like to think otherwise.
Let's take a moment right now to observe a particular peculiarity of
glider accidents. They nearly all involve the ground. This is
because air is relatively soft, and so for an accident to involve
damaging collision with the air, some pretty spectacular weather has
to occur, which scares away most of us. Such accidents have occurred,
but not often.
And there are hard objects in the air, which sometimes collide with
and damage gliders, but nearly all these hard objects contain brains
in one form or another, and detection systems, and genuinely want to
avoid each other, so such accidents are less common. I don't know
what your experience has been, but every time I've had a close
encounter with a bird while piloting an aircraft, the bird has been in
"aggressive avoidance mode." Airplane pilots are less observant. I
remember asking a macho local friend, after we both landed, "When the
glider is inside and below the Pitts and both are on base for the same
runway, which aircraft has the right of way?"
His answers were pretty humorous:
- "I called three miles out -- didn't you hear me?" [why didn't you
get out my way?]
- "I didn't hear you on the radio" [you must not have used one].
- "Well, I'm a lot faster" [why do you even bother to bring this
up?]
- "I didn't get in your way" [no blood, no foul; I landed on the
grass over-run before the paving began, while he was landing far
ahead on the pavement].
Most glider accidents involve collision with the ground because
- (a) there's a lot of it;
- (b) earth attracts gliders to it -- "ground" is what generates
"gravity;"
- (c) glider pilots must perform their most precise maneuvers real
close to the ground, e.g., the turn from base to final.
If we look carefully at the results of this study of French accidents,
we'll note that "mistakes from an inaccurate representation of the
situation or the status process" result in 27% of the accidents, 37%
of the injuries, and 73% of the fatalities. This somewhat cryptic
phrase means basically that the airman did not correctly perceive the
glider's current status, condition, or position relative to the air
flow or to the situation on the ground. Another phrase that says
something pretty similar is "spatial disorientation." More on that
later. A more instructive term might be "spatial misinterpretation."
Thus the pilot's inaccurate perceptions cause most of the fatalities
and many of the accidents. Why does such inaccuracy occur? It
happens not because pilots are negligent, but because our perceptions
have limitations and are subject to (undetected) error.
Let's say this again, differently. Pilot error--"inaccurate
representation of the situation" in Reason's rhetoric--occurs because
your body and your senses are working the way they are designed to
work, not because you are a dumb klutz. Well, OK, you and I might
both be klutzy under some circumstances, but spatial
"misinterpretation" usually occurs because your visual and vestibular
senses are working just the way they are supposed to and are prone to
certain predictable errors.
As an aside, the best way to overcome this inexactness is practice.
If you journey to a strange gliderport or fly an unfamiliar ship, do a
half-dozen pattern tows and practice landings with slightly different
approaches.
In addition, I want to show you some ways that the normal operation of
your senses, in usual flying tasks, can lead even the skilled pilot to
incorrectly perceive the glider's status or its relationship to the
ground. Understanding the circumstances in which this is prone to
happen can help you distrust your senses at the right times, and look
for confirming cues to sort out what's correct and what's real.
2.2 Accidents of Perception
First, let me mention three books that may help you study and
understand the illusions leading to confident error:
Aviation Medicine and other Human Factors for Pilots, Dr. Ross L.
Ewing, 3rd Int'l Edition, New Zealand Wings Limited, 1999, ISBN
0-908990-07-3 (info@nzwings.co.nz) PO Box 120, Otaki, New Zealand, tel
64-6-364-6423. This is a clear, concise text for the intelligent non-
physician. Highly recommended. The retail price is USD$21.00 plus
USD$2.55 postage: total USD$23.55.
And two textbooks, written for physicians, but understandable to the
educated layman:
Human Factors in Aviation, Earl L. Wiener and David C. Nagel, eds.,
Academic Press, 1988, ISBN 0-12-750031-6 (www.apnet.com), especially
Chapters 4, The Human Senses in Flight, by Herschel W. Leibowitz, and
9, Human Error in Aviation Operations, by David C. Nagel.
Fundamentals of Aerospace Medicine, second ed., Roy L. DeHart, ed.,
Williams & Wilkins, 1996, ISBN 0-683-02396-9 (www.wwilkins.com),
especially Chapter 11, Spatial Orientation in Flight, by Kent K.
Gillingham and Fred H. Previc.
Second, let me emphasize that, even if we understand intellectually
the perceptual illusions to which we pilots are prone, to realize in
the cockpit that "something is wrong" is very different from correctly
analyzing what is wrong and even that is well short of acting
correctly and doing this in time to avoid bonking the ground.
Third, it is important to realize that in severe turbulence or when
doing aerobatics, it is possible for your brain to become so
discombobulated that it is not even possible to read your instruments:
this is called "nystagmus," and prevents your eyes from focusing on
your instruments to read them and to understand what they are trying
to tell you or from focusing on the ground to understand your
orientation. This is rare, but has killed pilots, especially fighter
pilots in real or practice air combat.
Sometimes these illusions are dismissed by glider pilots as unlikely
to happen, for textbooks present them as problems that plague
instrument-airplane pilots, who are continually functioning in
conditions of reduced visibility or absent visual cues. But they can
and do affect glider pilots, only somewhat differently in visual
conditions; and we need to remember that glider pilots do sometimes
fly in cloud in other countries; and even in visual flight conditions,
that haze, smoke, dust, unexpected cloud formation, dirt on the
canopy, rain and dusk can significantly degrade outside visual
references, sometimes at inconvenient moments.
Canadian Air Force studies have shown that 24% of accidents in VFR
conditions in fixed-wing aircraft are related to spatial
disorientation. Note that's visual conditions, not instrument
conditions. A special study of Canadian Air Force rotorcraft
accidents that occurred when pilots were using night vision goggles in
VFR conditions revealed that 48% were related to spatial
disorientation. The simple lesson here is that even in VFR
conditions, any degradation of visual conditions doubles the accident
rate. This impresses me. It throws out the old notion that spatial
disorientation is for IFR pilots.
Let's stop for a bit to discuss the functional anatomy involved: the
inner ear and the eyes. "Seat-of-the-pants" flying, which dominates
soaring and gliding, requires fine coordination of the vestibular
system, chiefly involving the "inner ear," and the eyes.
The inner ear has three interconnected parts: the cochlea, which
parses sound into discrete frequencies; the semicircular canals, which
detect rotation in three axes, and the otolith apparatus -- the
utricle and saccule -- which detects gravity and linear acceleration.
The key to understanding the inner ear is that it detects change, not
the status quo. That is, it detects not whether you are turning, but
whether the rate of turn has changed. It detects not whether you are
moving or still, but whether your speed has changed; not whether you
are floating through space, but whether you got kicked in the rump.
The sensations of the inner ear are linked within the brain and cross-
checked against perceptions of neck position and visual perception, to
build a composite of your relationship to space, time, gravity, and
the cockpit. Each of these parts -- and the brain itself -- is
susceptible to predictable errors, to inherent limits, to disease, to
degradation from fatigue and dehydration, and to aging.
When these systems are working well, pilots can do amazing feats of
skill and precision; the problem is that pilots are often unaware of
subtle degradation, or underestimate more severe degradation of these
perceptual systems. Understanding the way they work and the illusions
we are susceptible to, and adapting our flying intelligently, can help
us avoid catastrophe.
Next, the eye: it has two visual functions:
- ambient vision and
- central vision.
Ambient vision is most important in maintaining spatial orientation
through detecting patterns and movement through peripheral vision. If
you doubt this, try flying your glider while looking through a pair of
paper tubes. Glue a pair of toilet-paper tubes to the front of your
sunglasses. Only with a safety pilot!
Focal vision is most important in object recognition through
processing of fine detail. It is not as important as ambient vision
in keeping track of where you are in space. Ambient vision knows
you're in a bank; focal vision knows the cloud up ahead has a tight
bottom.
Let's catalog the illusions of perception one by one, briefly.
2.3 Visual Illusions
Shape constancy. We expect all runways to have similar trapezoidal
shape when we're on final. Larger or smaller runways look nearer or
further than they "really" are, compared to the runway we're most used
to; an upsloping or downsloping runway likewise looks nearer or
further, respectively, than it really is. This illusion has led to
numerous overshoot or undershoot accidents, and it's not an easy one
to overcome, especially when the strange runway is almost like the one
at home.
Size constancy. Size is the main clue to distance beyond the 15-foot
range of binocular vision; we tend to flare high over a big runway and
low over a narrow one. A runway that slopes down makes us feel low,
so we tend to get high; one that slopes up makes us feel high, and we
tend to get too low. Some accidents have been caused by pilots
overcompensating for these illusions.
Aerial Perspective. Haze, fog, or rain, may obscure distant landmarks
that would otherwise give clues to distance. Mountain valleys provide
false convergence clues that can easily delude us. Flight in and
around steeply-sloped mountain valleys is particularly fraught with
illusions of perspective from the sloping terrain. It's safe to
assume that things are not as they seem: watch your airspeed indicator
and and look continually for visual clues for correct orientation such
as level cloud bases.
Absent focal cues. Smooth water and fresh snow are the classic
examples; glider pilots do not normally land on these surfaces, but
have done so by not realizing how close they really are. The lack of
texture makes it nearly impossible to judge distance either through
texture gradient (the usual mechanism past 15 ft) or through binocular
fusion (within 15 ft).
Absent ambient clues. A gust front has been approaching, and you
begin to transition to flare just as it crosses the airport boundary
and puts you into a turbulent dust cloud. Or you have lingered too
long in wave, and the evening is well along before your final glide
takes you over the darkening airport. You of course have no landing
light, and the asphalt is just a well between the glowing runway
lights. Or you are between thermals on a hazy day, and you notice a
small, blurry insect stuck on your canopy, which you suddenly realize
is another aircraft. An aircraft on a collision course does not
appear to move against the canopy! A clue is that the bug does not
grow!
Vection illusion. A fancy name for something we've all experienced.
You are in your car, waiting at a stoplight on an upsloping road. You
sense your car beginning to creep forward; you jam your foot on the
brake to keep from hitting the bumper of the car ahead; nothing
happens. The car next to you continues to creep backwards...
You are thermaling at a comfortably high altitude, in a 45-degree
bank. The glider pivots around its center of gravity, the inside wing
sweeping back across the terrain as you make tiny little turns; later,
having failed to make a low save over the factory next to the airport,
you turn from base to final at 200 ft agl. Despite your aggressive
45-degree bank, the glider makes huge turns, pivoting around some
point in the far distance, and seems to skid across the ground as it
speeds across the terrain. You make an S-turn back to line up with
the runway, keep the spoilers tucked in, and hope no one is there to
admire your performance.
There are three vection illusions described in this flight. Two are
opposite angular-vection illusions; one well above the pivot altitude,
one well below it. The angular-vection illusion in which we feel that
we are failing to turn sharply when low, below the pivot altitude,
contributes to over-ruddered, skidding turns and to spins on the turn
to final. The vection illusion of greater speed when low over the
ground may contribute to inappropriate slowing and to the stall that
permits the spin.
False Horizon. You are flying in wave, with lovely lenticular clouds
sloping up and away to your left. Your yaw string keeps drifting off
to the left across the canopy. What's happening? The lenticular
forms a false horizon that your ambient vision keeps trying to use.
Or you enter a canyon toward an off-field landing, and overshoot the
stub of straight road you had chosen for an off-field landing,
realizing too late that the canyon floor only seemed to be level, and
was actually sloping subtly away. Your buddy was in a similar
situation, only snagged the sagebrush on the way in, landing short
because the upsloping floor looked level.
False stabilization. When you are busy inside the cockpit, checking
charts or programming your GPS or final glide calculator, ambient as
well as focal vision may begin to depend on the stable cues of the
cockpit and canopy. Lack of clouds or other outside cues may mean
that there's nothing to contradict the false impression of stability.
You look up from your map to find the yaw string streaming crosswise
across the canopy and the right wing down thirty degrees.
2.4 Vestibular illusions
Vestibular illusions are much more important in instrument flying than
in visual flight, but even in visual flight conditions can lead to
seriously uncoordinated flight and can significantly delay our
recognition of dangerous attitudes. If you fly gliders in cloud,
understanding these illusions is important to safety.
Somatogyral illusion. The semicircular canals sense only change in
rotation, and the illusion is two-fold: when rotation begins, the
rotation is properly sensed, but when steady rotation is maintained,
such as with proper thermaling technique, in about ten seconds the
sense of rotation vanishes, even though the turn continues. This is
the first illusion; the second illusion is of false rotation in the
opposite direction when the turn is stopped. Often the Barany chair
is used to demonstrate these illusions. This illusion is prominent in
instrument flight, but not in visual flight because visual cues are
overriding. The famous graveyard spiral is a product of the
somatogyral (somato = body; gyro = turn) illusion, and occurs because
the pilot "corrects" for the false sensation of turning that is
provoked by stopping the initial, unintended, turn. This results in a
gradual turn in the opposite direction; the descent occurs because the
pilot reflexively seeks to keep the vertical G-force the same as in
level flight. This is what killed John F. Kennedy, Jr.--in VFR
weather.
Oculogyral illusion. During the somatogyral illusion, an isolated
object seen at a distance will seem to be moving with the falsely
perceived turn. As this involves the eyes, it's called "oculo." This
may cause the instrument panel to appear to briefly move when it
should not. This illusion is an interesting curiosity, but I don't
know that it has any special significance except to alert the pilot to
the simultaneous, more subtle and more hazardous somatogyral illusion
The Coriolis illusion. This one is important for glider pilots.
Here's the scenario: your body is turning at a steady rate, long
enough--ten to fifteen seconds--for the fluid motion to become stable
within the semicircular canal which is in line with that plane of
rotation. Then you raise or lower your head. A different
semicircular canal is abruptly lined up with the fluid flow, and
suddenly the fluid is flowing through another semicircular canal,
creating a sudden, strong sensation of turning in a different
direction. You instinctively respond to that sense, causing the
glider to change attitude to "correct" the illusion.
Does this happen? Of course, it does. It takes only about 10 seconds
for the fluid flow to stabilize in a semicircular canal. How long do
you remain established in a stable banked turn? Longer, sometimes for
many minutes when thermaling, but even in a turn from one pattern leg
to another the duration is longer than that.
What happens? You are in a stable banked turn, looking ahead, and...
- you look down to check a chart, or
- you look up to check traffic above you in the gaggle.
We are all well advised to keep track of the traffic around us. "Keep
your head on a swivel," is the byword. But checking for overhead
traffic is clearly dangerous if you've kept your attention forward
continuously for as long as ten seconds. And looking down at your
checklist or charts, flap handle or gear lever, likewise.
Remember, the key to this illusion is having your head's posture
stable for more than ten seconds. Continual head movement helps
protect against this illusion. In addition, one must have one's head
stable at just the right angle -- actually cocked downward slightly --
in order to experience it most vividly. There are clearly accidents
in which this illusion has been a factor.
Here's an example of how this works, from a 1998 accident, reported in
the December, 1998, and February, 1999, Soaring Magazine (quotations
are from those articles):
2.5 Coriolis Illusion Damages Glider Pilot
The pilot of the glider was circling above a ridge searching for lift,
and circling beneath a 1-34 in hopes of sharing a thermal. In a
report describing how he broke his glider and himself, the key
sentence is, "...not finding any lift under the 1-34, he craned his
head back to look directly overhead to center beneath the other
glider." We'll assume that he was making left turns, although the
direction is immaterial except to make the analysis clear.
Physiologically, the important point is that this pilot, an
experienced and competent fellow pilot, was in an established banked
turn at the moment he needed to look vertically. This would require
him to throw his head back and turn it to the right.
If he had been in the turn for as much as 15 seconds (probable, given
that this was thermaling flight), his vestibular system (semicircular
canals and otolith organs) would have stabilized.
When a pilot in a stable turn turns his head to the outside and tips
it back, an inevitable, strong sensation is created of banking more
steeply and diving.
When this pilot looked directly overhead, his visual references to the
cockpit and to the ground were dramatically changed and diminished.
Technically, this is "degradation of visual referents," which
predisposes to motion illusions.
To maintain a sense of remaining in a stable turn, he would have to
pull back on the stick and bank toward level. He would have been
strongly motivated to obey the seat of his pants by his sense that he
was close to the ridge. Whether he was 300 feet as he thought or 955
feet as his GPS readout indicated, is not material; the point is that
if the ridge "felt" close, the pilot would have been more motivated to
maintain coordinated-feeling flight than if he had been comfortably
high.
It is important to realize that these illusions feel right. There is
no confusion until something happens to contradict the illusion. To
continue, "At that point, he indicated that he might have become
disoriented, causing the stick to be pulled back excessively, and for
the ship to skid. It immediately went into a spin." Well, this is
the language of someone who was surprised, who is looking back at the
awful fact that a spin happened and trying to understand the cause.
It does not say, "The pilot said he became confused." It is the pilot
acknowledging that, because the spin happened, the aircraft could not
have been in the safe attitude he felt it to be in and which he was
trying to maintain.
In this particular case the pilot was flying a glider which doesn't
give much warning - buffet or shudder - of a stall, so he had no
opportunity to perceive the illusion that injured him until the stall
was fully developed.
I hope you do not think, just because this pilot crashed, that he was
incompetent, poorly trained, careless, negligent, or indulging in
deliberate risky thrill-seeking. In fact, the articles cite several
signs of careful planning for possible disaster and awareness of its
possibility. The fact is that someone just as careful and skilled as
you, could, while intending to be extremely careful, experience
exactly the same type of motion illusion and crash, with the same
humiliation, the same raised eyebrows, the same adverse presumptions
about pilot judgment and skill.
Now let's turn to another key fact in this incident. The GPS data
from the flight was analyzed. The pilot says, "The data shows that I
was flying straight and level for approximately 1 minute after making
the 180-degree turn in which I craned my head back to look up at the
1-34....So, the spin developed from some other reason rather than my
distraction with the 1-34."
The GPS data proves that the pilot did respond "appropriately" to his
vestibular sense, and did level out and straighten while looking up at
the 1-34; the physiologic point is that during this time he would have
felt as though he was continuing in a stable turn. If he had not had
illusion, his vestibular system would not be functioning properly.
Got that, guys and gals? The illusion is inevitable. It occurs
because the system is working. It occurs because cross-checks (visual
referents, tactile referents) are diminished. Everything feels right.
Suddenly something happens that shouldn't - a stall - and the pilot
must quickly re-orient. We hope. What about recognition and
recovery?
As the airplane quits flying, the pilot's vestibular system is
continuing to function normally, sending wrong information to his
cerebral cortex about the glider's motion, interfering with his
ability to recognize and recover from the spin. From the pilot's
point of view, something has happened, suddenly and unexpectedly.
He turns back to look "out the front window," and the message this
head movement sends to his cortex is that the glider has pitched up
and banked to the right. Meanwhile, the actual movement of the glider
has been to pitch his head down, and to turn it to the right or to
further to the left, depending on the spin rotation - or perhaps the
glider is not rotating; his head movement has only given him the
sensation of a spin and the glider is actually in a deep stall. In
this case, it will feel right to apply opposite rudder, which will
actually cause a spin.
Let's step aside from the vestibular illusions: please realize that
this is not a training session, where we expect a spin for learning
purposes. All the pilot knows at first is that the controls are slack
and the world is cockeyed. Has he had a mid-air with an unseen
glider? Has the elevator disconnected? It will take time to sort
this out, time that may not be available, given the alacrity and
enthusiasm with which gravity operates.
Now holding in mind that such a situation developed because of motion
illusions, what will overcome the illusion? Only a stable visual
reference. This may not appear until the spin is fully developed,
nose down and dropping. Prior to this, the sense of rotation may be
either exaggerated or wrong, and the pilot has no clue (more
precisely, has inadequate clues) that this perception is wrong.
Is this sufficiently clear? There are circumstances in which a stall-
spin is inevitable, and there are particular conditions under which
even a superb pilot will be genuinely incapacitated from recognizing
the pitch of the aircraft and its direction of rotation during those
few seconds in which recovery is aerodynamically possible. These
circumstances can arise in the normal conduct of glider operations:
thermaling "low" over ridges or during approach to landing.
Back to our story. The pilot concludes, "So, the spin developed from
some other reason rather than my distraction with the 1-34." He's
exactly right. Is it clear to you now what the "other reason"
probably was?
This sentence contains a common misconception: that it is
"distraction" that is the problem. It is not. We must "attend to
many cues," as the psychologists say, throughout flight, especially in
traffic. Every "cue" distracts from every other, and we don't know in
advance which "cue" may hide the teeth that bite us.
The problem occurs due to head movement in turns after holding it
stable for 10 to 15 seconds. Head movement, in a turn, always creates
a vestibular illusion. This illusion is usually over-ridden by
redundant correct sensations, chiefly visual ones. Unfortunately, to
avoid all risk of this illusion means not turning the head: not
checking for traffic, not checking ground reference points when
landing, not visually checking for flap, spoiler, and gear-handle
positions, not checking charts. Impossible. But another way to
decrease susceptibility is not to hold the head still for more than a
few seconds, so that the vestibular fluids never quite stabilize.
This is more realistic: Keep your head on a swivel, as pilots have
been told for decades for other reasons.
2.6 Coriolis Illusion Kills Jet Pilot
Another crash illustrates the risk of checking charts in the pattern.
A fighter was observed by a flight surgeon to be approaching a landing
in the early night. Just as it finished the turn to base, he saw the
cockpit interior light go on. Then the wings rolled up to vertical,
the nose dropped, and the fighter crashed, killing the pilot. What
happened? It was the coriolis illusion again.
The flight surgeon knew the fighter's cockpit layout, and realized
that when this light came on that the pilot was looking at a board
located downward and to his right. To look downward and to the right
at the completion of a stable turn to the left, creates the illusion
of pitching up and leveling off. Thus the pilot, with his eyes in the
cockpit and off the instruments, followed the "seat of his pants" into
dropping the nose and rolling to the left.
During the post-crash investigation he explained this mechanism, but
the base commander would not believe him, and said, "You can't be
right. There must have been a problem with the aircraft. I'll go out
and do the same maneuver myself and show you it won't cause an
accident."
The flight surgeon, pretty confident that he'd learned physiology
correctly, said, "Only if you take a safety pilot!"
That model of fighter is available as a two-holer; one was on base,
and so the challenge was on.
The commander flew the pattern, at night and in vfr conditions as
during the accident, and simply looked down and to the right at the
chart as he completed the turn to base. The safety pilot recovered
the aircraft less than 100 feet off the ground. QED.
A glass slipper can fall out of the sky from base or final just about
as quickly as a jet. It's not only warplanes that carry charts, that
have controls down and to the side at which we may wish to look.
Don't do it. Keep your eyes out of the cockpit during pattern turns.
Use peripheral vision to locate the spoilers and the gear lever, and
move your eyes, not your head. Use feel to check their position and
make sure they're locked.
2.7 Gravity Magnifies Tilt
Somatogravic illusion. This is an important illusion that can cause
slow airspeed on final. (Yes, "gravic" relates to gravity.) This
illusion is caused by a properly functioning otolith organ, which
senses gravity and linear acceleration.
Here's the deal: a forward acceleration and a tilting backward cause
exactly the same change in the otolith; a slowing and a tipping
forward also cause identical response. Which one seems actually to be
happening depends on your brain properly integrating vestibular
signals with visual cues.
You are on final in your glider. You sense you are high, and apply
full spoiler. This slows the glider abruptly, and the deceleration
causes a signal from the otolith organ, "Nose-down pitch change!"
Ground references are not fixed -- they're flowing backwards past the
nose -- so they don't correct the impression readily. If you simply
react confidently to this sensation with a nose-up pitch change, your
airspeed will diminish and you may quickly develop excessive sink.
You say, "Wait a minute, I can see the nose pitching down!" Sorry,
pal; that's a related illusion, the oculogravic illusion. The false
sense from our vestibular system causes us to "see" what we feel, and
momentarily prevents our vision from correcting the false sensation.
You are taking a winch or autotow launch. As you transition to climb,
there's a surge of acceleration as well as a dramatic increase in
nose-up pitch. The acceleration greatly magnifies the sensation of
nose-up pitch change, and if you fly by the seat of your pants you
will level off prematurely. The antidote is to discipline yourself to
look out at the wing angle and at the airspeed indicator instead of
what feels right.
This illusion is a special danger to airplane pilots taking off into
IFR or night skies. Many accidents have involved such an airplane
flying through the fog or the night into the ground, just a few miles
from the airport. Acceleration during the ten seconds after takeoff
from 100 to 130 knots creates only a 1.01 G gravitoinertial force, but
gives the unsuspecting pilot the sense of a nine-degree nose-up pitch
attitude. As single-engine aircraft typically climb at about 6
degrees, a seat of the pants correction yields a three degree descent,
exactly a normal instrument final-approach slope.
You can imagine the special danger in taking off at night or in fog
from an aircraft carrier, with the dramatic, 3 to 5 G acceleration of
a catapult, and the false sensation of nose-high pitch can last for 30
seconds or more after the acceleration slows. This has caused pilots
to nose down into the ocean.
This illusion can also lead to a form of graveyard spiral. If the
pilot maintains a sensation of constant G force while failing to
perceive a slow roll, the only way to maintain the proper G force is
with a descending turn. In 1978, a Boeing 747 left Bombay at night,
taking off over the dark sea under high overcast. The flight data
recorders indicated that the pilot, flying at night by the seat of his
pants instead of his instruments (which he misperceived to be
malfunctioning), maintained a constant G force of 1.0 +/- 0.1, as if
he were in a 10-12 degree climb; he actually leveled off as an
unperceived turn began and then descended in a spiral, crashing almost
inverted.
2.8 Gravity and Gliding Accidents
Inversion illusion ("Sub-Gravity"). In its extreme form, from which
this illusion has received its textbook name, the pilot feels the
aircraft has pitched upward and over on its back. The classic
situation happens when a jet fighter abruptly levels from a steep
climb and accelerates, especially in turbulence. The vestibular
system sends the message, "Hey, you're tumbling over on your back!"
and the pilot instinctively puts the stick full forward. The
resulting acceleration doesn't make the illusion any weaker! The
result may be a vertical dive into the ground, which is always down
there somewhere.
This is a dangerous illusion for glider pilots, for whom it evolves
differently than in this textbook depiction. The illusion for glider
pilots is of a dramatic nose-up pitch change (rather than actual
inversion), which occurs because of a sudden nose-down transition and
acceleration, producing less than 1 G on the pilot's body, a forward
rotation, and some degree of forward acceleration. Forward
acceleration is not essential to the illusion, but magnifies it
greatly. Thus, both the semicircular canals and the otolith organ
participate in the illusion, which can be very powerful and has killed
quite a few pilots.
Derek Piggott has thoroughly analyzed this illusion in his monograph,
Sub-Gravity Sensations and Gliding Accidents (1994, published by the
author). He does not explain the physiologic operation of the
vestibular system in this phenomenon (it is complex), but he describes
the situations well in which it arises, and shows exactly what pilots
do in response to these situations. He has observed that this
illusion is more likely to occur in low-G situations.
Piggott also shows vividly that fright or panic may completely mask
corrective sensory information, "locking" the pilot into the illusion.
And he does us a service in observing that the susceptibility of
pilots to this illusion, and the degree of panic they experience,
differ greatly between individuals. He suggests an approach to
training that identifies susceptible individuals and prepares them to
responds to the illusion.
The situations in which this illusion occurs in gliders are less
dramatic than in fighters but not less dangerous. First, consider
those situations in which you the pilot might make an abrupt nose-down
pitch change; in all such situations the glider will either accelerate
or stop slowing (which are equivalent to our otolith organ).
- Recovery from a cable break during a steep winch or autotow
launch.
- Abrupt recovery from a stall.
- Abrupt nose-down pitch in turbulence and wind shear.
- Pilot-induced strong pitch oscillations.
- Any time the stick is abruptly put well forward.
2.9 G-excess effect
This illusion's physiologic origin is complicated, but a simple
explanation will give you the idea. If you tilt your head while
sitting in a chair on the ground, your vestibular organs correctly
estimate the degree of tilt. However, if you are sitting in a seat in
an aircraft that is banked or swooping and tilt your head, the excess
G force magnifies the amount of sensed tilt. In a 45-degree bank, our
usual attitude, the turn's 1.5 G introduces a 5 to 10 degree error in
perceived bank when we tilt our head to look outside to check traffic
or terrain. The bank seems to level when we look to the inside of the
turn. If we react instinctively to this, as skilled and experience
pilots tend to do, the yaw string will be adrift when we look back.
Not a problem unless slow speed or turbulence puts you at the edge of
stall, at which point an incipient spin could develop. Perhaps you
looked out because you're in a gaggle. You see where I'm leading.
Has this caused mid-air collisions? I don't know. It clearly has
caused accidents in attack aircraft, for example, when the pilot looks
outside at the opponent during a 5-G tight turn.
2.10 The elevator illusion
Because the utricle is not exactly horizontal, vertical acceleration
causes a sensation of tilting. To go up in an elevator causes a
sensation of both climbing and tipping backward; to go down in an
elevator causes a sensation of descending and tipping forward.
So when you fly into a strong thermal, the aerodynamic pitch-up that
occurs is magnified falsely by the elevator illusion; and when you fly
into strong sink, the nose-down pitch change feels greater than it
actually is.
And if you level off abruptly during a descent, there is an immediate,
erroneous feeling that you raised the nose too much. In fact, if
pilots close their eyes immediately after leveling off, they resume a
descent at about 2/3 of their previous descent rate.
2.11 The Leans
Everyone who has ever flown in actual instrument conditions has
experienced the leans -- the persistent sense that the aircraft is
turning when the instruments say it is not. As you know, this sense
can be powerful and persistent. And the leans are not due to any
single vestibular or visual illusion, but can be caused by many
different stimuli. They contribute to erratic and uncoordinated
flying, and for glider pilots are a factor chiefly in cloud flying.
2.12 Spatial Disorientation
It is very important that you understand that "spatial disorientation"
does not mean "totally discombobulated." It means that you have
misinterpreted the glider's attitude, speed, or position with respect
to other ships or the ground, however slightly. This is why would I
prefer we would speak of "spatial misinterpretation," but the
terminology is frozen in tradition.
Spatial disorientation may affect us in three ways:
I - unrecognized. Of course, at first it is always unrecognized.
That's what "dis-" is all about. But fortunately you have flight
instruments and eyes, and a multitude of sensory inputs. So pretty
soon you realize that maybe your sensations aren't accurately
representing the real world.
But knowing "something" is wrong is far different from correctly
analyzing what is wrong and acting accurately and confidently and
expeditiously based on this analysis while your instincts are shouting
"NO!"
II - recognized. You have both recognized that something is amiss and
have correctly analyzed what the error is. Now we hope that you have
the training and skill -- and altitude -- to recover.
III - incapacitating. We must acknowledge that disorientation can
truly be incapacitating. In its most severe form nystagmus develops,
and the pilot's eyes are unable to focus or fixate on the visual cues
that will permit reorientation. Panic or fear may cloud reason,
delaying recognition or hindering analysis. Or we may fly into bad
visual conditions in which re-orientation is difficult or impossible.
In the end, incapacitation only lasts until we strike the ground...
2.13 Dynamics of Disorientation
Visual dominance and Vestibular suppression. Normally visual cues
dominate our interpretation of our orientation in space, and
vestibular cues are relatively suppressed. Disorientation occurs when
visual cues are reduced, vague, or ambiguous; or when, due to unusual
aircraft movements, vestibular sensations become obtrusively strong.
It's important for us each to acknowledge within our selves that our
bodies, functioning normally, can produce subtle or powerful false
sensations that seem right and valid. These illusions can best be
recognized by understanding what they are and looking for them.
Opportunism refers to the tendency of either the visual or vestibular
system to "opportunistically" -- reflexively, without conscious
decision -- fill a void in the welter of information that maintains
spatial orientation. This is a powerful tendency that operates
without regard to whether the opportunism provides a more correct
sensation than other simultaneous sensations.
Fixation refers to what happens when the pilot begins to concentrate
on one object to the exclusion of others. This typically happens when
something is going wrong; the neophyte glider pilot may fixate on the
yaw string in order to maintain coordination, on the vario to verify
lift, or on the airspeed indicator to ensure against variation. Less
often a pilot will fixate on traffic, clouds, or ground objects.
Fixation is a special danger because this stops head movement and
increases the pilot's susceptibility to vestibular illusion when head
movement resumes.
2.14 Motion Sickness
Motion sickness is a different sort of malfunction of the vestibular
system than the illusions we've discussed. This can occur from
turning movements, such as thermaling or riding a merry-go-round. The
strongest, most persistent motion sickness comes from sub-gravity
sensations such as weightlessness.
The important lesson for soaring pilots is to admit to ourselves that
motion sickness degrades our flying skills and judgment, and to
respond to it by getting out of the sky.