Operational
Aerospace Physiology *
A
Primer for the non-Aeromedical Professional
Aviation
Physiology
Introduction
Man's basic biological systems were designed to function at 1 ATA, on what is
basically a two dimensional coordinate system, in which the only acceleration
vectors are perpendicular to the ground. When
moving the human to the reduced
pressure, three dimensional arena of aviation, with its associated complex
acceleration vectors - often greatly increased in magnitude - the basic
physiological systems are pushed to, and sometimes beyond their limits of
functionality. It is the purpose of
this section to briefly outline some of these concerns and what they mean to
both the aircrew and to the clinician.
Hypoxia
"Hypoxia" has many subtleties of meanings to the medical community,
depending on the area of specialty. This
discussion will be limited the applications that are of particular concern to
the operational aviation community. Therefore,
in this light, hypoxia is defined as "an insufficient amount of oxygen
delivered at the tissue level" - with the caveat that oxygen supply to the
brain is the primary concern. Four
different "types" of hypoxia, categorized by the mechanism by which
they disrupt oxygen supply to the brain, are identified and briefed to aircrew:
Hypoxic (or Altitude), Anemic (or Hypemic), Stagnant, and Histotoxic.
Hypoxic
Hypoxia
This type of hypoxia can best be defined as "a deficiency in alveolar
oxygen exchange", resulting primarily from the reduced partial pressure of
oxygen as one goes to altitude. At
sea level, the alveolar partial pressure of oxygen (PAO2)
is approximately 100 mmHg. On the
standard oxyhemoglobin dissociation curve, this translates to approximately 98%
arterial oxyhemoglobin saturation. Through
research, healthy aircrew are found to be able to tolerate saturations as low as
87% without observable reduction in performance.
Again, using the dissociation curve, this can be found to occur when the
PAO2
equals 60 mmHg, which occurs at 10,000 ft of altitude. Because of this, there is a general oxygen use rule that
states that supplemental oxygen must be used by all aircrew when flying at a
cabin altitude exceeding 10,000 ft. This
rule, as well as exceptions/modifications to can be found in Chapter 8 of
OPNAVINST 3710.7 (General Naval Aviation Training and Operating Procedures
Standardization [NATOPS]).
As the altitude is increased above 10,000 feet, marked decrease in
performance can be observed, with the endpoint being loss of consciousness and
eventual death. A measuring tool
used in evaluation of hypoxia is the Time of Useful Consciousness (TUC), which
is defined as the amount of time that an aircrew member has to recognize his
hypoxic condition and still be capable of
self-treatment. The table below
identifies approximate TUCs for several benchmark altitudes:
|
Altitude (ft) |
Time of Useful Consciousness (TUC) |
|
Up to 10,000 |
Unlimited |
|
18,000 |
30-45 min |
|
25,000 |
3-5 min |
|
35,000 |
30-60 sec |
|
43,000 |
10-15 sec |
It should be noted that all of these are ranges, and that these represent
population averages; clearly a given individual may fall outside of these
guidelines. The greatest
significance of these figures is the non-linear rate in which the times
decrease. Any number of additional
factors, both medical and environmental, could alter (typically shorten) these
times. Some of these factors will
be discussed later in this article.
Prior to reaching the TUC, a number of recognizable symptoms may be present.
The most commonly reported symptoms (in no particular order) are:
light-headedness, dizziness, numbness, tingling, air hunger, hot/cold flashes,
fatigue, nausea, visual disturbances, euphoria, cyanosis, loss of coordination,
hyperventilation, and confusion. Actual
symptoms experienced vary widely from individual to individual, even when
exposed to identical environmental conditions.
All aircrew are trained in an altitude chamber, where hypoxia is induced
at a simulated altitude of 25,000 ft; typically aircrew will be familiar with
the symptoms, and will frequently self-diagnose hypoxia.
As virtually all of the above symptoms are perishable - they will resolve
upon introduction of oxygen and/or return to lower altitudes - the
land/ship-based clinician will have to assess the patient based on this
self-diagnosis.
Anemic
(Hypemic) Hypoxia
Although the terms have subtly different meanings "anemic" and
"hypemic" are used fairly interchangeably in the arena of aviation
physiology. The Navy has
historically embraced the term "anemic", whereas the USAF more
typically uses "hypemic". Only
the former will be used throughout the remainder of this article.
Anemic Hypoxia can best be described as "a reduction in the oxygen
carrying capacity of the blood" due either to physical or biochemical
problems. Physically, the concern is blood loss, whether from wounds or from
voluntary blood donation. As blood
loss and combat wound management are discussed elsewhere, those topics will not
be re-addressed in this article. Blood donation will be mentioned, as there is specific
guidance to the aircrew in the General NATOPS (cited above). Aircrew members are grounded for a period of 4 weeks
following the donation of a unit of blood if flying in actual combat or in the
shipboard environment. If flying
non-combat, shore-based operations, the grounding period is reduced to 4 days.
These periods are for the obvious circulatory re-hydration and
RBC production/maturation processes.
This grounding period is considered "self clearing" in that an
"up-chit" from a flight surgeon is not required prior to return to
flight status. Because of these
rules, the aircrew community should not be considered a viable source for blood
bank replenishment.
The primary biochemical concern of anemic hypoxia is that of CO interference
in the oxyhemoglobin formation process. The
natural 200x greater chemical affinity of hemoglobin to CO than to oxygen
results in the formation of carboxyhemoglobin, with the resulting displacement
of oxygen on the hemoglobin, leading
to a reduced amount of oxygen being transported to the tissues when CO is
present. In the aviation
environment the greatest sources of CO are from exhaust and from smoking.
As per NATOPS, smoking is prohibited aboard all Naval aircraft, although
there are no pre-flight restrictions. Short
of reaching clinical "CO poisoning" levels, symptoms linked to this
type of hypoxia are fairly limited to headaches (mostly from exhaust inhalation)
and a possible lowering in tolerance to other types of hypoxia.
Stagnant
Hypoxia
Stagnant hypoxia is best described as "a reduction in the blood flow to
the tissues", again with an emphasis on blood flow to the brain.
Because of the vertical nature of the human anatomy, the primary
mechanical cause of this is the effect of the gravity vector, either normal or
of increased magnitude due to flight profiles, on the heart-to-brain blood
column.
One aspect of stagnant hypoxia is non-specific to aviation and is based on
simple circulatory system mechanics. When an individual is sitting in one position for an extended
period of time, blood return from the legs can be compromised due to the lack of
the muscle pump effect. When this
occurs, blood will tend to pool in the lower extremities, reducing the amount
that return to the pulmonary system for re-oxygenation.
The resultant is a systemic lowering of oxygen levels and a slight
lowering of cerebral blood pressure. Symptomatically,
this results in headaches, loss of attention, drowsiness, and ultimately falling
asleep. Walking around is
considered the best "treatment" for this condition, and when that is
not possible due to aircraft constraints, isometric contractions of the
extremities, especially the legs, is recommended.
The other, more significant, aspect of stagnant hypoxia occurs when the
magnitude of the gravity vector is increased through flight operations
(typically referred to as "Pulling Gs").
This has been identified as one of the most significant aeromedical
threats to the tactical jet aircrew member, and as such, will be discussed in a
separate section of this article.
Histotoxic
Hypoxia
Histotoxic hypoxia is the reduction in the ability of tissues - in particular
the brain - to use oxygen, and is exclusively biochemical in nature.
As its name implies, the cause of this type of hypoxia is the presence of
various toxins in the system that impact the
oxygen metabolism of the brain. The
two chemicals in particular that are discussed with aircrew are cyanide and ETOH.
Cyanide can be released during the combustion of some plastics and
insulators used in aircraft construction; if there is an in-flight fire, cyanide
poisoning must be considered as a possibility.
Of much greater importance is the potential impact of alcohol on the
aircrew member. In both cases, the
toxin effects the cytochrome oxidase system, specifically cytochrome a3.
Although actual intoxication
at the time of flight is virtually unheard of, the residual (hangover) effects
of alcohol in aircrew members is a real operational concern.
NATOPs states that consumption of alcohol is prohibited 12 hours prior to
flight planning.
The rules go on to say that aircrew must be free of hangover effects.
Where the first half of this rule is easily quantifiable, the latter
portion is very subjective and open to wide interpretation.
Although anecdotal observation and conversation indicates a fairly
wide-spread disregard for these rules, especially the latter, there are no
empirical studies to support these observations.
Symptoms of this type of hypoxia are not aviation-specific and are
similar to any clinical presentation of cyanide exposure or alcohol usage.
Predisposing
Factors and Cumulative Effects of Hypoxia
Although discussed above separately, it is quite possible for multiple types
of hypoxia to be in effect simultaneously.
Some sources refer to the cumulative effects as being additive, others
multiplicative, but which ever the case, the combined effect may exceed the sum
of the individual parts. As an example of this, consider a transport crew member who
is a heavy smoker flying at 15,000 ft, slightly hung over, and near the end of a
4 hour mission. At this point in
time, the crew member is suffering from some anemic (residual CO) stagnant
(blood pooling) and histotoxic (alcohol) effects.
If the aircraft were to lose its cabin pressure at this time, hypoxic
hypoxia would be added to the mix. Although
each of the four aspects results in only a slight degradation in ability, the
combination of them will result in an aircrew member whose performance will be
far less than 100%.
Likewise, there are a number of factors which can predispose an individual to
hypoxia, in that they may become symptomatic more quickly or develop more severe
symptoms. Among these are:
Acclimatization (people living at higher altitudes have a better tolerance),
Physical Activity (more activity uses more oxygen), Decompression Rate (TUCs can
be reduce by 1/3 to 1/2 if the decompression is rapid/explosive), and
Dehydration (blood circulation impairment).
Prevention
and Treatment of Hypoxia
To a large extent, prevention of
hypoxia is common sense. Maintain
cabin pressure and avoid the various self-imposed stressors (smoking, alcohol,
medications, etc) which can lead to the various types of hypoxia.
There are special procedures for G-induced stagnant hypoxia that will be
discussed in a later section. The
aircrew are taught a basic five-step procedure to treat hypoxia if it is
suspected: (1) 100% oxygen, (2) Check oxygen equipment, (3) Control breathing
[to prevent hyperventilation], (4) Descend [to below 10,000 ft, if possible],
and (5) Communicate [with crew/other aircraft/ground].
As hypoxic hypoxia is primarily responsible for disabling conditions and
acute symptoms, and it typically clears within 10 seconds of breathing 100%
oxygen, the first step is clearly the most important.
Unless being caused by residual toxins in the system, a patient who
experienced hypoxia in flight will rarely present with any acute symptoms.
If acute hypoxia-like
symptoms are present post-flight, strongly consider Decompression Sickness (DCS)
as a possibility. DCS will be
discussed in detail later in this article.
Hyperventilation
As in any environment, hyperventilation is a possibility during flight.
The mechanism and symptoms are no different in the flight regime than in
any other. It is mentioned here in
order to bring attention to several flight-specific potential causes, as well as
some concerns regarding the symptoms.
In addition to being induced by anxiety (which can certainly be present in
aviation), a feature of the standard military oxygen regulator can induce
hyperventilation. As stated above,
100% oxygen is delivered to aircrew members at altitudes above 10,000 ft.
Because of the physics of gas exchange, at altitudes above 34,000 ft even
100% oxygen, alone, is not enough to maintain sufficient oxyhemoglobin
saturation; it must be delivered at an increased pressure.
Because this positive pressure delivery effectively reverses the
passive/active phases of the breathing cycle, excessively deep and/or fast
breaths may taken - sufficient to initiate a hyperventilation response. Anecdotally, it has been observed in the altitude chamber
that when a person becomes acutely hypoxic (due to altitude hypoxia) that they
may start to hyperventilate, presumably as a response to feelings of air hunger.
Frequently if a hypoxic student who is hyperventilating is placed on 100%
oxygen the symptoms will initially worsen (called oxygen paradox) before they
clear.
Of operational concern is the similarity in symptoms between hyperventilation
and acute hypoxia. (Many of the hyperventilation symptoms actually are hypoxia
symptoms due to the pH-induced oxyhemoglobin curve shift to the left and the
cerebral vasoconstriction resultant from the hypocapnia).
Because of the similarity in the symptom sets, aircrew are instructed to
treat both hypoxia and hyperventilation with the same procedure (described
above).
Boyle's Law states that the volume and pressure of a gas vary inversely,
given a constant temperature. What
the means in aviation is that as altitude increases (and pressure decreases),
the volume of any gases trapped in cavities of the body will expand.
As the relationship is linear, when the pressure drops by one half (at
18,000 ft), the volume will be approximately double (exactly double, if the
trapped gas was "ideal"). The four areas of concern are the middle
ear, the sinuses, the GI tract, and the teeth.
The Middle
Ear
The middle ear (containing the ossicles) is a small cavity bound with bone
everywhere except for the tympanic membrane (eardrum).
Air contained in the middle ear will expand as altitude increases,
pressing out against the eardrum, resulting in pain and possible physical damage
to the membrane. The middle ear is
also equipped with a small opening leading to the eustachian tube which in turn
opens out in the nasopharangeal cavity. It
is the presence of the eustachian tube that allows for "equalizing"
pressure changes; as the gas expands, the excess bleeds out through the tube,
allowing pressure on both sides of the eardrum to remain equal.
Upon descent, the opposite situation occurs.
As the pressure increases, the gas in the middle ear contracts, causing
the eardrum to be "sucked in" towards the middle ear. Again the eustachian allows for equalization by allowing air
to pass back into the middle ear from the nasopharangeal cavity.
In some cases, especially if there is congestion, air may not be able to pass
freely through the eustachian tube. This
condition is known as an "ear block", symptoms of which can range from
muffled hearing to excruciating pain. If
left untreated, rupture or perforation of the eardrum can be the endpoint.
During the ascent, when air must be expelled out of the middle ear, yawning,
chewing, or swallowing all may be effective in helping to open the eustachian
tube (by physically altering the shape of the opening into the nasopharangeal
cavity). On descent (which is generally when most ear blocks occur),
when air must be forced back into the middle ear, the above may also be useful,
but the definitive "treatment" is to perform a Valsalva Maneuver.
The procedures for this are: (1) Tip the head back 10 degrees, (2) Pinch
off the nose, and (3) With the mouth closed, exhale forcefully.
This effectively forces air back up the eustachian tube by increasing the
pressure in the nasopharangeal cavity. The
exhalation should be a short burst vice a lengthy force.
This can also be used as a diagnostic pre-flight procedure; if the ears
do not "pop" on the deck, then the individual should not attempt
flight that day. During altitude
chamber training, a spray decongestant (such as neosynepherin) is very
effectively used to aid in clearing ear blocks. Although use by aircrew is a violation of the NATOPs
self-medication rule, anecdotal evidence indicates that most aircrew carry Afrin
with them during flight, preferring to have to explain their actions to the
flight surgeon rather than to risk damaging their eardrums. This is actually done at the advice of most flight surgeons.
Sinuses
For all practical purposes, the problems associated with the sinuses during
flight are identical to those associated with the middle ears - except for the
location. All four of the sinuses
are potential sites for blocks to occur, however the frontal and maxillary are
by far the most common locations. Like ear blocks, sinus blocks are more likely to occur during
descent, and also like ear blocks, the Valsalva maneuver is the primary
"treatment" available to aircrew during flight.
All above comments about decongestants apply to sinus conditions as well.
Sinus blocks are more problematic than are ear blocks in two ways: the
potential for long-term tissue damage, and inability to check their condition
prior to flight. Even if an eardrum
perforates, the healing time is fairly rapid, with very few instances of
permanent damage. A severe sinus
block can cause mucous membranes to tear/separate, cartilage to break, and
extensive infections. The grounding
time can last months and cases of permanent grounding have been reported.
For this reason, aircrew are generally more concerned about sinus blocks
than ear blocks. The second concern
is that there is no definitive way of knowing if sinuses will be a problem
during flight. With ears, a
Valsalva maneuver can be performed on the ground.
No such check exists for sinuses. The
bottom line is that any indications of upper respiratory congestion are
considered to be incompatible with flying.
Gastrointestinal
(GI) Tract
Generally speaking, this is not considered a great threat to flight safety or
a great health risk to aircrew, but it is worth mentioning, at least to
first-time flyers, as it can, in severe cases produce considerable discomfort.
The presence of gases in the digestive tract is not arcane knowledge.
These gases will respond to Boyle's Law the same way in which will any
gas. The presence of higher levels
of water vapor in these gases make them somewhat more responsive than the
relatively dry gas found in the ears and sinuses. Without working through all of the math, an approximate
"gouge point" given to aircrew is that at 25,000 ft, any gases in the
digestive tract will be approximately three times their sea level volume.
The natural elasticity and release mechanisms of the GI tract minimize the
concerns for this expansion, although it can lead to discomfort.
Aircrew are briefed to naturally vent the gas as it expands, and to
perform abdominal massage if necessary. Avoiding
typical gas producing foods prior to flight can be preventative. There are some reports of gas expansion, and the resulting
discomfort, so severe that a vaso-vagal response is initiated resulting in
tacycardia and syncope. These
reports are anecdotal and quite possibly apocryphal.
Teeth
Gas expansion in the teeth is referred to as barodontalgia, and is very rare.
Poorly performed or broken-down dental work (fillings, crowns, etc) have
the potential for leaving small air pockets within the tooth which will respond
to Boyle's Law and will expand. Depending
on proximity, the expanding air can cause pressure on the nerves, inducing pain.
Alternatively, if the air pocket is in an external portion of dental
reconstructions, fillings can be loosened and actually fall out.
Although there are well documented cases of this, it is very rare.
When the mechanism for this problem is considered, one realizes that it can
only occur during the ascent phase of flight, when the gas is expanding.
There are, however, many reported cases of "tooth pain" during
descent. This pain is actually
referred pain from a block in the maxillary sinus, and should be treated as
described above.
Decompression
Sickness (DCS)
Although of greater concern to the undersea arena of operations, DCS is a
very real threat to aircrew, given the correct environmental conditions. In the
simplest definition, DCS is the formation of gas (primarily Nitrogen) bubbles in
the tissues of the body.
Physics
and Gas Bubble Formation
The exact mechanisms of gas bubble formation are very complex, and are still
not completely understood. It is clear that Henry's Law (the amount of gas that
will dissolve in a solution is directly proportional to the pressure above the
solution) plays an important role, but when the physics of gas behavior is
studied more closely, it is found that additional (not fully identified) factors
must be present.
It is generally believed that there are two requisite conditions for DCS to
occur: super-saturation of nitrogen in the tissues - a condition that occurs
when the body's natural off-gassing mechanism is not able to "keep up
with" pressure changes, usually when there is a 50% decrease in pressure
(i.e. exposure to altitudes above 18,000 ft), and the presence of micro-bubble
nuclei (seed bubbles). There is fairly strong empirical data to support the former;
there are virtually no reported cases of DCS occurring with exposures of less
than 18,000 ft, except for a few unusual cases with major predisposing factors
present (which will be discussed later). The
micro-bubble nuclei theory is widely accepted in both aviation and undersea
communities and accounts for observed bubble formation at altitudes lower than
predicted by application of pure gas law physics.
The most likely genesis of these micro-bubbles lies in naturally
hydrophobic segments of some membranes and the varying hydrostatic pressures in
synovial fluid during joint movement, although there is still considerable
ongoing discussion of this. Whatever
their origin, it is believed that these micro-bubbles cluster and/or grow in
size, during the pressure reduction, until bubbles large enough to be
symptomatic are formed.
It should be noted that the discussion of DCS does not involve bubbles
forming in the blood. Although
there may be some linkage between Venous Gas Emboli (VGE) and DCS, the DCS
symptom sets do not reflect this. Arterial
Gas Embolism (AGE) is a very real and potentially fatal condition that is caused
by an overpressurization of the lungs and results in air bubbles traveling
through the carotid artery to the brain. This
condition is discussed in more detail in the under sea portions of this CD ROM.
The standard classification of DCS symptoms identifies four manifestations of
DCS: the "Bends" (joint pain), Paresthesia (also called "skin
bends"), Chokes (pulmonary involvement), and Central Nervous System
(sometimes called the "staggers").
Classically, these have been grouped into two "types": Type I -
"Non life Threatening" and Type II - "Life Threatening",
however current literature is moving away from this classification.
A brief description of each of the 4 manifestations follows.
The Bends
The Bends, or joint pain, is the most common form of DCS, and accounts for 65
to 70% of reported aviation cases. The
bubbles form in the synovial fluid and produce a deep ache.
The large joints are most commonly affected, however any joint with a
synovium is a potential site.
By itself, the Bends is not life-threatening - but as with any suspected
DCS case, immediate medical management is required.
Paresthesia
Sometimes called "skin bends", this is the formation of bubbles in
the subcutaneous fat layer. Symptoms are typically a tingling, itching, crawling, or
burning sensation in a localized area. It
most commonly shows up on the trunk regions of the body, with some reports of
upper arm and thigh occurrences. This
form of DCS is considered the least dangerous and will frequently self-resolve
in a matter of minutes.
In some rare occasions, the sensation is accompanied by a red/purple rash.
This condition is known as cutis marmarota, and is considered to be of a
much more serious nature as it is indicative of a neurocirculatory effect of
bubbles within the system. Some
evidence indicates that 10% of patients presenting with cutis marmarota progress
to circulatory collapse if left untreated.
The Chokes
This is the rarest form of DCS in both aviation and diving, accounting for
less than 2% of reported cases. Although
rare, it is extremely serious in nature. The
cause of the chokes is bubble formation around the lungs, however a single
mechanism for this bubble formation is has not been universally accepted.
Chokes presents as a very specific triptych of symptoms: sub-sternal
chest pain, dyspnea, and a dry cough. If
left untreated, collapse is almost inevitable, possibly within minutes.
Central
Nervous System (CNS)
CNS DCS is present in 35 to 50% of altitude DCS cases where symptoms do not
self-resolve during descent. All
cases of CNS DCS involve bubble formation in the cerebrospinal fluid, either in
the spinal column or in the brain. Statistically, spinal column DCS is very rare in aviation
cases, but is quite prevalent in diving cases.
Just the opposite id true of cerebral DCS. The reason for this dissimilarity is not understood.
Symptoms of spinal column DCS are similar to any cord lesion, with loss
of feeling and/or motor control of a limb or limbs being the most common.
Symptoms of cerebral DCS are consistent with a non-specific brain lesion
and include, most commonly, dizziness, headache, disorientation, sensory
decrements, personality changes, memory loss, and unconsciousness.
In some reported cases, brain stem function has been compromised.
Regardless of how "mild" the symptoms may appear, immediate
medical management is essential for patient recovery.
One study indicated that 85% of patients who's symptoms resolved upon
descent had recurrence of symptoms post flight.
There are reported cases of permanent neural deficiency following CNS DCS
cases.
Of special note for all forms of DCS is the potential for delayed onset of
symptoms. An aircrew member can be
completely asymptomatic during the flight, yet have symptoms form hours later.
Questions to rule out DCS should always be included when evaluating an
aircrew member.
Predisposing
Factors and Prevention
The predisposing factor that is by far the greatest concern is SCUBA diving.
When diving, the tissues are become more highly saturated with nitrogen
than they are at a normal sea level equilibrium.
If an individual flys while in this condition, the altitude at which
super-saturation occurs will be lower, thus increasing the risk for bubble
formation. There have been reported
cases of DCS at altitudes below 10,000 ft following SCUBA diving. For this reason, NATOPS requires a 24 hour period between
SCUBA diving and flying. As a side
note, most civilian sport diver courses also teach this policy; there have been
reported DCS cases in commercial jet liners at a cabin altitude of 8,000 ft in
individuals who went straight from diving to the airport.
Remember, this is an aircrew regulation; other operational communities
may have different requirements.
Other factors that have been found to be "predisposing" in nature
are: dehydration, heavy exercise at altitude, joint injuries, and age.
There have long standing beliefs that body composition (body fat),
gender, and level of physical fitness are also factors, however there is little
empirical data to support these beliefs.
The only real tools for prevention are to avoid as many predisposing factors
as possible, and to maintain cabin pressurization.
If flying in tactical jet aircraft, breathing 100% oxygen is also
preventative, in that it helps purge some of the nitrogen out of the system,
thus reducing the chances for super-saturation.
Thirty minutes of breathing 100% oxygen can reduce the nitrogen levels in
the tissues by up to 40%. For this
reason, "pre-breathing" 100% oxygen is used in some altitude chamber
training profiles, and may be beneficial in some operational applications.
Treatment
The definitive treatment for all DCS cases is recompression.
Typically a standard Table VI is used, however many variables factor into
the decision made by the recompression team.
Occasionally the treatment is limited to breathing 100% while under
observation. When suspected during
flight, aircrew are instructed to go to 100% oxygen, descend to as low an
altitude as possible, land as quickly as possible, and see the flight surgeon
immediately. Occasionally a pilot
will declare an in-flight medical emergency and will have an ambulance waiting
at the flight line. Grounding time
following a DCS case is variable and will depend on the nature and severity of
the symptoms, as well as the presence of any residual deficiencies. Thirty days is typical, but the period could be longer.
Loss of consciousness due to high G-forces is one of the single greatest
physiological threats to tactical jet aircrew.
"Pulling Gs" is a vital element of air combat maneuvers;
something that all tactical aircrew manage on a regular basis.
Even highly trained and proficient aviators, however, do have occasional
problems. Although this article
will discuss theory and basic principles, EVERYONE who is going to fly in this
environment MUST have one-on-one training in the proper techniques for
maintaining consciousness during high G maneuvers.
G
Forces - The Basics
The basic principle of physics that is central to this entire discussion is
that of centripetal acceleration, or the vector that pushes from the center of a
circle to the outside of a circle during angular motion.
It is the force that holds water in a bucket if swung rapidly over head
and the force that causes things to slide across a car seat if a corner is taken
quickly. Because of the
aerodynamics of flight, this almost always translates to the vertical axis of
the body (pushing towards the feet) and
is called the Gz axis. The effect of this is the feeling of a multiplication of
the normal (downward or footward) pull of gravity - regardless of the actual
physical orientation of the aircraft.
The unit used in discussing G forces is the "G", which is defined
as the normal pull of gravity (32ft/sec2); "2 Gs" would be
double that pull, "3 Gs" triple, and so on.
As a general rule of thumb, the "average" human can tolerate up
to 3.5 Gs (sustained) without losing consciousness, although some individuals
could have a tolerance as high as 5 Gs or as low as 2Gs.
Additionally, an individual's tolerance may change from day to day,
depending on a wide variety of factors. Although
there are some predictors (discussed below), the only way to assess tolerance is
in a dynamic G environment.
The
Physiology of G Forces
In the simplest of terms, maintaining consciousness in any G environment is a
function of the blood pressure pushing the blood up to the brain with greater
force than the force of gravity pulling blood away from the brain.
The following briefly describes the effects of G forces on major
physiological systems.
Cardiovascular
System
As increased Gs are experienced, blood will tend to be pulled from the upper
portions of the circulatory system to the lower portions resulting in a drop in
blood volume above the heart. Simultaneously, the increased acceleration reduces venous
return from the lower extremities, thus reducing cardiac stroke volume.
As this happens, a concurrent drop in blood pressure is measured by the
carotid baroreceptors. The baroreceptors trigger a compensatory reflex response
which increases the heart rate, partially countering the lowered stroke volume,
in a attempt to maintain cardiac output. Ultimately,
the cardiovascular response will not be enough to maintain sufficient cardiac
output for retention of consciousness. Through
artificial means of increasing venous return and elevating the blood pressure to
the head, consciousness can be maintained.
These methods will be discussed later.
It should be noted that various EKG abnormalities have been observed during
centrifuge training/research on individuals exposed to high Gs, however a
discussion of those changes is beyond the scope of this article.
Pulmonary
Function
The primary effect of elevated Gs on the pulmonary system is in the
ventilation-perfusion relationship. As
Gs increase, blood is pulled to the lower portion of the lung, creating a
vertical profusion gradient; low perfusion at the top and high perfusion at the
bottom. Due to the relative
differences in hydrostatic pressure between blood and air, as the blood settles,
the alveoli in the lower portions of the lungs tend to be compressed - some to
the point of collapse. Conversely,
as the hydrostatic pressure drops in the upper portions of the lungs, there is
room for greater expansion (e.g. ventilation). The result of this is a cross
gradient; the area of highest ventilation has the lowest perfusion and the area
of lowest ventilation has the highest perfusion.
The end result of this is that approximately 50% of the blood that passes
through the lungs does not achieve a gas exchange with the alveoli.
Use of 100% oxygen during high G maneuvers can greatly reduce this
problem (lowering the number to approximately 15%), however there are other
problems associated with breathing 100% oxygen under high Gs - primarily that of
atelectasis.
Neurological Concerns: Greyout,
Blackout, and GLOC
Clearly the greatest threat to the tactical jet aviator when performing high
G maneuvers is losing consciousness. The cause of the loss of consciousness is the brain's
inability to function on the amount of blood that the circulatory system is
capable of supplying. Although
under certain conditions it is possible to proceed directly from consciousness
to unconsciousness, typically actual GLOC is preceded by one or two visual
phenomena - greyout and blackout.
The retina is especially sensitive to oxygen depletion - more so than the
brain - and as blood is pulled away from the head, the retina is frequently the
first to sense the oxygen deficiency. Intra-ocular
pressure may also be a factor in reducing blood flow to the retinal tissue as
the blood pressure to the eye decreases. During
early stages of retinal depletion, the peripheral vision is lost and there may
be an overall dimming of the visual image.
This condition is called Greyout, and is considered the first warning
sign that GLOC may be immanent. As
the depletion becomes more pronounced, central vision is also lost, resulting in
a condition called Blackout. It
should be noted that "blackout" is NOT a loss of consciousness, but
rather only a failure of the retina; the aviator will still have his mental and
motor faculties available.
Ultimately, if the G levels are maintained, oxygenation will fall below the
level required to maintain consciousness and GLOC will occur.
GLOC is a true loss of consciousness episode, with a subsequent period of
incapacitation.
Developing an accurate predictive time-line for the progression from one
phase to another is not possible given the number of general and idiosyncratic
variables. One generalization,
however, can be made. Due to the oxygen reserves in the brain tissue, a virtually
unlimited G level is tolerable for approximately 5 seconds (although this, like
most physiological responses, has a range across the population).
Beyond this, time until greyout, blackout, and GLOC becomes a function of
Gs pulled and individual tolerance factors.
Incapacitation
due to GLOC
Ultimately, incapacitation due to loss of consciousness is the endpoint of
excessive, unprotected Gs. The
overall period of incapacitation is usually considered to be two separate
phases: absolute incapacitation and relative incapacitation.
Absolute Incapacitation - Absolute Incapacitation (AI) is the period
of time when the aircrew member is actually unconscious.
Based on centrifuge studies, the average AI time is 12 seconds, with a
range of 9 to 22 seconds. The
primary symptom of this phase is unconsciousness.
Relative Incapacitation - Relative Incapacitation (RI) is that
recovery period of time following the regaining of consciousness before a pilot
is able regain his situational awareness; i.e. become functional as a pilot.
The period also averages 12 seconds, with a range from 5 to 40 seconds.
Symptoms of this phase are in many ways similar to any period after
regaining consciousness; disorientation, confusion, stupor, apathy.
Additional symptoms that may be present are amnesia of the event,
convulsive flailing, tingling, euphoria, anxiety, and nausea.
Even after the pilot has fully regained his faculties, there are some
symptoms that could persist for the remainder of the flight such as uneasiness,
disassociation, loss of will to fight, denial of the event, and depression.
Prevention
of GLOC
Through extensive centrifuge-based research, advancements in both hardware
and procedures have been made that allow aircrew to operate in a significantly
increased G environment.
The Anti-G
Suit
The anti-G suit (commonly know simply as the "G-suit") is a pair of
cut away trousers, containing 5 interconnected air bladders, that is worn over
the flight suit. The air bladders (located at the abdomen, thighs, and calves)
are connected to a valve (the "G valve") on the aircraft via a hose.
When the G-valve senses an increase in Gs on the aircraft, it
automatically opens, forcing air into the bladders, which in turn expand.
The expansion of the bladders compresses the tissues of the lower body,
reducing the amount of blood that can pool in the legs thus increasing both
cardiac output and upper torso blood pressure.
For obvious reasons, to function properly the G suit must be snuggly fit,
and requires custom sizing for each aircrew.
When properly fit the G-suit will increase tolerance by approximately 1
to 1 1/2 Gs.
Researchers are constantly looking for ways to make the G-suit more
effective. The most recent advance
is called Navy Combat Edge (NCE). NCE
combines a larger coverage G-suit, positive pressure breathing, and a chest
counter-pressure vest, all linked to the G-valve.
The purpose of the NCE is not to necessarily increase the maximum G
attainable (although this may occur), but rather reduce the fatigue level of the
AGSM (see below) and increase endurance. A
full technical discussion of the equipment exceeds the scope of this article,
but preliminary reports from aircrew indicate that "it makes 7 Gs feel like
5".
The
Anti-G Straining Maneuver (AGSM)
The AGSM is central to all GLOC prevention strategies and is one of the most
important skills that tactical jet
aircrew must master. Although
"AGSM" is the generally accepted "generic" term,
historically this procedure has
been known as the M-1, L-1, Grunting, Groaning, and currently "Hook"
maneuver (which will be used for the remainder of this article).
Interestingly, throughout all of these name changes the technique has
remained virtually unchanged. There are two basic components to the Hook
maneuver: isometric muscle contraction and forced exhalation.
Although the G-suit helps to reduce blood pooling in the legs, it has been
found that hard isometric contraction of the lower abdomen and leg muscles
greatly increases the benefits of the G-suit.
The recommended application of this component of the Hook maneuver is to
initiate contraction prior to the onset of Gs, and to hold it constantly until
the release of the Gs, regardless of the fluctuation of the actual Gs
experienced.
The forced exhalation component of the maneuver is where the name
"hook" originates. Technically,
this breathing is described as "a forced exhalation against a closed
glottis". As that can leave
aircrew confused, it was found that if the word "hook" said, without
the "k" (kind of a clipped "huh"), the glottis closes
appropriately. When this is
performed with a very hard diaphragmatic contraction, the inter-thoracic chest
pressure is increased which in turn increases the blood pressure to the head -
allowing for perfusion during increased Gs. The same increase in chest pressure, however, also impairs
venous blood return to the heart and can result in a compromise to the
circulatory system. For this
reason, unlike the isometric contractions of the legs which may be continued
indefinitely (limited only by muscle endurance), the chest pressure must be
released, very briefly, on a periodic basis to allow for venous return. To perform this release, the "k" from the
"hook" is sounded and a small, partial, air exchange is accomplished,
followed immediately by the next "huh". It must be understood that this is NOT a true breath, but
rather a brief release in pressure, ideally not lasting longer than 1 second.
Extensive research has been done on this issue, and repeated studies all
point to 3 to 5 seconds as the ideal length of time to hold the "huh"
before adding the "k". If
held longer than this, venous return becomes a problem; if done more frequently,
fatigue becomes a significant factor (when done at high G levels, this is
extremely tiring due to the number of muscles being used and the contraction
force required). The literature is
somewhat conflicted regarding the quantifiable benefits gained from the Hook
maneuver, but 3 to 4 additional Gs of protection seems to be typical.
The two components work together in the following way.
As soon as increased Gs are felt (ideally a few seconds prior to the
onset) the muscle contraction component should be performed.
This should accomplished to the maximum level obtainable by the muscles.
Once begun, it should not be released until all increased Gs are
released. IMMEDIATELY following the
contraction, the first "Hook" should be performed, also to the highest
level obtainable by the muscles. Once
the G loading is determined, the amount of force used on the Hook can be varied
as needed to prevent greyout from occurring.
The breathing component is repeated on a 3 to 5 second interval until all
increased Gs are released. Given a
correctly fit G-suit, a properly performed Hook maneuver, and practice, aircrew
can reach 9Gs for 30 seconds (in a centrifuge) without blacking out.
Factors
Effecting G Tolerance
There are numerous factors that can all effect an individual's G tolerance
level. Knowledge of these can also
help aircrew avoid GLOC. The
following list is representative, but should not be considered all-inclusive.
One must keep in mind that "tolerance" is actually a two
dimensional measure, incorporating both maximum G level and exposure time.
G Level - The higher the Gs experienced, the lower the tolerance and
the shorter the time until GLOC. As stated earlier, the basic G tolerance level for the
unprotected, untrained human (referred to as the resting tolerance) is
approximately 3.5 Gs. Anecdotally,
many experienced tactical jet aircrew have a resting tolerance between 4.5 and
5.0 Gs.
Duration of Exposure - The longer the exposure, the greater the
chances of experiencing difficulties. Beyond the 5 second oxygen reserve, there are no satisfactory
tools for predicting actual individual tolerance times.
Dehydration - Research has demonstrated that dehydration will lower G
tolerance.
Experience - The Hook maneuver is a learned skill that takes practice.
It naturally follows that more experience will lead to better proficiency.
There is are also anecdotal evidence
that when exposed to a high G environment on a regular basis that the body's
natural tolerance increases.
Currency of Experience - Although the mechanism is not understood,
aircrew almost universally report that their tolerance decreases if they have
been in a non flying status for a period of time (as short as a few days). This
may be the result of less effective Hook maneuvers ("use it or lose
it") and/or some unidentified physiological accommodation.
"Warm up" turns are typically completed by aircrew prior to all
actual high-G missions, which help them gauge their G tolerance for that day.
Illness - Acute illness will result in a lowered physical ability
which will translate directly to lowered performance on the Hook maneuver.
Extended illness will remove the aircrew from the aircraft for
days/weeks, and result in the problems discussed above.
Physical Conditioning - Due to the physical demands of the hook
maneuver, weight training is considered mandatory for any aircrew who regularly
operate in the high G environment. Both strength and endurance training is required for maximal
performance. In rough terms,
strength training will increase maximum G levels and endurance training will
increase duration of exposure times. Heavy
exercise just prior to flying is not encouraged, and the importance of
re-hydrating must be stressed.
Blood Pressure - Aircrew with low blood pressures may have a lower G
tolerance than those with higher blood pressures.
Aircrew are NOT encouraged to pursue a lifestyle that will increase their
blood pressures, however they are cautioned that extreme aerobic fitness
programs could have a negative effect on G tolerance.
Studies have shown that an aggressive weight training program can
"counter" the effects of an aggressive aerobic training program.
Spatial Disorientation (SD) is frequently cited as the single greatest threat
to aircrew, across the operational platforms. Defined as incorrectly perceiving
the position of the aircraft, relative to the earth's surface, SD can range from
a simple false sensation of motion to controlled flight into the terrain (CFIT).
This article will first look at basic sensory system function (and
dysfunction), some sensory illusions that are common to the flight environment,
a classification of SD, and guidelines for the prevention of and recovery from
SD.
Sensory
Systems
It should probably come as no surprise that the visual system provides the
most important cues for orientation; some sources say as much as 80% of all
orientation cues come from the eyes. The
vestibular system (semicircular canals and otolith organs) provides
approximately 15% of the cues, and the proprioceptive system supplies the
remaining 5%.
The Visual
System
It is not the purpose of this article to provide information about the basic
functional anatomy and physiology of the eye; that information can be found in
any standard text. There are a few
terms used in the arena of aerospace physiology that are not common, so they
will be addressed here.
Focal Vision - Anatomically, this is the central or foveal vision.
Compromising only 2 - 4% of the total field of view (FOV), this small
segment of the retina is responsible for all of the detailed vision and is what
is measured during visual acuity tests. From
the perspective of retinal micro-anatomy, this mode of vision uses almost
exclusively cone cells. Cognitively
speaking - and this is most important concept of focal vision - the highly
refined images fed to the brain from this mode require processing at the
conscious level. This means that
this sensory input must be thought about, and compared to previously learned
information for it to become usable. Most
notably, this mode of vision is used for all instrument flying, as well as other
important in-flight tasks such as target acquisition, distance/altitude
estimation, and landing. The
benefit of information coming from the focal vision is its reliability, but the
drawbacks are in the narrowness of the field of vision, and the requirement for
conscious tasking to interpret the data.
Ambient Vision - Anatomically, this is everything other than the
central, foveal vision, and is responsible many basic orientation and motion
cues. Unlike the focal vision which
uses exclusively cone cells, the ambient vision uses both cone and rod cells,
with an emphasis on the rod cells as the visual field progresses further to the
periphery. Cognitively speaking,
the ambient vision is not dependent on conscious processing; most of the
information from this mode is processed at the pre or subconscious level. This
is a two-edge sword. On the plus
side, this allows for a great volume of basic orientation and motion information
to be processed by the brain without having to use "scarce" conscious
processing resources, but on the negative side the ambient vision is very prone
to experiencing visual illusions. The
sense of motion one can feel when watching a wide-format film or when a car next
to you starts rolling backwards are two simple examples.
Under normal circumstances, the two visual modes are operating
simultaneously; the ambient flooding the brain with basic "gut-level"
orientation cues, and the focal filling in the details as it scans across the
visual field. If the ambient vision
is compromised (i.e. flying through heavy clouds or fog), all of the orientation
cues must come from the focal vision (i.e. instrument scan) which results in
both slower processing and a greater chance for task saturation.
If the focal vision is compromised (i.e. breakdown of instrument scan)
then all of the orientation cues will come from the ambient vision, which, due
to the number of illusions possible, could lead to disorientation.
Ultimately, both modes of vision, working together, are required to
maintain maximal orientation.
The
Vestibular System
The vestibular system is a series of primitive sensory organs, primarily
"designed" for a low acceleration, two dimensional world.
That they work at all in the highly dynamic flight environment is
amazing; it should come as no surprise that they are very prone to illusions.
As with the ambient vision, cognitive processing is done at the pre- or
subconscious level.
Semicircular Canals - The semicircular canals detect angular
accelerations and decelerations based on the relative motion of fluid in the
canals and the canal walls. As a
rotational movement is started, inertia causes the fluid in the canal to lag
behind which stimulates hair cells mounted to the side of the canal walls.
During a constant-rate turn, the fluid eventually "catches up"
with the canals, and no more motion cues are sent to the brain (even though the
rotation has not stopped). When the
rotation is stopped (especially suddenly), inertia causes the fluid to continue
move, again stimulating the hairs, creating a false sense of rotation in the
direction opposite to the original turn. The
technical term for this is a "somatogyral illusion"
Another function of the semicircular canals is to guide the tracking of the
eyes during head movements. A
neural connection between the semicircular canals and the ocular muscles directs
eye tracking as a function of semicircular canal stimulation (the vestibulo-ocular
reflex). This type of eye movement
is known as nystagmus, and is a "normal" physiological
response to head motion. When
semicircular canals send false information to the orientation centers of the
brain, they also send false information to the eyes, the result being the
appearance that the visual field is in motion (when it is not).
This is sometimes termed the "oculogyral illusion"
Otolith Organs - The
otolith organs are basically "tilt sensors".
Small calcium carbonate crystals (statoconia) imbedded in a mucosal mass
rest on a bed of hairs. When the
head is tilted, gravity moves the crystals across the hairs which in turn send
"tilt" information to the brain.
The inertia of the crystals, associated with linear accelerations or
decelerations can also cause movement across the bed of hairs resulting in
" false tilt" cues being sent to the brain; forward accelerations
being perceived as a "nose up" tilt and a forward deceleration being
perceived as a "nose down" tilt.
This is termed a "somatogravic illusion".
The
Proprioceptive System
This is frequently referred to as the "seat of the pants" sense,
and being the most primitive of all of the orientation systems it provides the
least amount of useful information for maintaining spatial orientation.
Working on the pre- or subconscious level, this "system" is
actually the collection of the various proprioceptors of the body that, through
pressure or stretch, give basic positional cues to the brain.
For spatial orientation, the basic cues deal with pressure and the assumed
pull of normal gravity; when seated, if pressure is felt on the buttocks it
indicates being vertical, if felt on the back it indicate being horizontal, etc.
As with otolith function, linear accelerations can produce false cues.
A forward acceleration will increase pressure on the back creating a
false sensation of being nose up. The
converse is true for decelerations. It
should be noted that these false perceptions of pitch reinforce the false
sensations that may come from the otolith organs.
Sensory
Illusions
Sensory illusions are simply misperceptions based either on confusing sensory
information or the malfunctioning of a sensory system.
The study of sensory illusions is, to a large extent, the study of
classification and example. Although
dozens of illusions have been named and categorized, they all come down to the
basic physiology discussed above. The
following provides a brief discussion of some of the most important of the
illusions.
Visual
Illusions
Shape/Size Constancy Illusions - Illusions based on assumed size or
shape of an observed object on the ground.
The most common examples of this involve runways of an unexpected size.
If flying over a runway that is smaller than "normal", it
"looks" smaller, and the natural assumption is that it is farther away
(false perception of being too high). The
"natural" correction for this is to reduce altitude - which could
place the aircraft at a dangerously low altitude. The converse is true of larger than "normal"
runways. Similar problems can occur
when judging altitude based on size of ground objects (i.e. trees and roads)
when the actual size of the objects is not known. Prevention of this illusion can be accomplished with a
thorough brief of runway and terrain features prior to flight.
Absent Focal Cues - This is not so much an illusion as it is a
situation of sensory deprivation. Because
an experienced pilot does frequently use ground references to judge altitude,
when flying over featureless terrain there is a significant reduction in visual
cues available for use. Examples of
this are sand, snow, and smooth water.
Absent Ambient Cues - As a large percent of the subconscious
orientation cues derive from the ambient vision (most importantly, the
perception of the horizon) if ambient cues are not available, the pilot must
depend strictly on focal cues - primarily instruments.
This significantly increases the task loading of the pilot.
Examples of this are flying at night and through fog or heavy clouds. The "Black Hole" illusion or approach - coming in
for a landing at night with no external visual cues but the runway lights - has
been cited as the cause of several mishaps.
"White-out" and "Brown-out" are two examples specific
to the rotary wing and Harrier communities. In both of these, vertical landings can create a thick cloud
of blowing snow or sand, suddenly obscuring virtually all outside visual cues.
Autokinesis - This is a visual illusion resulting from the normal
saccadic movement of the eyes. Under
daylight conditions, the presence of many visual cues obscures these small,
subconscious eye movements. At
night time however, especially if there is a single, small point light source (a
light on top of a tower, for example), the saccadic movement of the eyes can
create the illusion that the light source is actually moving.
There are anecdotal reports of pilots "joining up" on ground
structures thinking that they were actually other aircraft.
Conscious scanning of the eyes (vice staring at the source) usually
reduces this illusion.
Vection Illusions - Collectively, vection illusions are a false
perception of motion resulting from observing another object moving (usually in
the peripheral vision). The most
common example of a "linear vection" illusion is the sense of moving
forward when a car parked next to rolls backwards.
The perception of motion created in full visual flight simulators and
even wide format films also falls into this category.
An example of an "angular vection" illusion can be found in
various amusement park rides where a stationary walkway or track goes through
the middle of a rotating barrel, creating a strong illusion of rolling or
falling over.
False Horizons - Of all the illusions so far discussed, this is
probably the most frequent to occur. The
eye has a natural tendency to pick up a strong horizontal line and treat it as a
horizon. In the case of the real
horizon, this is an extremely beneficial tool in maintaining orientation, so
much so that there are systems currently under development that will project a
horizon on the inside of a cockpit, providing this subconscious level of
orientation input even at night. Unfortunately,
this same strong cueing can result in a very powerful illusion if the observed
line is NOT the true horizon. The
most common example of this is flying over a sloping cloud deck, although ridge
lines, coast lines, and at night isolated (well lit) highways have all be cited
as sources for this illusion. As
with most cases, cross-checking the instruments is the method of prevention and
recovery from this illusion.
Vestibular
Illusions
The "Leans" - The leans is probably the single most common
of all of the illusions discussed and is the primary example of a somatogyral
illusion. This illusion is the
result of misperceived semicircular canal cues, as described above.
While in a constant rate turn, the inertia-based stimulation ceases, and
the sense of turn (bank) is no longer perceived.
When the aircraft is rolled out of the turn, the continued movement of
the semicircular canal fluid creates the sensation of rolling into a turn in the
opposite direction. Because this
"feels" wrong, the pilot will tend to lean over in the seat to
"correct" for the feeling - thus the name.
Alternately, the pilot may re-enter the turn to make it "feel"
right. The oculogyral illusion, as
discussed above, can accompany these illusions. Other somatogyral illusions
include the "Graveyard Spin" and "Graveyard Spiral", which
are both exaggerated forms of the above and are very rare in modern aviation.
Use of instruments will help the pilot recover from any of these
illusions
Coriolis Cross-Coupling - This is an illusion of spinning or tumbling
that results from conflicting stimulation of the semi-circular canals.
The most common cause of this illusion is the movement of the head during
turning or spinning motions. When
this occurs, the semi-circular canals have to simultaneously respond to an
angular deceleration in one plane
and an angular acceleration in another plane.
The resultant effect can be one of turning or tumbling in an entirely
different plane than either of the two actually being experienced.
Inversion Illusion - This is a very serious example of a somatogravic
illusion (see above), and can occur during a level off following a high
performance climb by a tactical jet aircraft, typically at night or in the
clouds. As a pilot noses the
aircraft over to level flight, two acceleration vectors come into play: a
negative G vector pulling the pilot up from the seat, and a forward acceleration
vector pushing the pilot backwards. Without visual references, the net "feeling" of
this is rolling backwards out of the seat - what would happen if the aircraft
was steeply pitched up, starting to enter a loop.
Following the "natural" tendency to push the nose down will
only intensify the effect of the two acceleration vectors, and will put the
aircraft into a dive. If not
corrected (by flying the instruments) this clearly could, and has, led to ground
impact.
G-Excess Illusion - This is an unusual illusion in that it is an
otolith organ based illusion that involves turning maneuvers.
While in a turning maneuver, if the pilot looks to the inside of the
turn, the centripetal acceleration will cause the statoconia of the otoliths to
move back across the hair bed, creating a "pitch up" sensation.
While looking to the inside of the turn, this will result in the feel of
a reduction in the angle of bank. A
pilot will "correct" for this by increasing the angle of bank, which
will result in a loss of lift and altitude.
If the pilot turns his head forward, the resulting illusion will be of
excessive nose up pitch, which the pilot will "correct" by lowering
the nose, again losing altitude.
Mechanisms
and Classifications of Spatial Disorientation
It should be clear by now that there is a wide variety of potential causes
for spatial disorientation, and a wide range of effects - from simple
experiences of "vertigo" to incapacitating loss of control to
controlled flight into the terrain. What,
then, is the primary cause of disorientation?
If not a "cause", a common thread throughout all disorientation
situations is a breakdown in focal vision inputs to the conscious processing
centers of the brain; more simply put,
a breakdown in the instrument scan - typically caused by either complacency or
task overloading. Without the
dependable information from the instruments, the brain must rely on the
subconscious inputs from the ambient vision, vestibular, and proprioceptive
systems. As long as there are no illusions present, spatial
orientation will likely be maintained, but if an illusion does occur during this
period of scan break down, then spatial disorientation, to some extent, should
be expected.
Classifications
of Spatial Disorientation
Type I - Unrecognized Disorientation - Virtually all SD starts as
unrecognized. As indicated above, a
breakdown in the scan occurs, and an illusion is present.
For the duration of the scan break down, as long as the illusion is
present and the pilot flies the aircraft based on the false perceptions of the
illusion, unrecognized SD is being experienced.
Type I SD is generally considered to be the most dangerous, simply
because the pilot does not even realize that anything is wrong.
It is believed that many CFIT mishaps are the direct result of
unrecognized SD.
Type II - Recognized Disorientation - If Type I SD is the start of all
SD occurrences, then Type II could be considered the first step in all
successful SD recoveries. Using the
broadest definition of Type II SD, it occurs anytime that there is a mismatch in
sensory cues. This would clearly
describe the situation following the re-establishment of an instrument scan and
discovering that the aircraft was not doing what the pilot thought it was.
Another example would be that of a pilot who anticipated an illusion
before executing a maneuver; things may "feel" wrong, but by remaining
on the instruments correct control of the aircraft can be maintained.
The not entirely understood concept of visual (focal) dominance is
credited for the brain's ability to use focal vision inputs to override ambient,
vestibular, and proprioceptive inputs.
Type III - Incapacitating Disorientation - The least understood of the
three types, this is generally described as a breakdown in the communication
channels between the conscious and subconscious portions of the brain, resulting
in the pilot not being able to discern which sensory cues are accurate and which
are false. Not able to mentally process the mismatched sensory information, the
pilot becomes incapacitated - at least in regards to his ability to correctly
control the aircraft. Unresolved,
this type of SD will almost always result in a mishap (crash).
Some anecdotal studies indicate that this is primarily a concern in the
tactical jet community, although this may be due to the presence of ejection
seats, which will allow an extremely disoriented jet pilot an escape option not
available to other aircrew types.
Spatial
Disorientation Prevention and Recovery
Prevention
The most basic prevention technique is to maintain a solid instrument scan,
and if an discrepancy arises between the instruments and any other sensory
information, follow what the instruments say - unless there is a very real
reason to believe that there may be a mechanical malfunction in the instruments.
Reducing task loading can help prevent scan breakdown from occurring;
various techniques of compartmentalization can be used to this end.
To help reduce the incidence of illusions, a thorough pre-flight brief of
geographical features should be conducted.
Recovery
The bottom line to all SD is this: Fly the instruments! As
soon as the first hint of SD is suspected, the pilot must get on the instruments
and stay on them until the SD has been resolved.
If the disoriented pilot is in a multi-pilot aircraft, transferal of
control to the less disoriented pilot is recommended.
Identifying the illusion or cause of sensory mismatch can also help the
pilot focus on the instruments. If
a semicircular canal or otolith organ illusion is suspected, minimizing head
movements will help the illusion resolve itself.
Finally, if the SD cannot be resolved and an incapacitating situation has
developed, the pilot should egress (eject) from the aircraft while/if it is
possible.
Loss of Situational Awareness (LSA) is a very commonly used, if not well
defined term in virtually all discussions of aircrew human factors. Some sources
equate LSA with spatial disorientation, but in actuality, it is a much broader
concept than that. Spatial
disorientation, collectively, is a subset of
LSA - but there are many other ways in which a pilot can lose his over all
situational awareness, yet still be spatially oriented (i.e. he can be in
perfect straight and level flight, and know it, but be 50 miles off course).
The lack of a single definition can sometimes lead to confusion as to what
exactly is meant by the term. In it's most basic form, LSA is simply "losing
track" of some aspect of the mission.
In more technical terms, it can be viewed as a failure in attention, most
frequently - although not exclusively - due to task saturation. To better appreciate this, a brief discussion of cognitive
task management is appropriate.
Multiple
Task Management
Obviously, a human can perform multiple tasks simultaneously - whether it's
as basic as "walking and chewing gum" or driving down a busy
interstate, following a map, listening to the radio, and talking with a
passenger. In order to accomplish
multiple tasks, we have developed several strategies - overlearning,
integration, and task shifting - to aid in the process.
Central to all of these is that some tasks require the focus of our
conscious attention to perform, and some only require marginal attention. Each
type will be discussed briefly below.
Overlearning
Associated primarily with motor (vice cognitive) tasks, overlearning is the
process by which a task is learned so well, that it does not require conscious
thought to perform and can function in the margin.
A common example of this is basic driving skills.
At first every aspect of driving must be concentrated on, but with
practice, an experience driver does not have to consciously think about every
adjustment of the steering wheel, accelerator, etc.
The parallels to basic airwork involved with flying are obvious.
Every task that is moved to the margin, is one less that requires the
limited resources of attention focus.
Integrating
Also primarily associated with motor functions, integration is the process of
combining a group of related tasks into a single task.
Continuing with the driving example, manually shifting gears serves as a
good illustration. Clearly, there
are three separate tasks involved (clutch, shift, and accelerator), and in the
learning process each task must be concentrated on (as well as the coordination
between the three). As the learning
process progresses, the three tasks blend into a single task (shifting gears).
In this particular example, the integrated task may then become
overlearned, the net result being three attention-focused tasks becoming a
single attention margin task.
Task Shifting
Sometimes called "task swapping", this is the primary means
available for multiple cognitive task processing. As the name implies, the
mechanism for this process is the continuous shifting of the conscious attention
from one task to another, in a cyclical pattern. A basic assumption made is that
each task can function without constant attention, as long as it is tended to
periodically. Clearly, a pilot's
instrument scan is the most important example of this in aviation.
Also, each task will require a different amount of time to be attended to
appropriately; some may require only a second or two, 3 or 4 times a minute,
others may require 15 or 20 second time blocks.
It must be remembered that there are some tasks that require full
attention for their duration, and are not compatible with shifting.
Attention
Span and Task Saturation
Using a combination of the above techniques, multiple tasks can be
effectively accomplished. "Attention
Span", in the context of this discussion, indicates the maximum number of
tasks that an individual can effectively handle at a given time.
Frequently, the number of required tasks will fall below this level.
When the number of tasks required of an individual equals this number,
then the individual is said to be "Task Saturated" and the individual
can only assimilate a new task by dropping an existing one.
The reality of the flight environment is a dynamic, constantly changing
series of tasks, with new ones being added and old ones dropped, based on the
prioritization of a given task at a given time.
Prioritization,
Misprioritization, and Channelization
A human factors sage once said "Task saturation has never killed anyone,
task misprioritization has killed many".
Whether completely true or not, this statement does bring into focus the
final aspect of situational awareness - task prioritization.
Any time that multiple tasks are being performed, there is a hierarchy of
importance ranging from absolutely essential to optional.
Every time a new task is presented, the hierarchy must be reviewed and
possibly changed, depending on the nature of the new task.
These concepts are best understood by example.
Going back to a driving example, the following prioritization hierarchy may
be present: (1) Control the car,
(2) Obey the law, (3) Follow a map, (4) Talk with a passenger, and (5) Listen to
the radio. For the sake of this
example, assume that this represents even task saturation; all of these tasks
can be performed efficiently, no more can be added.
Now, the driver's cell phone rings, and he is presented with a choice;
either ignore the new task and maintain the currently functioning task load, or
answer the phone at the expense of one of the existing tasks.
If he chooses the latter, then a second decision must be made: which of
the existing tasks will be set aside in order to respond to the new one?
Correct prioritization will start on the bottom of the hierarchy and work
up. Once the nature of the call is
ascertained, then a reevaluation of the tasking hierarchy must be made, as well
as an informed decision regarding which task (s) must be eliminated.
As long as the mission essential tasks remain at the top of the list, and the
tasks of lesser importance remain at the bottom, then correct prioritization is
maintained, and the maximal level of situational awareness allowed by the
situation is maintained. In the
above example, the driver can turn off the radio, stop talking to his friend,
and answer the phone, then decide which of these tasks is the more important and
proceed from there. The problem
occurs when ALL of the existing tasks are mission essential, AND the new one is
also. When this occurs, some aspect
of situational awareness will be lost, the important decision being which
element is the least important.
Returning to the example, assume that the passenger is an important client
with whom the driver is trying to close a major contract, and that he is
listening to traffic reports because there has been an accident and he has to
get his client to an important meeting on time, and he needs the map to find the
location of the meeting. The phone
call is from his financial officer telling him that the Board of Directors is
NOT going to approve the 10% increase in advertising that the client is
requesting, threatening the whole contract... From the aspect of driving safety,
as long as (1), and to a slightly lesser extent (2) are maintained, there will
probably not be an accident (even though the overall outcome of his mission may
be effected). If, however the
driver misprioritizes and loses control of the car (or commits gross violations
of the law), an accident is a likely outcome.
Channelization occurs when a single task - either existing or new - becomes
so important that it eclipses virtually all other tasks.
If this task is, in fact, of crucial importance, channelization on it may
be the correct prioritization decision. If,
however, it is a task of lesser importance, then channelization becomes the
worst kind of misprioritization and almost inevitably leads to an accident.
Visiting our example one final time let's look at two different illustrations
of channelization, one correct, one
incorrect: (1) The traffic suddenly becomes very heavy at a construction
site with multiple lane merges and
detours. The driver correctly
fixates on the traffic situation (at the expense of his client and contract),
safely negotiates the construction zone, then returns to his client. (2) The
phone call is from the local hospital with news that a family member has been
involved in a serious accident and may not live. The driver becomes so fixated on the phone call that he
misses the construction warning signs and causes an accident himself.
Although the above has centered on a driving example, it should be fairly
clear how this could just as easily been a flying example (flying the aircraft,
obeying flight rules, following a chart, maintaining crew coordination with his
navigator, and listening to the radio for calls from the controllers).
Given all of the above, LSA can most simply, and accurately, be described as
the result of misprioritization during a period of task saturation.
One additional factor should be considered in a discussion of LSA, and that
is complacency. This is especially
dangerous on a "routine" mission or on one where there is very low
task loading. At these times, task
shifting (scanning) discipline may decrease, and although not due to
overloading, some tasks may slip out of appropriate attention focus.
This situation is amplified if an unexpected event occurs (i.e. aircraft
emergency) during this time of lowered scanning discipline, because it requires
a sudden prioritization of existing tasks coupled with the integration of the
new task. The shift from a
complacent low-discipline attitude to an attention-intensive multiple decision
making one can be very difficult and may require more time than is available to
correctly assess and prioritize all tasks.
This section was contributed by LCDR Brian D. Swan, MSC, USN
*Source:
Operational Medicine 2001, Health
Care in Military Settings, NAVMED P-5139, May 1, 2001, Bureau
of Medicine and Surgery, Department of the Navy, 2300 E Street NW, Washington,
D.C., 20372-5300 |
