8.  **Impact Acceleration Exposure:**

Definition of impact acceleration

Types of impact acceleration

Variables affecting impact acceleration

Causes of impact acceleration in aerospace operations

Symptoms and signs of impact acceleration

Predisposing and contributing factors to impact acceleration effects

Preventive methods and countermeasures (body position, seats,
restraints, inflatable bags, helmets)

Crashworthiness aspects of aircraft design

Impact acceleration, by its nature, happens fast. There is, therefore,
less time to observe and understand it. Some of the physical and
mathematical principles fundamental to its understanding can be
counterintuitive, particularly when reference frames get mixed up. An
objective understanding of the processes involved will result in
improved protection in the face of one of the most potentially severe
threats to occupants of both aircraft and terrestrial vehicles.

Selection for potential participation in an impact event is not commonly
carried out in civilian medical practice. In certain military
applications, candidates with a pre-existing medical condition may be
evaluated for their potential to operate in ejection seat equipped
aircraft. In civilian practice, "candidates" are commonly "selected" at
random by a combination of unforeseen events, which lead to an unplanned
termination of a flight. The outcome of that experience may depend
greatly upon the use and effectiveness of reasonably designed
protection. An aviator's use of the protection and the characteristics
of the protection available may depend upon the education provided to
aviators by aeromedical personnel and the analysis and recommendations
made by aeromedical personnel participating in aircraft accident
investigation.

Impact acceleration events occur not just in aircraft but also more
frequently in terrestrial vehicles simply because there are more of
them. The thoughtful aeromedical physician will recognize the potential
for decreasing morbidity and mortality among his treated population by
promoting appropriate use of available protection both in the air and on
the ground. Few other threats to health of the flying population can
compare to that of impact injury. It strikes the young and the healthy
more commonly and severely than any other disease of aeromedical
significance. The practitioner would do well to be thoroughly acquainted
with the potential for addressing this threat.

Aviation medical concern is commonly directed toward the prevention of
crashes by medical screening, physiological indoctrination, and other
training of pilots. Prevention of the impact must always be the first
line of defense and is addressed in many other areas of medical
education. This lesson turns our attention to the understanding of the
impact event when it occurs and how protection functions in a crash.

DEFINITIONS

We begin with a review of the tools necessary to understand impact
injury and prevention. This must include clear understanding of the
terminology as it is used to describe the impact event and injury
mechanisms.

ACCELERATION

Acceleration is simply a rate of change of velocity, which, in turn, is
a rate of change of position with respect to time. Acceleration,
velocity, and position are vector quantities with both direction and
magnitude. The magnitude of an acceleration is expressed in terms of
some velocity unit per unit of time, e.g., feet per second0 p0er second
or feet per second squared. Acceleration may also be expressed in terms
of g. One g is the magnitude of the acceleration experienced by an
unsupported object at the earth's surface as a result of the force of
gravity on that object. The value of one g is approximately 32.2 feet
per second squared where we must neglect the effect of any air
resistance. The unit of g is not a unit of force, but is in fact a unit
of acceleration. Acceleration can be produced by a gravitational force.

IMPACT ACCELERATION

Impact acceleration is a short-term acceleration, which is not sustained
long enough to result in any meaningful steady state or sustained
component in the mechanical response of the body. That means that an
impact acceleration must be of sufficient magnitude to produce at least
a transitory mechanical response. Some authors have defined impact as
being an acceleration event of less than one or two seconds. However,
the transitory response definition is more generally useful. Sustained
acceleration produces physiologic stresses, which take place over time.
Impact acceleration tends to produce mechanical stresses that are of
short duration and are potentially traumatic.

REFERENCE FRAMES

Perhaps the most critical issue in understanding an impact and the
response of a vehicle occupant is to observe it from an appropriate
viewpoint. Impacts can look very different depending upon whether they
are observed or envisioned from an onboard perspective or from a
relatively fixed perspective of an observer motionless on the earth. For
example, if you watch a rear-end collision recorded by a camera mounted
to the vehicle, you will see the head suddenly snap back. If you observe
the same event from a position standing on the sidewalk, you would see
the struck vehicle (and its onboard camera) suddenly thrust forward,
leaving the occupant's head behind. In the first case, one might be
tempted to consider protecting the head against a force from the front,
while from the second perspective one would be appropriately concerned
with providing a better means to accelerate the head forward with the
rest of the body.

The view recorded by the onboard camera in an impact represents an
accelerated reference frame, which always leads to the assessment of
what physicists call fictitious forces. If you are going to concern
yourself with real forces on an object observed in a reference frame,
you must use a reference frame that is not accelerated or rotating.

Newton's Third Law states that for every action there is an equal and
opposite reaction. The inertial reaction forces applied by an
accelerated body accelerated in the positive direction shown in the
figure will be directed oppositely. For example, a head accelerated in
the plus X direction by a headrest will apply an inertial reaction force
back against the headrest opposite the direction of acceleration.

FORCE

Newton's Second Law states that force on an unsupported mass will
produce an acceleration in the direction of the force that is directly
proportional to the magnitude of the force and inversely proportional to
the mass of the object. This relationship is expressed by the familiar
equation F = ma. Force and acceleration can be directly measured but
mass is deduced from the equation. The units of force are pounds-force
or newtons which are each expressed in mass units times acceleration
units. Force is not expressed in g.

ANGULAR MOTION

An object can undergo translational motion, which can be tracked in a
reference frame with x, y and z coordinates in three dimensions. An
object may also undergo rotational motion, which has to do with a change
in angle. An angular change may occur as a result of the object spinning
about its center of mass or angular change in another sense can be
described as a result of translational motion in a way which changes the
angle of its position vector. An object's motion may be completely
characterized by describing the translational motion of the center of
mass and the angular motion about the center of mass.

MOMENTUM AND ENERGY

The fundamental determinants of what authority an object has in a
collision are its mass and its velocity. An impact event changes the
velocity of a mass. It does that by applying force. Force over time
changes an object's momentum, which is expressed as its mass times its
velocity. For a constant force, force times the time over which it is
applied is called the impulse. The force times the distance over which
it is applied is called the work and it results in a change in the
energy of the mass, which is expressed as ½ mv^2^. Impact events may be
analyzed by the work-energy relationship and the impulse-momentum
relationship.

Fundamentally, the severity of an impact is characterized by its
velocity change and the time and/or distance over which that velocity
change must take place. If the velocity change is large and the time or
distance is short, the force must be high. Similar analyses can be
performed for angular momentum and angular energy.

Perhaps not immediately apparent is the interesting mathematical
observation that force-time computations (and therefore changes in
momentum) will look the same from any non-rotating, non-accelerating
reference frame. Force-distance or energy change computations, however,
will look different when viewed from different non-accelerating,
non-rotating reference frames, which happen to be moving at different
constant velocities. Therefore, different observers who assess the same
impact forces may see different energy changes and conclude that
different amounts of work have been done, but they will agree on how the
impact would be felt by a participant in terms of force and time.

As an example, consider an occupant of an airliner who runs forward and
collides with the cockpit door. His velocity change would translate into
a large energy change from the viewpoint of an observer on the ground,
but an observer on the plane would see the same force (and acceleration)
acting over a shorter distance, assessing the resulting energy change as
being much less. Both observers would reach the same conclusion about
momentum change and the effect on the occupant. These concepts are
important to avoid misunderstanding midair collisions for example.

STRESS-STRAIN

Total force on an object does not adequately characterize its injury
potential. Injury results from distortion of body structures beyond
their recoverable limits. This distortion, or strain, is produced as a
result of stress, which is not total force but force per unit area. The
same force applied by your thumb to a thumbtack will be appreciated
differently depending upon which end of the thumbtack you choose to push
on.

There are different kinds of stress that can be applied to a body part.
Compression or tension is probably most commonly considered, but you can
also apply bending stress (positive or negative) in two different
directions (front-back or left-right). Shear stress (oppositely directed
force pairs as applied by a pair of shears) may also be applied
positively or negatively in combinations of two directions. Finally,
torsion or twist can also be applied. There are therefore six potential
components of stress that can be applied to a structural member.

VISCOELASTICITY

Human tissue responds to stress by deforming. One component of this
deformation is analogous to that of a spring in which increasing stress
results in increasing strain in a linear relationship. Another component
involves the speed over which the stress application occurs. The
combined response is similar to the suspension elements in your car,
which include both springs and shock absorbers. The shock absorbers do
not resist displacement, but rather the velocity of a displacement. Real
human tissue acts that way and acts stiffer if you attempt to deform it
more rapidly. The behavior is called viscoelasticity.

COLLISIONS

A detailed description of collision physics is beyond the scope of this
lesson. Suffice it to say that when a collision occurs, the colliding
objects produce force on one another which results in positive or
negative accelerations of each object. The best way to describe the
event is to describe the acceleration as a function of time for some
relevant point in an object with respect to a fixed reference frame.
This acceleration-time profile is called the crash pulse. The velocity
change can then be determined by adding up the area under the curve of
the crash pulse. The area under an acceleration time curve amounts to
velocity change.

Since the work needed to produce the velocity change is related to the
square of the velocity, impact severity assessments appropriately
consider the square of the velocity change. In other words, a 20-foot
per second velocity change is four times more severe than a 10-foot per
second velocity change from an energy standpoint. To make the
assessments comparable, however, the energy change should be zero-based
to avoid the observer problem.

The same velocity change occurring over a long duration or a short
duration requires different amounts of force. Therefore, comparing
velocity changes as a severity assessment works only when the two
velocity changes occur over reasonably similar intervals. Analysis of a
crash often allows an estimate of the distance over which the crash must
have occurred and the initial velocity at impact. Armed with these data
an approximate crash pulse can often be determined. However, you must
remember that such calculations typically assess the event for the
center of mass of the vehicle. When a vehicle hits a fixed object and
deforms, the center of mass of the object gets a longer distance in
which to stop. The first part of the vehicle that strikes the fixed
object must stop much quicker, leading to different crash pulses for
different parts of the vehicle.

It is also important to note that different parts on an occupant's body
can undergo different crash pulses as well. An unsupported head, for
example, may continue at nearly the pre-impact velocity for some period
of time while other parts of the body may begin stopping with the
vehicle.

OCCUPANT KINEMATICS

We now have enough basic tools to begin assessing how occupants behave
in a crash. Occupant kinematics has to do with an occupant's motion
within a reference frame without regard to the forces that produce that
motion. This is precisely because forces on the occupant often do not
create the displacement of an occupant with respect to the vehicle.
Instead, forces are placed on the vehicle which change its motion and
the occupant continues for a time in the pre-impact path, resulting in
relative displacement of the occupant with respect to the vehicle.

Calculation of occupant kinematics is based upon Newton's First Law
which states that an object in motion remains in motion unless acted
upon by some force. During a crash event, forces ultimately act upon the
occupant, but do so later compared to those forces which act upon the
aircraft or other vehicle. Assessment of kinematics involves computing
the trajectory that the occupant would follow without the crash and then
computing the altered trajectory of the vehicle and its occupant as a
result of the crash.

The occupant's pre-impact trajectory will typically be modified in
different ways for different parts of the occupant's body. For example,
in a restrained occupant, the forces imposed by the restraints would
first alter the trajectory of the parts of the body upon which those
restraints act. Relatively unsupported parts of the body would follow
the "non-impact" trajectory for a longer period of time. During this
process, while the vehicle's velocity is being changed, the unsupported
occupant body part will initially continue with a relatively unchanged
velocity. This results in a velocity difference being built up between
unsupported parts of the occupant and structures within the aircraft.

For an unrestrained occupant, the whole body would continue moving
during the early part of the crash, and then various parts of the body
would come into contact with aircraft structure at different times and
at different velocities. Using the "two trajectory" computation of
occupant kinematics, estimates can be made as to the relative velocity
of various occupant body parts and the structures which they ultimately
contact.

Occupant kinematics in a real crash therefore depend upon how well the
occupant is coupled to the vehicle. The goal of the restraint system is
to couple an occupant well to his seat position so that forces can be
imposed on the occupant over a longer period of time rather than a
shorter period of time, with low initial velocity differences between
the occupant and the surfaces restraining the occupant.

Kinematic computations can be relatively complex. A common
misunderstanding of occupant kinematics is to assume that occupants
simply move toward the point of impact. That is clearly not the case.
Instead, occupants obey Newton's First Law as the aircraft motion is
modified. A revealing example would be to consider an occupant in an
aircraft which strikes a tree after overrunning the runway. If the tree
is struck in the middle of the right wing, occupant motion with respect
to the aircraft will tend to be generally forward and curving to the
left as the aircraft rotates. The occupant clearly does not move toward
the point of impact.

An additional caution involves the use of kinematic models. These may be
useful to assist in kinematic assessments, but they can lead to a false
sense of precision since they are heavily dependent upon a lengthy list
of assumptions that may or may not be valid for the application at hand.

An important use of kinematic calculations is the definition of
pre-impact positioning. If the impact point of a body part is well
defined on the basis of physical evidence, and if the dynamics of the
aircraft during the crash are well understood, back calculations can be
useful in estimating the pre-impact position of the body part.

ASSESSING INJURY MECHANISMS

The definition of an injury mechanism involves describing the process by
which the injury occurs. This first aspect of the assessment involves
defining the principle stress or stresses which proximately produce that
injury. Some injuries have physical characteristics that are indicative
of compression or tension, bending, shear, or torsion stress.

Most injuries in the real world involve some combination of all those
stresses, at least locally, but one or two often predominate and help to
define how the injury occurred.

Bending produces tension on one side of the bone and compression on the
other. From a biomechanical standpoint, bone tends to be weaker in
tension rather than in compression. Therefore, failure typically
originates on the side of the bone that is placed in tension. The
fracture then proceeds along two angled planes so that a third smaller
fracture fragment is pushed out of the compression side of the bend.
Such a fracture, seen on x-ray, allows the definition not only of
bending stress as the cause, but provides a means of determining the
direction of the bending.

Spinal fractures may also allow some definition of direction of stress
application. Compression fractures in the lumbar spine typically result
from compression in which the force is applied to the body from below
with the inertial reaction of the body providing a relatively smaller
force on the cervical spine supporting only the head and a greater force
on lower portions of the spine which are supporting larger amounts of
the body. Conversely, compression fractures in the cervical spine
typically result from forces applied from above with less total force
application being necessary to produce fractures since the cervical
spine is less well suited anatomically to support the larger mass of the
lower body than is the stronger lumbar spine. Thoracic spinal fractures
are relatively less common and typically require some set of
circumstances, such as seat geometry and/or restraining forces, that
combine to produce stress concentration in that region.

The aviation medical examiner attempting to assess how an injury
occurred in a crash may also find useful information by studying
radiographs, CTs and MRIs acquired by clinicians engaged in acute care.
Even when read as normal, such studies can provide useful data in the
form of soft tissue swelling or presence of foreign bodies in or upon
tissue. Definition of the location of a head impact, as can be seen for
example in Figure 6, may be crucial in sorting out a mechanism of
injury.

The general approach is to observe the physical characteristics of the
injury and then sort out the principal stress or stresses involved in
its creation by reference to the characteristics of the strained
structural material. Surface findings may yield clues to the area and
direction of application as well as information on the mechanical
characteristics of the object or structures by which the stress was
applied. Correlation is then sought by checking for signs of interaction
on the potential stressors such as restraint systems, instrument panels,
or other vehicle or external structure.

INJURY CRITERIA

A considerable body of literature has been devoted to assessing human
tolerance to potentially injurious stresses. This work has often led to
the postulation of various injury criteria to define the degree of
stress likely to lead to injury for various body regions.
Misunderstanding has been frequent in the application of these criteria.

One source of the misunderstanding relates to what the criteria mean.
There exists no fine line for any stress such that stresses above the
line are always injurious, while stresses below the line are always
safe. Instead, one finds an increasing frequency of injury with
increasing amounts of stress. It is a mistake to assume that a head
injury in an occupant implies an impact severity that is sufficient to
produce a Head Injury Criterion (HIC) of more than one thousand. In
fact, measurement of a HIC of one thousand in a test dummy may predict
that a significant fraction, but not the majority, of occupants in a
similar setting would sustain some appreciable head injury, such as
concussion or skull fracture. Furthermore, the HIC does not assess
rotational effects in its calculation nor was it intended to assess
sideward impact.

A second area of misunderstanding involves the assumption that injury in
an occupant implies that the vehicle impact was greater than some
published test impact in which a test dummy provided readings below the
associated injury criterion. Assuming the converse would similarly be a
mistake. Occupants may not behave like test dummies for a variety of
reasons including differences in size, weight, dynamic characteristics,
and pre-impact positioning. Furthermore, each impact is a bit different
as well. Differences in occupant pre-positioning, restraint tightness,
or structural deformations may lead to quite variable responses.

Therefore, injury criteria are helpful guidelines in understanding the
benefits of protective modalities and the hazards in particular types of
crashes. They cannot, however, be applied blindly to specific impact
events.

EVALUATING AN IMPACT EVENT

The examiner who has the opportunity to provide aeromedical assessment
in an aircraft accident investigation should attempt to understand the
injury causation process from end to end. This often requires some
consultation with other specialists in engineering and accident
reconstruction. The first step is typically to thoroughly understand the
injuries with particular attention to the medical evidence that
elucidates the mechanisms by which they occurred. The injuries then must
be placed in the context of the crash.

Vehicle dynamics during the crash are typically the province of the
accident reconstructionist, often resulting in assessments of velocity
change, principal direction of force, acceleration-time pulse shape and
duration, and assessments of any rotation of the aircraft during the
impact sequence.

As noted earlier in this text, accident reconstruction information
typically relates to the accelerations experienced at the center of mass
of the aircraft. Accelerations experienced at different seating
positions where a structural crush may have taken place can vary greatly
from the experience at the center of mass. Even without significant
crush, rotational dynamics during the impact can result in different
impact events for different seating positions relative to the center of
rotation.

EVALUATION OF THE VEHICLE AFTER THE IMPACT

Inspection of the crashed vehicle often yields critical information for
injury causation assessment. Even without quantitative accident
reconstruction, the aircraft wreckage and information from the scene can
provide useful qualitative information as to the principal directions of
crash forces experienced by the occupants. Aircraft crashes are usually
three-dimensional events so the relative proportions and timing of
vertical, longitudinal, and lateral crash loads may be decisive in
understanding injury outcomes.

Examination of occupant seating locations may yield helpful physical
evidence including evidence of load at restraint anchors, adjustments,
and webbing surfaces to assess not only restraint use, but also how it
may have been adjusted at the time of the crash. Examination of seat
structures, mounting locations, and surfaces around the seating location
may provide evidence of occupant loading or direct impact of occupant
body parts.

Current seat structural specifications for civilian aircraft require
structural capability to withstand longitudinal accelerations of 16g
with the occupant reacting forward with respect to the seat, 9g with the
occupant reacting down into the seat, and 3g for the occupant reacting
to the side. While these are dynamic specifications with some safety
factor involved, damage to seat structures cannot be used to simply
conclude that the crash loads were in excess of these numbers. The
specification relates to a seat occupant of a nominal adult male size,
but significantly heavier occupants or structural deformation at or
about the seat or variations in occupant restraint or initial position
may result in seat damage at lower levels.

It should be noted in passing that these numbers are well below human
tolerance potential in a number of circumstances. However, such
accelerations measured at seat locations represent severe crashes. For
example, forces placed on the nose of an aircraft sufficient to achieve
more than 16g at a fuselage seating location would typically produce a
dramatic amount of structural crush to the aircraft with associated
occupant compartment deformation. The reasonably well-behaved
environment in which extremes of human tolerance have been demonstrated
are not likely to exist inside an aircraft structure wherein extreme
loads are being sustained at occupant seating locations.

It should also be noted that dynamic foreshortening as a result of crush
during an impact event will typically be greater than that observed
statically after a crash. There is typically some elastic restitution
that makes the post-crash crush look less severe than actually occurred
during the crash.

MODELS AND CRASH SIMULATION TECHNIQUES

The use of models of both the crashed vehicle and its occupants is
becoming more common. Complex models of vehicle structures may provide
significant insights into how the structure may have responded to the
forces of the impact event. Where these models are based upon detailed
knowledge of the structures and where their responses have been
experimentally validated for the impact conditions experiences in the
accidents, these models can overcome many of the inaccuracies associated
with the simplified assumptions that have traditionally been used by
accident reconstructionists. However, these models, if misused or if not
adequately validated and accredited for use to simulate the response of
a specific vehicle, can be misleading and create a false sense of
accuracy of the accident simulation. An independent, knowledgeable
authority or agency, not the model developer, should accomplish
accreditation of such models.

Similarly, models of human body response or the impact response of
specific body segments must be experimentally validated for the intended
application. Unfortunately, many of the models of human response are
based upon a collection of data that are descriptive of only human
tissue or joint strength properties. In more extreme cases, the models
are based upon assumptions regarding the kinematic response of the human
or, worse yet, only the measured responses of anthropometric manikins.
Figure 7 illustrates the differences that have been observed between
volunteer human subjects and contemporary manikins used to evaluate
automotive and aircraft protection systems. Again, one must be cautious
in the application of such models in vehicle impact reconstruction and
in the development of conclusions about the mechanisms of injury.

Experimental validation for the impact conditions to be simulated and
independent accreditation of the models are essential elements that
should precede a thoughtful assessment of the accident causes and the
efficacy of protective devices using modeling and simulation technology.

PREVENTING IMPACT INJURY

The aeromedical examiner's goal in understanding impact injury is to
prevent future ones. It is clearly helpful to be able to relate a given
set of vehicle dynamics to the resulting occupant kinematics, the
stresses imposed as a result of contacts with vehicle, restraints, or
terrain and to understand how the biomechanical stresses imposed by
those contacts result in clinical injury. However, to prevent future
injury, the aeromedical examiner is often called upon to make an
assessment of what would have prevented a specific observed injury. In
order to do that, you must not only analyze the specific event, but also
analyze how that event would have been altered by some intervention. It
is not enough to simply say the stress needs to be lowered and invoke
some methodology for lowering stress. To encourage helpful
interventions, the thoughtful aeromedical examiner must understand how a
proposed intervention would work and whether or not it could practically
work in a given scenario, and what potential adverse effects it might
have in other scenarios.

The assessment of potential for injury mitigation and the means for its
accomplishment vary with the severity of the crash. For example, in a
relatively low severity crash with facial injuries incurred from
striking the instrument panel, questions might be directed at the
adequacy of shoulder harness use, adjustment, or performance. Similar
injuries in a more severe crash might have nothing to do with the
shoulder harness but would relate rather to foreshortening as a result
of structural crush between the instrument panel and the occupant
seating position. Such injuries might not even be amenable to mitigation
by eliminating the crush since the stiffer structures would result in
higher accelerations at the occupant seating position, potentially
exceeding tolerances for other injuries.

ENERGY ABSORPTION

Misunderstandings abound in the area of injury prevention. In
particular, the notion of energy absorption has been utilized as a
panacea that ultimately prevents the occurrence of injury. There are
some who think if you produce enough energy absorption you can keep
energy from being "channeled" to an occupant. Unfortunately, the laws of
physics limit what can be done. An occupant of a moving vehicle cannot
have his or her energy simply absorbed. It must be changed and since
energy is a function of mass and velocity, the only way you change the
energy of an occupant while preserving the occupant's mass is to change
the occupant's velocity. As we have seen, velocity is changed by
acceleration, and acceleration on a mass is produced by force.
Therefore, in an impact, the objective is to apply force to an occupant
to create the necessary velocity change in the least injurious fashion
possible.

All that energy absorption can do is to increase the distance over which
that force can be applied and decrease elastic rebound. An object
undergoing completely elastic rebound ends up getting twice the velocity
change of an object that hits and sticks. Therefore, energy absorption
approaches can attempt to decrease the "bounce back" and increase the
stopping distance to the maximum available. That maximum available
distance is limited by the size of the aircraft and the crushable
structure between the impact point and the occupant container. These
distances can be very small, particularly in cases where the impact is
from the side.

Aircraft are designed to provide some crushable structure, particularly
in the front, to give a longer stopping distance for a collision with an
object. The problem is that you have to select a stiffness for the
crushable structure which drives the design of the aircraft towards a
particular impact energy. For example, consider a rigid occupant cage
with an extremely stiff aircraft nose attached to the front in a
collision of low speed with a very stiff object. Resulting accelerations
would be very high. Then consider the same collision with aircraft nose
structure made out of tissue paper. Once again, the accelerations would
be very high, but simply delayed a little bit until the rigid occupant
container made contact. For a given impact energy you could define some
intermediate level of stiffness for the nose that would just complete
its crush at the point when the impact was over. However, such a nose
would not be optimized for a higher or lower energy impact since a lower
energy impact would not lead to the maximum stopping distance possible,
while the higher impact would bottom out and produce extremely high
acceleration towards the end or damage to the occupant compartment.

The current approach is typically to optimize the stiffness versus crush
space design around some severity level that is both relatively likely
and in which there is some significant risk of injury or death. That
clearly will result in too much stiffness at lower severity levels where
injury is less likely, and too little stiffness with typical damage to
the occupant compartment at higher severity levels where survival is
less likely. It is therefore, not sufficient in the analysis of a given
crash to say that the energy absorption was not optimized for that
crash. The question is, was it reasonably designed within the limits of
practicality.

PADDING

Another related issue is that of padding. The function of padding is
related to our previous discussion on energy absorption. Padding serves
three basic functions. It increases the area of force application, which
decreases the applied stress. This may benefit some injury modes and not
others. For example, padding may decrease the likelihood of skull
fracture, but may in fact increase the likelihood of neck fracture by
trapping the head and increasing stress on the neck.

A second effect of padding is to increase the stopping distance, which
can lower the magnitude of the applied force. Given the thickness of the
pad, its stiffness defines some optimum combination of impacting object
mass and velocity for which optimum protection can be supplied.

Finally, if the deformed padding does not have elastic rebound it may
decrease the velocity change by absorbing energy. However, the
underlying surface upon which the padding is applied may have a rebound
of its own and serve to obviate that effect.

Please note that the contact velocity for an impacting body part against
a pad is not the same thing as the velocity change of a crash. For
example, a head may be near a pad at the beginning of a crash and have a
relatively low contact velocity and then have the head coupled to the
vehicle through the pad for the majority of the velocity change. Padding
design, therefore, also presents a compromise in injury protection just
as did crushable structure for energy absorption during an impact.

Once again, one cannot simply make the suggestion that more padding or
more energy absorption would have been the answer for a specific impact
event.

PREVENTION APPROACHES

The investigation of an aircraft crash in which injury has occurred
necessarily turns at some point to the causes of the injury and what can
be done to prevent similar injuries in the future. Investigators have
often advanced specific, sometimes sweeping recommendations for change
in protective modalities that would provide seemingly obvious benefits
in the kind of crash being investigated. Sometimes the apparent benefits
are not real because they are based on misunderstandings of the physical
basis of impact injury as discussed previously. Even when actual
benefits would result from the recommended changes however, such
recommendations may still be inappropriate if they simultaneously
introduce other risks that would outweigh any benefit to be realized.
The attendant risks may be more subtle than the benefits. To appreciate
the overall result, one must understand both the physical basis of
impact injury and the nature of protection at the margin.

No practical impact protection system delivers optimum protection for
all occupants in all impacts. Any real protection system is the result
of a host of compromises among factors such as system weight, comfort,
mobility, and the ranges of occupant position, weight, and
anthropometric dimensions. In addition, real protection systems must be
designed for the potential range of impacts, including impacts from a
variety of directions at various levels of severity. Some beneficial
things you might want to do for one type of impact direction or severity
might be harmful in another and pose additional problems in normal
operations.

The aeromedical examiner should be well versed in the reasons for
employing available protection system for the flyers they advise. There
is an obligation to provide meaningful education to ensure that
occupants are well motivated to employ the available restraint systems
consistently and in accordance with recommendations. Particularly when
investigating aircraft crashes, the astute aeromedical examiner will
also employ the underlying fundamental principles in understanding how
the injuries occurred so that recommendations coming out of
investigations will be objectively based and broadly applicable.