FEDERAL AVIATION ADMINISTRATION (FAA)

AEROSPACE MEDICINE SELF-ADMINISTERED COURSE

SECTION II, 2.4 WHOLE-BODY VIBRATION EXPOSURE

Suzanne D. Smith, PhD

Air Force Research Laboratory

Wright-Patterson Air Force Base, OH

LESSON OBJECTIVES: This lesson provides a summary of the effects and
assessment of whole-body vibration in aviation. In addition to basic
definitions, the section describes both structure-borne and airborne
vibration and the sources of internal and external vibration in
aerospace environments. The contribution of specific vibration exposure
characteristics on human body biodynamics, comfort and health, and
visual and motor performance are discussed. The goal is to insure that
the course participant understands the potential effects of aerospace
vibration on the human operator and has specific knowledge on the
current guidelines available for assessing these effects. A summary of
vibration mitigation strategies is also included in the lesson.

PRE-TEST QUESTIONS:

1\. What frequency range is associated with the major human body
resonance in the vertical direction?

A. 10 -- 15 Hz

B. 4 -- 8 Hz

C. 1 -- 2 Hz

d\. Greater than 20 Hz

Correct Answer: B. 4 -- 8 Hz

2\. Aircrews exposed to aerospace vibration may experience the
following:

A. Blurred vision

B. Reduced motor control

C. Discomfort

D. Back aches and pains

E. All of the above

Correct answer: E. All of the above

3\. Conventional cushioned seats tend to decrease the transmission of
low frequency aircraft vibration and increase the transmission of high
frequency aircraft vibration to the occupant.

A. True

B. False

Correct Answer: B. False

4\. The primary internal source of structure-borne aircraft vibration is

A. Aircraft Noise

B. Turbulence

C. Propulsion System

D. A and C

Correct Answer: C. Propulsion System

**Section II, 2.4 Vibration Exposure**

[Definitions]{.underline}

Vibration is oscillatory motion in dynamic systems. A dynamic system
possesses mass and the capability of relative motion between parts of
the system (elasticity). The human body is such a dynamic system and can
therefore be affected by vibration. Vibration can include oscillatory
motion that repeats itself in a given time period (periodic). The
simplest periodic motion is sinusoidal motion. Vibration can also be
aperiodic, such as occurs with shock or transient motions. Random
vibration is described by its statistical properties. The statistical
properties of stationary random vibration can be estimated to be time
invariant, whereas non-stationary vibration changes with time and is
unpredictable. Most vibration encountered in aviation is random or
aperiodic.

In aviation, humans can be exposed to vibration via the transmission of
oscillatory motion from vibrating structures in contact with the body
surface (structure-borne). The body can also be exposed to vibration via
the transmission of sound pressure waves in high noise environments
(air-borne). Structure-borne vibration is the primary concern for the
crew and passengers of aircraft. Airborne vibration is the primary
concern for ground crews exposed to high levels of aircraft engine
noise.

Vibration can be characterized by its frequency, direction, and
amplitude. For simple sinusoidal vibration, the frequency of the motion
is defined as the number of complete cycles of motion occurring in a
unit of time. The reciprocal of frequency is the period or time
associated with one cycle of the motion. The unit of frequency is the
Hertz (Hz), which is 1 cycle per second. Random vibration includes
multiple frequency components and is described by its frequency content
or frequency spectrum. Frequency is a critical phenomenon for a dynamic
system. When a sinusoidal excitation force is applied to a simple one
degree-of-freedom system (mass-spring-damper), the system will initially
vibrate at its natural frequency (free vibration) as well as at the
excitation frequency. If a damper is present, the motion at the natural
frequency will die out (transient response) while the motion at the
excitation frequency will continue (steady state) as long as the force
is present. If the excitation force is at the same frequency as the
natural frequency of the system, resonance occurs. The resultant
oscillations will continue to build unless damping is present, usually
with catastrophic consequences. The damper will limit the oscillations
to a finite size or amplitude. Resonance can also occur during random
vibration that includes the natural frequency of the excited system.
When a dynamic system is complex or includes multiple sub-systems
(multiple degrees-of-freedom), all associated with a particular natural
frequency, each sub-system can be excited if the associated natural
frequency is included in the excitation. The resultant oscillatory
motions can be quite complicated since the motions of one sub-system can
affect the motions of connected or coupled sub-systems.

A vibrating system can move in translation, rotation, or both.
Translation is oscillatory motion along a straight line and is typically
defined along three perpendicular or orthogonal axes, fore-and-aft (X),
lateral (Y), and vertical (Z). Rotational motion occurs when a body or
system oscillates or rotates about an axis. Roll occurs when the
vibration is about the X axis; pitch occurs when the vibration is about
the Y axis; and yaw occurs when the vibration is about the Z axis. For
the human body, the X axis is from back to front, the Y axis is from
side to side, and the Z axis is from the buttocks to the head (seated)
or feet to the head (standing). The human body can translate or rotate
in or about a single axis, or in a combination of up to six axes (X, Y,
Z, roll, pitch, yaw).

Amplitude is the measure of oscillation about a position at rest and can
be described as a displacement, velocity, or acceleration. Typically,
human vibratory motion is described in terms of acceleration in g or
meters per second squared (m/s2). One g is equal to the acceleration due
to gravity or 9.81 m/s2. Amplitude can be described as a peak or
peak-to-peak value during simple sinusoidal vibration. With random
vibration, a time-averaged or root-mean-square (rms) value is commonly
used.

[Sources of Vibration in Aviation]{.underline}

Internal and external sources of vibration exist in aircraft. The
primary internal source is the propulsion system. In helicopters and
propeller-driven aircraft, vibration is generated at very

![](media/image1.wmf){width="6.25in" height="2.0in"}distinct frequencies
associated with the rotor speed and blade passage frequency. The rotor
speeds associated with helicopters can result in relatively low
vibration below 10 Hz. The blade passage frequencies associated with
three and four-bladed helicopters can fall below 40 Hz. Other
propeller-driven aircraft have higher rotor frequencies. For example,
the Navy E-2C Hawkeye has a rotor speed of 18.4 Hz. With four blades,
the blade passage frequency is equal to four times the rotor speed or
73.6 Hz. Figure 1 illustrates an example of the rms acceleration
measured on a flight officer seat during E-2 Hawkeye operations. The
figure shows the distinct vibration associated with the rotor speed and
blade passage frequency (Smith, 2006).

The vibrations associated with the rotor speed and blade passage
frequency can be felt by the occupant and can be quite annoying. Jet
engines operate at relatively higher speeds, minimizing the potential
for generating low frequency, structure-borne vibration. However, during
engine

run-up and ground-based maneuvering, ground operations and maintenance
personnel can be exposed to airborne vibration.

Air turbulence is the major external source of structure-borne
vibration. Up to approximately 10,000 m or 33,000 ft, weather and
thermal effects contribute to air turbulence. At altitudes above 10,000
m, wind shear between moving air masses can cause clear-air turbulence.
At altitudes below 500 m or 1,600 ft, local heating and cooling between
the air and ground can cause ground turbulence. Military aircraft and
helicopters can be affected by ground turbulence since they can operate
at these low altitudes. Ground effects can also generate vibration as
the consequence of coupling between the downwash and blades (Guignard
and King, 1972). Buffeting is the vibration of a vehicle as it interacts
with the air, generating aerodynamic forces on the structure. These
forces can excite various modes of vibration or resonances of the
aircraft. Bad weather, thermal disturbances, and clear-air turbulence
can result in buffeting in commercial jet aircraft. Buffeting can occur
at low frequencies and be transmitted to the occupant via the floor and
seat of the aircraft. On rare occasions, buffeting in commercial
aircraft can be great enough to affect the control of the aircraft.
Significant buffeting can also occur during aerial combat maneuvers
(ACMs) in high performance military jet aircraft. While acting as a cue
to the pilot, such buffeting can cause involuntary motions of the body
and head and can affect the operation of critical equipment (Smith and
Smith, 2006).

[Human Body Biodynamic Responses to Vibration]{.underline}

As mentioned previously, the human body is a dynamic system and can be
affected by vibration. Numerous studies have been conducted to evaluate
the response characteristics of the human dynamic system. The primary
goal has been to determine the frequency location and magnitude of the
human body resonances associated with the highest body motions. These
studies have primarily focused on the effects of vertical axis
vibration. Frequency response transfer functions are used to evaluate
human resonance(s). One type of frequency response transfer function is
non-invasive and involves measuring the transmitted force of the human
body against a supporting structure, typically a seat mounted on load
cells. The ratio between the transmitted force at the seat and the input
velocity at the seat is called the driving-point impedance. Impedance is
a complex number and is described by the magnitude of the ratio and the
phase angle between the two measurements. Plots of impedance vs
frequency are then used to identify regions of high transmission and
resonance in the body. Peaks in the magnitude of the plots and rapid
changes in the phase of the plots are indicative of a body resonance.
The major impedance peak of humans occurs between 4 and 8 Hz and defines
the vertical whole-body resonance of the human. It is generated due to
the relative motions between anatomical structures located in the upper
torso and shoulder girdle, including the soft tissue and organs in the
upper thoraco-abdominal region. These relative motions cause the
transmission of force back to the measurement site. At higher
frequencies, the impedance tends to decrease, although other smaller
peaks have been reported and associated with other anatomical structures
such as the spine and legs. Figure 2 illustrates the driving-point
impedance of a subject for several postures. Included in the figure is
the impedance response of a pure or rigid mass (mω) and of a simple
mass-spring-damper structure (one degree-of-freedom) used to model the
primary

![](media/image2.jpeg){width="5.6in" height="3.7118110236220474in"}

resonance peak. The natural frequency, ω (in units of radians/s or 2πf
where f is in units of cycles/s or Hertz), mass, m, and stiffness
coefficient, k, for a one degree-of-freedom system are related by ω
=(k/m)½. From this simple representation for the human, it can be shown
that the erect posture results in stiffening of the body (k increases
with ω.) The body response is only linear as a first approximation.
Increasing the acceleration preload, such as occurs during sustained
accelerations in military jet aircraft, can cause the body to stiffen
even more, resulting in a higher resonance or natural frequency.
However, under normal gravity, increasing the vibratory acceleration can
reduce the body stiffness, causing a downward shift in the resonance
frequency by 1 to 2 Hz. As demonstrated by Figure 2, the human can
voluntarily affect his/her vibration response.

Transmissibility is another frequency response function and is defined
as the ratio between the motion measured on the body (such as the head,
chest, or spine) and the input motion at the supporting structure.
Typically, the motion measured is acceleration using miniature light-
weight accelerometers. Transmissibility usually refers to the magnitude
of the ratio but can include phase information. For the head, plots of
the transmissibility show peaks occurring in the vicinity of 4 to 8 Hz
for exposures to vertical vibration, the motion being influenced by the
major whole-body resonance. Other resonances have been associated with
other body regions depending on the location of the accelerometer. In
addition to the sensitivity of the human body to low frequency
vibration, other structures are sensitive to higher frequency vibration.
Resonance of the eye has been reported to occur between 20 and 70 Hz.
This can have a significant effect on visual performance as described
below.

While vertical vibration has been the major concern in aviation, the
human body is known to be sensitive to horizontal motion around 1 to 2
Hz. The effects of multi-axis vibration have been studied only minimally
but are becoming more common with the availability of safe but
sophisticated multi-axis vibration platforms and improved methods for
quantifying actual vibration environments. In propeller-driven aircraft,
the vibration associated with the propulsion system may be the greatest
in the horizontal plane as shown in Figure 1. In addition, vibration in
one axis can produce body motions in other directions. For example,
exposure to either vertical or fore-and-aft vibration can cause both
types of motion in the head in the form of head pitching. Off-axis head
and body orientations can cause the transmission of vibration in other
axes. More complex analysis techniques are available for estimating the
multi-axis transfer functions or transmissibilities of the human. These
techniques can be used to estimate the contributions of vibration in a
given direction to the responses in another direction (off-axis
effects).

As suggested by Figure 2, posture can play a major role in the
transmission of vibration in the body. The coupling of the human body
with interfacing equipment can also have significant effects on the
transmission of the vibration. During whole-body vibration in aircraft
flight environments, the vibration of the aircraft is usually
transmitted to the human body via the seating system. Various components
of the seating system can affect this transmission. For example, the
materials used in conventional seat cushions tend to amplify the
vibration of the upper torso and head in the vicinity of vertical
whole-body vibration (4 -- 8 Hz), but also tend to dampen the motions at
higher frequencies as compared to sitting on a rigid seat. Newer
rate-sensitive foam cushions, such as those being introduced into
military aircraft, can reduce the low frequency effects to some extent.
Another important factor is the amount of relative motion exhibited
between seat components, such as the seat pan, seat back, and seat base,
as well as any motion between the seat base and any seat adjustment
mechanism or rails. These motions can increase the transmission of
undesirable vibration to the occupant. Passive suspension systems have
been designed to attenuate vehicle vibration in the vicinity of body
resonance (4 -- 8 Hz) by reducing the peak response of the coupled
seat/human to 2 Hz or below, producing relatively large motions at these
lower frequencies. Active suspension systems are designed to stabilize
the seat motion at low frequencies. Both passive and active suspension
seats are typically used in rail or ground vehicles, but not in
aircraft, partly because of the added weight of the seating system. New
materials may provide for the potential use of these seating
technologies in aircraft. Helmets can directly affect the motion of the
head, particularly those helmets with added weight that may also shift
the center-of-gravity of the head and helmet system. The use of
sophisticated helmet-mounted equipment for improving flight control and
tactical activity in military aircraft requires consideration of these
adverse effects. The effects of vibration on head or helmet-mounted
equipment are further discussed below in Visual Performance.

The mismatch between the acoustic impedance of air and the human body
surface prevents significant amounts of acoustical energy from entering
the body, particularly at higher frequencies. Below 1000 Hz, more
acoustical energy is absorbed in the form of transverse shear waves.
High intensity noise between 100 and 1000 Hz can cause tissue vibration
and is felt via the somatic mechanoreceptors (Guignard and King, 1972).
Below 100 Hz, intense noise can cause vibratory motion in the chest,
abdominal wall, viscera, limbs, head, and the air-filled and gas-filled
body cavities. Recently, a study has shown peak accelerations in the
chest between 60 and 100 Hz during exposure to military jet aircraft
noise (Smith, 2002). The results coincided with an earlier report of
resonance in the chest wall and air-filled lungs occurring around 60 Hz
(von Gierke and Nixon, 1976).

[Human Physiological and Subjective Responses to Vibration]{.underline}

![](media/image3.jpeg){width="4.790157480314961in"
height="3.3751968503937007in"}The mechanical stresses imposed on the
body during vibration exposure can potentially affect bodily functions
and may cause tissue damage in practically all parts of the body.
Fortunately, most exposures to vibration in aviation remain below such
critical levels. Acute exposures to whole-body vibration of intensities
voluntarily tolerated by human subjects up to the limit of severe
discomfort or pain have not resulted in demonstrable harm or injury.
However, continuous exposures to these levels may cause bodily damage.
Tolerance has been shown to decrease with longer exposures. Figure 3
illustrates the short-time, one-minute and three-minute tolerance limits
reported by healthy adult males exposed to vertical vibration (Magid,
Coermann, and Ziegenrucker, 1960). The figure shows that the lowest
tolerance occurred in the range of 4

to 8 Hz, and that lower tolerance occurred with the longer exposures.
Figure 4 depicts the frequencies associated with the most severe
symptoms reported between 1 and 20 Hz.

Vibration at lower frequencies can be sufficient to affect respiration.
The involuntary movement of air into and out of the lungs has been shown
to increase minute volume, alveolar

![](media/image4.jpeg){width="3.3751968503937007in" height="3.0in"}

ventilation, and oxygen consumption. During some experimental exposures,
hyperventilation was suggested by the decrease in PCO2 and clinical
signs of hypocapnia. Dyspnea can result from short exposures to high
amplitude vibration.

Changes in cardiovascular functions, including arterial blood pressure,
cardiac index, heart rate, and oxygen consumption index, have all been
shown to be dependent on the amplitude and

frequency of the vibration. In general, the cardiopulmonary responses to
vibration in the frequency range of 2 to 12 Hz resemble the response to
exercise (Hood, et al., 1966). For prolonged exposures to vibration, the
most commonly reported chronic health symptom

is back pain or back disorder. This symptom has been reported for other
occupations with minimal vibration exposure. The spinal column is the
major pathway for the transmission of vibration in the upper torso of
the seated occupant. Repeated exposures to higher levels of vibration
may affect the mechanical integrity of the musculoskeletal components,
causing injury. The highest incidence of back pain or back disorder is
reported by helicopter pilots (Pope, Donnermeyer, Wilder, and Hundal,
1983). However, the importance of posture in these pilots renders it
difficult to determine the extent to which vibration contributes to
these symptoms.

Studies conducted to investigate the effects of airborne vibration
(Mohr, Cole, Guild, and von Gierke, 1965) have shown that the most
common symptom reported for noise exposures below 150 dB was mild to
moderate chest vibration. From 50 to 100 Hz, pure tone exposures caused
subjects to reach the limit of voluntary tolerance at 50 Hz (153 dB), 60
Hz (154 dB), 73 Hz (150 dB), and 100 Hz (153 dB) based on the
significance of the reported symptoms. Symptoms included mild nausea,
giddiness, subcostal discomfort, cutaneous flushing and tingling (around
100 Hz); coughing, severe substernal pressure, choking respiration,
salivation, pain on swallowing, hypopharyngeal discomfort, and giddiness
(60 Hz and 73 Hz); and headache (50 Hz). In the study mentioned
previously (Smith, 2002), the peak chest acceleration occurring between
60 and 100 Hz was shown to increase with increases in the noise level.
The physiological consequences of extreme and prolonged exposure to
airborne vibration are not clear.

Involuntary body movements that can occur with vibration exposure are
superimposed as disturbances on the active control motions of the human
operator. In the case of aircraft pilots, involuntary motions of the
hand and/or foot can have undesirable consequences when the amplitudes
of the motions are greater than that required to control the vehicle. In
general, laboratory manual tracking experiments have shown that tracking
errors increase in the 2 to 12 Hz frequency range at vertical seat
accelerations above approximately 0.05 g rms. The main decrement with
vertical vibration occurs in the vicinity of whole-body resonance
(usually near 4 Hz). Likewise, the main decrement with horizontal
vibration occurs in the vicinity of 1.5 to 2 Hz. The tracking error
tends to be the largest in the direction of the vibration stimulus. With
multi-axis vibration of substantial amplitude in more than one
direction, the disturbance can be quite complex. Other factors also
affect the magnitude of the disturbance, including body position or
posture, the type of control (velocity or force control), the amplitude
of the control motion, and body support. The severity of the vibration
can be controlled to some extent by the operator's strategy. For
example, during short bursts of turbulence, pilots may postpone
corrective motor activity until the burst is over. With sustained
turbulence, low frequency vibration can cause inappropriate control
activity resulting from "pilot-induced oscillations." The pilot adds to
the aircraft instability through inappropriate motor responses or the
misinterpretation of kinesthetic cues.

[Visual Performance]{.underline}

Vibration can introduce relative motion of the eye with respect to an
observed object or display, causing difficulty in reading instruments
and performing visual tasks in aerospace environments. The greatest
visual performance degradation occurs when the object or display
vibrates. The least degradation occurs with vibration of both the
operator and object or display (Moseley and Griffin, 1983). Compensatory
eye movement can be effective in minimizing visual performance
degradation during low frequency vibration. With head rotation, the
vestibulo-ocular reflex causes the eye to move in the opposite direction
of the vibratory motion, stabilizing the line-of-sight to the observed
object or display. This compensatory eye movement has been shown to be
effective up to 8 Hz, with some evidence of stability out to 20 Hz
(Griffin, 1990). Although compensatory eye movement is not sufficiently
effective during head translation, exposure to translational vibration
at lower frequencies is expected to produce both translation and
rotation of the head. Unfortunately, with head- or helmet-mounted
displays, the movement of the projected display with the head renders
the compensatory eye movement ineffective in stabilizing the
line-of-sight to the projected image (Wells and Griffin, 1984). Reading
error has been shown to be greater with the use of a helmet-mounted
display than with a panel-mounted display (Furness and Lewis, 1978).
Relative motion between the head and the head- or helmet-mounted system
(slippage) can further degrade visual performance. Stabilization
techniques and algorithms may be effective in minimizing the reduction
in performance. Degradation in head-slaved tracking tasks can also occur
when sufficient levels of low frequency vibration cause involuntary head
vibration. Such motions can occur during maneuvering of fighter
aircraft. Extreme head orientations can further increase the head
motions, causing even further degradation (Smith and Smith, 2006). At
higher frequencies, the eye resonance described above can result in
blurred vision. Since the body dampens the transmission of higher
frequencies to the head, relatively high levels of vibration or direct
contact of the head with a vibrating surface are needed to produce
visual blurring.

[Whole-Body Vibration Exposure Assessment]{.underline}

Based on laboratory and field experiences, whole-body vibration
standards have been developed that provide guidelines for assessing the
effects of vibration exposure. The International Organization for
Standardization (ISO) provides the ISO 2631-1: 1997 entitled "Mechanical
vibration and shock -- Evaluation of human exposure to whole-body
vibration -- Part 1: General requirements" that is widely used by the
international community. This document describes methods for measuring
structure-borne whole-body vibration. Informative annexes are included
that give guidance on possible effects of vibration on comfort,
perception, health, and motion sickness based on current knowledge in
these areas. The American National Standards Institute (ANSI) accepted
the ISO 2631-1: 1997 as a nationally-adopted international standard
(NAIS) in 2002 (ANSI S3.18-2002). The ISO 2631-1: 1997 and ANSI
S3.18-2002 recommend that accelerations be measured in the three
orthogonal directions relative to the seated or standing individual (X,
Y, and Z). The primary acceleration measurements should be obtained at
the location where the body is in contact with the vibrating surface.
The main supporting surfaces for the seated occupant include the seat
pan, seat back, and feet. Triaxial acceleration pads are typically used
for collecting these data. The accelerations are weighted (either in the
time or frequency domain) based on the frequency sensitivity of the
human body as mentioned previously. Various frequency weightings are
described in the ISO and ANSI standards. There are commercially
available meters that incorporate weighting filters. The overall
weighted root-mean-square (rms) acceleration is determined in each axis.
Multiplying factors are applied depending on the location and direction
of the vibration, and the type of assessment (health or comfort). The
assessment is made based on the highest weighted seat pan acceleration
level. For assessing health risk, the highest weighted acceleration for
the associated exposure duration is compared to the Health Guidance
Caution Zones given in Annex B of the ISO 2631-1: 1997. Below the lower
boundary of the zone, health effects have not been clearly documented or
observed. Within the zone boundaries, caution is indicated for potential
health risks. Above the upper boundary, health risks are likely. The
current 1997 standard does suggest that, if vibration in two or more
directions is comparable, then the multiplying factors are applied and
the vibration total value (VTV) or vector sum is calculated. In addition
to a caution zone based on the weighted rms acceleration (second power),
a caution zone is also provided based on the weighted fourth power
vibration dose method or VDV. The VDV represents a cumulative exposure
and is more sensitive to peaks in the exposure. It is noted that the
emphasis of these standards is on the health risk to the lumbar spine.

The ISO and ANSI standards also provide guidance on the effects of
vibration on comfort and perception (Annex C). The frequency-weighted
rms acceleration is calculated in each direction at the main supporting
surfaces. The Vibration Total Value (VTV) is calculated at each
supporting surface. A total VTV can be determined from the root
sum-of-squares of the VTVs associated with those surfaces affecting
comfort. The values are compared to the comfort reactions expected in
public transport (ISO 2631-1: 1997, Annex C).

The American Conference of Governmental Industrial Hygienists (ACGIH,
2002) also provides guidance on human structure-borne vibration
exposure, although the emphasis is on land vehicles and heavy equipment.
Threshold limit values (TLVs) are given in the horizontal and vertical
axes using the weighted rms accelerations calculated in one-third octave
frequency bands via a family of curves that are both time and frequency
dependent. These curves are based on previous versions of the ISO and
ANSI standards. The threshold limit value (TLV) refers to that limit
under which most workers may be repeatedly exposed with minimum risk to
health (particularly with reference to back problems). The ACGIH
emphasizes that the TLVs should be used as a guide for controlling
vibration exposure and should not be used to define boundaries between
safe and dangerous levels.

There are also human vibration exposure standards associated with the
military and other foreign countries. In particular, the European Union
established its human vibration directive in 2002 (Directive
2002/44/EC). Most of these standards use the ISO guidelines for
measuring the vibration exposure.

All of the standards described above are periodically reviewed and
revised by experts as new knowledge is acquired on human exposure
effects. It is imperative that an assessor is familiar with these
revisions and applies the most appropriate assessment methods. It is
also cautioned that the current standards may not adequately reflect the
severity of higher frequency vibration, particularly in propeller
aircraft. Much of the research has focused on lower frequency vertical
vibration in the vicinity of human body resonance. The standards should
be used to target potentially harmful exposures. In addition, health and
safety personnel should investigate any persistent concerns or
complaints from aircrew and other occupants regarding vibration.

[Vibration Mitigation]{.underline}

The most ideal way to reduce human vibration is to eliminate the
vibration at the source. The second method is to isolate the human from
the source of vibration. It may be quite difficult and even costly to
eliminate the vibration at its source, particularly if the vibration is
caused by the interaction of the aircraft with the environment
(turbulence and vehicle resonance). In propeller-driven aircraft,
vibration may be mitigated to acceptable levels by insuring proper
propeller balance and synchronization. In isolating the human from the
source, occupants could be located in an area of the aircraft where the
vibration is not as severe (usually away from the propeller plane). This
may not be feasible in small aircraft or in the case where the cockpit
vibration is significant. Current vibration damping and isolation
techniques, including special cushion materials, may be used to minimize
high frequency vibration. Designing processes and mechanisms for
reducing low frequency vibration in the region of greatest human
sensitivity is still a challenge. Insuring that the various seat
components (seat base, seat back, attachment and seat adjustment
mechanisms) are not adversely contributing to the vibration transmitted
to the human is very important. The effects of such components on
increasing the vibration transmitted to the human may be quite
surprising. The use of seat cushions that evenly distribute seated
pressure and promote good posture could significantly improve comfort in
localized regions of the body. The discomfort, fatigue, and potential
visual effects of higher frequency vibration could be mitigated via
damping and isolation of controls, displays, and seats. In some cases,
unstable low frequency body motions may be minimized by rigid coupling
between the body and contact surfaces to prevent repeated impact and
possible injury. However, with higher frequencies, it may be more
appropriate to remove the coupling between the body and contact surface,
such as removing the head from contact with a vibrating headrest. In the
case of helmet-mounted displays, mitigating head motion at low
frequencies is also a challenge. Computer software and active feedback
mechanisms may be used to reduce the adverse effects on performance. As
a final note, one important practice is to educate the pilot and
occupants on potential adverse vibration conditions in the flight
environment and their effects on human body motion, comfort, and
performance. Being aware of these effects could go a long way in
achieving appropriate actions to minimize the risk to health and safety.

[Acknowledgment]{.underline}

The information contained in this document has been extracted from the
third edition of Fundamentals of Aerospace Medicine (R. L. DeHart and J.
R. Davis, eds.), Chapter 7, Vibration, Noise and Communication, by S. D.
Smith and C. W. Nixon (See reference below).

[References]{.underline}

American Conference of Governmental Industrial Hygienists (ACGIH). Two
thousand two (2002) TLVs and BEIs  based on the documentation of the
threshold limit values for chemical substances and physical agents and
biological exposure indices.

American National Standards Institute, Inc (ANSI). Mechanical vibration
and shock--Evaluation of human exposure to whole-body vibration--Part 1:
General requirements. ANSI S3.18-2002.

Coermann, RR. The mechanical impedance of the human body in sitting and
standing position at low frequencies. ASD Technical Report 61-492, 1961.

Furness TA and Lewis CH. Helmet-mounted display reading performance
under whole-body vibration. Proceedings of the United Kingdom Informal
Group on Human Response to Vibration, National Institute of Agricultural
Engineering, Silsoe, Bedfordshire, 18-20 September 1978.

Guignard JC and King PF. Aeromedical aspects of vibration and noise.
AGARD-AG-151, 1972.

Griffin MJ. Handbook of Human Vibration, Academic Press Ltd., London,
1990; 128-134.

Hood WB, Murray RH, Urschel CW, et al. Cardiopulmonary effects of
whole-body vibration in man. Journal of Applied Physiology, 1966;
21(6):1725-1731.

International Standards Organization (ISO). Mechanical vibration and
shock -- Evaluation of human exposure to whole-body vibration -- Part 1:
General requirements. ISO 2631-1:1997(E).

Magid EB, Coermann RR, and Ziegenruecker GH. Human tolerance to whole
body sinusoidal vibration. Aerospace Medicine, 1960; 31:915-924.

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POST-TEST QUESTIONS

5\. Pilot-induced oscillations that can add to aircraft instability are
caused by

A. Inappropriate control activity

B. Air turbulence

C. Postponement of control activity

Correct Answer: A. Inappropriate control activity

6\. The greatest visual performance degradation occurs with vibration of
the

A. Object or display alone

B. Operator alone

C. Both operator and object or display

Correct answer: A. Object or display alone

7\. Compensatory eye movement is most effective in minimizing visual
performance degradation under what conditions?

A. Low frequency vibration of an observed object or panel display

B. Low frequency vibration of a projected head- or helmet-mounted
display

C. Both A and B

D. High frequency vibration of a projected head- or helmet-mounted
display

Correct answer: A. Low frequency vibration of an observed object or
panel display

8\. The primary standard providing guidelines for assessing the effects
of structure-borne vibration exposure is

A. AFOSH STD 48-19

B. ISO 2631-1: 1997

C. ANSI S3.18-1979

D. ISO 2631-2: 1997

Correct answer: B. ISO 2631-1: 1997