Flight Helmets: How They Work & Why Wear One - USAARL Report

USAARL Report No. 93-2
Flight Helmets: How They Work
and Why You Should Wear One
(Reprint)
John S. Crowley
Biomedical Applications Research Division
.
and
Joseph R. Licina
James E. Bruckart
Biodynamics Research Division
October 1992
United States Army Aeromedical Research Laboratory
Fort Rucker, Alabama 36362-0577
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Department of the Army position,
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or decision,
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~dSSifiCdtiOfl)
Helmets:
How They Work and Why You Should Wear One
_.
12. PERSONAL AUTHOR(S)
Final
Reprinted with permission from WordPerfect
of Air Medical Transport,
Vol II,
pp 19-26,
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in Journal
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18. SUBJECT TERMS (Continue on reverse if necessary
COSATI CODES
FIELD
SUB-GROUP
GROUP
Publishing
Corporation,
1992.
c
Helmets,
flight
safety,
head injury,
dd
khtiti
bY
aviation
blotk
mmk)
medicine
06/05
23104
19. ABSTRACT (Conthe
on reverse if necessary
dnd
McfItifY
by
block
fWmkr)
Flight
helmets have been in use since the early days of aviation,
yet, debate continues
regarding their benefit.
The most common cause of death in aircraft
accidents is head
injury,
and several studies have concluded that helmets can provide significant
protection.
This review chronicles
the development of the modern helicopter
crew helmet,
and presents arguments and data supporting their use.
Throughout histo?y,
man has
worn head protection
in response to the threat of head injury.
Such a w
has limitations
and drawbacks, but in helicopter
aviation,
it is effective
and worthwhile.
,
20. DISTRIBUTION
Chief,
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)D Form 1473,
JUN 86
OF ABSTRACT
Information
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SECURITY CLASSIFICATION OF THIS PAGE
UNCLASSIFIRD
CLINICAL &PORT
~
is
Flight Helmets:-How
They Work and Why
ShOuldWear One
You
I
John S. Cqwley, MD, MPH; Joseph R. Licln8, MSS; Jamss E. Brucksrt, MD, MPH
Introduction
?kE MOST COMMON CAUSEOF DEATH IN
accidentsof all typesis head
injury.‘” Over the years, several
strategieshave been employed to
reducethe incidenceof headinjury.
Theseincludethe useof lap belt and
shoulderharnessrestraintsystems,
andmakingthe cockpitlesslethalby
eliminating sharp knobs and other
protrusions,increasingthe amount
of spacearound the occupant,and
padding surfaces likely to cause
injury.4~5
This paperreviewsanother
classicbut surprisinglycontroversial
approach-the flighthelmet.
aircrdt
Helmet History
.
Protective helmets have been
around as long as armed conflict.
Originally,their mainpurposewasto
protect the head from blows from
primitive weapons;later, helmets
helpedstopcrossbowboltsandmusket balls.These early helmetsdidn’t
permit much mobility of the head
and neck-one medievalarmor helmetwornby CharlesV weighedover
40 lb.6
.In’1908, while flying with Orville
Wright, an unhelmetedArmy pilot
Figure 1. World War I experimental
helmet, circa 1918. (Reprinted from
Dean B: Helmets and Body Armor in
Modern Warfare. 2nd ed, Tuckahoe,
MY., Carl J. Pugliese, 1977.)
namedThomasSelfridgebecamethe
first poweredairplanefatalitywhen
he sufferedlethalheadinjuriesin the
crash of a Wright Flyer.7
Subsequently,a few Americanaviators, including Lt. Henry “Hap”
Arnold(laterGeneralArnold),began
The authors above work at the U.S.
wearing
leather footballhelmets to
Army Aeromedical Research Laboratory
protect
the
skull from injury in case
at Fort Rucker, Ala. John S. Crowley and
of
a
crash.8
In Great Britain, many
James E. Bruckartare majors in the U.S.
Army Medical Corps and senior flight early aviatorswore modified hard
surgeons;Joseph R. Licina is the safety motorcycleracing helmetsto avoid
and occupationalhealth manager.
injury.g
The Journalof Air Medical Transport l August 1992
1
Figure 2. The APHQ Helmet (official
U.S. Army photograph).
During World War I, allied
instructors
andstudentscontinuedto
wear theserigid-stylehelmets,while
mostoperational
pilotsoptedfor soft
leather flying helmetsthat afforded
betterheadmovementandsomeprotectionfrom wind and cold,but preciouslittle crashprotection.Late in
the war, it wasrecognizedthat aviatorsneededmoreballisticprotection,
.and a 2 lb steel flight helmet was
designed(Figure1js6
Flight helmetsfor bombercrews
in World War II were similarly
intendedfor ballisticprotection@rimarily from flak), and many lives
19
Figure 3. The SW-4 Hslfnst (offkid
U.S. Army photograph).
Figure 4. British inventor W.T. Warren in 1912 demonstrating
__
c&u&y
were savedby thesesteel-plated
helmets.6Towardthe end of WorldWar
II, helmets were also designedfor
impactprotection,mainly for use in
new jet aircraft. Test pilots were
r encounteringseverebuffetingwhen
fly@ at high speedsin turbulence,
incurringhead injuriesfrom contact
with the aircraftcan~py.~FMecting
the headduringan ejectionsequence
alsowasa growingconcern.
Thus,a p@losophy
of headprotection for fixed-wingaviatorsevolved
(andenduresto this day) that relied
on in-flight escape to protect the
fixed-wingaviatorloomcrashforces.
For the rotary-wingpilot, who must
“ride out”the entire crashsequence
inside the aircraft, head protection
requirements. are much more
demanding.
Head Injury Protection
Deceleration is expressed in
meters per second,2or “g,” where
one g is equalto the forceof gravity
on the earth’ssurface.The g forceof
an impactdependson initialvelocity
andavailablestoppingdistance.
Head tolerance to a focused
impact(bonebreakstrength)ranges
from30gfor the noseto NO-200gfor
onesquareinchof frontalbone.4The
head can tolerate more diffuse
impact forces of 300-400g without
20
a rprlng-
of Quadrant/Flight Inbmational).
skull fractureor concussion.
Helmet ing locally procured helmets, it
designersendeavorboth to insulate becameevidentthat headprotection
the headfrom penetratinginjuryand waseffectiveandnecessary.
Table 1 depictsan evolutionof the
alsoto reduceglobalhead decelerahelicopterflighthelmetoverthe past
tionforcesto the 300grange.
The ideal flight helmet should 35 years. New, specializedhelmets
weighlessthan4.4 lb, andthe center with integratedvisualdisplays(used
of gravityof the helmet-headcombi- by Apachepilots)are not includedin
nation should match that of the thisdiscussion.
7Jae
AF’H-5 Helmet.With headprounhelmetedheadas closelyas possible. A heavy or unbalancedhelmet tection as the primary driver, the
will rapidly cause fatigue or neck Army adopted the Navy’s Aircrew
pain and could affect performance. ProtectiveHelmet (APH-5)for wear
The helmetshouldbe assmoothand in 1959 (Figure 2), although some
streamlined as possible, to avoid Army pilots had beep wearing this
cockpit entanglementsand reduce helmetsince1954.liar
the effectof tangentialimpacts.
The APH-5 providedimpact proApartfrom distributingthe impact tectionby combininga compressible
force and providingenergy-absorb- liner with a hardshellconstructedof
ing substance,
a goodhelmet fulfills resin-stiffenedlayers of fiberglass.
a variety of other functions. Understandardtestconditions,
a helHelicopters are notoriously noisy, meted headform experienced less
and a helmetcan protecthearingas than 25Og.l* Separatefoam sizing
well as facilitatecommunication.It padswere providedin three thickalso may serveas a platformfor an nessesto facilitatecustomfitting.
oxygen mask or specializednight
The retentionsystemconsistedof
visionequipment.
a padded nylon chin strap screwsecuredto the shell on both sides
Modern Flight Helmets
andfastenedviaasnap.DuetolimlIn the decade following World t&ions of this single-snapfastener
War II, headprotectionwasnotwide system,chin strapstrengthwasonly
ly availablefor U.S. Army helicopter 159lb.
pilots.However,asaccidentstatistics AlthoughtheNH-5 was,in generbegan to demonstrate fewer head al, well-received,
aviatorscomplained
injurieswhen aviatorscrashedwear- that the helmet was too hot, too
2
TheJouInnlofAirMedicalT~
l AlJgustl9Q2
4 !
I’
‘W.I
heavy, and too tight.13These prob- assemblyprovidesa comfortablefit
lems, combined with other design withoutheadcontactwith any partof
deficiencies,spurredthe searchfor a the Styrofoam shell. During an
impact,the nylonstrapselongate(up
replacement
The SPH-4 Helmet. In the mid- to 2%) andthe clipsbend,providing
196Os,the Army determinedthat the impactattenuationprior to headconAPH-5 providedinadequatehearing tact with the styrofoamshell. Drop
protection,particularlyin the low fre testsshowthatthe SPH-4limitshead
quency soundrange (75-2,000 Hz). decelerationto 300g. Although this
Subsequent
testsof the Navy’snewer representsa 50gincreasein transmltsound protective helmet (SPH-3) ted energy comparedto the perforproved it superior to the APH-5 in manceof the APH-5,it is still below
soundattenuation,earphonedesign, the 300-4OOgthresholdfor concusthe suspensionsystem,microphone, siveinjury.’
The retentionsystemconsistsof a
andeaseof fit After severalmoditicationsimprovingthe crashworthiness nylon/cotton assemblyholding the
andretentionof the helmet,the SPH- eat-cups,chin strap,and nape strap,
3 was acceptedby the Army as the forming a circular harness around
the neck. -Aswith the APH5, chin
SPH-4(F&n-e 3).
The SPH4 providestwo layersof strapstrengthin earlyversionsof the
impact protection via a hard fiber- SPH4 was limited to 150 lb because
glassclothandresinshellanda hiih- of a single-snapfastener.The chin
densitysingle-piecestyrofoamliner. strapcomfortpad of the APH-5 was
A third layer of protectionincludesa downsizedto providea closerfit for
suspensionsystem consisting of a the SPH-4,anda maximumallowable
leather-covered
nylonheadbandwith chin strap elongation of 1.5 in
three intersecting crown straps improvedhelmetretention.‘Ihe SPHattachedto the shellwith metalclips. 4 employs the M-87 microphone,
Properadjustmentof the suspension which greatly reducesvoice distor-
The Journal
ofAirMedical
Transport l August 1992
3
tion,14and a singleacrylicvisorwith
plasticoutercover.
Studiesshowedthat aviatorspreferredthe SPH-4overthe APH-5with
respectto fit, comfort,noiseattenuation, and the communication
system.” As crash experiencewith
the SPH-4 accumulated over the
years, several improvementswere
made. For example, the chin strap
was modified to improve helmet
retentionby usingstrongermate&l,
and by installingtwo snapson one
side and a screwpostattachmenton
the other.
The SPH-dB Helmet. Despitethe
overall successof the SPH-4, it did
not provide ideal head protection.
Army epidemiologists
notedthathelicopter crash victims wearing the
SPH-4were still at hiih risk for two
principal types of head injury: concussion and basilar skull fracture.
The latest upgrade to the SPH-4,
termed the SPHAB,greatlyreduces
the risk of these injuries. Global
impact protectionwas improved by
reducing the density of the
polystyreneliner (to allow the foam
21
to compress more easily) and
increasing the liner thickness (to
increasestoppingdistance)
.I5
The elevatedrisk of basilarskull
fracturein SPH4 wearerswastraced
to the rigid plastic earcups.16Onefourth of all impacts to the SPH-4
occurto the ear-cupregion, and the
lack of energy attenuation in the
earcupallowedexcessiveforceto be
transmittedto the baseof the skull.
The new SPH4B includesa thinner
plasticearcup and more liner foam
alongthe sidesof the helmet, allowing energyfrom a lateralblow to be
dissipatedby fracturing the helmet
earctlp.17
Otherchangesin the new SPH4B
helmetincludea mod&d chin strap
andyoke assemblyto improveretention of the helmet, a thermoplastic
liner to improvecomfortand fit, and
a Kevlar’“” shell (Table 1). These
modificationsresultin a new helmet
that is .5 lb lighter than its predecessor,the SPH-4.
Ihe SPH-5 Helmet. A civilianversionof the Army’s SPH4B helmet is
calledthe SPH-5.It is similarto the
SPH4B in weight and performance,
but the Kevlarshellof the SPH4B is
replaced with ballistic nylon and
graphite(Table1).
The Alpha Helmet. Developedby
HelmetsLimitedof St. Albans,Great
Britain,the Alphahelmetwasintended to be usedby helicopterpilo&and
fixed-wingpilots.The significantdii
ferencebetweenthis helmetand the
SPH seriesis the foamliner,whichis
integralto the helmet shell, providing extra stiffness with minimal
weight18
Fixed-Wing Helmets. Helmets
designedexclusivelyfor usein fixedwinga&raft shouldbe carefullyevaluated before purchase. Many of
these lightweight helmets are
designedto withstandejection and
windblastforces,but will not provide
sufficientenergy attenuationto prevent injury in a rotary-wingaccident
sequence. In addition, helmets
designedfor use in a fixed-wingaircraft may not providesufficientprotectionfrom the low frequencynoise
often encountered in helicopter
flight18
Helmet Effectiveness
Althoughprotectiveflight helmets
were scornedby some early flight
safety authorities,lg others were
strongbelievers.GraemeAnderson,
in his 1919 aviation medicine textbook,20reported that of 58 training
accidentsin his experience,student
pilotswere savedfrom headinjuryin
15.
“Over and over again the author
has seenpilotsthrown out who owe
their escapefrom more or less seriousheadwounds,to their safetyhelmets.” Most pre-World War I
aviators, on the other hand, were
unconvinced
of their benefitlg
The early helmet developeroften
tested his own designs,sometimes
beforean amusedand skepticalau&
ence(F&me 4), andsometimes
by hitting himselfon the headwitha mallet
in the privacy of his laboratory.8
Modem helmetengineersuseprecise
ly calibrateddrop towersand other
devicesto assess
helmetperformance.
However, the most compelling evidenceregardinghelmeteffectiveness
is actualcrashinjurydata
Table 1
Characteristics
of Helicopter
Helmet Detail
APHQ
SPH-4
SPH-46
SPH-5
ALPHA
Year Fielded
1958
1970
1991
NA
NA
WeighP
3.5 lb
3.3 lb
2.8 lb
2.8 lb
2.8 lb
Visor Type
Acrylic single
Acrylic single
Polycarbonate
Polycarbonateb
dual
Polycarbonate
dual
Shell Material
Fiberglass
Fiberglass
Kevlar graphite
Nylon/graphite
Kevtar graphite
Styrofoam Liner
Thickness
0.5 inc
0.4 in
0.6 in
0.6 in
0.75 in
Suspension
System
Leather-covered
foam pads
Three-strip
sling
Thermoplastic
liner
Thermoplastic
liner
Sling/pad
Impact
Protection
2509
3009
1809
l6Og
~WI
Chin Strap
Strength
150 lb
150 lb
440 lb
440 lb
450 lb
Earcup Type
Eclipse-shaped
soft foam
6mm flat flange
rigid plastic
3mm contour
rigid plastic
crushable
3mm contour
rigid plastic
crushable
NA
l
Weight indudes medium-sizedhedmet.viaor. and communicationaatmmbly.
b!Singbandduahrisor
ayabm!3amavaiMe.
~linaviaaplitintothmeL3t3uba
dtidmetedheadimpausanateurfaoefrom6-nfreefan.
22
Flight Helmets
4
.
1
1
c
/’
There have been two studies of emergencymedicalservices(EMS)
helmet effectivenessin helicopter aviation programs used flight helaccidents-onein 1961andthe other mets,and29%usedfire-retardant
uniin 1991.The first studyexaminedthe forms. Reasons cited for helmet
effectof the Army’s APH-5helmeton non-useincludedbadpublicrelations,
injury severity during the period high costs,and uncertaineffectiveHoffmanand Shiiskie report1957-1960.13
Fatalheadinjurieswere ness.22
foundto be 2.4 times more common ed in 1990 that helmet use had
among unhelmeted occupants of increasedto 21%,noting that 5%of
potentiallysurvivablehelicopteracci- responding programs reported at
dents than among helmet-wearing least one in-flight injury that could
occupants.The author credited the havebeenpreventedby helmetuseF3
then-newAPH-5 helmet with saving
265livesduringthe studyperiod.
The 1991 study comparedcrash
occupantswearingthe later helmet,
the SPH-4, with unhelmeted occupantsof severe,but potentiallysurvivable helicopter accidents from
1972-Bs1 In the crashesstudied,the
risk of fatalheadinjurywas6.3 times
greater in unhelmeted occupants
compared with those wearing the
SPH-4 (pcO.01).Unhelmeted occupantsriding in the rear of the crash
aircraftwere at even higher risk of
fatal head injury (relative risk=7.5;
p&01). This latterfindingis particularly relevantbecausecivilianflight
medicalpersonnelgenerallyride in
the rearof the helicopter.
Sincethesestudiesare basedsolely on U.S. Army accident data, the
issueof externalvalidityshouldfirst
be addressed, that is, can these
resultsbe appliedto civilianaviation?
Althoughmuchcivil helicopterflying
is obviouslydifferent from tactical
militaryaviation(controlledairspace,
high altitude, busy airports), some
civilian flying is very similar. Since
civil aviationinjury dataare lacking,
it does appear reasonableto apply
these military data to civilian helicopter scenarioswith similar flight
profiles.
.
Reluctance to Use Flight Helmets
Despite the acceptanceof flight
helmets in military helicopter aviation, and the recommendationsof
numeroussafetyagencies,the civilian
rotary-wing community has been
slowto embraceheadprotectionand
otheraviationlife supportequipment,
suchasfire protectiveflight suitsand
flightgloves.In 1939Kruppareported
in JAMT that only 13%of civilian
The Joumal of Air Medii
Transport
August 1992
5
These concernsaboutpublicrelations and the patient’s emotional
state, while well-intended, ignore
three things: (1) a regular aircrew
member’s level of risk over a career
spanningseveralyears is far higher
thanthe patient’srisk duringa single
flight; (2) if there shouldbe a crash,
consciousflight crewmemberswill
be of better use to the patientthan
would an unconsciouscrew; (3) by
providinga headsetfor the patient,
reassuringcommunicationcan actuallybe enhanced.”
Conclusions
Throughout history, man has
worn head protectionin responseto
the threatof headinjury.Sucharmor
haslimitationsanddrawbacks,but in
helicopteraviationit is effectiveand
worthwhile.All personnelregularly
participating in helicopter flight
(civilian or military) should be
equippedwith protectiveheadgear.
Acknowledgment
The views, opinions,and/or find-
ings contained in this report are
thoseof the authorsand shouldnot
be construed as an official
Department of the Army position,
policy,or decision,unlessso designatedby otherofficialauthority. n
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6
The Journal of Air Medical Transport August 1992
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