USAARL Report No. 93-2

Flight : 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 Notice Qualified reouesters Qualified requesters may obtain copies from the Defense Technical Information Center (DTIC), Cameron Station, Alexandria, Virginia 22314. Orders will be expedited if placed through the librarian or other person designated to request documents from DTIC. Chanse of address Organizations receiving reports from the U.S. Army Aeromedical Research Laboratory on automatic mailing lists should confirm correct address when corresponding about laboratory reports. Disposition Destroy this report when it is no longer needed. Do not return to the originator. Disclaimer The views, opinions, and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy, or decision, unless so designated by other official documentation. Citation of trade names in this report does not constitute an official Department of the Army endorsement or approval of the use of such commercial items. Reviewed:

CHARLES A. SALTER Director, Biomedical Applications Research Division Rej..qased for publication:

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6a.NAME OF PERFORMING ORGANIZATION 6b. OFFICE SYMBOL 7r. NAME OF MONITORING ORGANIZATION U.S. Army Aeromedical Research (If l pplkable) U.S. Army Medical Research and Development Laboratory Command SCADDRESS (Uty, State, and ZIPCook) 7b. ADDRESS (City, stdtd, dd ZIP CO&) P.O. Box 577 Fort Detrick . Fort Rucker, AL 36362-5292 Frederick, MD 21701-5012' k. NAME OF FUNDING/SPONSORING Bb. OFFICE SYMBOL 9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBER ORGANIZATION f/f dpPkd&?)

k ADDRESS (City, stdtd, dfld ZIP Co&) 10. SOURCE OF FUNDING NUMBERS PROGRAM PROJECT WORK UNIT ELEMENT NO. NO. 3M1627 SK . ACCESSION NO. 06027878 878875 Al? 382 I 1. TITLE (he/u& Suufity ~dSSifiCdtiOfl) Flight Helmets: How They Work and Why You Should Wear One _. 12. PERSONAL AUTHOR(S)

Final 16. SUPPLEMENTARYNOTATION Reprinted with permission from WordPerfect Publishing Corporation, Published in Journal of Air Medical Transport, Vol II, pp 19-26, 1992. c

17. COSATI CODES 18. SUBJECT TERMS (Continue on reverse if necessary dd khtiti bY blotk mmk) FIELD GROUP SUB-GROUP Helmets, flight safety, head injury, aviation medicine 23104 06/05

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 , 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.

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Flight Helmets:-How

~ They Work and Why is You ShOuldWear One

I John S. Cqwley, MD, MPH; Joseph R. Licln8, MSS; Jamss E. Brucksrt, MD, MPH

Introduction ?kE MOST COMMON CAUSEOF DEATH IN aircrdt accidentsof all typesis head injury.‘” Over the years, several strategies have been employed to reducethe incidenceof head injury. These includethe use of lap belt and shoulderharness restraint systems, and makingthe cockpitless lethal by eliminating sharp knobs and other protrusions,increasing the amount of spacearound the occupant,and padding surfaces likely to cause injury.4~5This paperreviews another classicbut surprisinglycontroversial approach-the flight helmet.

Helmet History Protective helmets have been around as long as armed conflict. Figure 1. World War I experimental Figure 2. The APHQ Helmet (official Originally,their main purposewas to helmet, circa 1918. (Reprinted from U.S. Army photograph). protect the head from blows from Dean B: Helmets and Body Armor in primitive weapons; later, helmets Modern Warfare. 2nd ed, Tuckahoe, During World War I, allied helpedstop crossbow bolts and mus- MY., Carl J. Pugliese, 1977.) instructorsand studentscontinued to ket balls.These early helmetsdidn ’t wear theserigid-style helmets, while permit much mobility of the head namedThomas Selfridge became the most operationalpilots opted for soft and neck-one medievalarmor hel- first poweredairplane fatality when leather flying helmets that afforded . met worn by CharlesV weighedover he sufferedlethal head injuries in the betterhead movement and somepro- 40 lb.6 crash of a Wright Flyer.7 tectionfrom wind and cold, but pre- .In’1908, while flying with Orville Subsequently,a few American avia- ciouslittle crashprotection. Late in Wright, an unhelmeted Army pilot tors, including Lt. Henry “Hap” the war, it was recognizedthat avia- Arnold (laterGeneral Arnold), began torsneeded more ballistic protection, The authors above work at the U.S. Army Aeromedical Research Laboratory wearing leather football helmets to .and a 2 lb steel flight helmet was at Fort Rucker, Ala. John S. Crowley and protectthe skull from injury in case designed(Figure 1js6 James E. Bruckartare majors in the U.S. of a crash.8In Great Britain, many Flight helmetsfor bombercrews Army Medical Corps and senior flight early aviators wore modified hard in World War II were similarly surgeons; Joseph R. Licina is the safety motorcycleracing helmets to avoid intendedfor ballisticprotection @ri- and occupationalhealth manager. injury.g marily from flak), and many lives

The Journalof Air Medical Transport l August 1992 1 19 Figure 3. The SW-4 Hslfnst (offkid Figure 4. British inventor W.T. Warren in 1912 demonstrating a rprlng- U.S. Army photograph). __ c&u&y of Quadrant/Flight Inbmational). were savedby thesesteel-plated hel- skull fractureor concussion.Helmet ing locally procured helmets, it mets.6Toward the end of World War designersendeavor both to insulate becameevident that head protection II, helmets were also designed for the headfrom penetratinginjury and waseffective and necessary. impactprotection, mainly for use in alsoto reduceglobal head decelera- Table 1 depictsan evolutionof the new jet aircraft. Test pilots were tion forcesto the 300grange. helicopterflight helmetover the past r encounteringsevere buffeting when The ideal flight helmet should 35 years. New, specializedhelmets fly@ at high speedsin turbulence, weighless than 4.4 lb, and the center with integratedvisual displays (used incurringhead injuriesfrom contact of gravityof the helmet-headcombi- by Apachepilots) are not includedin with the aircraftcan~py.~ FMecting nation should match that of the this discussion. the headduring an ejectionsequence unhelmetedhead as closelyas possi- 7JaeAF H-5’ Helmet.With headpro- alsowas a growingconcern. ble. A heavy or unbalancedhelmet tection as the primary driver, the Thus, a p@losophyof headprotec- will rapidly cause fatigue or neck Army adopted the Navy’s Aircrew tion for fixed-wingaviators evolved pain and could affect performance. ProtectiveHelmet (APH-5)for wear (and enduresto this day) that relied The helmetshould be as smoothand in 1959 (Figure 2), although some on in-flight escape to protect the streamlined as possible, to avoid Army pilots had beep wearing this fixed-wingaviator loom crashforces. cockpit entanglementsand reduce helmetsince 1954 .liar For the rotary-wingpilot, who must the effectof tangentialimpacts. The APH-5 providedimpact pro- “ride out” the entire crashsequence Apartfrom distributingthe impact tectionby combininga compressible inside the aircraft, head protection force and providingenergy-absorb- liner with a hard shellconstructed of requirements. are much more ing substance,a good helmet fulfills resin-stiffenedlayers of fiberglass. demanding. a variety of other functions. Understandard test conditions, a hel- Helicopters are notoriously noisy, meted headform experienced less Head Injury Protection and a helmet can protecthearing as than 25Og.l* Separate foam sizing Deceleration is expressed in well as facilitatecommunication. It pads were provided in three thick- meters per second,2or “g,” where also may serveas a platformfor an nessesto facilitatecustom fitting. one g is equalto the force of gravity oxygen mask or specialized night The retentionsystem consisted of on the earth’s surface.The g forceof visionequipment. a padded nylon chin strap screw- an impactdepends on initialvelocity secured to the shell on both sides andavailable stopping distance. Modern Flight Helmets andfastenedviaasnap.Duetoliml- Head tolerance to a focused In the decade following World t&ions of this single-snapfastener impact(bone break strength) ranges War II, headprotection was not wide system,chin strapstrength was only from 30gfor the noseto NO-200gfor ly availablefor U.S. Army helicopter 159lb. one squareinch of frontalbone.4 The pilots.However, as accidentstatistics Althoughthe NH-5 was,in gener- head can tolerate more diffuse began to demonstrate fewer head al, well-received,aviators complained impact forces of 300-400g without injurieswhen aviatorscrashed wear- that the helmet was too hot, too

20 2 TheJouInnlofAirMedicalT~ l AlJgustl9Q2 heavy, and too tight.13These prob- assemblyprovides a comfortablefit tion,14and a singleacrylic visor with lems, combined with other design withouthead contact with any part of plasticouter cover. deficiencies,spurred the searchfor a the Styrofoam shell. During an Studiesshowed that aviatorspre- replacement impact,the nylonstraps elongate (up ferredthe SPH-4over the APH-5with The SPH-4 Helmet. In the mid- to 2%) and the clipsbend, providing respectto fit, comfort,noise attenua- 196Os,the Army determinedthat the impactattenuation prior to head con- tion, and the communication APH-5 provided inadequatehearing tact with the styrofoamshell. Drop system.” As crash experiencewith protection,particularly in the low fre testsshow that the SPH-4limits head the SPH-4 accumulated over the quency sound range (75-2,000 Hz). decelerationto 300g. Although this years, several improvements were Subsequenttests of the Navy’s newer representsa 50g increasein transmlt- made. For example, the chin strap sound protective helmet (SPH-3) ted energy comparedto the perfor- was modified to improve helmet I4’ ! proved it superior to the APH-5 in manceof the APH-5, it is still below retentionby usingstronger mate&l, soundattenuation, earphone design, the 300-4OOgthreshold for concus- and by installing two snapson one the suspensionsystem, microphone, siveinjury. ’ side and a screwpost attachment on ‘W.I and easeof fit After severalmoditica- The retentionsystem consists of a the other. tions improvingthe crashworthiness nylon/cotton assemblyholding the The SPH-dB Helmet. Despite the and retentionof the helmet,the SPH- eat-cups,chin strap,and nape strap, overall successof the SPH-4, it did 3 was acceptedby the Army as the forming a circular harness around not provide ideal head protection. SPH-4(F&n-e 3). the neck. -Aswith the APH5, chin Army epidemiologistsnoted that heli- The SPH4 providestwo layersof strapstrength in earlyversions of the copter crash victims wearing the impact protection via a hard fiber- SPH4 was limited to 150 lb because SPH-4were still at hiih risk for two glasscloth and resinshell and a hiih- of a single-snapfastener. The chin principal types of head injury: con- density single-piecestyrofoam liner. strapcomfort pad of the APH-5 was cussion and basilar skull fracture. A third layer of protectionincludes a downsizedto providea closerfit for The latest upgrade to the SPH-4, suspensionsystem consisting of a the SPH-4,and a maximumallowable termed the SPHAB, greatly reduces leather-coverednylon with chin strap elongation of 1.5 in the risk of these injuries. Global three intersecting straps improvedhelmet retention. ‘Ihe SPH- impact protectionwas improved by attachedto the shellwith metal clips. 4 employs the M-87 microphone, reducing the density of the Properadjustment of the suspension which greatly reducesvoice distor- polystyreneliner (to allow the foam

3 The Journalof AirMedical Transport l August 1992 21 to compress more easily) and SPH4B in weight and performance, Helmet Effectiveness increasing the liner thickness (to but the Kevlarshell of the SPH4B is Althoughprotective flight helmets increasestopping distance) .I5 replaced with ballistic nylon and were scorned by some early flight The elevatedrisk of basilarskull graphite(Table 1). safety authorities,lg others were fracturein SPH4 wearerswas traced The Alpha Helmet. Developedby strongbelievers. Graeme Anderson, to the rigid plastic earcups.16One- HelmetsLimited of St. Albans,Great in his 1919 aviation medicine text- fourth of all impacts to the SPH-4 Britain,the Alphahelmet was intend- book,20reported that of 58 training occur to the ear-cupregion, and the ed to be usedby helicopterpilo& and accidentsin his experience,student lack of energy attenuation in the fixed-wingpilots. The significantdii pilotswere savedfrom head injuryin earcupallowed excessive force to be ferencebetween this helmet and the 15. transmittedto the base of the skull. SPH seriesis the foam liner,which is “Over and over again the author The new SPH4B includesa thinner integral to the helmet shell, provid- has seen pilotsthrown out who owe plasticearcup and more liner foam ing extra stiffness with minimal their escapefrom more or less seri- alongthe sidesof the helmet, allow- weight18 ous headwounds, to their safetyhel- ing energy from a lateralblow to be Fixed-Wing Helmets. Helmets mets.” Most pre-World War I dissipatedby fracturing the helmet designedexclusively for use in fixed- aviators, on the other hand, were earctlp.17 wing a&raft shouldbe carefullyeval- unconvincedof their benefitlg Other changesin the new SPH4B uated before purchase. Many of The early helmet developeroften helmet includea mod&d chin strap these lightweight helmets are tested his own designs,sometimes and yoke assemblyto improvereten- designedto withstand ejection and beforean amusedand skepticalau& tion of the helmet, a thermoplastic windblastforces, but will not provide ence(F&me 4), andsometimes by hit- liner to improvecomfort and fit, and sufficientenergy attenuationto pre- ting himselfon the headwith a mallet a Kevlar’“” shell (Table 1). These vent injury in a rotary-wingaccident in the privacy of his laboratory.8 modificationsresult in a new helmet sequence. In addition, helmets Modem helmetengineers use precise that is .5 lb lighter than its predeces- designedfor use in a fixed-wingair- ly calibrateddrop towers and other sor,the SPH-4. craft may not provide sufficientpro- devicesto assesshelmet performance. Ihe SPH-5 Helmet. A civilianver- tectionfrom the low frequencynoise However, the most compelling evi- sion of the Army’s SPH4B helmet is often encountered in helicopter denceregarding helmet effectiveness calledthe SPH-5.It is similar to the flight18 is actualcrash injury data

Table 1 Characteristics of Helicopter Flight Helmets

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 Polycarbonate dual dual

Shell Material Fiberglass Fiberglass Kevlar graphite Nylon/graphite Kevtar graphite

Styrofoam Liner 0.5 inc 0.4 in 0.6 in 0.6 in 0.75 in Thickness .

Suspension Leather-covered Three-strip Thermoplastic Thermoplastic Sling/pad System foam pads sling liner liner

Impact 2509 3009 1809 l6Og ~WI Protection

Chin Strap 150 lb 150 lb 440 lb 440 lb 450 lb Strength

Earcup Type Eclipse-shaped 6mm flat flange 3mm contour 3mm contour NA soft foam rigid plastic rigid plastic rigid plastic crushable crushable

l Weight indudes medium-sized hedmet.viaor. and communicationaatmmbly. b!Singbandduahrisorayabm!3amavaiMe. ~linaviaaplitintothmeL3t3uba dtidmetedheadimpausanateurfaoefrom6-nfreefan.

22 4 There have been two studies of emergencymedical services(EMS) These concernsabout public rela- helmet effectiveness in helicopter aviation programs used flight hel- tions and the patient’s emotional accidents-one in 1961and the other mets,and 29%used fire-retardant uni- state, while well-intended, ignore in 1991.The first studyexamined the forms. Reasons cited for helmet three things: (1) a regular aircrew effectof the Army’s APH-5helmet on non-useincluded bad public relations, member’s level of risk over a career injury severity during the period high costs,and uncertain effective- spanningseveral years is far higher 1957-1960.13Fatal head injurieswere ness.22Hoffman and Shiiskie report- than the patient’srisk duringa single found to be 2.4 times more common ed in 1990 that helmet use had flight; (2) if there shouldbe a crash, among unhelmeted occupants of increasedto 21%,noting that 5%of consciousflight crewmemberswill potentiallysurvivable helicopter acci- responding programs reported at be of better use to the patient than dents than among helmet-wearing least one in-flight injury that could would an unconsciouscrew; (3) by occupants.The author credited the havebeen prevented by helmetuseF3 providing a headsetfor the patient, then-newAPH-5 helmet with saving 265 livesduring the studyperiod. The 1991 study compared crash occupantswearing the later helmet, the SPH-4, with unhelmeted occu- pants of severe, but potentiallysur- vivable helicopter accidents from 1972-Bs1 In the crashesstudied, the risk of fatalhead injury was6.3 times greater in unhelmeted occupants compared with those wearing the SPH-4 (pcO.01).Unhelmeted occu- pantsriding in the rear of the crash aircraft were at even higher risk of fatal head injury (relative risk=7.5; p&01). This latter finding is particu- larly relevant becausecivilian flight medical personnelgenerally ride in the rear of the helicopter. Sincethese studies are basedsole- ly on U.S. Army accident data, the issueof externalvalidity shouldfirst 1 be addressed, that is, can these resultsbe appliedto civilianaviation? 1 Althoughmuch civil helicopterflying is obviously different from tactical militaryaviation (controlled airspace, high altitude, busy airports), some civilian flying is very similar. Since civil aviationinjury data are lacking, it does appear reasonableto apply c these military data to civilian heli- copter scenarioswith similar flight / ’ profiles. . Reluctance to Use Flight Helmets Despite the acceptanceof flight helmets in military helicopter avia- tion, and the recommendations of numeroussafety agencies, the civilian rotary-wing community has been slowto embracehead protectionand other aviationlife supportequipment, suchas fire protectiveflight suitsand flightgloves. In 1939Kruppa reported in JAMT that only 13% of civilian

The Joumal of Air Medii Transport August 1992 5 reassuringcommunication can actu- Ala., U.S. Army Board for Aviation Accident Research:M-87, M3.3, USABAARWeekly Summa- ally be enhanced.” Reseal&. 1961. ry 1971,13(1):l2. 3. Sha&an DF. %anahanMO: Injury in U.S. 15. HaleyJL Jr., HundleyTA: &c& dhelmet Army helicoptercrashes, Ott 1979-Sept1985. / mndrwtionon impacteneqy attenuation. [Poster Conclusions Tmuma 1989.29~415422. abstract],New Orleans. La. AerospaceMedical Throughout history, man has 4. GlaisterDH: Head injuryand protection.In: Association,1978. worn head protectionin responseto AviationMcdicne. Em&g J. King P. (eds). 2nd 16.Shanahan DF: Basilarskull fmcture in U.S. ed, Imdon, Bu@nvorths,1988, pp 174-184. Armyairaaftao4es1tsUS.USAARLRcportNo. the threatof head injury.Such armor 5. Anonymous:Forewarn ed and forearmed. 8511. Fort Rucker,Ala., U.S. Army Aeromedical haslimitations and drawbacks,but in [Editorial].77re AenqUane 1913; 53471-2. ReseamhLaboratory, 1985. helicopteraviation it is effectiveand 6. Dean B: Helmetsand Boa)Annor in Modem 17. PalmerRw: SPH4 aircrewhelmet impact worthwhile. All personnelregularly Wa&tv. 2nd ed, Tuckahoe,N.Y.. CarlJ. Pugheae, protection improvements 19701990. USAARL. 1977,p 228. Repoti No. 91-11, Fort Rucker.Ala., U.S. Army participating in helicopter flight 7.,CombsH: Kill Devil Hill. Englewood,Cola., AeromedicalResearch laboratory, 1991. (civilian or military) should be TernstylePress Ltd. 1979,p 304. 18. GamblinRw: Flight helmet user require equippedwith protectiveheadgear. 8. SweetingCG: Cm&at&fag Qthing. smith- mentsandhowtheyarexhieved.Imf+uc&fngsof sonianIn&ution Ress, Washington D.C., 1984, the Twenty-Fipk Annual SAFE Symposium. Acknowledgment pp 71-79.. NewhaU,Calif.,SAFE~1967,pp235240. 9. Greer L, Harold A: FQing Clothing-7be 19.Rob&on DH: & DangetvusSky: A Hisb The views, opinions,and/or find- .%nyofitsDevclopment.Shrewsbmy,Engkmd,Air-ty of Aviation Medicine. Seattle, University of ings contained in this report are life PublishingLtd, 1979,p 15. Wash&ton presS 1973. those of the authorsand shouldnot 10.New Helmet.Awful oig 1958,4(11):15. 2O.AndexsonHG:MedicalandSu*gicoAs@da be construed as an official ll.BynumJA:UserevaMationsoftwoaimew of Aviatior, London, Oxford University Press, protectivehelmets. USAARU Report 69-1, Fort 1919,p 162. Department of the Army position, Rucker.Ala., U.S. Army Aeromedical Research 21. CrowleyJS: Should helicopter frequent fly- policy, or decision,unless so desig- vi& 1968. erswearheadprote&m?Astudyofhelmeteffec- natedby other officialauthority. n 12. Societyof AutomotiveEngineers: Motor 6venessJoccllpMed1991;33:76669. vehicle instrumentpanel laboratoryimpact test 22. KruppaRM:Airmedicalsafety4fonow-up procedure-headarea SAE RecommendedFmctice. smvey.jAirMed Tmnsport1989; 4(10):1@21. References !!iAEJ921b. Pennsyhrani SAE 1979, p 34.133 23. Hoffman DJ. ShinskieDW: Evaluationof 1. McMeekin RR:AiraatI accidentinvest& 34.134. helmet wearing by medical helicopter services tion. In: Fundamentalsof AerospaceMedicine. 13. U.S. Army Board for Aviation Accident [Ab&act].jAfrMuf7hz@orr1~9(9):79. DeHart RL, (ed). Philadelphia,Lea & Febiier, Research:Army AvMtion AccidentExperience: 24. Foster-BalonJ. Effects of patientheadset 1985.pp 762614. ReportNo. HF 461, Fort Rucker,Ala, USABAAR, utiliaationduring aeromedicalhelicopter trans- 2. Beareh AAz Army Erpcriencewith Crash 1961. port. [Abstract]. / Air Med Transport 1990; Injuriesand ProtectiveEquipment. Fort Rucker, 14. U.S. Army Board for Aviation Accident 9(9)91.

26 6 The Journal of Air Medical Transport l August 1992