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					                                                                                                 WF 16

                                  EVALUATING THE BIRDSTRIKE THREAT
                                  TO A1RCRAFT WTNDSITIELDSYSTEMS -
                                     A PROBABILTSTIC APPROACH]

                                                     SpeelDan
                                                 Ralph
                                                   USAF
                                      Flight  Dynanlcs Laboratory
                                    Wright-Patterson    AFB, Ohio, USA


       Fteure 1-  The purpose of this report is to briefly                      describe and
illustrate   a iecent effort to analyze lhe potential  risk                     of birdstrlke
danage to an aircraft   windshield  system,

      ligure 2 - This devetopnent affords us another weapon for use 1n our
bird \rar battles.   The technique results   ln predlctire  the probability of
birdstrlke   danage thru statlstlcal  considerarion   of 4 facrors:  Number of
inpacts, bird \reight, impact velocity,    and inpacr codponent strength,

     Fieure 3 - The approach vhich v111 be taken 1n descrlbing                           thls tech-
nlque is to briefly  describe rhe staristical inodel forDulatlon,                         descrlbe
the components of the node1, apply the technique  and suinarlze                          the flndings


      Figure 4 - The techflique was fonnulated by Dr. John Halpin, USAF,
during his efforts to assess the adequacy of vindshleld systeD blrdstrike
ptotection being designed lnto a new aircraft.     Subsequent nodel dewelop-
nrent, analysis and docunentation were perfornred by the Universlty  of
Dayton Research Insrirure (UDRI). The efforr at UDRI was sponsored by
the Air Force Flight Dynamics Laboratory,    Princlpal lnvestigators at
UDRIwere: A. Belens, B. West, and M. Turetla.

      Iigure 5 - Regardless of preventive neasures, birdsrrikes !1111 occur.
USAF  experience is that there are abour 35 birdstrlkes   per 100,000 fllght
hours. It is obvlously unrealistic    to protecr againsr all 1eve1s of potenrial
birdstrikes, so the general approach, as you all nell know, is to pick a
?lotection 1eve1 vhich is felr to reasonably represent the rlsk to be en-
countered. Hotrever, once rhis desired protectlon level is esrablished,
we ltust consider tro questions:  rrhrharhappens tf I hit a bigger bird ar
a slower speed or a sr0a11erbird at a higher speed?I and,'kow likely
is such an occurrence?rr The answers ao these tso questions are, of course,
a functlon of such things as geographical location, alrcraf!     alrlrude,
alrcraft nission, and aircraft deslgn. An abiliry     to aralytlcally   assess
this rlsk to a tr:ansparency systen, from birdstrike   hazards other than
those fohhich    it was designed, is truly needed. Tr is this need which
plomptedthe nork being reported on coday.

     I i g u r e 6 - l { h a t a r e lhe factors which influence qrhethe. a birdstrike
'iII result in danage ro t h e w i n d s h i e l d s y s t e r o ? I n t u l t i v e t y , the probability
of damage    due to a single b i r d s t r i k e is a funcrion of the bird weieht.                  the


  oral P r e s e n t a t i o n o u t l i n e    for use a! the Birdstrike  Cordnitree Europe Meeting
 ln The H a g u e , N e r h e r l a n d s      d.UTing 22-26 october 1979.
lnpact velocity,  and the strength of the lnpact point.        Carrying this intui-
tion one step further,   the probability    of birdstrike   danage during a flighr
is then the probability   of danage due to a single blrdstrike       times the
expected nuDber of birdstrikes.     It was hypotheslzed that if each of these
factors could be ldentlfted    ln a statistical    fashion,  there should be
             of Dathenatically   conbtning thed.

       Flgure 7 - The flode1 formulated by Dr. Halpin was based on the prenlse
that the expected nunber of birdstrikes           can be predlcted     and thar damaae
lrlll  resutt when the kinetic        energy fron a birdstrike     is greater than sone
crltical    tevel.     It ras assuned that a velocity      distribution    for the air-
craft    and 'eight    distribution    for the birds could be conbined ro predicr a
probability     dlstrlbutlon      for kinetic  energy.   By assuning lhat the abillty
of any given windshietd         systern to defeat a birdstrike     is essentialty    related
to impact kinetic        energy and impact location,     we can ldentlfy    the proportion
of the {indshi€ld        systen which will    be daraased as a functlon    of birdstrike
ktnetlc    energy 1eve1.       Mathematically   conblnlng the kinetic     energy prob-
abifity    distributton     with the critical    strength leve1 distribution      vill
produce the probability         of danage due to a single birdstrlke.         combining
the expected nunber vith the probabillty            of daoage yields    the expected
nunber of damaging birdstrlkes.

       Iigur:e 8 - Let's lake a brief     exanination    of how these terns can be
quantified.     The flrst    one is the expected nunber of strikes.        The equation
is not new to those of you who have worked with this problen.             D represents
the btrds per unit volune through vhich the aircraft           is flying   and AVT
represents    rhe volurne sn,ept by the cornponent of concern.       Ir is inportant
to note that in conpuling lhe values tor these terns, there is a necessity
to give ser:ious attenEion ro the environnent of concen.             Ir nusr be rele-
vant to the specific       problem being addressed and must be comon for each
of the terns.      The expected nunber of danaging birdstrikes,        as opposed to
lhe expected number of strikes,        is then the expected nunber of strikes
tiines the probabilily      that any one strike   vi1l result    in daMge.    The prob-
abllity    of a birdstrike    causing danage is obtalned by an integration       \rhich
considers the probability        thac a specific  klnetic   energy w111 occur, and
t\e probrbility     thar darage Li I I occur rr th ,L kjnetic erergy.

      Figute 9 - It is this comJrucation of the probability            of danage shlch
represents Dr. Halpin's    major contriburion.       l"l3rhenarically,    this tern
relates   the probability  that a specific     KE IJilI occur and the probability
that danage w111 result    at this KE. It conblnes the bird welght disrri-
butlon,   f(w), the inpact veloclry    distribution,      f(v),  and determines rhe
probability    that the resulttng  kinetic    energy will     exceed the critical
value necessary to result     in darnage.

       Figure 10 - Let's now take a look at how this technlque was applied
to 3 situation     vhich would seeD iepresentacive    of the necessity      to assess
the risk of birdstrike      damage. lde have an exisrinA    aircraft    vhere mission
profiles     are betng revised to inprove low-altitude     proficiency.      Those
r:esPonsible fo! flight     safety were concerned with the inc.ease in bird-
strike   risk due to the increased lov-altitude     flying   time.     Infornation
\tas needed which they could use in considering      both the effects       of additional
flight    tine in the birdstrike   enviroomenr and the effects       of improving
rtndshleld systen blrdstrlke    resistance'    In short' what is the blrdsttike
                           aiicraft   due to increased lo\t_allitude flight   tlne'
,f"t to ttl. crev and lhe
                                                                         to reduce
ana vtrat noartications of the windshield svsten would be required
thls !lsk,
                                                                    to use the
        Ftsure 11 - We krow that there are four faclors required
                upp.ouch: Ebe expected number    of birdstrikes'  the blrd ueisht
orol.iii".i"
ii"iiit"ir.",    iire werocrrv of impact distribution   and the $indshleld svsten
".."";ir' ar"i'rt".ron.     Let's take a brief look at hor values were conputed
for these four Para0etels'
                                                                     Birdstrlke
       fisure 12 - Faclor 1, Expecteal i{unber of Birdstrtkes'
..-tF"t          ,ni" speclfic   alraraft were on hand so these were used arong
                                                                             the
"iii-"p.r.ir.""r      records of velocitv bv altirude band to establish
                                       pe! 100,000 fllght hours --The  equation
expectea ntortter of birdstrlkes
;ii;;;      i;-;;;"    .t .".."e.    veiocltv and per:centas€ or r11sht time below
                                                                         increased
i.OOO:.*       so that it coulJ be used lo consider the effects of
low-altitude loisslons.

       Fieure 13 - Iactor 2, Bird Weight Discribution'            Birdscllke.re'otds
fo. .Eif"p."lfl"        alrcraft   were aaain used in esEabllshing the bird reight
                                                                                  of
aisttit"tion       characterislics . rhe records shoq a sPeciftc quanlitv
                                                         A certain   pe'centage of this
blrdstrikes occurring over a peraod of tine'
ouantltv was ldentlftable         as to tvpe of bird'      This racio of identiflable
                      assumeito exist for the entire quanritv'
                              l                                        Bird $eight
i;;-;";h";
                                                                            rhe r€surtlna
;;";;;         ;;;; "." "".a co esrablish the welghr dlstribution'
""*ririi""-Jt".rrt"tion         curve for rhis biril  !,,elshr is as shown in the fisure.

        Fieure 14 - Iaclor 3, Velocity Distribullon        oPeraEional records
 tor tt'Tfipeciflc      alrcraft were used  Eo esrablish the rePresentatiwe cumu-
                                   pxofile shown in lhe figure'      The equation
 lattve velocltv dlsttlbutlon
 i.r tt. "".". a11ows consideration       of the effects on the overall distri-
                                                               flighls'
 irif."   ar'.."gt' increasing the quantitv of lo\t-altitude
                                                                                      Based-on prior           birdstrlke
        Fieure 15 - Iaclot             4, strength Distribution'
 t""tiffi-t       tnr" "i..i"i.          ir"J"r'r!rd svsEem, strensth discribution plofile
                                                           a
                                                                      ot tairure
 """ *ip"*a       by assunlng thal faltures vere representative
 r.-" tiit.tf"    .rle.gy f".'"f.  Based on current   \tindshield svsten sEate-of-the-
                                                                            and wisi-
 ..a, i""f"ai"g       operationaf consideratlons such as cosl' weight '
                           possible design nodificarions    were eslablished'    strength
 iriity,    " *ri*'.r
 dlstrilutlon     profiles for tfrese svsce'   nodlficaEions uere estinated as sho!'n'
                                                                 1et's see v'hat the
 Noo that sll four iactors are rnatienatically defined'
  technlque predlcted.
                                                                                                 penetratldns for
        llsure 16 - Predicted windshleld systen birdsttlke
                                                    Note thal thele are four different                            wersions
 u r.offiroa                   ".. "r'.*-.
                                                                                                        with,lultiP1e
 "f .r"l,io""l        the existing nlssion, anil lhe existing nission
 i""i.t""."        ;f lo\r-altltuAe flight;'                    Note also that thele ate 4 dJfferent
                                                                                                and.then 3 oodlfi-
 r"i"i"""      "t lhe windshield svsteo; the existinB svsten
 cations of that o.lstlng systeE' Since I hawent                                    t presented you with the
                                                                                                                         nbers
 e x a c t n u ! $ e r s g h i c h \ ' e r e u s e d h a P p l y i n g t h e t e c h n l q u e t h e s p e c i f i c n usignificant
                                                                                             trends and the
 have 1ltt1e oeanlng to you other ti1u"- tftut tnty shos
inprovenents attainable   through any one of the sysren nodifications.    DesiAn
                                            o
otEions whlch will allou inpLemenLation f any one or tle noailt.ations are
currently  being pursued.   Final selection   oi]1 be nade b! about mid-1980.

       Figure 17 - But hoq realistic        are rhese predicrions?       A predicrion
technique such as this produces results            which are only as realistlc      as
are the inputs.       ExaDining roode1 sensilivity       ro lnputs reveated tha! resulrs
can vary quite significanrly.          The approach used to get a measure of hov
realistic    were the predicrions,      rras ro rake a backlrard glance ar how,e11
this model would have predict€d         losses over a ren-year period ending uirh
the current period.       These loss predlcrions        could then be compared wirh
the actual losses and hopefully         rhe results     would indicare   sorne deAree
of realisn    berween nodel and acrual results.           Flight  recor:ds for rne ren-
year period for this specific        aircraft     revealed rhar the canopy experienced
17 birdstrikes    rirh   roughly 6-8 of rhese being recorded as ,'failures.,'
The Findshield    experienced 29 birdstrikes         nith roughly 1-2 of rhese being
recorded as "failures.r'       The "failures"      were representative    of Denerration
or signlficant    fracture    and parrial     loss of a portton of the canopy.
Resufting probabiliries       of failure    were as shor"{r on the figure.      Applying
the probibllistic      rechnique for rhe sane rine period resulted          in a p!e_
dicted probability      of failure   as shom.       The correlarion    between the actual
and the predicted qas rarher reassurang.

       FiCgg lq - Ue have a risk assessnenr technique which is avaitabte
for use in rhe conrinuing bird wa! bartle.              The technique is sensitive
to bird density,       bird relghr,    inpac! vetoclty,    and inpact area strength
distributions,        This sensitiwiry     has borh negative and positive      inptlca-
tions.     On the negarive slde ir pernits         eirher accidental   or inrentional
bias to distort       the resulrs.     On rhe posirive    side ir pernrits consideration
of specific     alterations    to the inpur dara and rhus, atlows recoAnltton           ot
the overall     problen conplexlry.        The abitiry   ro derive Ehe prerequisite
distributions      fron operalional     dara inproves confldence in the resulEing
risk assessnent as rhese distributions,            if selected vith cale, can rruly
represent the speciflc        operarional    enwironnent of conceh.       ApplicaEaons
where all foui distributions          nusr be hyporhesized,     such as foi a new aircraft
in a nev role,      rhus inwolve the highest risk of inproper interpretarion
of results.      Applications     involvlne   tower rlsk in interprering      the resutrs
are those where fever distributions           nusr be hyporhesized,    as opposed ro
derived fron operational         data.   For exaDple, chansing operating locarlons
with a given aircraf!        and nission profile      or changins nission profile
without changing aircraft         or operating locarion.      Inwestisations    have shoLn
r\at the sensi' iviLy of the 'F.nniquo can be reduced. if ;esired, rhroLsh
future 'nodel developnenrs,         Unrit such ilevelopnent is acconplish;d,        hovever,
it is felt     that the technlque does hawe current applicarlon           providing
reallstic    disrributions     for rhe,:our    factors are establistred-represenrrng
a comon operarionsl         environnent of concern.
BACKUP FIGURES
      A. Characterisric   disrribution  shapes.    Weighr and velocity  distri-
butlon are of rhe l,Ielbufl typei str:enarh dlsrriburion   is of 1in€ar
      B.   Conputarion   of probabiliry    density   funcrion   of   impacting   kinetic
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