Current aircraft design practices to minimize the hazard from rotor bursts are described. The consequences of non-contained engine failures and the impact of rotor burst protection systems on aircraft design are discussed

Mccormick, R. B.

English

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ROTORBURSTPROTECTIONCRITERIAt' !MPLICATIONS
RALPH B, MCCORMICK
BOEING COMMERCIAL AIRPLANE COMPANY
- ABSTRACT-
Due to the high energycontentof the rotatingcompressorand the turbine
assembliesin a turbineengine,the possibilityof an engineburstwas
recognizedas a potentialhazardfromthe earliestdevelopmentdays. Recog-
nitionof the potentialfor engineburst has led to definitiveFAA certifica-
- tion regulationsand specificconsiderationsin the designof currentaircraft.
This designphilosophyis continuedtodayand historicallyprovedvery effec- /
tire. However,rotorburst protectionmust be consideredan importantelement
of overallaircraftsafetyand continuedeffortto reducethe frequencyand
_ minimizethe consequenceof non-containedrotorfailuresis justified. This
paperreviewscurrentaircraftdesignpracticesto minimizethe hazardfrom -'
rotorbursts,and discussesthe consequencesof non-containedenginefailures
and the impactof rotorburstprotectionsystemson aircraftdesign.
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1978002125-044
https://ntrs.nasa.gov/search.jsp?R=19780002128 2020-03-20T12:23:14+00:00Z
INTRODUCTION
The high energycontentof the rotatingcompressorand the turbineassemblies
in a turbineengineand the possibilityof an engineburstwas recognizedas
a potentialhazardto turbinepoweredaircraftfrom the earliestdevelopment
days. For this reason,the FAA has developedstringentregulationsfor engine
certificationwhich requiresspecialtestingto substantiaterotorintegrity.
Recognitionof thispotentialfor enginebursthas also led to definitiveFAA
certificationregulationsand specificconsiderationsin the designof current
aircraft.
This designphilosophyis continuedtodayand historicallyhas proved very
effective. The U. S. commercialair carrierrecordof one fatalityattributed
to non-containedrotorfrageentsin over 400 millionturbineenginehoursof
flyingshowsthat enginerotorfailureis, in fact,statisticallya very small
hazardto the welfareand safetyof commercialaircraftpassengers.
However,to furtherimproveflightsafety,it is necessaryto continuallylook
for meansof eliminatingor reducingthe potentialfor accidents,including
the non-containmentof turbineenginefragments. Considerationof the potential
hazardof an enginerotorburst is an importantfactorin overallaircraft
safetyand continuedeffortsto reducethe potentialhazardis warranted.
CURRENT DESIGN PRACTICES
I
Althoughconsiderableeffortis beingexpendedby enginemanufacturersto reduce
the numberof enginerotorfailures,it is believedthat the ratewill not be
• reduced to zero. Rotor failures may continue to occur at a rate near the
current level of approximately one non-contained failure pet million engine
hours. Therefore, continued effort to minimize the hazard to the aircraft of
non-contained engine fragments is required. !
Currentdesignpracticesto minimizeti:ishazardincludeconfiguringthe air-
craft to reduce the risk of: (1) loss o_ additional thrust, _2) fuel fed
fires, (3) loss of critical systems, and (4) loss of structural integrity.
These objectives are accomplished by: (1) controlling the relative location
and spacing of engines and critical systems, (2) use of redundant systems,
(3) use of dual load path structure, and (4) use of fire protection systems.
/ In addition,where configurationpeculiarityindicates,considerationis given
to special shielding of critical components. The application of these concepts f
is of course very dependent upon the basic airplane configuration. The success
:' of this design approach is a matter of record. ,
CONSEQUENCEOF AN ENGINE BURST
Boeing has recently completed a study to identify the consequenceof engine
non-contained failures and to determine if there is a correlation between damage
i 38
1978002125-045
severityand fragmenttype. Infomationon aircraftdamageresultingfrom
non-containedenginefragmentswas obtainedfromFAA,NTSB,and CAA reports
{ReferencesI through5). All currentBoeingaircraftmodelswereincludedin
the studyas wellas availabledataon DC-8,DC-9,DC-IO,CV880,cvggo,and
L-lOllaircraft.Damagedatawerecollectedon 366jet enginenon-containments
thatoccurredbetweenJanuary1964and February1976.
For thisstudy,"non-containment"was definedas the releaseof inte_l parts
of an enginewithsufficientforceto punctureor splitthe engineoutercase
withor withoutfragmentspassingthroughthe case. _nerally,non-containments
thatinvolvefragmentsthatexitthe nacellewiththe potentialto damage
aircraftstructureotherthanthe affectednacelleareof primaryconcern.
However,the broaddefinitionof non-containmentusedin thisstudywas selected
to giveconsiderationtoall non-containedoccurrences.
Variousotherstudies{References6 and 7) haveexaminednon-containedengine
failuresfromthe standpointof the causeof failure. The Boeingstudywas an
atteq_tto analyzethe consequenceof non-containedfailureswithrespectto
the hazardto the aircraftand its passengers.Sinceall commercialaircraft
certified underFARPart 25 are capable of continued safe operation after the d
loss of thrust from one engine during any phase of flight, this study was
concernedwith damageto the aircraft other than the affected nacelle.
A methodwasdevelopedwhich attempted to relate the aircraft damagecausedby ,'
an engine non-containment to the potential hazard to the aircraft resulting from
that damageand to the class of fragment causing the damage. The method
generated a "relative damageseverity rating" for each occurrence. This rating !
was in the form of a numberby which the hazard associated with one occurrence .
could be comparedto that of another occurrence. The rating has no absolute ( _
meaning. It was offered only as an aid in relating occurrences to each other.
The relative damageseverity rating is a subjective measureof what could have
happenedin a particular occurrence, given the actual damagecausedby the |
. engine non-containment. Thus, it is a meansof identifying the potential
hazard. Since it is subjective, each occurrence could be rated differently by
different analysts. However, it was felt that by applying the samecriteria
by the sameanalyst to all occurrences, a reasonable picture of the criticality
could be obtained. After the numerical values for the relative damageseverity
were determined for each occurrence, the data were divided into four general
categories shownin Table 1. It is apparent that Categories 1 and 2 damage
severity presents no hazard to the welfare and safety of the commercialairline
/ passenger. It is also apparent that if a meaningful reduction in the hazard
/ to the aircraftfrom non-contained rotor failureis to be achieved, Categories
3 and 4 type damagemust be significantly reduced.
3
It should be recognized that Boeing has no first handknowledgeof the vast
majority of these occurrencesandwhile for purposesof this study the informa-
tion reported and the conclusions Peachedby the investigating authority or
; operator involved are assumedto be correct, they maynot be so.
The results of the study are summarizedin Figure 1. Of the 366 non-contained
occurrencesclassified, 283 were judged to have causedminor damageseverity,
53 moderatedamageseverity, 19 significant damge severity, and 11 extreme
39
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1978002125-046
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damage severity. These data indicatethat a relativelysmall percentageof
engine non-containedfailuresresult in significantor extreme damage severity.
Only limited data was availablecovering the number of blades released,the
size of the rim or disk segmentor how many pieces were involvedin a failure.
For this reason only general categoriesof fragmentsize were used in plotting
the data. The fragmentcategoriesused are: single blade, multiple blades,
rim segments,and disk segments. Only occurrenceswhere measurableaircraft
damage occurred and where the fragmentclass was known are plotted. The
figure shows that the majorityof non-containedoccurrenceswith high damage
severity ratings (significantor extreme) involved large fragmentswith high
energy levels (rim or disk segments)while very few smaller fragments•(blades)
were involved in significantor extreme damage severity. Thus to measurably
reduce the hazard of non-containedrotor fai'uresby the use of containment
would requirecontainmentof the majorityof the large high energy fragments.
These analyses indicatedthat the majority of non-containedengine bursts released
fragmentswith relativelylow energy levels. Although the installationof
increasedcontainmentcapabilitycould significantlyreduce the number of non-
containedengine bursts to which the aircraft structureand system are exposed,
reducingthe number of non-containedburst does not directly imply an equivalent
reductionin hazard to the aircraft. The hazard to the aircraft is a function
of fragmentsize and energy. Containmentof only the low energy fragments
would not significantlyreduce the hazard to the aircraft and could result
in a significantweight penalty.
' IMPACT OF .ENGINE BURST PROTECTION
'
Substantialdesign effort has been expended by aircraft companiesto retain a
high degree of flight safety and at the same time minimizethe penaltiesto
the aircraftdue to current design practicesfor engine burst protection. Any
considerationof changes to the current design practices,such as increasing
the containmentcapabilitymust be evaluatedon the basis of overall improvement
in flight safety. The impact on air carrieroperatingcost must also be deter-
mined.
Improvingthe engine fragmentcontainmentcapabilityor providingdeflection
capabilityin order to further protect vital aircraft areas impacts almost a11
aspectsof aircraftdesign. The impact could include: nacelleweight,
airframeweight, nacelle performance,nacelle and engine thermalbalance, engine
maintainability,aircraftweight and balanceand aircraftstructure including
/ flutter. The amount of impact is dependent upon the configuration and the
design of the specificaircraftbeing considered.
Increasing the containment capability presents an additional problem. Containing
fragments within the engine case can cause increased damageto the rotor system
and cause the release of more and larger fragments. This in turn could result
in a greater hazard t.o the aircraft than the release of the initial fragment.
.
CONCLUSIONS
The Boeing study of non-contained turbine engine rotor failures resulted in the
fol 1owtn9 concl usi ons:
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; . *, s _.Jll,,: ? e #,41 .r _:
1978002125-047
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1. Current design practices to: (1) reduce the numberof turbine
engine rotor failures, and (2) to minimize the hazard of a
rotor failure to the aircraft has resulted in an outstanding
safety record.
2. There was significant or extreme damageseverity to individual
aircraft in a relatively small percentageof engine non-contain-
ments.
3. The majority of non-containedoccurrenceswith high damage
severity ratings (significant or extreme) involved large fragments
(rim or disk segments)with high energy levels. Containmentof
only low energy fragments (blades) would not have significantly
reducedthe hazard to the aircraft.
4. Any measureto reduce the hazard of non-contained engine fragments
must be evaluated in terms of overall aircraft safety. In addition,
economiceffects must be evaluated.
REFERENCES
j-
r
I. NationalTransportationSafetyBoardReportNumberNTSB-AAS-74-4
VolumeII,OPcamber18, 1974:TurbineEngineRotorDiskFailure
CaseHistor';:J
2. NationalTransportationSafetyBoardReport:Fan/Compressor/Turbine
DiskFailure,Ig63-I967.
3. NationalTransportationSafetyBoard:Briefsof Accidents/Incidents,
A11 TurbineEnginesSelectedCause/Factor,U. S. CivilAviation
• 1964-1974.
4. FederalAviationAdministrationAFS-140January1, 1974,
Tabulationof UncontainedRotorDiskFailureCausein Civil
Engines.
S. Civil Aviation Authority (United Kingdom): January 10, 1975:
Tabulation of IJ. S. UncontainedEngine Failures, 1963 through
September1974.
J
6. Nationa] Transportation Safety Board Report NumberNTSB-AAS-74-4,
VolumeI; December18, 1974: Special Study - Turbine EngineRotor
Disk Failures.
7. Naval Air Propulsion Test Center Report NAPTC-PE-67,NASA/CR-134855,C
; Rotor Burst Protection Program: Statistics on Aircraft GasTurbine
Engine Rotor Fatlures that Occurredin U. S. CommercialAviation
during 1974, September1975.
TABLE1. - DAMAGESEVERITY.
• AIRCRAFTDAMAGECLASSIFIEDBYRELATIVEDAMAGESEVERITY
1,MINOR- DAMAGETOAFFECTEDNACELLE,NICKS
ANDDENTSINAIRCRAFTSTRUCTURE
2,MODERATE- DAMAGETOSECONDARYSTRUCTUREAND
SYSTEMS
3.SIGNIFICANT- DAMAGETOAIRCRAFTPRIMARYSTRUCTURE
ANDSYSTEMS,MINORINJURIES
4,EXTREME- HULLLOSS,FATALITIES
lm
0
Extreme o
damage
mwDverily o(11)
CD
0 C_ oO|
o oo
Relative Significant
m._Oewvemydamage Q _o
m,o,i_ o _ oo09) o
; , 0
__.. ,° oO°./ ,mr.y(,_)
_ Minor o ° oooooo0 ooo0
Nved_ o(:m) oo o
_.. Single. _ Rim Oi_
.... blldel _ legrnentlugnwlntl
FrlWmmm
FlgureL - RelatlveDamageS verltyVersusFragmentSize,
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1978002125-049
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DISCUSSION
E. Witmer, MIT-ASRL
Ralph, you talked about the occurrence of secondary damage that might
develop as the result of primary fragment release. Are you suggesting that
perhaps the use of deflectors rather than containers might be a preferable
alternative?
R. McCormick, Boeing
I didn't mean to imply that. I suppose that may be a consideration.
I intended to suggest that perhaps small fragments would do less damage by
exiting out than if we contained them in the engine.
Unknown Questioner
Are you in a position to do more of a systems study of the effect of
containment on these items that you talked about: flutter, increased weight,
fuel consumption, those sorts of things?
R. McCormick, Boeing
We haven't done that type of study because we haven't looked at a
containment system installation in an aircraft and we have no immediate
plans to do so.
J
J
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] 978002] 25--050


Rotor burst protection criteria and implications

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