The results of studies on the risk due to the use of carbon fibers in aircraft are assessed. Assumptions such as additional fire protection in the aircraft, new structural concepts, and the development of unique carbon composites are evaluated. Some findings from the national risk profile including equipment vulnerability and economic impact are discussed

Huston, R. J.

English

NASA Technical Reports Server

Perspective on the Results 
Robert J. Huston 
NASA Langley Research Center 
This paper is an effort to put some of the things that we have found from the 
NASA study into perspective. I would like to start out by covering some issues 
that were not covered in detail in our discussions (Figure 1). One of the ques- 
tions that we were originally asked was: is additional aircraft protection re- 
quired? You should have noticed that air transport aircraft were included in 
the calculations of the risk profile, and, in fact, in the specific examples 
that were cited, you noted that the expected losses at the various airports were 
quite low. Independently, we have had Boeing, Lockheed, and Douglas analyzing 
each of their own aircraft, using fiber dissemination footprints that have been 
calculated for crash fires and explosions on an airport where quite high expo- 
sures have been predicted. We found, in the analysis of the aircraft systems, 
that the key issue that prevented aircraft from being considered perhaps totally 
invulnerable was the fact that occasionally aircraft avionic bay doors would be 
open. On those occasions, we found that the interior exposure was sufficient to 
fail one or two pieces of avionics equipment in a fleet of aircraft that were 
sitting on the ground where an aircraft accident had occurred. That fact sug- 
gests that some precautionary measures may be prudent. If you have an aircraft 
crash fire releasing carbon fibers on an airport it makes sense to close the 
doors on the aircraft and not allow interior exposure to carbon fibers. 
We have also looked at fire-release strips as a possible cause of power inter- 
ruptions. Our calculations show that only at a probability of one chance in a 
thousand will we be able to get power interruptions from the kind of composite 
strips that were generated at China Lake and Dugway Proving Ground tests. This 
kind of failure, incidently, is equivalent to the interruption that occurs when 
a squirrel shorts a powerline to ground. 
The generation of composite strips and, in fact, the amount of carbon fiber de- 
bris that is actually found after a fire, suggests very clearly that carbon fi- 
ber debris should be cleaned up. Dr. Bell's paper, Reference 1, showed the mass 
balance distribution of carbon fiber from burned composites. The amount of 
debris or residue was quite large, in fact, many times the amount of free fiber 
that is released by the burn. Therefore, prudent practice would suggest that 
the aircraft owner go in and clean up the composite debris of an accident to 
prevent a secondary release of fibers. At this time it is the responsibility 
of an aircraft owner to clean up an accident so the extension of that require- 
ment to include composite debris would be normal practice. 
It should be pointed out that some unique carbon composites could be developed 
in the future. The term unique here applies to a carbon composite which has a 
release characteristic different from the average. Dr. Bell's paper described 
one composite with such a characteristic. It is possible that in the future, 
as additional carbon composite concepts are developed, other unique composites 
could turn up but we have also developed some test methods that are appropriate 
213 
https://ntrs.nasa.gov/search.jsp?R=19800010925 2020-03-21T02:25:14+00:00Z
for evaluating the fiber release characteristics. We think that these methods 
will be used to analyze the potential release from new concepts. 
As for the assumptions inherent in this study, Figure 2 lists those that we 
feel result in a conservative final answer. Our analysis assumptions have 
included one percent release for a crash fire and 3 and a half percent release 
for a crash explosion. This assumption was based on the amount of material 
that we had found to be released from specimen tests. Based on the data re- 
cently acquired from the outdoor tests, we see considerably less release than 
that. Therefore we feel that our estimate is quite conservative here. 
In the case of the explosive release model that A. D. Little uses (Reference 
2), the analysis is considered to be conservative, that is it gives excessive 
fiber concentrations because it is really a ground-level fiber release and 
not an explosive plume that is carried up in the developed fire plume. 
Our analysis at this point ignores the filter effect on fiber length. Dr. 
Elber's paper (Reference 3), showed that not only did filtration reduce the 
number of fibers that entered into either cases or buildings, but that it also 
provided an effective shortening of the fiber length spectrum. This phenomenon 
has a strong effect on the failure rate, as shown by Mr. Taback's paper (Refer- 
ence 4). Therefore the fibers that enter a filtered enclosure are not as 
damaging as the fibers exterior to that enclosure. Our analysis has not in- 
cluded that effect. 
We have based our equipment vulnerabilities on tests done with 1970 technology 
avionics and electronics. And as we see the future, electronics, avionics, 
and computer design and packaging is aiming towards low power, well-protected 
circuitry. Some of the more recent aircraft designs are utilizing totally air- 
conditioned avionics bays, primarily to increase the effective reliability of 
avionics. This effect is not considered in our analysis. 
The equipment failure model over-predicts the failure rate for multi-fiber sen- 
sitive equipment. Test data described by Mr. Taback (Reference 4) shows the 
multi-fiber effect on equipment failure rates. There is a lot of equipment 
that we know requires multiple fiber contacts to initiate failure. In fact, 
anytime the electrical contact spacing exceeds the fiber length, shorting can 
occur only when two or more fibers bridge the space. From a practical sense, 
for this analysis, we don't think it is practical to try to even identify the 
multi-fiber sensitive equipment. 
A final item on this list of conservative assumptions is that the analysis is 
made assuming that no precautions are taken against a known carbon fiber acci- 
dent. Earlier, I pointed out that aircraft avionics can be effectively pro- 
tected, when a crash fire occurs in an airport, by simply closing the avionics 
bay doors to prevent penetration by carbon fibers. Also, Mr. Taback (Refer- 
ence 4) pointed out that in the worst case, the range of a scanning beam M.L.S. 
landing aid would be effectively reduced by 45% if an approaching aircraft was 
flown directly down the center of a fire release?i?arbon fiber plume. Obviously, 
even present operating rules would not allow a second aircraft to approach an 
214 
airport directly over a burning aircraft where the crash cause was undetermined 
and could have been related to the operating condition of the landing aid. 
These examples of prudent precautions will obviously be expanded if cost effec- 
tive measures are found suitable even with the current level of projected risk. 
There are limitations in the analysis (Figure 3). One is that the data sample 
is small. We have only about 250 specimen burns in which to quantify the fiber 
release data. In addition, we have five outdoor tests in the most recent Dugway 
Proving Ground experiments that quantify the carbon fiber release. It is a 
small sample of data upon which to base our analysis. In addition, the total 
variety of equipment that we have examined for fiber vulnerability, either in 
the chamber here at Langley, at the Army Ballistic Research Laboratory, or in 
the work that has been done by the Department of Commerce Bureau of Standards, 
consists only of about 150 pieces of equipment. We have had to have quantified 
the vulnerability of all of the equipment considered nationally from a very 
limited number of tests. We are.aware of and have considered other data in our 
analysis which is not available to the public. 
One limitation that is important to recognize is that the structural concepts 
tested are considered typical of carbon composites of the future, Basically 
we have made the assumption that the structural concepts that we have available 
to us today are those which are going to be used in composite applications on 
aircraft built in the future. 
We have not considered redissemination for the reasons that Dr. Elber (Reference 
3) presented in his paper. In addition, a test in a clean wall and floor room 
showed that even a fan recirculating fibers increased exposure only about a 
factor of two. Both results indicate redissemination should not be a signif- 
icant problem. 
We believe that an accident clean up should include the accident debris, and 
as an assumed prudent measure, that would be a limitation of this analysis. 
It should be pointed out that this analysis is limited to accidental release 
from civil aircraft and to damage done to residential, public, utility, com- 
mercial and industrial installations. What has been left out? The answer is 
very simple, what has been left out is the potential risk from military air- 
craft crashes and the potential damage to military sites. That is the subject 
of Department of Defense analyses. 
It is appropriate to review the key findings of this study (Figure 4). We 
know that the release of fibers does require agitation of the composite residue. 
We see, from the evidence that we have, that a substantial portion of the 
carbon fiber is oxidized away. We see, from our specimen tests, that about 
one percent of the available fiber is released, but in our outdoor tests, we 
found only about 0.13 percent released or about a factor of eight conservative 
from our assumptions in our risk calculations. One final point is that most 
released single fibers are very short. This point should be amplified be- 
cause we have had some specific examples of released fibers where the averages 
are longer than what we used in our analysis. A look at the sensitivity of 
215 
the risk estimates to the average fiber length is appropirate. If we normalize 
the risk at the 3 mm length used in the risk calculations, we can study the 
effect of fiber length on the risk. In our large scale tests in the Dahlgren 
shock tube, we found a mean fiber length of 2 millimeters. Now perhaps that 
is because of the extreme agitation of operating on the composite material 
with the rotisserie. However, in the outdoor tests we found a mean fiber 
length, in the worst case, of about 5 millimeters. Now, what is the effect of 
the mean fiber length on the mean exposure level for failure? Israel Taback 
(Reference 4) pointed out several possibilities. One possibility is that E 
bar is proportional to one over the length. Another possibility is that E 
bar is proportional to one over the length cubed. Figure 5 shows the effect 
of these two variations in the mean exposure to failure on the normalized risk 
assuming all equipment follows either one law or the other. The notation 
indicates the lengths obtained from the large scale tests. It is appropriate 
to point out that at least we have bracketed the large scale test data. The 
actual value of the risk could vary at the most, as a result of fiber length, 
by a factor of 2. Considering the fiber release characteristics as a whole, 
including percentage released as well as length, it would appear that we 
over-estimated the potential damage by somewhere between a factor of 4 and a 
factor of 8. 
If we review the key findings in the area of vulnerability of equipment, 
(Figure 6), we find that the damage done by released fibers is not as severe 
as once thought. The reason is that equipment vulnerability is lower for the 
currently acceptable structural fibers than for the very highly graphitic fibers 
once considered. If highly graphitic fibers (possessing modulus at elasticity 
two times that obtainable with current structural fibers) are found acceptable 
in the future, the base vulnerability level would have to be reevaluated. We 
find from our test data that vulnerability of domestic and industrial electron- 
ics and avionics is very low. The vulnerability of industrial electronics and 
avionics is low primarily because they are conformly coated and well protected. 
In addition our industrial power arc studies show that power vulnerability is 
low and unlikely to cause damage when properly fused. We have found that 
110 volt motors and home appliances can not be hurt by carbon fibers. 
Our present assessment of the risk, using current estimates of the carbon fiber 
release from civil aircraft and conservative vulnerability data, indicated that 
the expected annual cost is insignificant (Figure 7). A comparison of the 
results from the A. D. Little and the OR1 studies (Figure 8) shows quite rea- 
sonable agreement. While the details of the risk profiles are slightly differ- 
ent, the expected annual cost for air transport for either is under $500. 
Adding the expected dollar loss from crash fires of carbon composite general 
aviation aircraft adds about 50% to the mean cost. Compared with the mean 
cost of the aircraft accidents the expected annual cost is an insignificant 
number. The FAA study of 1966-1975 aircraft accident costs (Reference 5) 
showed that the costs of air transport aircraft accidents range from less 
than a million dollars to nearly fifty million dollars (non-fire accidents 
are included). The mean cost of those accidents where the aircraft sustained 
at least substantial damage ranged from 5 million dollars for small jet air- 
craft to in excess of 10 million dollars for large jet aircraft. Therefore, 
216 
considering the annual number of aircraft crashes, the $500 annual potential 
damage from released carbon fiber must be compared with annual aircraft crash 
costs of nearly 100 million dollars (based on 1974 dollars). Considering that 
comparison,the worst case, low probability event is basically a low cost event. 
Figure 8 shows that there is only about one chance in 2000 of exceeding 
$150,000 damage annually. Our studies show that there is no need for additional 
protection of civil aircraft avionics, and the potential shock hazard is in- 
significant, hence risk to life should not be a consideration in carbon fiber 
applications. 
In conclusion, we have some work to do (Figure 9). We have completed the 
planned work on the agreed to schedule. We will publish this conference 
proceedings, complete and publish the analysis of our large scale outdoor 
tests, and complete our final NASA summary report. NASA efforts in the 
carbon fiber hazard area will be completed when all NASA studies are 
published. 
References 
1. Bell, Vernon L.: Release of Carbon Fibers From Burning Composites: 
Assessment of Carbon Fiber Electrical Effects. NASA CP-2119, 
1980 (Paper 3 of this compilation). 
2. Fiksel, J.; Rosenfield, D.; and Kalelkar, A.: Assessment of Risk Due 
to the Use of Carbon Fiber Composites in Commercial and General 
Aviation. Assessment of Carbon Fiber Electrical Effects. NASA 
CP-2119, 1980 (Paper 9 of this compilation). 
3. Elber, Wolf: Dissemination, Resuspension,'and Filtration of Carbon 
Fibers. Assessment of Carbon Fiber Electrical Effects. NASA 
CP-2119, 1980 (Paper 4 of this compilation). 
4. Taback, Israel: 
Shock Hazards. 
Evaluation of Equipment Vulnerability and Potential 
Assessment of Carbon Fiber Electrical Effects. 
NASA CP-21.19, 1980 (Paper 5 of this compilation). 
5. Fallon, William L.: Cost Analysis of Aircraft Accidents. 
Proceedings of the 31st Annual International Air Safety 
Seminar, Flight Safety Foundation, Inc., Nov. 1978, 
pp. 72-82. 
217 
ADDITIONAL AIRCRAFT PROTECTION IS NOT REQUIRED 
PRECAUTIONARY MEASURES ARE PRUDENT 
FIRE RELEASED STRIPS HAVE LOW PROBABILITY OF CAUSING DAMAGE 
CARBON FIBER DEBRIS SHOULD BE CLEANED UP 
UNIQUE CARBON COMPOSITES COULD BE DEVELOPED IN THE FUTURE 
TEST METHODS ARE DEVELOPED FOR FIBER RELEASE EVALUATION 
Figure l.- issues not covered in detail. 
ANALYSIS ASSUMPTIONS 1% FIRE, 3 l/2% EXPLOSIVE RELEASE 
EXPLOSIVE RELEASE MODEL GIVES EXCESSIVE FIBER EXPOSURES 
ANALYSIS IGNORES FILTER EFFECT ON FIBER LENGTH 
EQUIPMENT VULNERABILITIES BASED ON 1970'S TECHNOLOGY 
MODEL OVERPREDICTS FAILURE RATES FOR MULTI-FIBER SENSITIVE EQUIPMENT 
NO PRECAUTIONS TAKEN AGAINST KNOWN CF ACCIDENT 
Figure 2.- Conservative assumptions. 
218 
I 
-- 
THE DATA SAMPLE IS SMALL 
STRUCTURAL CONCEPTS TESTED ARE TYPICAL OF THE FUTURE 
REDISSEMINATZON IS NOT CONSIDERED 
ACCIDENT CLEAN UP INCLUDES COMPOSITE DEBRIS 
ANALYSIS LIMITED TO: 
ACCIDENTAL RELEASE FROM CIVIL AIRCRAFT 
DAMAGE TO RESIDENTAL AND PUBLIC, UTILITY, COMMERCIAL AND INDUSTRIAL INSTALLATIONS 
Figure 3.- Limitations in analysis. 
0 REQUIRES AGITATION OF RESIDUE 
0 SUBSTAilTIAL PROPORTIONS ARE OXIDIZED AWAY 
0 1% OR LESS RELEASED AS SINGLE FIBERS IN SPECIMEN TESTS 
. 0,13% OR LESS RELEASED IN LARGE-SCALE OUTDOOR BURNS 
0 ,MOST RELEASED SINGLE FIBERS ARE VERY SHORT 
Figure 4.- Key findings in carbon fiber risk analysis. 
Release of fibers by fire. 
219 
10,o 
NORMALIZED 
RISK LO 
081 
. MEAN FROM LARGEST MEAN 
SHOCK TUBE TEST OUTDOOR TEST 
1 
rot - 
L3 
RISK CALCULATIONS 
L 
1 1 
1 3 10 
MEAN FIBER LENGTH, FIBERS>1 mn 
Figure 5.- Sensitivity of risk calculation to average released fiber length. 
0 LOW FOR CURRENT STRUCTURAL FIBERS 
0 HIGHER FOR HIGHLY GRAPHITIC FIBERS 
@ VERY LOW FOR DOMESTIC, INDUSTRIAL ELECTRONIC AND AVIONICS 
0 LOW FOR POWER SYSTEMS 
0 ZERO FOR llO-VOLT MOTORS AND HOME APPLIANCES 
Figure 6.- Key findings in carbon fiber risk analysis. 
Vulnerability of equipment. 
220 
0 THE USE OF CONSERVATIVE STIMATES OF ACCIDENTAL CF RELEASE FROM CIVIL AIRCRAFT AND 
CONSERVATIVE VULNERABILITY DATA INDICATES THAT: 
0 THE "EXPECTED ANNUAL COST" IS INSIGNIFICANT 
0 THE WORST CASE, LOW PROBABILITY EVENT IS A LOW COST EVENT 
0 NO ADDITIONAL PROTECTION OF CIVIL AIRCRAFT AVIONICS IS REQUIRED 
0 THE POTENTIAL SHOCK HAZARD IS INSIGNIFICANT - HENCE RISK OF LIFE IS NOT 
A FACTOR 
Figure 7.- Key findings in carbon fiber risk analysis. 
Present assessment of risk. 
1 
10-l 
PROBABILITY 10-2 
OF 
TOTAL COST 
EXCEEDING 
X DOLLARS 10-3 
IN ANY YEAR 
10-4 
10-S 4 
1OZ 103 104 105 lob 
TOTAL COST PER YEAR, X DOLLARS 
Figure 8.- National annual risk profile. Carbon fiber released from 
commercial air carrier accidents. (1993 CF usage.) 
221 
0 REMAINING WORK 
PUBLISH CONFERENCE PROCEEDINGS 
FINAL ANALYSIS OF LARGE-SCALE OUTDOOR TESTS 
FINAL NASA REPORT ON PUBLIC RISK DUE TO ACCIDENTALLY RELEASED CF FROM CIVIL AIRCRAFT 
@ NASA EFFORT WILL BE COMPLETE UPON PUBLICATION OF ALL TEST AND STUDY RESULTS 
Figure 9.- Conclusion of carbon fiber risk assessment. 
222 


Perspective on the results

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