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Full text of "A study of carburetor/induction system icing in general aviation accidents"

5-1920 



NASA CR- 143835 



A STUDY OF CARBURETOR/INDUCTION SYSTEM ICING IN 
GENERAL AVIATION ACCIDENTS 



Richard W . Obermayer and William T . Roe 



MANNED SYSTEMS SCIENCES 
Northridge, Calif. 91324 



March 1975 



I^ATioNAL TECKNICAL 
INFORMATION SERVICE 

U S Depstmenl of Commorce 
Springfi.W, VA. 22151 



Prepared for 



NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
Flight Research Center 
Edwards, Calif. 93523 



1. Report No. 

NASA CR-143835 



2. Government Accession No. 



3. Recipient's Cataiog No. 



4. Title and Subtitle 

A STUDY OF CARBURETOR/ INDUCTION SYSTEM ICING IN 
GENERAL AVIATION ACCIDENTS 



5. Report Date 
March 1975 



6. Performing Organization Code 



7. Author(s( 

Richard W , Obermayer and William T .Roe 



8. Pertofmins Organization Report No. 



10. Wwk Unit No. 



9. Performing Organization Name and Address 

Manned Systems Sciences 
8949 Reseda Boulevard 
Northridge, California 91324 



IT, Contract or Grant No. 
NAS4-2191 



12. Sponsoring Agency Name and Address 

National Aeronautics and Space, Administration 
Washington, D. C. 20546 



13. Type of Report and Period Covered 
Contractor Report - Final 



14. Sponsoring Agency Code 



IS. Suppiementary Notes 



16. Abstract 



The purpose of this study was to perform a current assessment of the frequency 
and severity of carburetor/induction icing in general -aviation accidents. The 
available literature and accident data from the National Transportation Safety Board 
were collected. A computer analysis of the accident data was performed. Between 
65 and 90 accidents each year involve carburetor /induction syBtem icing as a prob- 
able cause/factor. Under conditions conducive to carburetor /induction icing, 
between 50 and 70 percent of engine malfunction /failure accidents (exclusive of 
those due to fuel exhaustion) are due to carburetor/induction system icing'. Since 
the evidence of such icing may not remain long after an accident, it is probable 
that the frequency of occurrence of such accidents is underestimated; therefore, 
some extrapolation of the data was conducted in this study . The problem of 
carburetor /induction system icing is particularly acute for pilots with less than 
1000 hours of total flying time. The severity of such accidents is about the same 
as any accident resulting from a forced landing or precautionary landing . About 
144 persons, on the average, are exposed to death and injury each year in acci- 
dents involving carburetor /induction icing as a probable cause/factor. 



PRICES souEo 10 mm^ 



17. Key Words (Suggested by Author(s)) 

Carburetor ice 
Induction system icing 
Icing accidents 



19. Security Qassif. (of this reporti 
Unclassified 



18. Distribution Statement 



Unclassified - Unlimited 



20. Security Classif, lof this page) 
Unclassified 



21. - 



Category: Q3 



22. Price* 



*For sale by the National Technical Information Service, Springfield, Vir^ginia 22151 

1 



TABLE OF CONTENTS 

Page 

SUMMARY 1 

INTRODUCTION 2 

SURVEY ANALYSES 3 

Search for Accident Data 3 

Literature Search , , ...... 4 

Trends in the Future Use of Carburetor-type 

Aircraft 8 

THE COMPUTER DATA BASE 9 

DATA ANALYSIS 20 

Examination of Aircraft and Engine Makes Under 
Conditions Conducive to Carburetor/Induction 

System Icing ....,.....,..,.,.... 20 

Probable Cause/Factor by Year 20 

Composite Carburetor/Induction Ice Cause/Factor , . 22 

Pilot Experience as a Factor in Carburetor/ 

Induction System Icing Accidents ......... 24 

Composite Indicator to C/I Ice Conditions 25 

Analysis of Accidents Occurring Under 

Conditions Conducive to C/I Ice ..-....,. 29 

Severity of Carburetor/Induction System 

Icing Accidents 29 

DISCUSSION 32 

The Difficulty of Diagnosing Carburetor/ 

Induction System Icing .............. 32 

A Context for Data Interpretation ......... 32 

Severity of Carburetor/Induction System Icing ... 33 

CONCLUSIONS . 34 

REFERENCES ................. 35 

SELECTED BIBLIOGRAPHY 35 

APPENDIX A: INFORMATION SOURCES 39 



HI 



Preceding page blank 



LIST OF FIGURES 

Figure No. Page 

1 Aircraft Analysis Sheet . 10 

2 DOT Icing Probability Curves 26 

LIST OF TABLES 
Table No. Page 

I Accident File Contents . - , . , 13 

II Accident File Codes 15 

III Comparison of Aircraft Make Sensitivity 
to Carburetor/Induction System Icing 
Conditions 21 

IV Comparison of Engine Make Sensitivity 
to Carburetor/Induction System 
Icing Conditions , 21 

V Accidents with Icing as Probable 

Cause/Factor by Year .,.,.. 23 

VI Composite C/I Ice Cause by Year 23 

VII Composite C/I Ice Cause by Investigator's 
Judgment that Conditions are Conducive 
to C/I Ice 24 

VIII Effect of Pilot Experience on C/I Ice as 

Accident Cause/Factor . . , 25 

IX Number and Percentage of Accidents that 
Occurred Under Conditions Appropriate 
for C/I Ice by Year . 27 

X Number and Percentage of Accidents that 
Occurred Under Conditions Appropriate 
for C/I Icing by Cause Determination ... 28 

XI Accident Cause/Factor by Composite 

Indicator of C/I Conditions 30 

XII Aircraft Damage Due to C/I Icing 30 

XIII Number of Accidents Involving Injury 

Due to C/I Icing 31 

XIV Number of Occupants Involved in C/I Icing 

Accidents 31 



IV 



A STUDY OF CARBURETOR/INDUCTION SYSTEM 
ICING IN GENERAL AVIATION ACCIDENTS 

By Richard W. Obermayer and William T. Roe 



Manned Systems Sciences, Inc. 
Northridge, California 



SUMMARY 

The purpose of this study, was to perforin, a current 
assessment of the frequency and severity of carburetor/induction 
icing in^general-aviation accidents. The available literature 
and accident data from the National Transportation Safety Board 
were collected. A computer analysis of the accident data was 
performed. Between 6 5 and 90 accidents each year involve 
carburetor/induction system icing as a probable cause/factor. 
Under conditions conducive to carburetor/induction icing, 
between 50 and 70 percent of engine malfunction/failure 
accidents (exclusive of those due to fuel exhaustion) are due to 
carburetor/induction system icing. Since the evidence of such 
icmg may not remain long after an accident, it is probable 
that the frequency of occurrence of such accidents is under- 
estimated; therefore, some extrapolation of the data was 
conducted in this study. The problem of carburetor/induction 
system icmg is particularly acute for pilots with less than 1000 
hours of total flying time. The severity of such accidents is 
about the same as any accident resulting from a forced landing or 
precautionary landing- About 14 4 persons, on the average, are 
exposed to death and injury each year in accidents involving 
carburetor/induction icing as a probable cause/factor. 



INTRODUCTION 



The conditions in the carburetor and induction system of 
aircraft engines are such that ice can be formed, even on sunny, 
warm days. There are ordinarily some indications to the pilot 
of the onset of such icing such as a decrease in RPM or manifold 
pressure. If the pilot does not take appropriate timely action 
engine malfunction/failure may occur, and at times airstart of 
the engine may be difficul-t or impossible- The pilot must be 
aware of the possibilities of carburetor/induction icing and 
routinely make proper use of carburetor heat controls. The 
pilot must also be aware that use of carburetor heat controls at 
the wrong times can also lead to difficulty. 

It is generally known that carburetor/induction system 
icing causes a number of accidents each year. The purpose of 
this study is to make a current assessment of the frequency and 
severity of the problem, to provide a basis for determining if 
corrective research should be conducted. 

A broad definition of "carburetor ice" will be used in this 
report. Generally, the term carburetor/induction system ice 
will be used in this report to indicate icing throughout the 
fuel, carburetor, or induction systems while in operation. 
Carburetor/induction system icing consists of three categories: 

(1) Impact ice, which occurs when super-cooled moisture- laden 
air strikes elements of the induction system. Build-up of such 
ice occurs when operating in snow, sleet, rain, or clouds. 

(2) Throttle icing, which occurs when moisture in the induction 
air condenses and freezes due to the expansion cooling and 
lower pressure as the air passes the restriction imposed by the 
throttle. (3) Fuel-evaporation icing, which occurs when cooling 
of the fuel mixture by fuel vaporization causes entrained 
moisture to freeze. 

The current study collected available literature and 
accident data, and subsequently analyzed the accident data to 
determine the frequency of occurrence and severity of accidents 
for which carburetor/induction system icing was a probable cause 
or factor. The study included the following specific tasks: 
(1) establishment of available data sources, (2) search of the 
available literature, (3) collection of data from specific 
individuals and organizations, and organized data bases of 
accident records, (4) formation of a computer data base of 
relevant accident data, and (5) performance of an analysis of 
these data and a statistical summary. The results of these 
efforts, along with discussion and conclusions, are presented in 
this report. 



SURVEY ANALYSES 

Three types of searches were conducted during the course of 
this study: (1) a search for accident data bases relevant to 
the study of carburetor/induction icing in general aviation, 
(2) a search for published literature pertinent to aviation 
accident statistics, accident investigation methods, carburetor/ 
induction system icing, engineering for icing protection, and 
analyses of pilot error-related accidents, and (3) a follow-up 
survey to determine the extent of the future use of carburetors 
in aircraft design. 

Search for Accident Data 

Based on a survey of information available in the NASA 
Flight Research Center Library, an initial list was made of 
federal aviation agencies, safety institutions, flight schools, 
recognized authorities, foreign agencies, and manufacturers of 
airframes, engines and carburetors. These sources were queried 
by mail or telephone: Did they have relevant information? 
Were they aware of data existing elsewhere? Who else would they 
suggest we contact? In this way, data were collected and 
additional sources were discovered. . When no new sources were 
uncovered, the process was stopped, and it was assumed that 
complete coverage had been achieved. A list of the persons and 
organizations contacted is presented in Appendix A. 

The search for accident data extended throughout the United 
States, Canada, and Mexico. While specific documents were 
elicited from many sources, it became clear there is but one 
organized general aviation accident data base which is maintained 
by the National Transportation Safety Board. No organized data 
base IS maintained in Canada and Mexico. In this country, both 
the National Transportation Safety Board (NTSB) and the Federal 
Aviation Agency (FAA) conduct accident investigations, but the 
maintenance of a data base is the responsibility of NTSB. 

As a result, the efforts of this study subsequently 
concentrated on familiarization with the NTSB data base and with 
acquisition of a subset of the data base suitable for this study. 
Speific training information for accident investigators was 
provided by the Transportation Safety Institute, Oklahoma City, 
Oklahoma (ref. 1) and personal contact established with NTSB 
personnel. A description of the data base' contents is provided 
in references 2, 3 and 4. 



Literature Search 

The Southern California area is host to many fine libraries 
which were available for the conduct of this study. In 
particular, the University of California, Los Angeles; the 
University of Southern California, California State University, 
Northridge; and Los Angeles Pierce College have all been 
extensively searched. The National Aeronautics and Space 
Administration Flight Research Center Library provided many 
aviation-specific documents these other sources could not. As 
might be expected, the literature pertinent to the topic of this 
study is not large. The References and Selected Bibliography of 
this report presents the results of the search of these 
libraries. 

Many of these documents influenced the conduct of this 
study: however, three are selected for specific coimnent in this 

report; 

Special Study; Carburetor Ice in General Aviation ( Ref. 5) .- 

The authors cite that: ^'During the latest 5-year period for 

which complete data are available, there was a total of 360 
general aviation accidents involving carburetor ice as a probable 
cause or factor. There were 40 fatalities and 160 persons 
injured, 40 of them seriously, in these accidents. The number of 
persons exposed to death or injury in these accidents was 6 36; 
47 aircraft were destroyed and 313 others substantially damaged." 

The blame for these accidents is laid squarely on the pilot, 
stating that these can be attributed to the pilot in virtually 
all cases. Further, carburetor icing is considered by NTSB as 
one of the "unnecessary" causal factors in general aviation 
accidents, since "carburetor icing can be avoided because the 
means to preclude it are readily available." While it may be 
true that the pilot has the means to prevent carburetor icing, 
consider the following complex procedures which are recommended 
m this document to avoid such icing: 

"1. Periodically check carburetor heat systems and controls 
for proper condition and operation. 

"2. Start engine with carburetor heat control in the "cold" 
position, to avoid possible damage to the carburetor heat system. 

"3. As pref light item, check carburetor heat availability 
by noting heat "on" power drop. 

"4. When the relative hiimidity is above 50 percent and the 
ambient temperature is below 299.8*^K (80°F), use carburetor 
heat immediately before takeoff. In general, carburetor heat 
should not be used during taxi because of possible foreign 



matter entry when intake air is unfiltered in the "alternate" or 
carburetor heat "on" position. 

"5. Conduct takeoff without carburetor heat unless extreme 
carburetor icing conditions are present, when carburetor heat may 
be used if approved by aircraft manufacturer, and when conditions 
are such that there will still be ample power for takeoff without 
incurring engine overheat damage. 

"6. Remain alert for takeoff for indications of carburetor 
icmg, especially when the relative humidity is above 50 percent, 
or when visible moisture is present. 

"7. With supplemental instrumentation, such as a carburetor 
air temperature gage, partial carburetor heat should be used as 
necessary to maintain gage temperatures to forestall icing. 
Without such instrumentation, use full heat but only intermit- 
tently if considered necessary. 

"8. If carburetor ice is suspected of causing a power loss, 
immediately apply full heat. Do not disturb throttle initially, 
since throttle movement may kill engine if heavy icing is 
present- Watch for further power loss to indicate effect of 
carburetor heat, then rise in power as ice melts. 

"9. In case carburetor ice persists after a period of full 
heat, gradually move throttle to full open position and climb 
aircraft at maximum rate available in order to obtain greatest 
amount of carburetor heat. if equipped with mixture control, 
adjust for leanest practicable mixture, (approach this remedy 
with caution - although carburetor ice generally serves to 
enrich mixture, the reverse can be true; if the engine is lost 
through excessive leaning, an airs tart might be impossible with 
an iced induction system) . 

"10. Avoid clouds as much as possible. 

"11. In severely iced conditions, and when equipped with 
mixture control, backfiring the engine can sometimes be effective 
in dislodging induction system ice. With carburetor heat 
control "off", lean engine while at full throttle (observe 
caution note in No. 9, above). 

"12. Consider that carburetor icing can occur with ambient 
temperature as high as 310. 9°K (lOO^F) and humidity as low as 
50 percent. Remain especially alert to carburetor icing 
possibilities with a combination of ambient temperature below 
294.3 K (70 F) and relative humidity above 80 percent. 
However, the possibility of carburetor ice decreases in the 
range below 273. 1°K (32^F) , This is because of (a) lessened 
humidity as the temperature decreases, and (b) at around 263- 7°K 
(15 f) any entrained moisture becomes ice crystals which pass 



through the induction system harmlessly. It should be remembered 
that if the intake air does contain these ice crystals, 
carburetor heat might actually cause carburetor icing by melting 
the crystals and raising the moisture-laden air to the carburetor 
icing temperature range. 

"13. Prior to closed- throttle operation, such as for a 
descent, apply full heat and leave on throughout throttled 
sequence. Periodically, open throttle during extended closed 
throttle operation so that enough engine heat will be produced 
to prevent icing. Be prepared to remove carburetor heat if 
go-around is initiated. 

"14. Return control to "cold" position immediately after 
landing. If carburetor heat should be further required, observe 
ground operation precaution in (4), above." 

The above routing use of carburetor heat, and the awareness 
of possible icing conditions in the induction system, are 
probably not appreciated by any but the more experienced pilots. 
Consequently, this special study by NTSB was produced to improve 
pilot awareness, attention, and/or carefulness to reduce the 
number of accidents of this type, 

Aiycraft Carburetor Icing Studies (Ref. 6) .- The National 
Research Council of Canada conducted an investigation into the 
prevention of carburetor icing in aircraft by the use of 
gasoline-soluble inhibitors. In addition to, or instead of, 
using gasoline additives, the possibility of treating interior 
components of the carburetor on which ice is formed was 
investigated. 

An engine test procedure was developed that (1) produced 
sufficient carburetor ice to affect engine performance, and 
(2) allowed engine performance to be monitored. It was possible 
to select temperature, humidity, and throttle plate settings to 
give the most severe icing conditions. 

The importance of this study is that it established a basis 
for hoping that the best possible solution to the problem of 
carburetor icing can be achieved — that of eliminating the 
problem. In this study the use of a teflon-coated throttle 
plate was studied and found to virtually eliminate any ice 
formation on the plate. The use of ethylene glycol monome thy 1 
ether at 0.10 - 0.15 percent by volume in the gasoline and the 
teflon-coated plate was shown to prevent both carburetor and 
fuel system icing. Coating the shaft at the point where it 
enters the barrel wall may be required to eliminate all ice 
deposits. 



) 



Icing-Protection Requirements for Reciprocating-Engine 
Induction Systems (Ref. 7) .- This study provides an extensive 
and comprehensive discussion of the various types of icing 
associated with the generic term of "carburetoi: icing", and, 
from these investigations, establishes criteria for safe 
operation and for design of new induction systems. 

The opinion is reinforced that "the symptoms associated 
with induction-system icing are not always discernible and 
recognition of icing conditions generally requires considerable 
operational experience." And, "The icing problem remains a 
serious hazard, despite the fact, that the induction systems on 
most licensed aircraft are capable of providing sufficient 
protection in all weather conditions if th^ protection is 
correctly applied and at the proper time" .,'' Further , a study by 
Weick (ref . 8) of light-airplane power plant failures in 1947 is 
cited, in which it is indicated that "34 percent of the forced 
landings were caused by ineffective carburetor heat or by non- 
application or improper application of carburetor heat" . This 
analysis was based on one-seventh of the total 9 2-53 privately 
operated airplane mishaps recorded by the Civil Aeronautics 
Board for the year. 

The methods and techniques for this research are presented, 
including both airborne and ground-based test installations. 
The temperature and the pressure of the air supplied to either 
the airborne or ground-based induction system were carefully 
controlled and measured at the carburetor inlet. Moisture 
content of the air was regulated by steam jets to control 
humidity and by water sprays to provide water in excess of 
saturation. Instrumentation was provided to measure rates of 
air flow, fuel flow, and simulated rain injection, as well as 
carburetor and supercharger pressures, carburetor metering 
pressures, and other variables affected by icing. 

This study also reflects on a possible approach to the 
carburetor icing problem: displaying information to the pilot 
which will allow him to more readily detect and diagnose 
carburetor/induction system icing. It is pointed out that 
numerous instruments have been designed for this purpose, 
including (1) interruption of a light beam focussed on a 
photocell, (2) the blocking of special air passages to upset a 
pressure-differential system, (3) a comparison of pressure drop 
through 'an ice-free air passage and the induction system, and 
(4) the change in capacitance between charged plates due to 
icing. 

The authors conclude that none of these ice detectors is . 
entirely satisfactory because of the difficulty in locating the 
sensing elements in positions in which they will register any 
type of icing. Further, many detectors have been erratic and 



indicate falsely. The authors feel that "a suitable ice detector 
must be an extremely reliable instrument to retain pilot con- 
fidence." In general, the use of ice-warning devices had been 
unsatisfactory and none had been accepted for general use at 
that time (circa 1950). 

Trends in the Future Use of Carburetor- type Aircraft 

While fuel-injected engines retain the danger of induction 
system icing, the danger of. icing in a carburetor is removed. 
High-performance engines using fuel injection are available in 
general aviation. If fuel-injected engines were to eventually 
exclude carburetor designs from future aircraft, then the 
nature of the carburetor/induction system icing problem could 
change. To test this hypothesis, and to collect data on future 
trends in aircraft manufacture, a questionnaire was sent to the 
major airframe manufacturers to determine their future plans. 
Unfortunately, the attempt was fruitless, and no meaningful data 
were collected in regard to this question. Informal communica- 
tion with the four major engine manufacturers (Continental, 
Franklin, Avco-Lycoming, and Pratt & Whitney) indicate a desire 
to increase production of fuel-injection- type engines, but the 
situation is not expected to change in the near future. 
Examination of existing aircraft designs and lists of available 
engine types for future design indicates that the carburetor -type 
aircraft will predominate in the near future. 



THE COMPUTER DATA BASE 

The computer data base was formed using data provided on 
ffiagnetic tapes by the Safety Analysis Division, Bureau of 
Aviation Safety, National Transportation Safety Board, Department 
ot Transportation. Data were requested on all accidents involv- 
ing engine malfunction/failure in general aviation from 1968 
JvIf^Tnil III '^^^ magnetic tapes provided were compatible with 

zne IBM 360 computer system on which they were produced, and 
therefore required conversion to run on the CDC Cyber 70 computer 
system at the NASA Flight Research Center. 

4-K 7^^ information potentially contained for each accident in 
the data base is shown in Figure 1 in the form of the analysis 
Sheet which is completed by the accident investigator and 
subsequently key-punched for computer entry. While only 55 of 
over 350 data items are mandatory for each accident,, the total 
amount of data is large and much is unnecessary for this study, 
consequently, to increase the efficiency of analysis, the data 
base was reduced in content and number of accidents. 

4-u ^ff^t, the content of each accident record was reduced to 
those Items of information relevant to the current study. The 
41 Items selected are presented in Table I. The codes used for 
those items which are not entered directly as a numerical 
quantity are given in Table 11. 

Secondly, only accident records for the years 1969 through 
19 73 were retained (five years). Accident records for the few 
accidents which occurred outside the continental United States 
were also discarded, as most of these contained very little 
information. Also, only accident records for fixed-wing 
aircraft with reciprocating engines were retained. It was 
believed that the problems of rotary-wing aircraft were 
inconsistent with the problems of fixed-wing aircraft. Only 
reciprocating- engine- type aircraft accidents were retained, since 
the term carburetor ice" does not apply to turbine engines. 

Lastly, accident records which involved fuel exhaustion were 
removed from the data base since (1) a large number of engine- 
malfunction/f allure accidents are due simply to running out of 
gasoline (approximately 170 accidents per year) , and (2) such 
accidents are irrelevant to a study of carburetor icing. The 
final data base contained 3555 records of the original 5930 ' 
records. 

S ummary - The final data base of 3555 records is character- 
ized by the following conditions: 



^ARD I^O. 00 IDEHIIFriNG MfOnMATION 






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Figure 1. (page 1 of 3) 



10 



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Figure 1. (page 2 of 3) 



11 



SPEClAt DATA 



fAKD NO. ZJ AEIIAL tifWlKAltOt* 



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Figure 1- (page 3 of 3) 



Aircraft Analysis Sheet (from Manual of Code Classifications, 
Aircraft Accidents and Incidents, National Transportation 
Safety Board, Department of Transportation, Washington, D.C., 
June 1970, Third Edition). 



12 



u> 











TABLE I 






■s 




ACCIDENT FILE CONTENTS 




NO, 


LABEL 


CHAR 


FORMAT 


NAME 




001 


FILENO, 


5 


F5.0 


FILE NUMBER 




002 


MONTH 


2 


F2.0 


MONTH OF ACCIDENT 


^ 


003 


YEAR 


2 


F2.0 


YEAR " " 




004 


HOUR 


2 


F2.0 


HOUR 




005 


LOC 


2 


F2.0 


LOCATION OF ACCIDENT 


^ 


006 


ACMAKE 


3 


F3,0 


AIRCRAFT MAKE 




007 


ACMOD. 


2 


F2.0 


AIRCRAFT MODEL 




008 


ENGMAKE 


2 


F2.0 


ENGINE MAKE 




009 


ENGMOD. 


2 


F2.0 


ENGINE MODEL 




010 


NO.ENG. 


1 


Fl.O 


NUMBER OP ENGINES 




Oil 


DAMAGE 


1 


Al 


AIRCRAFT DAMAGE 




012 


KIND 


l' 


A1,1K 


KIND OP PLYING 




013 


PHASEl 


2 


A1,1X 


PHASE OF 1ST ACCIDENT {TYPE = 


= ENG.MALF./FAIL.) 


014 


TYPE2 


1 


Al 


TYPE OF SECOND ACCIDENT 




015 


PHASE2 


2 


A1,1X 


PHASE OF SECOND ACCIDENT 




016 


ALT. 


5 


F4.0,1X 


ALTITUDE OF OCCURRENCE 




017 


PLAWD 


1 


Al 


FORCED LANDING 




018/019 


CIRCUM1/C1RCUM2 


2 


2A1 . 


EMERGENCY CIRCUMSTANCES 




020 


TERRAIN 


1 


Al 


TERRAIN 




021 


TOTHRS 


5 


F4.0,1X 


TOTAL HOURS - PILOT 




022 


PLT.CNTRL 


1 


Al 


PILOT AT CONTROL 




023 


INJURY 


1 


Al. 


INJURY INDEX 





TABLE I 



(Page 2 of 2) 



NO. 


LABEL C 


024 


TOTABRD 


025 


PARTPWR 


026 


TOTPWR 


027 


SKYCOND 


028/029 


PRECIP1/PRECIP2 


030 


DE^^^TSPRD 


031 


TEMP. 


032 


INDAIR 


033 


UNDET 


034 


FUELHTR 


035 


CARBICE 


036 


FUELDEICE 


037 


CONDCI 



038 


AIEQUIP 


039 


ICEFUEL 


040 


ICECARB 


041 


ICE.IND 



AR 


FORMAT 


3 


F3.0 


I 


Al 


1 


Al. 


1 


Al 


2 


2A1 


2 


F2.0 


3 


F3.0 


1 


IX 


1 


Fl.O 


1 


Fl.O 


1 


Fl.O 


1 


Fl.O 


1 


Fl.O 


1 


Fl.O 



NAME 



1 Fl.O 

1 Fl.O 

1 Fl.O 

1 Fl.O 



TOTAL ABOARD 
PARTIAL POWER LOSS 
COMPLETE POWER LOSS 
SKY CONDITIONS 
PRECIPITATION (2 CODES) 
DEW POINT SPREAD 
TEMPERATURE 
- NOT APPLICABLE - 
PROBABLE CAUSE/FACTOR: 



INDUCTION AIR/PREHEAT 
CONTROLS, MISUSE 

UNDETERMINED CAUSE 

FUEL HEATER 

CARB. DE-ICING SYSTEM 

FUEL DE-ICING SYSTEM 

CONDITIONS CONDUCIVE 
TO CARB. ICE 

IMPROPER USE: ANTI- 
ICE, DE-ICE 

ICE IN FUEL 

ICE IN CARBURETOR 

ICE IN INDUCTION 
SYSTEM 



TABLE II 
ACCIDENT FILE CODES 



AIRCRAFT DAMAGE 

D - DESTROYED 

S - SUBSTANTIAL 

M - MINOR 

N - NONE 

a - UNKNOWN/NOT REPORTED 

KIND OF FLYING 

A - INSTRUCTIONAL 
B - NONCOMMERCIAL 
C - COMMERCIAL 
D - MISCELLANEOUS 

TYPE OF ACCIDENT 

A GRND/WATER LOOP/SWERVE 

B DRAGGED WING, POD, FLOAT 

C WHEELS -UP 

D WHEELS DWN LNDG - WATER 

E GEAR COLLAPSED 

F GEAR RETRACTED 

G HARD LANDING 

H NOSE OVER/DOPWf 

I ROLL OVER 

J OVERSHOT 

K UNDERSHOOT 

L COLLISION WITH A/C 

M COLLISION - GRND/WATER 

PHASE 



N 

P 

Q 
R 
S 
T 



COLLISION - 



BIRD STRIKE 
STALL 

FIRE OR EXPLOS 
AIRFRAME - FAIL 
ENGINE - TEARAWAY 
U ENG MALF/FAIL 

V PROP/ROTOR FAIL 

W PROP/ROTOR ACC - PERS 
X JET INTAKE EXH - PERS 

Y PRpP/JET/ROTOR BLAST 
Z TURBULENCE 



A - STATIC 

B - TAXI- 

C - TAKEOFF 

D - INFLIGHT 

E - LANDING 

B - UNKNOWN/NOT REPORTED 



15 



TABLE II 



(Page 2 of 4) 



TERRAIN 



A - MOUNTAINOUS 

B - HILLY 

C - ROLLING 

D - LEVEL, FLAT 

E - FROZEN 

F - ROCKY 

G - SANDY 



H 


- 


DENSE WITH TREES 


I 


- 


CITY AREA 




J 


- 


PLOWED 




K 


- 


WATER 




Y 


- 


OTHER 




8 


— 


UNKNOWN/NOT 


REPORTED 



PILOT AT CONTROLS 

A - PILOT IN COMMAND 
B - COPILOT 
C - DUAL STUDENT 
D - CHECK PILOT 



E - NONE 

F - BOTH PILOTS 

Y - OTHER 

g - UNKNOWN/NOT REPORTED 



INJURY INDEX 

F - FATAL 

G - SERIOUS 

M - MINOR 

N - NONE 

3 - UNKNOWN/NOT REPORTED 

PARTIAL/COMPLETE POWER LOSS 

A - ONE ENGINE 

B - TWO ENGINES 

C - THREE ENGINES 

D - FOUR ENGINES 

a - UNKNOWN/NOT REPORTED 



16 



TABLE II - . (Page 3 of 4) 

FORCED LANDING - PRECAUTIONARY LANDING 
CODE 

A FORCED LANDING ON AIRPORT/SEAPLANE BASE HELIPORT 

B FORCED LANDING OFF AIRPORT ON LAND 

C FORCED LANDING OFF AIRPORT ON WATER (DITCHING) 

D PRECAUTIONARY LANDING ON AIRPORT 

E PRECAUTIONARY LANDING OFF AIRPORT 

a UNKNOWN/NOT REPORTED 

EMERGENCY CIRCUMSTANCES 

CODE 

A LOW ON FUEL 

B SMOKE IN COCKPIT 

X PASSENGER DISTURBANCE 

D FALSE FIRE WARNING 

E LATERAL CONTROL PROBLEM 

F PITCH CONTROL PROBLEM 

G DIRECTIONAL CONTROL PROBLEM 

H ADVERSE/UNFAVORABLE WEATHER 

I APPROACHING DARKNESS 

J SUSPECTED OR KNOWN AIRCRAFT DAMAGE 

K SUSPECTED MECHANICAL DISCREPANCY 

L DOOR/PANEL OPEN 

M AIRFRAME BUFFET 

N UNUSUAL NOISE 

P PHYSICAL CONDITION OF PASSENGER 

Q FUMES IN CABIN 

' R PROP/ENGINE VIBRATION ' 

% UNKNOWN/NOT REPORTED 



17 



TABLE II 



(Page 4 of 4) 



SKY CONDITION (Local weather at time of 'accident) 
CODE 



A 


CLEAR 




B 


SCATTERED (Above 1,000 feet) 


C 


SCATTERED (Below 1,000 feet) 


D 


BROKEN 




E 


BROKEN/LOWER SCATTERED 




F 


OVERCAST 




G 


OVERCAST/LOWER SCATTERED 




H 


PARTIAL OBSCURATION 




I 


OBSCURATION 




% 


UNKNOWN/NOT REPORTED 




PRECIPITATION OF ACCIDENT SITE 




CODE 






A 


HAIL (A) H 


SNOW SHOWERS (SW) 


B 


SLEET (E) I 


THUNDERSTORM (T) 


C 


DRIZZLE (L) J 


FREEZING DRIZZLE (ZL) 


D 


RAIN (R) K 


FREEZING RAIN (ZR) 


E 


RAIN SHOWERS (RW) L 


NONE 


F 


SNOW (S) % 


UNKNOWN/NOT REPORTED 


G 


SNOW GRAINS (SG)/SNOW 





PELLETS (SP) 



18 



(1) Included accidents for the years 1969, 1970, 1971, 
1972, and 1973. 

(2) Included only engine malfunction/failure accidents. 

(3) Included only fixed-wing, reciprocating engine accidents 
which occurred within the continental U.S. 

(4) Excluded accidents due to fuel exhaustion. 



19 



DATA ANALYSIS 



The analyses performed on the computer data base consisted 
primarily of crosstabulations of variables selected from the 41 
variables included in the data base, and subsequent statistical 
analysis of the resulting comparisons. The primary tool used for 
this purpose was the Statistical Package for the Social Sciences 
(SPSS) (ref. 9). SPSS is an integrated package of programs for 
statistical analysis, data base management and file manipulation. 
Using these programs, selected variables may be transformed or 
receded, new variables created through computations based on 
existing variables, and statistical procedures executed. This 
package of programs provides efficient analysis of a given data 
base with minimal programming effort. In addition, special- 
purpose statistical programs were used, and hand calculations 
performed in selected cases. The principal results of all 
analyses are summarized in the following sections. 

Examination of Aircraft and Engine Makes Under Conditions 
Conducive to Carburetor/Induction System Icing 

As part of an initial examination of the data base to 
determine data base variables important to the study of 
carburetor/induction system icing, aircraft makes and engine 
makes were compared under conditions conducive and not conducive 
to carburetor/induction system icing. One of the cause/factor 
variables included in the data base was an indication of whether 
the accident investigator considered the conditions conducive to 
carburetor icing. For the present it is assumed that the 
conditions are not conducive to icing if the accident investiga- 
tor did not indicate such conditions existed; the validity of 
this assumption will be discussed later. 

The results comparing aircraft makes are presented in 
Table III, while the results for engine makes are given in 
Table IV. As no practical or statistical significance exists 
between the major manufacturers of either airframes or engines, 
no distinction between makes was carried through the subsequent 
analyses. These results contradict informal reports that 
specific manufacturers produced designs more or less prone to 
carburetor icing difficulties. 

Probable Cause/Factor by Year 

Selected causes/factors, indicated by the accident 
investigator, are tabulated in Table V by year. The causes/ 
factors selected are: (1) induction air, preheat cockpit 
controls, (2) power plant failure for undetermined reasons, 
(3) anti- icing, de-icing, and carburetor de-icing systems. 



20 



TABLE III 
COMPARISON OF AIRCRAFT MAKE SENSITIVITY TO 
CARBURETOR/INDUCTION SYSTKM ICING CONDITIONS 





NO. OF ACCIDENTS THAT OCCURRED UNDER - 


PERCENT OF TOTAL 
ACCIDENTS THAT 
OCCURRED UNDER 
CONDITIONS CON- 
DUCIVE TO ICING 


MRCRAPT 
MAKE 


CONDITIONS 
CONDUCIVE 
TO ICING* 


OTHER 
CONDITIONS 


A 

B 
C 
D 

E 
F 
G- 
H 
Other 


13 

17 

126 

7 

11 

97 

5 

8 

32 


88 

298 

989 

43 

84 

935 

39 

73 

690 


12.9 

5.4 

11.3 

14.0 

11.6 

9.4 

11.4 

9.9 

4.4 



*As defined by the accident investigator. 



TABLE IV 

COMPARISON OF ENGINE MAKE SENSITIVITY TO 

CARBURETOR/ INDUCTION SYSTEM ICING CONDITIONS 





NO. 


OF ACCIDENTS THAT OCCURRED UNDER - 


PERCENT OP TOTAL 
ACCIDENTS THAT 














CONDITIONS 




OCCURRED UNDER 


MRCRAFT 




CONDUCIVE ' 


OTHER 


CONDITIONS CON- 


MAKE 




TO ICING* 


CONDITIONS 


DUCIVE TO ICING 


A 




6 


86 


6.5 


B 




144 


1376 


9.5 


C 




153 


1437 


9.6 


D 




7 


246 


2.8 


E 




3 


37 


7.5 


Other 




3 


57 


5.0 


Total 


316 


3239 


8.9 


*As def 


ined 


by the accident 


investiqator. 





21 



(4) weather conditions conducive to carburetor/induction system 
icing, (5) improper use, or failure to use, anti-icing de-icing 
equipment, (6) ice in fuel, (7) ice in carburetor, and (8) ice in 
the induction system. More than one (usually five or more) 
cause/factor may be indicated by the investigator for a given 
accident. 

A number of results accrue from examination of Table V: 

(1) There is a statistically significant trend for 
accidents within the selected data base to decrease over the five 
year period examined (P<.05). 

(2) Relatively few accidents involve identifiable failures 
of the controls and systems associated with carburetor/induction 
system icing. 

(3) There are a large number of powerplant failures for 
undetermined reasons . 

(4) There are approximately 65 accidents per year, on the 
average, for which evidence of ice is found in the fuel or 
induction system. 

(5) The number of cases for which ice in the carburetor is 
cited as a probable cause/factor often exceeds the number of 
cases for which it is cited that conditions are conducive to 
carburetor/induction system icing. Apparently the investigators 
may omit indicating that the conditions are conducive to 
carburetor icing when other, more specific, evidence exists. 

Composite Carburetor/Induction Ice Cause/Factor 

It was noted in Table V that a number of probable causes/ 
factors may be indicated by the accident investigator either 
singly or in combination. A composite cause/factor was formed to 
avoid this confusion. The composite indicator was defined as a 
probable Carburetor/Induction Ice Cause/Factor _if any of the 
following cause/factors were noted, by the accident investigator: 
(1) induction air/preheat controls, or (2) the carburetor 
de-icing system, or (3) improper use of anti-ice/de-ice 
equipment, or (4)~Tce in the carburetor, or (5) ice in the 
induction system. 

The composite carburetor/induction ice cause/factor (C/I Ice 
Cause/Factor) is tabulated in Table VI over the five year period 
included in this study. While the number of carburetor/induction 
system ice accidents may appear to decrease over the five year 
period (as did the total number of accidents in the data base) 
the number of accidents in years 1969 and 1970 are not statis-^ 
tically significant from the number in years 1972 and 1973 
(P<.33). Based on the composite index, the number of such 
accidents averages 7 4 per year. 



22 



TABLE V 
ACCIDENTS WITH ICING AS PROBABLE CAUSE/FACTOR BY YEAR 



PROBABLE CAUSE OF ACCIDENT 



Misuse of induction air, 
preheat cockpit controls 

Powerplant failure - 
reason undetermined 

Carburetor de-icing system 
malfunction 

Conditions conducive to 
carburetor/induction 
system icing 

Improper operation - 
failed to use anti- 
icing/de-icing equipment 

Ice in fuel 

Ice in carburetor 

Ice in induction system 



Total 



YEAR 



1969 1970 1971 1972 1973 



Number of Accidents 



5' 
206 
1 

79 

85 
4 

82 
2 




228 

7 

47 

58 

3 

59 

1. 



1 
203 

1 

70 

64 

7 

70 

3 



1 

158 

2 

65 

69 

8 

63 

1 



1 

173 
3 

55 

50 
4 

52 
4 



Total 



760 



730 



730 



661 674 



8 

968 

14 

316 

326 
26 

326 
11 



3555 



TABLE VI 
COMPOSITE C/I ICE CAUSE BY YEAR 







YEAR 






1969 


1970 


1971 


1972 


1973 








Number 


of Accidents 




Total 


WAS C/I ICE 

CAUSE OF 
ACCIDENT?* 


No 
Yes 


666 

94 


659 
71 


658 
72 


587 
.74 


615 
59 


3185 

370 


Total 


760 


730 


730 . 


661 


674 


3555 



♦Composite C/I Cause: Misuse of induction air/preheat controls, 
carburetor de-icing system malfunction, improper use of ' 
de-icing system, ice in carburetor, or ice in induction 
system. 



23 



Also, as previously noted in Table V, Table VII indicates 
that a large nvunber of carburetor/induction system icing acci- 
dents are not noted by many investigators as occurring when the 
conditions are conducive to icing; consequently/ a better 
indicator of the conditions appropriate to such accidents must be 
constructed. 



Pilot Experience as a Factor in Carburetor/Induction 

System Icing Accidents 

Pilot experience, in terms of total number of flying hours, 
is cross-tabulated with the composite C/I Ice Cause/Factor in 
Table VIII. The percent of carburetor/induction ice accidents 
decreases with the total number of pilot hours; however, the 
decrease is statistically significant (P<.05) only for pilot 
hours greater than 1,000 hours. Stated in other terms, only 
pilots with over 1,000 hours experience seem to possess the skill 
and knowledge to perform significantly better in avoiding 
carburetor/induction system icing than pilots with less than 1,000 
hours total flying time. About 68% of the carburetor/induction 
icing accidents involve pilots with less than 1,000 total hours. 



-TABLE VII 
COMPOSITE C/I ICE CAUSE BY INVESTIGATOR'S JUDGMENT 
THAT CONDITIONS ARE CONDUCIVE TO C/I ICE 





Were Conditions Conducive to C/I 
Ice? (Investigator's Judgment) 






No 


Yes 






Number of Accidents 


Total 


WAS C/I ICE No 
CAUSE OF 
ACCIDENT?* Yes 


3173 
66 


12 

304 


3185 
370 


Total 


3239 


316 


3555 



♦Composite C/I Ice Cause; Misuse of induction air/preheat 
controls, carburetor de-icing system malfunction, improper 
use of de-icing system, ice in carburetor, or ice in induction 
system. 



24 



TABLE VIII 
EFFECT OF PILOT EXPERIENCE ON C/I ICE AS 
ACCIDENT CAUSE/FACTOR 



Pilot 
Experience 

Hours 


Total 
Accidents 

for 
Experience 
Range 


Cause of 


Accident 


C/I Ice 


Other 


Niomber of 
Accidents 


Percent of 

Total 
Accidents 


Number of 
Accidents 


Percent oi 

Total 
Accidents 


10 to 100 


477 


70 


14.7 


407 


^ 85.3 


100 to 500 


1000 


125 


12.5 


875 


87.5 


500 to lOOC 


519 


55 


10.6 


464 


89.4 


>1000 


. 1559 


120 


7.7 


1439 


92.3 


>10 


3555 


370 


10.4 


1559 


89.6 



Composite Indicator to C/I Ice Conditions 

As previously noted, there is reason to believe that condi- 
tions are appropriate for carburetor/induction system icing more 
often than indicated by the accident investigators. A composite 
indicator (Conditions Appropriate) was constructed based on the 
following: (1) conditions were considered appropriate by the 
accident investigator, or (2) there was an indication of 
precipitation at the accident site, or (3) the temperature and 
dew point were within the profile presented in Figure 2. The 
temperature/dew point profile of Figure 2 is generated by the 
Department of Transportation (taken from Gardner and Moon, 1971); 
the solid line profile was that used in the computer implementa- 
tion and is to be considered an approximate indicator of the 
existence of conditions appropriate for carburetor icing. 
Unfortunately, the existence of temperature/dew point data was 
rare in the NTSB data base. 

The number of accidents occurring when conditions are 
appropriate for icing according to the composite indicator are 
presented in Table IX. Approximately 18% of the accidents in the 
reduced data base used in this study occurred when the conditions 
were conducive to carburetor/induction system icing. Based on 
these data, one may estimate the maximum number of, accidents due 
to this cause at about 128 per year. 

It is interesting to note in Table X that about 13% of the 
accidents for which the cause is undetermined occur when the 
conditions are appropriate for carburetor/induction icing. 



25 



ICIN6 PROBABILITY CUWV^S 

i 1 1 L 







Figure 2. DOT Icing Probability Curves. (The heavy solid 
line is the profile implemented in the computer 
analysis . } 



26 



TABLE IX 
NUMBER AND PERCENTAGE OP ACCIDENTS THAT OCCURRED UNDER 
CONDITIONS APPROPRIATE FOR C/I ICE* BY YEAR 



(YGar(s) 


Total 

Accidents 

for 

Year(s) 


Were 


! Conditions Appropriate for 
C/I Ice? 


Yes 


No 


Number of 
Accidents 


Percent of 

Total 
Accidents 


Number of 
Accidents 


Percent of 

Total 
Accidents 


1969 


760 


126 


16.6 


634 


83.4 


1970 


730 


134 


18,4 


596 


81,6 


1971 


730 


121 


16.6 


609 


83.4 


1972 


661 


144 


21.8 


517 


78.2 


1973 


674 


113 


16.8 


561 


83.2 


1969 to 












1973 


3555 


638 


18.0 


2917 


82.0 



*Conditions appropriate for C/I ice: Indicated by accident 
investigator, temperature/dew point within profile, or 
precipitation at accident site. 



27 



TABLE X 
NUMBER AND PERCENTAGE OF ACCIDENTS THAT OCCURRED UNDER 
CONDITIONS APPROPRIATE FOR C/I ICING* BY 
CAUSE DETERMINATION 





Number of 






Accidents 


Percent 


Number of accidents for which cause was 






determined 


2587 


100 


Number of these accidents for which 






conditions were appropriate for 






C/X icing 


511 


14.4 


Number of these accidents for which 






conditions were not appropriate for 






C/I icing 


2076 


85.6 


Number of accidents for which cause 






was not determined 


968 


100 


Number of these accidents for which 






condition was appropriate for 






C/I icing 


127 


13.1 


Number of these accidents for which 






conditions were not appropriate for 






C/I icing 


841 


86.9 



*Conditions appropriate for C/I ice: Indicated by accident 
investigator, temperature/dew point within profile, or 
precipitation at accident site. 



28 



Analysis of Accidents Occurring Under Conditions 
Conducive to C/I Ice 

Table XI presents an analysis of accidents occurring during 
conditions conducive to carburetor/induction icing. At least 
319 out of a total of 638 accidents are attributable to 
carburetor/induction icing, or about 50%, Another 127 accidents 
occurring under appropriate conditions are due to undetermined - 
reasons. Thus a total of 446 accidents might be due to such 
icmg causes, or an estimated maximum of 70%; it might be 
expected that about 50% of these are due to carburetor/induction 
'■^^"?:. Consequent ly, it may be estimated that, when the 
conditions are app ropriate, between 50% and 70% arf^ dn^ tn 
carburetor/ induction system icincr" ~ 

Severity of Carburetor/Induction System Icing Accidents 

It should be clear that, while a number of accidents may 
have^involved carburetor/induction system icing as a cause/factor, 
the flights eventually terminated with a forced landing and 
possible collision with a ground object. As is the case with 
other engine/malfunction failure accidents, approximately 80% 
resulted m substantial damage to the aircraft, and the 
remainder resulted in destroyed aircraft (see Table XII) 
However, as indicated in Table XIII, 230 of 370 accidents (62%) 
resulted m no personal injury and only 14% resulted in serious or 
±atal injury. The distribution of occupants involved in C/I icina 
accidents is presented in Table XIV; the net result is that 
between 720 (for C/I ice conditions) and 977 (including possible 
accidents with undetermined reasons) persons have been involved 
""I? such accidents over the five year period considered in this 
study {between 144 and 195 per year) . 



29 



TABLE XI 
ACCIDENT CAUSE/FACTOR BY COMPOSITE INDICATOR 
OP C/I CONDITIONS 



■ 


NUMBER OF ACCIDENTS 


TOTAL 


CONDITIONS APPROPRIATE 


ACCIDENT CAUSE/FACTOR 


FOR C/I ICE? 


C/I ICE 


NO C/I ICE 


UNDETERMINED 




Yes 


319 


192 


127 


638 


No 


51 


2025 


841 


2917 


Total 


370 


2217 


968 


3555 



TABLE XII 
AIRCRAFT DAMAGE DUE TO C/l ICING 





NUMBER OF ACCIDENTS 


TOTAI 






AIRCRAFT DAMAGE 





MINOR 


SUBSTANTIAL 


DESTROYED 


NOT REPORTED 


C/I ice 


1 


309 


60 





370 


Dndetermined, 












with C/I ice 












conditions 





80 


47 





127 


Other 





2425 


632 


1 


3058 


Total 


1 


2814 


739 


1 


3555 



k 



30 



TABLE XIII 
NUMBER OF ACCIDENTS INVOLVING INJURY DUE TO 

C/I ICING 





NUMBER OF ACCIDENTS 


TOTAL 


Accident Cause 


INJURY INDEX 


None 


Minor Serious 


Fatal 


UnJcn. 


C/I Ice 


230 


S7 34 


19 





370 


Undetermined, 












with C/I Ice 












Conditions 


51 


38 22 


16 





12 7 


Other 


1803 


652 347 


255 


1 


3058 


Total 


2084 


777 403 


290 


1 


3555 



TABLE XIV 
NUMBER OF OCCUPANTS INVOLVED IN C/I ICING ACCIDENTS 



1 . 1 


NUMBER OF ACCIDENTS 


TOTAL 


Accident Cause 


NUMBER ABOARD 


SIR* 1 2 3 4 5 6 >6 


C/I Ice 

Undetermined, 
with C/I Ice 
Conditions 

Other 

Total 


143 154 38 27 4 2 2 

51 51 13 7 2 3 

1 1350 1030 289 281 59 23 25 
1 1544 1235 340 315 65 25 30 


370 

127 
3058 
3555 



Accident Cause 


TOTAL PERSONS INVOLVED 
IN ACCIDENTS 


C/I Ice 


720 


Undetermined, 
with C/I Ice 
Conditions 


257 


Other 


6105 


Total 


7082 



*Not reported 



31 



DISCUSSION 



The Difficulty of Diagnosing Carburetor/Induction System Icing 

A fundamental problem affecting the study of the severity 
of carburetor/induction system icing in general aviation acci- 
dents is the inherent difficulty of diagnosing carburetor/ 
induction icing. If ice has caused an accident, the ice will 
melt and the resulting water may evaporate; so, unless the 
investigator is astute and has an initial strong suspicion of 
carburetor icing, the cause/factor associated with the accident 
may not be properly identified. 

Since this type of accident may be so easily mis-diagnosed, 
it is believed that the results of this study underestimate the 
magnitude of the problem. A broad definition of "carburetor 
icing" has been taken, and some reasonable extrapolations to 
include the probable number of "undetermined" accidents which 
may be carburetor icing have been made in this study. Neverthe- 
less, it is believed that the estimates made in this study are 
quite conservative and that a number of additional accidents 
could have included carburetor/induction icing as a probable 
cause/factor. Further, the findings of this study concentrate on 
the accident data available; a large number of forced landings 
occur each year which are not included in the accident data base. 

A Context for Data Interpretation 

Over a five-year period analyzed in this study, an average 
of 65 carburetor ice accidents per year are identified by the 
accident investigators, and one may extend this estimate to 
approximately 90 probable carburetor/induction icing accidents 
per year. That is, there are about that many general aviation 
accidents per year which involve carburetor/induction icing; 
however, a niimber of other probable cause/factors are generally 
also involved. 

Compared to the approximately five thousand total general 
aviation accidents per year, and the approximately one thousand 
engine malfunction/failure accidents per year, the less than one 
hundred carburetor/induction accidents per year may seem small. 
On the other hand, it must be recognized that carburetor/ 
induction icing can only occur when the weather conditions are 
appropriate. Viewed in this context, the picture changes, for 
when the weather conditions are conducive to icing, 50 percent 
to 70 percent of all engine malfunction/failure type accidents 
(which are not due to fuel exhaustion) include carburetor/ 
induction icing as a probable cause/factor. That is, 
if the weather conditions are conducive, if the engine 



32 



malfunctions or fails with fuel still available to the engine, 
then carburetor/induction icing is involved more than half of 
the time . 

Further, carburetor/induction system icing is a widespread 
problem. Such icing can occur anywhere, even at temperatures 
over BlO.g'^K (lOO^F), and is equally likely throughout the 
year. Pilots with long experience become involved in such 
accidents, although more than two-thirds of carburetor/induction 
system icing accidents involve pilots with less than 1000 hours 
of total flying time. Apparently, any pilot, wherever he/she 
may be flying, at any time of the year, may encounter carburetor/ 
induction system icing. 

Severity of Carburetor/Induction System Icing 

Of course, as with many other accident causes/factors, the 
injurious and damaging aspects of an accident for which 
carburetor/induction icing is a probable cause/factor are 
associated with the terminating forced or precautionary landing. 
One hundred, and forty-four persons per year, on the average, are 
involved in carburetor icing accidents, about 4 of these 
accidents involve fatalities, about 7 per year involve serious 
injury, and almost all of these accidents involve substantial 
damage or destruction of the aircraft. Consequently, carburetor/ 
induction system icing accidents are just about as serious as 
any other accident involving a forced landing or precautionary 
landing. Other types of accidents occur more often: for 
example, accidents involving fuel exhaustion occur about twice 
as often as those involving carburetor/induction icing; but 
when weather conditions are conducive, carburetor/induction 
system icing is the most common cause for engine malfunction/ 
failure. 



33 



CONCLUSIONS 



Carburetor/induction system icing is a serious 
threat existing in general aviation operations. Given that 
fuel is available, under the proper weather conditions, which 
occur frequently and regularly throughout the year, carburetor/ 
induction icing is the most common cause of engine malfunction/ 
failure in general aviation. Overall, this cause amounts for 
about 10 percent of engine malfunction/failures, making it a 
major cause for concern. Carburetor/induction icing is a 
particular threat to the pilot with less than 1000 hours of 
total flying time. 

While engineering analysis was not included in the current 
study, the review of the literature indicates a strong 
possibility that this problem may be solvable. The problem 
may yield to (1) efforts to eliminate icing through 
carburetor/induction system design and fuel additives, and/or 
(2) efforts to either increase pilot awareness through improved 
display of icing information or automation of the pilots 
function for the control of icing countermeasures . The payoff 
would be high: 65-90 accidents per year could be eliminated 
along with the attendant death, injury and damage. 

The analysis of accident data is severely limited for 
investigation of a problem as insidious and difficult to detect 
as carburetor/induction icing. Further investigation of the 
problem is recommended using flight or ground-based facilities 
permitting the control of induction icing. Under controlled 
conditions, the ability of the pilot to detect, recognize and 
take proper action can be accurately assessed. It is suspected 
that given such data it will be found that the findings of this 
study, which label carburetor/induction icing as a major problem, 
have underestimated the magnitude and frequency of carburetor/ 
induction system icing occurrences. 



34 



REFERENCES 



1. Transportation Safety Institute, Basic aircraft accident 

investigation procedures and techniques . Oklahoma City, 
Oklahoma: Department of Transportation, October 19 71. 

2. National Transportation Safety Board, Manual of code 

classifications: Aircraf-t accidents and incidents . 
(3rd Ed.) Washington, D.C.; Department of Transportation, 
June 1970. 

3. National Transportation Safety Board, Manual of aircraft 

and engine code classifications . Washington, D.C. : 
Department of Transportation, June 1973. 

4. Bureau of Aviation Safety, User instructions: ADP programs . 

Automated aircraft accident and incident information 
system . Washington, D.C: National Transportation Safety 
Board, November 1971. 

5. Bureau of Aviation Safety, Special Study: Carburetor ice in 

general aviation . Washington, D.C: National Transporta- 
tion Safety Board, NTSB-AAS-72-1, January 1972. 

6. Gardner, L. and Moon, G. Aircraft carburetor icing studies . 

Ottawa, Canada: National Research Council of Canada, 
Fuels and Lubricants Division, 1971. 

7. Coles, W.D-, Rollin, V.G. and Mulholland, D.R. Icing - 

protection requirements for reciprocating-engine induction 
systems . Washington, D.C: U.S. Government Printing 
Office, 36th Report of the National Advisory Committee for 
Aeronautics, Technical Report 9 82, 1950. 

8. Weick, F.E. Powerplant failures--causes and cures. Aviation 

Week , 7 March 1949, 50(10), 21-25. 

9. Nie, N.H., Bent, D.H., and Hull, C.H. Statistical package 

for the social sciences (SPSS) . New York: McGraw-Hill, 
1970. 



35 



SELECTED BIBLIOGRAPHY 

ACCIDENT INVESTIGATION METHODS 

A method of determining the danger of airplane icing . 

Charlottesville, Va.: Army Foreign Science and Technology 
Center. FSTC-HT-23-1818-73, 16 July 1974 (AD 781 221/7GA") 

Association for the Aid of Crippled Children, Behavioral 
Approaches to Accident Research . New York, 1966. 

Dresser, Robert F. New Approach to Air Safety Statistics . 
Federal Aviation Agency, Washington, D.C. 

Haddon, W. , Jr., Sauchman, E.A., and Klein, D. Accident Research : 
Methods and Approaches . Harper & Row Publishers, New York, 
1964. 

Johnson, W.M. "Aircraft accident investigation in Canada", 
Canadian Aeronautics and Space Journal , 15 , Pp 113-116. 

AVIATION ACCIDENT INVESTIGATION 

Bureau of Aviation Safety. A Preliminary Analysis of Aircraft 
Accident Data; U.S. Civil Aviation , 1970. National 
Transportation Safety Board, Washington, D.C, NTSB-APA-71-1, 
1970. 

Gough, Melvin N. Study of Current Information Pertinent to 

Aviation Accidents (or Incidents) , Final Report - Contract 
NAS4-866, October, 1966, PR-1909-16 - 1/251 PPC 61, NASA 
Flight Research Center, Edwards, California. 

Sparrow, S.W. Airplane Crashes: Engine Troubles - A Possible 
Explanation . NACA TN55, 1921. 

AVIATION SAFETY 

Hoekstra, H.C., Huange, S.D. Safety in General Aviation . Flight 
Safety Foundation, Inc., Arlington, VA, 1971. 

CARBURETOR ICING REPORTS 

Aircraft Ice Protection , FAA Advisory Circular No. 20-73, April 
21, 1971. 

Barnard, D.P. and Scott, E.H. Weather or Stall-Carburetor Icing 
Control . The Standard Oil'To^^ (Ohio), SAE Annual Meeting, 
January 1959. 



36 



Carburetor Icing , Ethyl Technical Note, Ethyl Corp. 

Carburetor Icing Weather Data for the United States , En jay 
Chemical Co. 

Gardner, L. and Moon, G. Aircraft Carburetor Icing Studies . 

National Research Council of Canada; Fuels and Lubricants 
Division, Ottawa (Ontario), 19 71. 

"Look out for carburetor ice". FAA Aviation News , August 1967. 

Perkins, P.J., Lewis, W. , and Mulholland, D.R. Statistical Study 
of Aircraft Icing Probabilities at the 700- an d 500- 
Millibar Levels over Ocean Areas in the Northern Hemisphere . 
National Advisory Committee for Aeronautics Technical Note 
3984, Lewis Flight Propulsion Laboratory, Cleveland,. Ohio, 
May 1957. 

Richter, K. "Carburetor heat: how to use it". Carburetor 

Icing - A Problem ? The Standard Oil Co. (Ohio), SAE Paper 
448b, January 1962. 

Use of Carburetor Heat Control . Lycoming Division Service 
Instruction No. 1148A, December 8, 1967. 

GENERAL AVIATION REFERENCES 

Gillespie, A.C. (ed.) 19 74 Aviation Almanac , National Aviation 
Consumer Association, New Jersey, 1973. 

World Aviation Directory . Public Transportation and Travel 

Division, Ziff-Davis Publishing Co., Washington, D.C., 1974. 

DeMuth, T.P., Jackson, H.R. and Test, L.J. Carburetor Icing 

Tests in the Laboratory and in Service . Atlantic Refining 
Co., SAE Paper 448c, January 1962. 

Investigation of Icing Characteristics of Typical Airplane 

Engine Induction Systems . NACA TN No. 1970, February 1949. 

Schuel, H.J. and Burt, J.G. "New tests on carburetor icing". 
E.I, duPont Co., Petroleum Refiner , November i960. 

The Aircraft Engine and its Operation . Installation Engineering- 
Pratt & Whitney Aircraft, May 1947. 

NTSB SPECIAL STUDIES 

Special Study; Carburetor Ice in General Aviation . National 

Transportation. Safety Board, NTSB-AAS-72-1, Washington, D.C. 
1972. 



37 



Special Study; U.S. General Aviation Accidents Involving Fuel 
Starvation; 1970-1972 . National Transportation Safety 
Board, NTSB-AAS-74-1, Washington, D.C, , 1974. 

PILOT ERROR 



Aircraft Design-Induces Pilot Error . 
Washington, D.C. 



Civil Aeronautics Board, 



Kowalsky, N,B. , Masters, R.L., Stone, R.B. , Babcock, G.L. , and 
Rypka, E.W. An Analysis of Pilot Error-Related Aircraft 
Accidents . Department of Aerospace and Environmental 
Medicine, Lovelace Foundation, Albuquerque, New Mexico, 
June 1964, NASA CR-2 444. 



38 



APPENDIX A; INFORMATION SOURCES 



GENERAL SOURCES 



AEROSPACE SAFETY RESEARCH AND DATA INSTITUTE {ASRDI ) 

AIRCRAFT OWNERS ASSOCIATION 

AIR FORCE FLIGHT DYNAMICS LABORATORY (AFFDL) 

AIRWORK SERVICE DIVISION 

ALLEN AIRCRAFT (DIVISION OF AAR CORP.) 

AMERICAN NATIONAL . STANDARDS INSTITUTE. 

AMERICAN SOCIETY FOR NONDESTRUCTIVE TESTING 

AMERICAN SOCIETY OF SAFETY ENGINEERS 

ANDREWS & ASSOCIATES 

ATLANTIC AVIATION CORPORATION 

AUTOMOTIVE ENGINEERING (SAE JOURNAL) 

AVCO (LYCOMING DIVISION) CORP. 

AVQUIPO INC. 

BORG-WARNER CORP. (MARVEL- SCHEBLER DIV, ) 

BUREAU OF AVIATION SAFETY (NTSB: WASH., D.C.) 

CALIFORNIA AVIATION COUNCIL 

CANADIAN CURTIS S-WRIGHT LTD. 

CATERPILLAR TRACTOR CO. ^ 

CHANDLER EVANS CONTROL SYSTEMS (DIV. OF COLT INDUSTRIES) 

CHEMICAL PROPULSION INFORMATION AGENCY 

CHRYSLER CORP . , DEFENSE SPACE GROUP 

CIVIL AEROMEDICAL INSTITUTE 

COLT INDUSTRIES 

COMBUSTION INSTITUTE 

COOPER- BESSEMER 

E.I. DU PONT CO. 

ETHYL CORPORATION , 

PAA SAFETY DATA BRANCH FLIGHT STANDARDS TECHNICAL DIVISION 

FEDERAL AVIATION ADMINISTRATION (LOS ANGELES) 

FIELD AVIATION CO., LTD. 

FLIGHT SAFETY FOUNDATION, INC. 



39 



FRANKLIN GNO CORP. 

GUGGENHEIM CENTER FOR AEROSPACE HEALTH AND SAFETY 
JET AVIATION CORP. 

JOHNS HOPKINS APPLIED PHYSICS LAB 
LARSON AERODEVELOPMENT 
LEAVENS BROS . , LTD , 

MINISTRY OF (CANADIAN) TRANSPORT ACCIDENT INVESTIGATION 
NATIONAL AIRCRAFT ACCIDENT INVESTIGATION SCHOOL (NAAIS) 
NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH 
NATIONAL SAFETY COUNCIL 

NATIONAL TRANSPORTATION SAFETY BOARD (NTSB) 
LOS ANGELES, CALIFORNIA 
WASHINGTON, D,C. 

ELSON AIRCRAFT CORPORATION 

NONDESTRUCTIVE TESTING INFORMATION CENTER 

ONAN DIVISION, ONAN CORPORATION 

REGIONAL INFORMATION AND COMMUNICATION EXCHANGE 

SOUTHWEST RESEARCH INSTITUTE 

STANDARD OIL COMPANY (OHIO) 

THE OHIO STATE UNIVERSITY: 

DEPARTMENT OF AERONAUTICAL AND ASTRONAUTICAL ENGINEERING 
DEPARTMENT OF AVIATION 

TRANSPORTATION SAFETY INSTITUTE 

TRW EQUIPMENT, INC. 

TELEDYNE CONTINENTAL MOTORS, AIRCRAFT PRODUCTS DIVISION 

TRANSAERO INC. 

UNITED AIRCRAFT OF CANADA, LTD. 

WHITE SUPERIOR, DIVISION OF WHITE MOTOR CORPORATION 

UNIVERSITY OF ILLINOIS: 
STAN ROSCOE 
JESSE STONECIPHER 



40 



ACTUARIAL SOURCES 

ABBOTT, WHITE, AND CO., INC. 

AERO COVERAGES, INC. 

AERO INSURANCE AGENCY 

AIR ASSOCIATES, INC. 

AIRBANC OF AMERICA, INC. 

AIRWAY CASUALTY CO. 

ALEXANDER & ALEXANDER, INC. 

ASSOCIATED AVIATION UNDERWRITERS 

AVEMCO AIRCRAFT INVESTMENT CORPORATION 

AVEMCO CORPORATION 

AVIATION INSURANCE SERVICE 

AVIATION OFFICE OF AMERICA, INC. 

RICHARD J. BERLOW & CO. , INC. 

J.H. BLADES & CO., INC, 

BOWES & CO. , INC. 

C&A FLIGHT CREW INSURANCE, INC. 

CESSNA FINANCE CORP. 

C.l.T. LEASING CORP. 

COMMERCIAL CREDIT EQUIPMENT CORP. 

CONNECTICUT GENERAL LIFE INSURANCE CO. 

CONTINENTAL ASSOCIATES, INC. 

DELTA AGENCIES, INC. 

ELECTRA FINANCIAL GROUP 

FAIRFAX UNDERWRITERS SERVICES, INC. 

G. SHANNON GROVER 

HARLAN INC, OF PENNSYLVANIA 

HOLTON INSURANCE AGENCY, INC. 

INA REINSURANCE CO. 

INSURANCE CO. OF NORTH AMERICA 

INTERNATIONAL AIRCRAFT SALES 

INTERNATIONAL AVIATION UNDERWRITERS, INC, 

KUDLICH GENERAL INSURANCE AGENCY, LTD. 

MANN-KLINE, INC. 

NATIONAL AVIATION UNDERWRITERS 



41 



PACIFIC INTERNATIONAL UNDERWRITERS 
PARKER & CO. INTERNATIONAL, INC. 
PETER J. MC BREEN & ASSOCIATES, INC. 
THE SOCIETY OF ACTUARIES 
TEX-WIDE INSURANCE 
TRANSWORLD INSURANCE BROKERS 
WRIGHT & CO. 

AIRFRAME MANUFACTURERS 



AERONCA, INC. 

AERO RESOURCES 

AERO SPORT 

AMERICAN AVIATION CORPORATION 

ATLANTIC AVIATION 

AVCO-LYCOMING 

BEECH AIRCRAFT CORPORATION 

BELLANCA AIRCRAFT CORPORATION 

CESSNA AIRCRAFT CO. 

CONTINENTAL 

ENSTROM CORPORATION 

EVANGEL AIRCRAFT CORPORATION 

FAIRCHILD INDUSTRIES, INC. 

FRANKLIN 

MAULE AIRCRAFT CORPORATION 

NORTH AMERICAN ROCKWELL CORPORATION 

PIPER AIRCRAFT CORPORATION 

PITTS AVIATION ENTERPRISES 

PRATT & WHITNEY 

PERSONNEL CONTACTED 

WILLIAM ALLEN: FAA ACCIDENT PREVENTION & ANALYSIS 
VINCENT BROPHY: FAA 

JAMES CONSTANTINE: NTSB ACCIDENT RECORDS SECTION 
JOHN CRAWFORD: BUREAU OF AVIATION SAFETY 



42 



ROBERT CRESTON: FAA: TRANSPORTATION SAPETY INSTITUTE 

GENE KING: FAA, DOT 

DAVE PARKER: NTSB 

JOHN REED: NTSB 

BOB SHAW: NTSB 

JIM WEBSTER: FAA FLIGHT STATISTICS BRANCH 

GREGG VAN BRUNT: FLIGHT SAFETY COUNSELLOR 



43