Document
r b 1 s c *+ INTERNATIONAL COMNITTEE ON AIRCRAFT FATIGUE AN ASSESSMENT OF FGPUTED LOADS ON GENERAL AVIATION AND TRANSPORT,ATRpR*m 1 I'
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Chief, Flight Mechanics & Technology Division a Joseph W. Jewel, J r ! 1 - Aerospace Technologist GPO PRICE ' Paul A . Hunter I Aerospace Technologist CFSTI PRICE( I Hard copy (HC) ff 653 July 65 1 ' Papes -&+be' presented at the 5th I.C.A.F. SYMP@XCuM" f , I , AIRCRAFT FATIGUE - DESIGN, OPERATIONAL AND ECONOMIC A S P E C T S : d
Melbourne, Australia i 22-24 Mayl 1967
/ - i l t
I
/ National Aeronautics and Space Administration
Langley Research Center, Hampton, Virginia r AN ASSESSMENT OF REPFATED LO& ON~GENERALAVIATION AND TRANSPORT AIRCRAFT By Philip Donely, Joseph W. Jewel, Jr., and Paul A. Hunter NASA Langley Research Center SUMMARY An assessment is made of recent repeated loads data from short-haul jet transports and several general aviation airplanes. The jet transport data indicate t h a t except f o r check f l i g h t maneuvers the load histories a r e essen- t i a l l y independent of operator and airplane type. General aviation data show a large amount of s c a t t e r i n the repeated load history. The use and geographical location of operations may be the primary means of specifying the repeated loads environment.
INTRODUCTION About the time engineers interested i n repeated loads f e e l t h a t they can provide the fatigue s p e c i a l i s t with stable and valid information, someone e i t h e r develops a new a i r c r a f t or a new use f o r a i r c r a f t . O n this basis the demand is always present f o r additional collections of information or refinements i n past results. The current changes a r e the introduction of small jet transports i n t o short-haul operations and the increasing diversity and u t i l i z a t i o n of general aviation a i r c r a f t . I n both cases potential problems are created by applica- tions of a i r c r a f t not f u l l y anticipated i n the past.
A t the fourth ICAF symposium i n 1965, M r . Coleman presented an excellent Since t h a t time data summary on repeated loads on transport airplanes ( r e f . 1).
have become available t o augment t h i s summary i n regard t o the load expectancy of the small jet transports and t o permit some assessment of the effect of t h e operator and geographical environment. In regard t o repeated loads on transports, then, the present paper w i l l up-date the information reported i n /A* reference 1.
.-----”* , A ’ There has been l i t t l e information available on general aviation a i r c r a f t , but NASA and F A A i n a cooperative e f f o r t have been collecting data for t h i s The slow progress i n obtaining information i n t h i s category f o r some 3 years.
area i s due primarily t o the diverse nature of,general aviation. A t t h i s time, available information w i l l be presented as a preliminary guide t o the uses, t o the load experience of representative operations, and w i l l include an assessment of the data collection process f o r such operations.
L-5232 J c SYMBOLS incremental acceleration, g units
an
maximum incremental acceleration, g units incremental acceleration corresponding t o l i m i t load factor, g units %LLF Mach number corresponding t o maximum operating l i m i t speed %O l i m i t gust load factor ng l i m i t maneuver load f a c t o r
nm
S wing area, sq f t design maneuvering speed, k t s vA design cruising speed, k t s vC design diving speed, k t s vD maximum s t r u c t u r a l cruising speed, k t s ' N O never-exceed speed, k t s VNE Mach nmber corresponding t o the maximum operating l i m i t speed $0 derived gust velocity, ft/sec ude airplane weight, l b
w
GENERAL C O N S I D E M T I O N S Transport Aircraft i s t o ensure t h a t changes i n The continuing saurpling of a i r l i n e operations use of a i r c r a f t and type of a i r c r a f t have not introduced serious discrepancies i n the load histories. There i s also a need f o r continuing study t o evaluate A n example of changes the influence of a i r l i n e practices on load histories.
t h a t may affect the fatigue l i f e i s i l l u s t r a t e d when the jet transport i s used i n short-haul operations, where it w i l l be spending more time i n a turbulent environment than would be inferred by results obtained from the intercontinental , J . 1 , In regard t o a i r l i n e practices, f o r example, landing impact loads have Questions of t h i s type varied more widely between operators than a i r c r a f t .
i f , i n the long run, we are t o make r a t i o n a l decisions as require examination t o design f o r fatigue. While many e f f o r t s have been made t o resolve these and s i m i l a r questions, at present our only recourse i s t o do additional work since answers have not been found.
Another consideration t h a t requires examination i s the f a c t that, f o r example, United States a i r c r a f t are designed and b u i l t f o r the American environ- ment and according t o the United States philosophy, yet are used i n other environments and operated by nationals with other philosophies. One does not expect the Northern European Operations t o be i n the same environment as oper- ations i n the Tropics.
By the same token, examination of airworthiness discus- sions between nations indicates philosophical differences, although the objec- t i v e (a safe airplane) i s the same.
General Aviation Aircraft A t t h i s stage i n data collection, the questions t o be answered a r e many.
Certainly a major question i s how t o c l a s s i f y operations.
While the transport operations represent an organized e f f o r t and well-defined operations, general aviation represents many individual operations of almost a l l types and sizes of a i r c r a f t . It appears that classification by type of airplane may not be satis- factory since as performance has improved a given type may be used as an execu- t i v e transport, trainer, or air taxi.
Another distinguishing feature of general aviation may be the classifica- t i o n of f l i g h t regimes. as a t r a i n e r m y not permit the The a i r c r a f t used c l a s s i c a l climb, cruise, descent segregation of the transport. It is probable w i l l require an approach s i m i l a r t o the t h a t many general aviation operations military concept of mission and nonmission operations. Such an approach may be required f o r multiuse a i r c r a f t and perhaps f o r survey a i r c r a f t .
A factor f o r consideration is the wide variety of p i l o t experience and The a i r l i n e p i l o t satisfies specific requirements as p i l o t training involved.
t o training and currency. The general aviation p i l o t ranges from the Sunday afternoon once-a-month experience t o the professional p i l o t on a busman's holiday. I n some way representative p i l o t images w i l l have t o be established if any generalized load spectra are t o have meaning.
A serious problem, at least i n the United States, is the question of sample s i z e and bias which may apply i n other countries as w e l l . A n optimistic e s t i - mate i s t h a t the current e f f o r t represents l e s s than 0.1percent of general aviation and one cannot be sure t h a t a l l classes of operations are covered. On the matter of bias, it should be obvious that volunteer participation means a The data collected then should show more mature and b e t t e r than average p i l o t .
less severe load experience than i f some of the l e s s responsible individuals were participating .
..
INS-WATION AND DATA EVALUATION Since members of the symposium may not have convenient access t o refer- ence 1, the material used by M r . Coleman t o describe the N A S A a c t i v i t i e s has been reproduced verbatim. It is applicable t o both the transport and general amlation operations as a l l data are collected and evaluated the same way.
Instrument a t ion The data t o be discussed were obtained primarily w i t h NASA VGH and V-G recorders, which are described i n detail i n references 2 and 3, respectively.
Consequently, only a brief description of the recorders and the type record obtained i s given below.
VGH recorder.- A picture of the VGH recorder is shown i n figure 1. The recorder consists of three major components: the recorder base, the attached film recording drum, and the acceleration transmitter. The transmitter i s i n s t a l l e d near (usually within 5 f e e t ) the center of gravity of the airplane, whereas the recorder base may be mounted at any convenient location within the airplane. The i n s t a l l e d weight of the VGH recorder i s 20 t o 25 pounds.
An i l l u s t r a t i v e VGH record is shown i n figure 2. It i s a time-history normal acceleration. From record of indicated airspeed, pressure altitude, and t h i s record, it i s possible t o make detailed counts of the normal acceleration peaks caused by various sources such as gusts, maneuvers, and ground operations, and t o determine the associated airspeeds and altitudes.
V-G recorder.- A picture of the V-G recorder i s shown i n figure 3 . It weighs l e s s than 5 pounds i n s t a l l e d and is usually mounted within 5 f e e t of the center of gravity of the airplane.
A n i l l u s t r a t i v e V-G record is shown i n figure 4. It is an envelope of the maximum positive and negative accelerations experienced throughout the airspeed range during the period (usually approximately 200 f l i g h t hours for commercial airplanes and 60 hours f o r general aviation airplanes) covered by the record.
Record Evaluation Detailed methods used f o r evaluating the VGH and V-G records are given i n references 4 and 5. Consequently, only a brief explanation of the methods of evaluating t h e records i s given in t h e following sections.
V%H records.- The sketch i n the l e f t of figure 5 i l l u s t r a t e s the method of evaluating the VGH records. The steady f l i g h t position of the acceleration trace is used as a reference from which t o read the incremental acceleration peaks which equal or exceed a selected threshold value. Only the maximum value of the acceleration is read f o r each crossing of the reference. The selected threshold values range from kO.09g t o *0.40g, depending upon the airplane type and the source of the accelerations being evaluated. For each acceleration peak evaluated, the corresponding values of airspeed and a l t i t u d e are also I n addition, the airspeed and a l t i t u d e at 1-minute intervals are evaluated.
read t o provide data on the airspeed operating practices and the a l t i t u d e s flown. The acceleration data a r e sorted according t o source (gusts, maneuvers), flight condition (climb, cruise, and descent), and by altitude.
V-G records.- The sketch i n the right of figure 5 i l l u s t r a t e s the manner As indicated, only one maximum positive and one of evaluating the V-G records.
negative acceleration increment from the reference are evaluated from each record. Generally, it is not possible t o determine the source (i.e., gusts or maximum accelerations on a V-G record. Consequently, the maneuvers) of the V-G acceleration data are not generally sorted aecording t o the source, but rather are given as combined data representing in-flight accelerations.
NOTE: For many of the early transport airplanes, the maximum accelerations on Because the V-G records were ascribed t o gusts rather than maneuvers.
of t h e r e l a t i v e l y high response of these airplanes t o gusts, t h e assump- t i o n w a s considered t o be valid. For several types of current transports and f o r general aviation airplanes, however, detailed data from VGH records indicate that the assumption may not be valid since maneuver accelerations may be as high as gust accelerations.
Method of combining VGH and V-G data.- Because VGH data samples'are gen- s m a l l (approximately 1000 f l i g h t hours), they do not provide r e l i a b l e e r a l l y estimates of the frequency of the large accelerations. They do, however, pro- vide detailed information on the smaller accelerations and the sources of these accelerations. Conversely, the larger samples of V-G data do not provide detailed information on the sources of the accelerations, but do give r e l i a b l e The two types of data estimates of the frequency of the large accelerations.
are complementary and may be combined t o obtain an estimate of the t o t a l i n - flight acceleration experience.
The method of combining the VGH and V-G data i s i l l u s t r a t e d i n figure 6.
The figure shows the cumulative frequency distributions per mile of flight of gust and maneuver accelerations as determined from the E H data sample, the maximum accelerations from the V-G data, and the t o t a l in-flight acceleration distribution obtained by summing the ordinate values of the maneuver, gust, and V-G acceleration distributions.
SCOPE Scheduled J e t Transports Table I lists the general characteristics of the jet transports f o r which data have been analyzed.' Airplanes I t o VI1 are the large transcontinental and IX, and X I 1 1 are the small short-to- intercontinental transports while VIII, medium-haul a i r c r a f t w i t h two or three engines. Almost a l l the data f o r the large a i r c r a f t have been reported i n reference 1 and e a r l i e r publications. For the small j e t transports the samples have been evaluated recently.
Table I1 i s a surmnary of operations by airplane and operator. For defini- t i o n the table includes the average f l i g h t duration, altitude, and the percent of time spent i n climb, cruise, and descent. It i s of i n t e r e s t t o note t h a t - the average f l i g h t time and a l t i t u d e f o r the large j e t s a r e about 3 hours and 32,000 f e e t while the corresponding values f o r the small a i r c r a f t a r e 1 hour and about 25,000 f e e t . Another significant difference between the large and sm;all a i r c r a f t is the f a c t t h a t large a i r c r a f t spend about 75 percent of t h e i r f l i g h t time i n cruise as compared w i t h about 40 percent f o r the smaller a i r c r a f t .
The operations of the large a i r c r a f t include operations i n almost every part of the free world by both United States and other operators. I n the case of the small short-haul j e t s , three of the f i v e operations are within the con- t i n e n t a l United States while two operations represent an European and an Australian operation. In connection with the intercontinental operations, the recorded data can include any part of the world while the short-haul j e t s are r e s t r i c t e d by range t o more localized geographic areas.
As a matter of convenience, t h e amount of flight operations spent i n check or training f l i g h t s i s included i n table I1 f o r l a t e r reference. The category of check flying a l s o includes f l i g h t s following overhaul or modification t o the airframe. N o attempt has been made t o s o r t the information on a more specific basis than noted.
General Aviation Table I11 i s a l i s t i n g of the pertinent a i r c r a f t included i n the sampling program even though r e s u l t s w i l l not be presented f o r every type of a i r c r a f t .
The table lists f i v e categories which define i n a rough way the primary u t i l i - zation. Table I11 also l i s t s the number of V-G and VGH installations and the hours of data currently on hand i n each case.
Since the categories such as 'single-engine executive" and "personal" a r e not e n t i r e l y descriptive, the types of operations included i n each category are as follows: Twin-engine executive:
Charter f l i g h t - cargo and personnel
Business f l i g h t - company and individual
Instrument check f l i g h t - training f o r instrument card
Instructional - check-out f o r multiengine
Single-engine executive:
Charter f l i g h t - cargo and personnel
Business f l i g h t - company and individual
Instrument check flight - training f o r instrument card
Instructional - check-out f o r heavier airplane
Personal :
Flying Club - airplane flown by from 3 t o 21 m e m b e r s . Used f o r pleasure
flying, instruction, o r business
Individual - used f o r pleasure and business
Company owned - airplane rented t o individual f o r business or pleasure
flying, also aircraft used as check-out f o r heavier airplane Instructional:
Training - a l l instrumented airplanes owned by flying schools. U s e d as
basic t r a i n e r s f o r private license. Also used by student a f t e r solo f o r cross -country Commercial survey:
Pipe l i n e p a t r o l - patrols flown from 230-300 feet above ground t o check f o r
leaks o r breaks i n the pipe l i n e Forest p a t r o l - patrols flown 1300 f e e t above t e r r a i n f o r f i r e spotting.
When f i r e i s spotted, descents are made t o 200-300 feet t o check condition of t e r r a i n around f i r e
Fish spotter - patrols flown 1500-2000 f e e t above water. Occasional descents
are made t o 300 t o 300 feet.
Figure 7 i s a map showing the distribution of the installations throughout the continental United States. The solid symbols indicate a VGH i n s t a l l a t i o n while the plain symbol is a V-G recorder installation. Most geographic sec- tions of t h e country are represented i n the sampling, t o the extent t h a t instruments are flying i n 37 of the 48 domestic s t a t e s . A s can be seen from figure 7 not a l l classes of operations are represented i n each locality.
Table I V shows the t i m e spent i n each f l i g h t condition, average f l i g h t time, and the a l t i t u d e and airspeed distributions according t o category. In contrast t o the j e t transports the a l t i t u d e s axe below 20,000 f e e t f o r a l l air- c r a f t , and except f o r airplane T-2 the average a l t i t u d e is below 10,000 feet.
Comparison of transport airplane I with airplane S-12 ( single-engine executive) emphasizes t h e influence of a l t i t u d e since airplane I spent about 4.0 percent of the f l i g h t time i n rough air while the single-engine executive spent some 76 percent of the time i n rough air. Similar comparisons f o r the other cate- gories can also be made.
DISCUSSION Scheduled Transport Operations General.- Inspection of a l l the data at hand indicates t h a t the only f l i g h t phases which have not e n t i r e l y stabilized are the loads i n landing impact and check-flight maneuvers. Ground loads, gust accelerations, and operational maneu- vers a l l appear t o be independent of operator, geography, and a i r c r a f t type within the jet category. There has been some concern t h a t operators c o d d be a significant factor. A l l attempts t o find significant differences have been negative.
In regard t o landing impact accelerations, although t h e s c a t t e r is great, only one sample shars a wide discrepancy. A t the present time it i s not possi- b l e t o s t a t e whether t h i s is due t o operating techniques or the airplane characteristics.
The check-flight load h i s t o r i e s as noted i n reference 1, follow no rational pattern. In broad terms, U.S. operators do some two t o three times as much but the amount varies widely between air- a check-flight flying as other nations, lines. As w i l l be shown l a t e r , the severity of the maaeuver loads also varies widely f o r the same equipment but different operators.
Ground loads.- Review of ground loads data, that is, taxi, take-off and landing roll-out loads, indicates that f o r different equipment and operations, the overall h i s t o r i e s are essentially the same (fig. 8). Inspection of other data, f o r the three separate phases, indicates that the landing run-out imposes higher loads than e i t h e r the take-off run or taxiing. Since the data of fig- ure 8 are on a per f l i g h t basis, a single distribution may be suitable f o r a l l a i r c r a f t i n the jet transport category.
Impact accelerations, - Figure 9 summarizes landing acceleration data f o r
a l l operations and f i v e airplane types. The basic data sorted according t o operator showed l i t t l e or no scatter, figure 9, except f o r airplane X I I I .
Since airplane X I 1 1 is f a i r l y new i n the inventory, one might expect a more severe environment, but airplanes V I 1 1 and IX are also f a i r l y new and show l e s s than average load experience. Until the severe load history f o r airplane X I 1 1 can be explained, it does not appear feasible t o suggest a single curve. The severe loading could be due t o some airplane characteristic or t o the training practices of the a i r l i n e .
Figure 10 shows that three operators of identical equipment had the same landing acceleration histories. A s noted e a r l i e r it was thought t h a t geography and national traits might have some significance which was not borne out by the data, since two operators are from countries other than the United States. The airplane IX is a short-haul j e t introduced a few years ago t h a t appears t o have good handling q u a l i t i e s i n the approach.
figure 1 1 indicates a significant difference In contrast t o figure 10, between two operators of large j e t s flying the same equipment. Subsequent study indicated t h a t it w a s the general practice of one operator t o use a fixed descent r a t e without flare, while the operator with l e a s t severe load history Subsequent e f f o r t s by the f i r s t operator trained the crews t o flare on landing.
a reduction i n load experience by changes i n landing technique.
resulted i n
Turbulence. - Since rough air is the natural environment of the airplane
i n many papers, the general aspects are well and has been thoroughly discussed known. Figure 12 shows the amount of rough air flown at different a l t i t u d e s f o r the short-haul j e t transports. The general distribution and s c a t t e r are i n keeping w i t h past experience and it would be expected that the r e s u l t s pre- For comparison w i t h the data from other sented i n reference 1 a r e applicable, load sources the gust acceleration distribution w i l l be included i n l a t e r figures .
Operational maneuvers.- Figure 13 summarizes a l l maneuver data available and indicates that frequency distribution i s essentially the same regardless of airplane. Such an observation might be expected since operational maneuvers are basically specified by terminal area and ATC routings rather than by the crew. Since most changes i n direction are on a standard pattern, the load experience should be essentially the same.
The deviations f o r two short-haul operations are f o r samples l e s s than 1000 flight hours while a l l other samples w i t h a s c a t t e r of about 2 t o 1 represent samples varying from 18,000 t o 9,000 hours of operation by the large j e t s . A s sample size increases it is expected that the small j e t w i l l tend t o approach the other curves reducing the overall s c a t t e r t o about 2 t o 1.
The data shown are primarily f o r U.S. operators, and involve operations i n a high density environment. It i s probable t h a t f o r some areas of the world where t r a f f i c density i s low, the maneuver h i s t o r i e s would be somewhat l e s s severe.
Check-flight maneuvers.- Figure 14 shows the mean and the extreme distri- butions of check-flight maneuver loads from some 16 operations involving both the new and the older j e t transports.
The r e s u l t s indicate a s c a t t e r of from 15 t o 20 t o 1. For a cumulative frequency of 10-5 per mile, the mean check- f l i g h t acceleration i s O.9g as compared t o 0.6g f o r operationalmaneuvers.
Also at 10-3 per mile the maximum and minimum accelerations a r e 1.04g and O.72g, respectively. This type of operation produces many large loads on the airframe and it does not appear feasible t o suggest a single distribution.
Inspection of the time spent i n check f l i g h t s , table 11, indicates a wide 8.7 t o 0.7 percent of the t o t a l f l i g h t variation between operators from about time. While some of the variation i n loads could be ascribed t o the variation i n time, inspection of individual operations also indicates wide variations i n the severity of the maneuvers. Variations f o r one operator ranged from 6.5 t o 3.6 percent of the t i m e while another operation indicated variations f r o m 8.7 t o 1.8 percent. Some o f these variations r e f l e c t the t r a n s i t i o n from training on new a i r c r a f t t o routine operations since the percentage i s highest f o r the new airplanes. A s a point of interest, the two lowest times are f o r other than U.S. operators.
Summation of acceleration experience.- Four of the many samples are sum-
(b), (c), and (a), t o show the r e l a t i v e importance of
marized i n figures l 5 ( a ) , the different load sources-. Two samples representing intercontinental opera- tions and two short-haul operations a r e shown. The r e s u l t s indicate that the check-flight maneuver tends t o be the most significant source of repeated loads f o r three of the four operations. I n one case, airplane X I I I , the landing impact accelerations tended t o predominate. Since each load source can be a different s t r u c t u r a l component, it i s not possible t o assess c r i t i c a l f o r t o source, but it i s appztrent that all elements m u s t be* fatigue damage according considered i n the repeated loads assessment.
While a limited assessment of the influence of airplane type, operator, and geography has been made, the only significant differences appear t o be i n the landing impact and check-flight maneuver accelerations. It i s probable t h a t the reasons f o r the single unusual landing load experience will be found through further analysis, but accounting f o r the check-flight load histories may not be practical. I n the case of check flights, the load history seems t o depend t o a high degree on a i r l i n e training practices and policy and it i s not it t o a technical problem f o r solution.
possible t o reduce General Aviation General.- Inspection of sample V-G envelopes f o r each category, figure 16, indicates consistent exceedance of the design cruise speed and some V , increase i n positive load factors f o r t h e instruction and commercial survey categories as compared t o the other three. the records Except f o r a f e w peaks,
indicate negative accelerations only s l i g h t l y below zero g . Figure 16 (a) shows
more peaks at high negative g than the other categories but the character of the record indicates t h a t the largest peak at 160 knots i s due t o a gust.
Records from other a i r c r a f t indicate more violent maneuvers than shown on the figure, including one t h a t showed exceedance of and both the positive and VD negative design l i m i t load factor. Insufficient data a r e on hand, however, t o place such records i n the proper s t a t i s t i c a l perspective. From the crude image t h a t emerges f o r the operations, a 3.0g positive load factor i s t o be expected; and the operators do not appear t o be concerned with excess speed.
Figure 1 7 i s a composite p l o t of the cumulative frequency distributions
f o r the basic V-G data. The abscissa i s the r a t i o of the maximwn acceleration increment divided by the design l i m i t load factor increment from 1.Og. This ratio, which w i l l be referred t o as the acceleration fraction, w a s selected since l i m i t load factors f o r general aviation airplanes designed t o meet the requirements of reference 6 vary widely. The incremental value measured from 1.Og w a s used t o avoid d i f f i c u l t i e s with values near zero g. For values of the acceleration fraction l e s s than 0.4, the shape of the distribution curves i s not significant since it i s highly dependent on the number of records and the number of hours represented by each record.
Figure 1 7 indicates t h a t the cumulative frequency distributions of the
acceleration fraction a r e symmetrical and essentially the same f o r a l l cate- gories. Since the positive design l i m i t load factor i s somewhat higher than the negative load factor, the symmetry indicates some tendency f o r the positive accelerations t o be higher as might be expected f o r maneuvering a i r c r a f t . The bias i s not very strong since inspection of t a b l e I11 indicates differences l i m i t load factors of about 20 t o 30 percent.
between positive and negative Since t h e individual curves of figure 17 a r e e r r a t i c because of data limitations it i s not possible at t h i s time t o extrapolate the results t o the t o t a l popula- tion of general aviation.
The landing acceleration data shown i n figure 18 indicate as might be expected, t h a t the accelerations are most severe for the instructional category.
The commercial survey and "twin" executive show the l e a s t severe load h i s t o r i e s For comparison with the "single" executive and personal only s l i g h t l y higher.
as the with figure 18, the extremes f o r the jet transports have been superposed record dashed l i n e s . A t a probability l e v e l of 0.01, the best general aviation i s more than 0.2g above the lower l i m i t f o r the transports while the most severe history (instructional) i s about 0.15g above the worst j e t transport history. The s c a t t e r of 3 t o lbetween the lower four curves is considered t o be reasonable since records from individual a i r c r a f t i n a category can vary by factors from 10 t o 1 0 0 .
The several factors t h a t influence the load histories are the pilot, the airplane characteristics, and the landing-gear characteristics. Consideration of airplane and p i l o t characteristics indicates t h a t f o r the high performance a i r c r a f t the wing loadings are high, about 30 pounds per square foot, and decrease f o r instructional airplanes t o about 10 pounds per square foot. Since the more expensive a i r c r a f t such as the "twin" executive probably have commer- c i a l or experienced pilots, and the l i g h t instructional a i r c r a f t have the least experienced pilots, the variations i n wing loading and p i l o t experience would tend t o exaggerate the differences i n load experience. By the same token, the large a i r c r a f t have the more sophisticated landing gear while the instructional w i l l tend t o have the more elemental landing gear which could also affect the landing load history. The resolution of these questions w i l l have t o a w a i t more information and analysis.
Twin-engine executive.- The twin-engine executive a i r c r a f t have an average f l i g h t time of about 1 hour and the cruise a l t i t u d e f o r piston-engine a i r c r a f t i s about 5500 f e e t . Examination of one sample from a twin turbopropeller air- c r a f t indicates the same average f l i g h t t i m e but the average cruise a l t i t u d e is about 15,000 f e e t . For these a i r c r a f t t h e amount of rough air varies from about 45 percent of time f o r the low cruise a l t i t u d e t o 30 percent for, the cruise a l t i t u d e of l 5 , O O O feet. These figures are at l e a s t twice as great as the values f o r transport operations but w i l l require more definition as the sample s i z e increases. Since general aviation would be expected t o be predom- inantly a daylight operation as compared t o scheduled transport, the increased exposure t o rough air m y be accounted f o r by operations during the roughest part of the day.
Gust velocities, figure l g ( a ) , appear quite consistent f o r the two twin- engine executive a i r c r a f t . For airplane T-7, the high negative gust velocities up t o 48 f e e t per second appear t o be a "rare" event and the t a i l of the dis- tribution may follow the trend of t h e data at lower load levels as further data a r e acquired. Comparison of the distributions i n the reliable range (from 8
t o 30 f p s ) indicates a gust experience about 4 f e e t per second l e s s than f o r
transport a i r c r a f t and the curves are almost identical for the sample airplanes.
The difference between the transport and executive gust experience could be ascribed t o the f a c t t h a t the transport goes on schedule i n most weather condi- tions whereas the l i g h t twin i s probably operated mainly i n t h e daytime and under more selective weather conditions, o r the difference may be due t o sample size.
The maneuver accelerations i n figure lg(b) emphasize the unsymmetrical experience f o r positive and negative loads and the apparent practice of rather Comparison of figures l g ( b ) and ( e ) shows t h a t f o r these gentle maneuvers.
operations t h e gust accelerations are more severe than the maneuvers. A t an acceleration fraction of 0.3 the accelerations due t o turbulence would be about 10 times more frequent than the maneuver accelerations. For airplane T-2 the , I r a t i o i s about 2 t o 1 due i n part t o the higher wing loading of the turbine- powered a i r c r a f t , table 111, and i n p a r t because of flight at a higher altitude.
If the values f o r landing impact of figure 18 are considered, the landing impact appears t o be a less severe environment than f l i g h t f o r the airframe, although it could be c r i t i c a l f o r particular airplane components.
Single-engine executive.- These a i r c r a f t show an average flight time of about 1 hour, with average operating a l t i t u d e of about 6000 f e e t and about The high percentage of t i m e i n 76 percent of t h e f l i g h t t i m e i n rough air.
rough air may also be explainable i n terms of f l i g h t during the most turbulent hours of t h e day. The largest sample represents operations i n mountainous sections of the United States and contains a fair amount of "bush" operations.
Comparison of figure l9(a) with figure 20(a) indicates t h a t the gust experience is about 2 feet per second l e s s than f o r the twin executive. Inspec- t i o n of a smaller sample from operations i n the plains states shows t h a t f o r such operations the tendency is f o r a somewhat l e s s severe gust history. Other things being equal, the reduced severity of the gust velocity distributions suggests more fair weather flying than f o r the twin-engine executive a i r c r a f t .
The maneuver load distributions, figure 20(b), a r e more severe than f o r the twin-engine executive and indicate perhaps two operations since the curves a r e concave downward at the high end. Comparison with figure l9(b) shows the a c t i v i t y i s about 10 times t h a t f o r the twin. Another notable feature of fig- ure 20(b) i s the high incidence of negative maneuvers. In contrast, another sample of single-engine executive operations produced only one negative accel- eration i n 138 flight hours, and positive maneuvers a t a frequency of about one-thirtieth t h a t of figure 20(b). Discussions with the operator of the air- craft, whose data are presented i n figure 20, indicate t h a t many of the oper- ations involved carrying sportsmen i n t o mountainous areas t o landing sites which required "dragging" the s t r i p before touchdown. Brief inspection of commercial survey operations using t h e same airplane type shows the same prob- a b i l i t y of the larger maneuver loads but about 20 t i m e s as many of the more moderate loads. Referring back t o figure 17, which indicates the large load probability is essentially independent of category, one must conclude t h a t l i t t l e or no relation w i l l e x i s t between the extreme values and the frequency of repeated loads.
Comparison of the gust and maneuver acceleration fractions indicates t h a t at moderate load levels, about 0.3, the two load sources w i l l be of equal importance f o r t h e single-engine executive category. The s c a t t e r between samples previously discussed r a i s e s the question of whether the category i s homogeneous and whether any refinement i n load spectra may require a more detailed breakdown of the operations.
-.
Personal a i r c r a f t . - Operations by airplane P-14 indicate about 32 percent of the t i m e i n rough air with an average operating a l t i t u d e of about 2500 feet.
"he f l i g h t duration of some 35 minutes i s the shortest f o r a l l categories.
With an average f l i g h t speed of about 100 miles per hour t h i s would imply t h a t most f l i g h t s take place within about 60 miles of the home airport. The amount of rough air experienced is at about the r i g h t level i n comparison with the other categories f o r the operating altitude.
Figure 21(a) shows that, f o r the limited sample of gust velocities, the experience is somewhat more severe than the single-engine executive operations.
The maximum gust velocity experience of 28 f e e t per second would imply gust accelerations corresponding t o an acceleration fraction of about 0.55 whereas figure 21(c) indicates a maximum acceleration fraction of about 0.35. On t h i s basis it would appear that the more severe gusts w e r e encountered at low speeds, probably well below the structural cruising speed.
Figure 21(b) indicates a rather severe maneuver environment, particularly As i n the other categories there i s the negative acceleration distribution.
bias toward positive maneuver accelerations as might be expected. Comparison with figure 20(b) indicates a more severe maneuver load history than f o r the at a probability l e v e l of 10-3, about single-engine executive operations, and as f o r the commercial survey.
the same acceleration frequency Figures 21(b) and ( c ) indicate that f o r the sample studied, the maneuver loads would produce more repeated loads than the rough air i n the range of interest. Since t h e flight t i m e is only 35 minutes, the landing accelerations could be a significant feature of the repeated load history f o r the airplane.
Instructional.- The 115-hour sample from airplane 1-18 indicates, as might be expected, a large amount of f l i g h t time, 75 percent, i n rough air since the average operating a l t i t u d e w a s only 1500 feet. These operations were 'of very short f l i g h t duration amounting t o about 40 minutes.
The gust velocities, figure 22(a), experienced i n these operations were If it i s assumed t h a t quite low with a maximum value of 16 f e e t per second.
basic training is primarily a fair weather operation, then f l i g h t s close t o the airport would experience a great deal of l i g h t t o moderate turbulence since operations would be a t the lower altitudes.
are a l s o quite moderate with The maneuver accelerations of figure 22(b) the acceleration fraction having maximum values of about 0.4. Comparison with the gust accelerations of figure 22(c) indicates t h a t the maneuvers would be the prime source of repeated f l i g h t loads although neither load source appears t o provide a severe environment. When the limited sample i s viewed i n terms of it appears t h a t it may not be e n t i r e l y representative the V-G data of figure 17, and there i s a d i s t i n c t p o s s i b i l i t y that, as a category, instructional flying may show more s c a t t e r between operations than the other categories.
Commercial survey. - The commercial survey (the sample is f o r pipeline
operations) i s characterized by spending 97 percent of the t i m e i n turbulence, an average operating altitude of 1200 feet, and f l i g h t t i m e s of about 3 hours.
Since most of the f l i g h t operations are at a l t i t u d e s of 200 t o 400 feet, the continuous exposure t o turbulence i s not surprising. The long average f l i g h t time is characteristic of commercial operations t h a t involve spotting ground objects. In t h e case of t h e pipeline a i r c r a f t (airplane C-19) the average f l i g h t speed i s 89 knots and the design cruising speed i s 104 knots.
Such operations are conducted i n WR weather since v i s i b i l i t y i s a prime requisite of the mission.
The gust velocity distribution, figure 23(a), i s t h e most severe of the general aviation experience due t o the almost continuous exposure t o rough air.
A t large gust velocities, t h e experience matches t h a t of the twin executive but f o r lower values, 8 t o 20 f e e t per second, t h e frequency of occurrence is higher than f o r the twin. A t 16 f e e t per second the commercial survey airplane expe- riences about 6 times as many gusts as the twin. Comparison of the trends shown i n figures 23(a) and l9(a), i f continued, would indicate t h a t f o r larger samples the maximum gust velocities f o r the twin would exceed those f o r commercial sur- vey operations. A possible reason f o r t h i s trend i s t h a t the VFR requirements of survey work indicate a minimum exposure t o convective cloud a c t i v i t y while the twin executive would be expected t o penetrate such cloud a c t i v i t y during transport type operations.
The maneuver accelerations, figure 23(b), indicate a very strong bias toward positive load factor, and a very high frequency of maneuvers. Since survey work involves banking, turning, and c i r c l i n g f l i g h t t o avoid obstacles t o follow the l i n e and t o check f o r leaks, a high incidence of positive maneuvers would be expected. The shape of the distribution curve f o r positive accelera- t i o n fractions would indicate t h a t very large maneuver loads would not be expected and is, of course, borne out by t h e data of figure 17 based on V-G recordings. A t 20 percent of t h e l i m i t load factor, the maneuver frequency i s about 100 times more frequent than f o r e i t h e r the twin- o r single-engine executive categories.
Comparison of figures 23(b) and ( c ) indicates that, f o r the survey type of operation, maneuver loads would be the prime source of repeated loads. Despite the p r a c t i c a l l y continuous operation i n rough air the imposed gust loads f o r airplane C-19 are about one-hundredth of the frequency at an acceleration frac- tion of 0.4. Since the f l i g h t s average about 3 hours as compared t o 1 hour f o r the executive operations the frequency of landing impact accelerations w i l l a l s o be l e s s by a factor of about three. O f the categories studied, the commercial survey is potentially the most severe environment from a repeated loads st andpoint.
Comparison of Categories Most general aviation a i r c r a f t , because of speed limitations, are probably best categorized by the geography surrounding the home station, and by the usage As of the a i r c r a f t , than by the categorization selected i n t h e present paper.
further samples a r e collected it may be feasible t o determine more suitable categories, but a t the present time data are not available t o define the differ- I n operations t h a t are primarily commercial i n character, ent environments.
such as the commercial-survey and twin-engine a i r c r a f t , it appears t h a t the operations are single purpose and the load distributions should s t a b i l i z e quite w e l l .
The image t h a t emerges of the general aviation p i l o t is, i n the main, a man with a large investment i n equipment who i s interested i n t h i s investment The apparent lack of concern f o r speeds rather t h a n - i n taking chances.
beyond V , creates the impression t h a t the p i l o t has not been taught the sig- i s probably not familiar with ni-ficance of the s t r u c t u r a l design speeds and F A R 23 o r 25, references 6 and 7.
While it is s t i l l too early t o t e l l a great deal, the gust environment i s different from the transports as t o the amount of rough air encountered and the Except f o r the twin executives, the impression i s t h a t the maxi- gust severity.
mum gust velocities encountered will be less than those f o r transports, but the amount of rough air and number of encounters with moderate turbulence w i l l be greater. For the twin executive the r e s u l t s lead one t o believe t h a t i n the long run the gust environment w i l l approach t h a t f o r transport a i r c r a f t except f o r some increase i n the amount of rough air, which would be most significant f o r repeated loads experience. The l e a s t severe gust experience has been with instructional a i r c r a f t which apparently i s primarily a fair-weather operation.
The more severe maneuver loads environment appears t o be generated by the commercial survey and single-engine executive classes. While the large load experience i s not outstanding f o r these categories, the frequency of occurrence of moderate maneuvers i s very high.
The landing impact experience appears t o be r e l a t i v e l y stable and orderly i n t h a t instruction i n basic f l i g h t technique creates the greater number of large loads while the other four categories indicate essentially the same load experience.
D a t a Collection f o r General Aviation data on general aviation i s a discouraging experi- The collection of loads A s compared t o s i m i l a r collections of transport data, the major problems ence.
a r e the individual operations and t h e i r number. The current U.S. program amounts t o about 0.1 percent of t h e general aviation f l e e t and w a s planned t o sample both the repeated and large load experience. In 3 years of operation 100 t o TOO hours of data per instrument with the collection r a t e varies from V-G and personal a i r c r a f t being the lowest. Comparison of data hours f o r the VGH recorders indicates about twice as many hours per instrument f o r the V-G recorder. A s might be expected, the simpler the instrument the b e t t e r the collection. The r e s u l t s a l s o indicate t h a t commercial or semi-commercial operators do a much b e t t e r job than the individual owner.
Current operations involve an e f f o r t of about 4 man years per year and a cost per year of about one hundred and twenty thousand dollars. The cost f i g - ure amounts t o about t e n dollars per data hour with about half the cost being i n instrument maintenance, calibration, and adjustment. In the 3-year period some 90 days of t r a v e l has been involved t o v i s i t the locations of figure 7 f o r s o l i c i t i n g cooperation and improving the collection, In retrospect, if man- power were available the amount of t r a v e l would be doubled o r t r i p l e d t o keep the program moving. The current substitute i s very extensive use of the t e l e - phone f o r l i a i s o n and follow-up. The need f o r extensive promotion arises from the low flying hours per year of many a i r c r a f t , the d i f f i c u l t y i n maintaining enthusiastic cooperation over long periods of time, and the changes brought about ' . I by the sale or trade of a i r c r a f t . In many cases an owner w i l l trade or s e l l the a i r c r a f t after only a few hundred hours have been acquired.
The three c r i t i c a l problems i n extensive data collection programs have been: 1. The lack of uniformity and capacity of e l e c t r i c a l supply systems i n general aviation a i r c r a f t 2. The i n s t a l l a t i o n and weight limitations f o r the smaller a i r c r a f t 3 . The nuisance e f f o r t required t o handle the records and necessary bookkeeping The first two problems have been solved on an individual basis, but the record collection and bookkeeping i s s t i l l a serious problem, particularly f o r has been found. Record handling and VGH installations, and no simple solution t h e cooperation of the collection a r e the limiting factors i n maintaining operat or.
For the data collected t o date, the evaluation of the VGH records taxes our manpower and f a c i l i t i e s even though it is semi-automatic. If, as i n the case of transport a i r c r a f t , a 1- o r 2-percent sample were required, the data I n evaluation and analysis w i t h current methods would swamp the investigator.
the long run the larger sample w i l l be required and automatic evaluation w i l l be a must, or extremely simple instrumentation such as the V-G recorde?, o r counting accelerometers, w i l l have t o be accepted with the attendant reduction i n the amount of detailed information obtained.
CONCLUDING NZMARKS The infomation on the jet transport category indicates a remarkable con- gust and maneuver loads, but that check flying s t i l l shows sistency i n landing, a large degree of scatter. Results t o date indicate that i n contrast t o expec- tations the h i s t o r i e s of repeated loads show a high degree of independence of operator and geographical location.
The picture of repeated load experience on general aviation a i r c r a f t indi- cates wide variations and d i f f i c u l t i e s can be foreseen i n sorting the operations The categories used i n the present study according t o homogeneous categories.
will probably have t o be changed on the basis of the evidence presented. While the evidence i s inconclusive it appears that geographical location and airplane use w i l l be predominant factors f o r most categories. The results also indicate l i t t l e if any relation between the frequency of the extreme and the small repeated loads.
1. Coleman, Thomas L . : Trends in Repeated Loads on Transport Airplanesi To be published in Proceedings of 4th ICAF Symposium, "Fatigue Design Procedures," Munich, Germany, June 1965.
2 . Richardson, Norman R . : NACA VGH Recorder. NACA TN 2265, 1 9 5 1 .
3 . Taback, Israel: The NACA V-G Recorder. NACA TN 2 1 9 4 , 1950.
4. Walker, W. G., and Copp, Martin R . : Summary of VGH and V-G Data Obtained
From Piston-Engine Airplanes From 1 9 4 7 to 1 9 5 8 . NASA TN D-29, 1 9 5 9 .
5. Staff of Langley Airworthiness Branch: Operational Ekperience of Turbine
Powered Commercial Transport Airplanes. NASA TN D-1392, 1962.
6. Anon. : Airworthiness Standards; Normal Utility and Acrobatic Category Airplanes. Federal Aviation Regulations, Part 23, Feb. 1965.
7 . Anon.: Airworthiness Standards; Transport Category Airplanes, Part 25, Feb. 1 9 6 5 .
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0 CHECK FLIGHT MANEUVER
A LANDING IMPACT
tl GROUND INDUCED
lom3,
CUMULATIVE 10-4
FREQUENCY
PER M I L E
-
-
-
I I I I
I I
.2 .4 . 6 .8 1.0 1.2
I NCREMENTAL ACCELERATION, g
(d) Type I1 airplane.
Figure 15.- Concluded.
, a-
>
W
z-
>
h >a-
A
S-12 AN5 13 1684 HOURS
I-----
LOAD
FACTOR
INDICATED AIRSPEED, KNOTS
-; I i (b) Single-englne executive.
Figure 16.- Continued.
W 7-
v ) > f x
f=) I d
> -
s
I
> -
?
I
a
n
C-19 6806 HOURS
'NE
' A ' C
'r
LOAD
FACTOR
-4
60 80
INDICATED AIRSPEED, KNOTS
(e) Commercial survey.
Figure 16. - Concluded.
d r-i eu d* r-i cv * v ) v) I L L
'90
I - C- m e I
o o O a A 0
r-i I eu r-4 I tt
6 I I I I i
J-I M Ln I I > r-l
lo-*
PROBABILITY
IO-)
- TYPES OF NO. OF
AIRPLANES LANDINGS
TWIN-ENGINE EXECUTIVE (PROP) 3 935
S I NGLE-ENG I NE EXECUTIVE 3 541
3 866
b INSTRUCTIONAL
INCREMENTAL ACCELERATION, g
Figure 18. - Landing impact accelerations experienced by General Aviation airplanes.
c v r - I-!-&-
0 0
1 I I I
cv cr\ F-4 'd L n ' I I I I I 0 0 0 0 P-4 t--l F-4 P-4 F-4 . .
I c( de +I (T3 +I * +I d +I N .
+I
-
z
--I" CY L L I W cv b I e
I-1- I-
CY
a
I
El
+
$
d +I N * LL +I L I -
111 m <
CT cv I I I
I ? (ItL
0 0 0 0 d 111 4 I--I
TYPE FLIGHT
AIRPLANE HOURS
10-1
D s-12 262
CUMULATIVE
FREQUENCY
PER M I L E
-24 -16 -8 0 8 16 24
DERIVED GUST VELOCITY, FPS
(a) Gust velocity distribution.
20.- Gust velocity and in-flight acceleration experience f o r single-engine Figure executive operations.
1~
+I u3 +I d +I cv .
+I &
v-
-
t -
a
C Y LL d A u +I I
i m
G o (u +I Io +I d +I N .
+I I--I N
i CT\
I 0 0 0 r-l I--Iw I--I >t.
n
- 0 3
P c 7 W
AIRPLANE FLIGHT
TYPE HOURS
0 P-14 276
10-1
CUMULATIVE
FREQUENCY
PER MI LE
I I I
-24 -16 -8 0 8 16 24 a 32
DERIVED GUST VELOCITY, FPS
(a) Gust velocity distribution.
Figure 21.- Gust velocity and in-flight acceleration experience f o r personal operations.
do +I
i (TT
+I \o .
+I w +I cv +I I4 +I +I
z '
\o I c -1-I tr 3-1
+ o
eu +I
A I R PLANE FLIGHT
TYPE HOURS
0 1-18 115
CUMULATIVE
FREQUENCY
PER M I L E
lo -5 -32 - -24 -16 -8 0 8 16 24
DERIVED GUST VELOCITY, FPS
(a) Gust velocity distribution.
Figure 22.- Gust velocity and in-flight acceleration experience f o r instructional operations.
U I
A I R PLANE FLI O H T
10-
10-
CUMULATIVE
FREQUENCY
PER M I L E
10-
t -
10-
-32 -24 -16 - 8 ' 0 ' 8 16 24 32
DERIVED GUST VELOCITY, FPS
( a ) Gust velocity distribution.
Figure 23.- Gust velocity and in-flight acceleration experience f o r commercial survey operations.
? - h L v> . .
CY '9 => x I--.
I c3 9-l LL
O Z
CY L a u3
O Z
I n d d N aL
I T
I I I 0 0 0 0 A d F-l F-l d r-l NASA-Langley, 196'7