Data from airline digital flight data recorders provides relevant statistical data for estimating fatigue life consumption of the current airliner fleet and for design criteria updating for future designs. The data indicates real operating effects due to the autopilot, i.e., gust response frequency peak increase by 2 or 3 times, and the existence of the low frequency low amplitude limit cycle motion in altitude hold. The extension of more data types for ground operations is considered. Onboard processing of simple data types is also considered

Crabill, N. L.

Morris, G. J.

English

NASA Technical Reports Server

THE NASA DIGITAL VGH PROGRAM--EARLY RESULTS 
Norman L .  Crabill and Garland J .  Morris 
Langley Research Center 
SUMMARY 
NASA has recently revived the V G H "  measurement program for  a i r l  ine 
transports to  f u l f i l l  a need w i t h i n  the a i r l i ne  industry for  r e a l i s t i c  data 
describing f l i gh t  operations of  a i r l  ine transports. 
used by transport designers t o  make bet ter  estimates of fatigue l i f e  con- 
sumption of current a i r c r a f t  and t o  update design c r i t e r i a  for future a i r -  
c r a f t .  
s tar ted i n  1977. Several examples of the s t a t i s t i c a l  outputs are reviewed 
t o  i l l u s t r a t e  the types of analyses and resul ts  becoming available. 
These data will be 
This program, using digi ta l  data recorded on magnetic tape was 
I NT RO DU CT I ON 
NASA has recently revived the "VGH" measurement program for  a i r l i n e  
transports t o  f u l f i l l  a need w i t h i n  the a i r l i ne  industry for  r e a l i s t i c  data 
describing f l i g h t  operations of a i r l  ine transports. Data needed include 
operating a l t i tude ,  airspeed, weight, loads, and control usage. These data will 
be used by transport designers t o  make better estimates of fatigue l i f e  
consumption of current a i r c r a f t  and to  update design c r i t e r i a  for  future 
a i  r c r a f t  . 
NACA/NASA has his tor ical ly  provided such data, s ta r t ing  i n  the 1930's 
w i t h  the NASA "VC" program which gave velocity and load factor from 
operating a i r c r a f t  fo r  d i rec t  comparison with the designer's VG diagrams. 
Later, s t a r t i n g  i n  the 1 9 5 0 ' ~ ~  a new dimension of a l t i tude  was added i n  the 
NACA/NASA-suppl ied film recorder, which provided time h i  s to r ies  of velocity, 
load factor ,  and height which were read u p  and analyzed principally by manual 
processes. 
i n  1971, due to  changing p r io r i t i e s  w i t h i n  NASA. 
changes ( r e f .  1). I t  was decided to  determine i f  data from dig i ta l  recorders 
a1 ready exis t in? on many wide-body a i r c r a f t  for  accident investigation 
purposes could be ut i l ized t o  f u l f i l l  the Dig i t a l  VGH Program objectives. 
More parameters would be attempted, since they are  readily available. 
present report gives several examples of the s t a t i s t i c a l  outputs evolved for  
a Lockheed L-1311 a i r c ra f t  using data obtained i n  routine f l i g h t  operations 
i n  1973, and compares them with resu l t s  from 1978 3perations. In addition, 
These analogue recorder programs for  a i  rl ine transports ceased 
In 1977, rJASA revived the program fo r  a i r l i n e  transports w i t h  some 
The 
61 3 
https://ntrs.nasa.gov/search.jsp?R=19810010543 2020-03-21T09:02:34+00:00Z
the report  discusses several in te res t ing  operational e f f ec t s  obtained from an 
inspection of the  time h is tor ies .  
The service areas f o r  the two data sets discussed a r e  given i n  
figure 1. The o ldes t  data s e t  was obtained f o r  f l igh ts  flown i n  February 
through May 1973 along the e a s t  coast  w i t h  Chicago and San Juan added. 
T h i s  set consis ts  of 83 f l i g h t s ,  about 200 hours of f l i g h t  time, and about 
169 000 km (91 000 nautical miles), The data tapes f o r  this 1973 data set 
were obtained i n  1977 and were used t o  develop the analysis  techniques, 
computer programs, and output formats. 
th rough  July 1979 and i s  fo r  much the same routing, b u t  with the addition of 
Mexico City, Acapulco, Sea t t le ,  and Portland, f o r  918 f l igh ts ,  about 1600 
hours, and 1 300 000 km (700 000 nautical miles). 
a r e  f o r  the same airplane obtained continuously, o r  as nearly as possible. 
The newest set covers March 1978 
For each data s e t ,  the data 
DISCUSSION 
The data a re  recorded on existing Digital Flight Data Recorders on a 
25-hour loop-tape on a i r l i n e r s  i n  routine a i r l i n e  service ( f igure  2). 
a i r l i n e  company reads out the data once o r  twice a week, t ranscr ibes  the 
selected parameters in to  an IBM compatible format, and sends i t  and the  
s u p p o r t  data (weights and t r i p  length from f l i g h t  logs) t o  NASA. NASA plo ts  
these data and ed i t s  them both manually and automatically and processes them 
i n t o  two basic s t a t i s t i c a l  outputs--loads s t a t i s t i c s  and f l i g h t  p ro f i l e  
s t a t i s t i c s - - a s  described i n  reference 1 and i l l u s t r a t e d  in f igures  3 and 4. 
T h e  matrix of s t a t i s t i c a l  data types output f o r  this f i r s t  program phase i s  
shown in f igure  4. Not a l l  these data types will  be discussed i n  this paper. 
Instead, t h i s  paper wil l  show typical s t a t i s t i c a l  outputs f o r  a l t i t u d e ,  
weight, airspeed, and accelerat ion,  as  shown by the numbers 1, 2, 3, and 4 
on figure 4. 
planes a re  expected t o  be published w i t h i n  the next year ,  i n  separate reports .  
The 
Complete s t a t i s t i c a l  r e su l t s  f o r  the L-1011, B-727, and B-747 a i r .  
Maximum A1 t i  tude  Per F1 i g h t  
a1 t i t ude  per f l i g h t  was about equally d i v i d e d  between 8892-10516 and 10516- 
12040 meters (30-35 000 and 35-40 000 f e e t ) .  For the la rger  sample of 918 
f l i g h t s  s t a r t i ng  i n  1978 over a broader route s t ruc ture ,  the 10516-12340 m 
(35-40 000 f e e t )  a l t i t u d e  was by f a r  the most popular.and two f l i g h t s  ac tua l ly  
went t o  12800 m (41 000 feet) .  A t  the 10516-12040 m (35-40 000 f e e t )  maximum 
a l t i t u d e  leve l ,  the 2-2.5-hour t r ip  was most prevalent f o r  both sets of  data.  
About 17 percent of a l l  trips had peak a l t i t u d e s  l e s s  than 5944 m (19 500 feet)  
in 1978-1979. 
The 83 f l i g h t s  of the 1973 operations,  f igure  5, show the maximum 
Landing Fuel 
Landing fuel mass is  given f o r  the L-1011 operation i n  figure 6 fo r  the  
83 f l i g h t s ,  February to  Nay 1973, and f o r  918 f l i g h t s ,  1978-1979 operations. 
61 4 
In  1973, the most popular trip of 2-2.5-hours duration almost always landed 
with 13,608-22 680 kg (30-50 000 lbs)  of fuel.  
popular landing fuel mass by a factor of 2 was 4536-13 608 kg (10-30 300 lbs )  
for  the same 2.5 hour t r ip  length. However, the 1.0-1.5-hour t r i p  landed 
with more fuel , 13 608-22 680 kg (30-50 000 l b s ) ,  twice as often as i t  d i d  
w i t h  the minimum 4536-13 608 kg (10-30 000 lbs ) .  The 13 608-22 680 kg (30- 
50 000 lbs )  landing fuel mass was s t i l l  the most probable mass for  a l l  t r ip 
lengths. The dis t r ibut ions of trip duration for  a l l  fuel masses in 1978-1979 
show more shorter trips than i n  1973. The differences i n  trip duration 
distributions for  the 83 f l i gh t s  and the 918 f l i gh t s  i n  1978-1979, shown in 
figures 5 and 6,  can be at t r ibuted to  the effects  of sample s i ze  and possibly 
other factors such as changes i n  fuel cost  and ava i lab i l i ty .  
In 1978-1979, the most 
Landing Flap Usage 
The time spent a t  various airspeed intervals i n  each f lap  detent i s  given 
in figure 7 fo r  83 landings of the L-1011-1 a i r c r a f t  i n  1973 operations; f lap 
placard speeds are a lso noted. These data are actual f lap surface position 
indications collected into standard "bins" or detents of 5 to  8 degrees. 
can be seen tha t  f lap deployment above the placard speed is  minimal, the 
general trend i s  for broad speed distributions in small detents, and narrow 
distributions close to  the placard speeds i n  the landing detents. 
I t  
Loads 
The measure of load on the a i r c r a f t  i s  the body-axis normal acceleration 
a t  the center of gravity. 
technique employed i s  the "level crossing method" as opposed to  the "peak 
count method" of the previous NACA/NASA VGH Program. 
L-1011-1 operation are  given in figure 8 and compared w i t h  previous wide-body 
data extracted from the data s e t  reported in reference 2. 
As explained i n  reference 1, the basic counting 
Sample resul ts  for the 
The previous wide-body resul ts  are  within a factor of 2 of the present 
resul ts  a t  a given load increment. However, a t  a given counting r a t e ,  the 
loads a re  within 10 to  15 percent. However, since they were n o t  derived 
exactly the same way as the present resul ts ,  the comparison must be made 
cautiously. The previous wide-body resul ts  are total  peak counts per hour 
obtained by adding separately determined maneuver counts and g u s t  counts t o  
get the total  counts; the present resul ts  are total  level crossing counts 
per hour obtained direct ly  from the t race of total  normal accelerations. 
Results i n  reference 3 indicate the level crossing technique can sometimes 
give up to  twice the crossing ra te  of the peak count method. 
The present resu l t s ,  figure 8, provide d a t a  a l l  the way t o  zero load. 
A t  these low levels ,  the counts are  considerably higher than a t  the 0.2-g 
level previously available,  as would be expected. 
61 5 
Examination of many power spectra of center-of-gravi ty normal 
acceleration showed the presence of sharply peaked responses just  below 1 hz, 
whereas, the a i r c r a f t  short  period response (or g u s t  response frequency) i s  
character is t ical ly  0.2 to  0.5 hz for  the L-1011 w i t h  s t ick  fixed. From the 
f l i gh t  conditions, i t  was surmised tha t  autopilot  operation was involved; 
however, the 1973 data s e t  did not include autopilot  s ta tus .  When the 1978 
data s e t  was obtained w i t h  autopilot  s ta tus ,  i t  was possible to  d e a r l y  see 
that  the high frequency response was occurring w i t h  autopilot  on; as i n  
figure 9. 
Figure 9 shows power spectra of L-1011 c.g. normal acceleration plotted 
a f t e r  the manner of reference 4 ,  t o  show the re la t ive  power under each peak. 
T h u s ,  the vertical  scale is  l inear  with power spectral density times 
frequency versus log frequency horizontally. The measured data a re  for  auto- 
p i lo t  off  and on under similar operating conditions. For autopilot  o f f ,  the 
pe'ak response i s  a t  about 0.3 hz, which i s  the a i r c r a f t  stick-fixed short 
period frequency. 
surmised t o  be the short  period response with autopilot  on. 
calculations by the manufacturer and NASA confirmed this frequency shift .  
For autopilot  on, the peak i s  a t  about 0.8 hz, which was 
Subsequent 
There are  a t  l ea s t  two effects  of t h i s  s h i f t  to  be noted. F i r s t ,  the 
higher frequency motion i s  considered t o  be less  disturbing t o  the passengers 
according t o  reference 5, which indicates 0.2 to  0.7 hz as the c r i t i c a l  
motion sickness region. Secondly, the average zero crossing ra te  of in te res t  
to  fatigue analysis i s  increased 10 t o  20 percent when the response i s  
integrated across a l l  frequencies. 
Autopilot Operation 
To assess the importance of these effects  o f  autopilot operat-lon, 
s t a t i s t i c s  on autopilot  usage were compiled from 400+ hours of the 1978-1979 
data se t .  The resul ts  are  given i n  f igure 10 and indicate for  the L-io11 
that  the most frequently used a l t i tude  band for  autopilot  turn-on i s  ~ 2 8 9 6  
to  4420 m (10-15 000 f t )  i n  the departure, b u t  the most popular turn-off 
a l t i tude  i s  i n  the l a s t  1372 m (5000 f t )  i n  descent. 
the cases, i t  was on a t  touchdown; i n  about 8 percent o f  the f l i gh t s ,  i t  was 
not on a t  a1 1 .  T h u s  , the L-1011 autopilot  was used approximately 75 percent 
of the time i t  was operating. 
In about 5 percent o f  
Detailed examination of the normal acceleration trace showed the 
occasional presence of a low-amplitude low-frequency osci l la t ion.  This 
"1 imit cycle" phenomenon i s  i l lus t ra ted  in figure 11. Peak-to-peak 
amplitudes averaged about .07 g's. This low-amplitude low-frequency motion, 
associated with the a l t i tude  hold mode according t o  the theoretical studies , 
is below the region associated with passenger discomfort (reference 5) and i s  
not considered a factor i n  fatigue l i f e  consumption. 
noted on the 727 and 747 a i r c r a f t ,  f igure 1 2 ,  w i t h  the smaller shorter-range 
a i r c r a f t  experiencing i t  more than 20 percent of the time, and the large long- 
range type less  than 10 percent of the time the autopilot  i s  on. 
estimated the e f fec t  of t h i s  l imi t  cycle i s  to  increase the fuel consumption 
a few tenths of 1 percent. 
Similar e f fec ts  were 
I t  i s  
61 6 
CONCLUDING REMARKS 
Data from a i r l i n e  d ig i t a l  f l i g h t  data recorders can provide relevant 
s t a t i s t i c a l  data f o r  estimating fa t igue  1 i f e  consumption of the current  
a i r l i n e r  f l e e t  and f o r  design c r i t e r i a  updating f o r  future designs. In 
addition, the data have indicated real operating effects due t o  the au topi lo t ,  
i . e . ,  g u s t  response frequency peak increase by 2 o r  3 times, and the existence 
of the low-frequency low-amplitude l i m i t  cycle motion i n  a l t i t u d e  hold. 
Extension of the program t o  more data types f o r  ground operations is planned, 
along w i t h  acquis i t ion of DC-10 data. Finally,  on-board processing o f  simple 
data types is being considered. 
is sought and received. 
Throughout the program, industry feedback 
REFERENCES 
1. Morris, Garland J . ;  and Crabi l l ,  Norman L , :  Air Transport Flight 
Parameter Measurements Program - Concepts and Benefits. 
Aero-Space Congress, October 1980. 
Comparison of VGH Data from Wide-Body and Narrow-Body Long-Haul 
Turbine-Powered Transports. 
SAE 1980 
2. Zalovcik, J .  A . ;  Jewel, Joseph W . ,  J r , ;  and Morris, Garland, J . :  
NASA TN D-8481 , July 1977. 
3. Crooks, W .  M . ;  Hoblit, F.  M.; Mitchel, F. A , :  Project  HICAT. High  
A1 t i  tude C1 ear  Air Turbulence Measurements and Meteorological 
Correlations. AFFDL TR 68-127, Vol , 1, Final Report, March 13, 1967 
t o  July 31, 1968. 
4. Houbolt, John C . :  Design Manual f o r  Vertical Gusts Based on Power 
Spectral Techniques. AFFDL-TR-70-106, pp. 11 and 1 2 ,  December 1970. 
5. Aeromedical Aspects o f  Vibration and Noise. AGARDOGRAPH No. 151, Chap. 5, 
November 1972. 
61 7 
Figure 1 .- Location of s e rv i ce  areas 
f o r  L-1011 opera t ions .  
DIGITAL FLIGHT DATA RECORDER AND 
TRANSDUCERS 25 HOUR LOOP TAPE 
AIRLINE READOUT AND TRANSCRIPTION: TWICE PER WEEK 
AIRLINE SUPPORT DATA 
Figure 2.- D i g i t a l  VGH program data source.  
61 8 
LaRC COMPUTER PROCESSED TIME H I  STORIES 
LANGLEY 
SOFTWARE 
0-0 - COMPUTER AND - 
m 1111 I I 
COUNTS/ hr PERCENT OF TIME 
ACCELS AND FOR ALTITUDE 
A I RSPEED 
FLAP POSITION 
ALTITUDES, ETC. 
PERCENT OF FLIGHTS 
M A X  ACCELS AND GUST 
Figure 3.- Data processing and analysis. 
FLAP - FOR TAKE-OFF 
DETENTS AND LANDING 
WE I GHTS 
ALT. ABOVE 
GROUND 
AIRSPEED 
- 
FOR 
DETENT 
- EACH 
- 
FOR TAKE-OFF AND LANDING 
FOR EACH DETENT 
' M A X  NORMAL ACCEL EACH 
DETENT VS AIRSPEED 
- 
TIME OF FLAP CHANGE FROM 
TAKE-OFF AND LANDING 
- 
- 
EQUIV. AIRSPEED AT' 
FLAP DI3ENT CHANGE 
- 
- I VEL. FOR ALT INTERVALS I 
WE I GHTS 
AIRSPEEDS 
ALTITUDE 
i W K  LLIIVlti. L t V .  I-LI 
AND DESC. V S  ALT. 
TOTAL, CLIMB, LEV. FLT 
AND DESC. V S  ALT. 
SEE WEIGHTS 
AND AIRSPEED 
Figure 4.- Statistical data outputs. 
61 9 
FL I GHT 
D URAT I ON, 
hr 
6- 
5 -  
4 -  
0 
TOTALS 
3 -  
2 -  
1- 
-_ 
ALTITUDE, m - - 11.2 13.6 I 12.4 1 115.6142.0)34.8 I I Im-1 
(a) 1973 operations, 83 flights. 
FLIGHT 
DURAT 1 ON, 
hr 
6 
5 
4 
3 
2 
1 
0 
-152 1372 28% 4420 5944 7488 8992 10516 12040 13564 
ALTITUDE. rn 
TOTALS] . 2  1 3.3 ] 4.3 1 8 . 5 ' )  7.6 1'2.7 115.6157.2 1 . 3  ] 
m.5 4.5 9.5 14.5 19.5 24.5 29.5 34.5 39.544.5. 
ALTITUDE, ft 
(b) 1978-1 979 operations, 91 8 flights. 
TOTALS 
1939.71 
10 
Figure 5.- Percent of flights to maximum altitude per 
flight versus flight duration. 
6 20 
5 .  
TOTALS 
ALL MASSES 
6'1973 OPS -I83 FLIGHT; 
I I 
FL I GHT 
DURATION, 
h r  
TOTALS1 3.6 1 64.8 I 26.4 I 4.8 199.61 
10 30 50 70 
LANDING FUEL MASS, Ib 
(a) 1973 operations, 83 flights. 
LANDING FUEL MASS, kg TOTALS 
4536 13,608 22,680 31,752 40,824 ALL MASSES 
LANDING FUEL MASS,  Ib 
(b) 1978-1 979 operations, 91 8 flights. 
Figure 6.- Landing fuel mass versus flight duration. 
6 21 
TIME IN AIR SPEED INTERVAL AND IN FLAP DETENT X 100 
TIME IN FLAP DETENT PERCENT TIME =' 
100 
0 
0 
0 PERCENT TIME 
=2.8hr 
TIME @ FLAP 18- 
= .4hr  
TIME@ FLAP le- 
= 2.7 hr 
TlME@FLAP4' A i i l I ' E  I =1.6hr 
l a  1 
0 50 100 150 200 250 300 
CALIBRATED AIRSPEED, knots 
Figure 7.- L-1 01 1 flap position versus airspeed 
during landing. 
+L-1011 1973 
200HOURS 
*LEVEL CROSSINGS 
*TOTAL ACCELERATIONS 
4-747 197l(REF.Z) - 75M) HOURS 
PEAKCOUNTS 
MANEUVER AND GUST 
ACCELERATIONS 
10- 
-.6 -.4 -.2 0 .2 . 4  .6  
INCREMENTAL NORMAL ACCELERATION, g UNITS 
Figure 8.- Loads exceedances. 
622 
CENTER -OF-GR AV I TY NORMAL ACCELERAT I ON 
20 
AUTOPILOT STATUS - 
Figure 9.- Effects of autopilot on L-1 011 measured 
power spectra. 
L-1011 FOR 400 + HOURS OF REVENUE FLIGHTS 
1978 
ALTITUDE FOR ALTITUDE FOR 
I N  I N  
CLIMB DESCENT 
A LT I TU DE 
Id TURN-ON TURN-OFF 
9.5 
-. 5 
0 10 20 30 40 50 0 10 20 30 40 50 
PERCENT OF FLIGHTS 
2,896 
- 152 
SUMMARY 
0 AUTOPILOT WAS "ON" ABOUT 75% OF THE TIME 
0 AUTOPILOT WAS "ON" AT TOUCHDOWN FOR 5% OF THE FLIGHTS 
0 AUTOPILOT WAS NOT USED ON 8%OFTHE FLIGHTS 
Figure 10.- Autopilot on-off s ta t i s t ics .  
623 
1 
I I I I I I I I I I I I I I I I I I I I I I nl-rlll- 
- 
-c- -.c- - 7 LALT ITUDE 
- 
- 
NORMAL ACCELERATION, g 
g UNITS 0 vm-+-h- 
-. 4 k L  IM IT n M  IT CYCLE- 
0 AP ON -75% 
PERCENT @ L I M I T  CYCLE 
AMP L ITU DE FREQUENCY OF TIME AP ON 
60 OF TIME 
AT EACH 40 OCCURS -15% 
20 
0 .04 .08 .12 .16 0 .02 .04 .06 .08 .10 
PEAK-TO-PEAK AMP 
g, VERG-1.0 
FREQUENCY, Hz 
Figure 1 1  .- Autopilot "limit cycle" experience. 
DOMINANT PEAK-TO-PEAK % TIME 
LOAD, g AUTOPILOT ON WHICH ,-REQ. HZ 
%TIME 
A'RPLANE AUTOP ILOT ON HAVE LIMIT CYCLE 
B -727 
L-1011 
B -747 
56.42 23.69 .06 .068 
75.14 15.14 .07 .07 
92.12 7.48 .066 .074 
Figure 12.- Summary of autopilot-on and limit cycle times. 
624 


The NASA digital VGH program, early results

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server