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SESSION I I A PAPER I I A - 4 ( N A S A - T R - X - 6 94 5 5 ) GSNEk.9 L A V I B T I O N AIP TRAFFIC P A T Z L F N SAFLtrY A ! i P L Y S I S ( K A S A ) 22 p YC $ 3 25 CSCL 176 U ric 1 a s 5 3 / 2 1 24337 GENERAL AVIATION A I R TRAFFIC PATTERN SAFETY ANALYSIS LOYD C. PARKER NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WALLOPS STATION W a l l o p s Island, V i r g i n i a P r e s e n t e d a t THE SYSTEM SAFETY SOCIETY SYMPOSIUM TUESDAY, JULY 1 7 , 1 9 7 3 GENERAL A V I A T I O N A I R TRAFFIC PATTERN SAFETY ANALYSIS Loyd C. Parker NASA Wallops S t a t i o n ABSTRACT This paper describes a concept f o r e v a l u a t i n g t h e general a v i a t i m mid-air c o l l i s i o n hazard i n uncontrolled terminal airspace. Three- dimensiona: t r G f f i c p a t t e r n measurements were conducted a t uncontrol- l e d and c o n t r o l l e d 3 i r p o r t s . Computer programs f o r data reduction, storage r e t r i e v a l and s t a t i s t i c a l analysis have been developed.
I n i t i a l general a v i a t i o n a i r t r a f f i c p a t t e r n c h a r a c t e r i s t i c s are presented. These p r e l i m i n a r y r e s u l t s i n d i c a t e t h a t patterns are h i g h l y divergent from t h e expected standard pattern, and t h a t p - - i - t e n procedures observed can a f f e c t the a b i l i t y of p i l o t s t o see and avoid each other.
I NTRODUCT I ON Numerous reports' have been w r i t t e n which characterize t h e mid- a i r c o l l i s i o n hazard. I n general, m i d - a i r c o l l i s i o n s occur i n un- control l e d terminal airspace, i n v o l v e tw general a v i a t i o n a i r c r a f t , occur i n t r a f f i c patterns when both a i r c r a f t are i n approach t o landing on f i n a l , under VFR conditions, on a weekend and a t low convergence angles and rates o f closure. M i d - a i r c o l l i s i o n r e p o r t s u s u a l l y contain the phrase " p i l o t s f a i l e d t o see-and-avoid." This hazard may be characterized by the f a c t o r s shown i n Figure 1. Mid-
I I
LOOKED BUT FAILED Figure 1.-Mid-air c o l l i s i o n f a c t o r s a i r collisiors occur because pilots f a i l to look, l o o k b u t do not see, a n d cannot see because o f view testrictions. Preliminary d a t a obtained by P , E . 9 . Rico, Federal Aviation Adrnin+stration ( F A A ) , indicates t h a t YFR genera? aviation pilots spend approximately 50 o f their t o t a l flight time looking outside the cockpit. I n the terminal a r e a , however, t h i s d a t a indicates the L L time spent i n a i r
search i s approximately 40 . Other studies' ' have shown t h a t
even when a pilot looks for a known aircraft a t a distance greater t h a n a mile, his probability o f detection may b~ ;ery low unless he l o o k s longer t h a n several seconds. A t ranges less t h a n one mile, detection i s almost certain i f the pilot looks and the other d i r - c r a f t i s w i t h i n h i s view field. P i l o t s are beinri encouraged to scan properly and t o increase their attention twtrd detecting other aircraft in the termlnal area. I t is our conclusion t h a t , i n many cases involving a mid-air collision i n the t r a f f i c pattern, a t least one o f the pilots involved--and p o s s i b l y b o t h pilots--were unable to see one another d u r l n g the c r i t . -a1 l a s t mile o f closure because o f vision envelope restrictions, the pattern flown and the maneuvers involved. The objective o f the s t u d y being conducted i s t o evaluate the present uncontrolled patterns flown a n d t o deterntine the improvements I D a p i l o t ' s ability t o see another aircraft (if he looks) f o r various changes in t h e t r a f f i c pattern concept.
DATA SYSTEM I t was determined i n 1971 through an extensive literature search t h a t a i r t r a f f i c pattern measurements o f the uncontrolled environ- ment wew essentially non-existent. An MPS-19 tracking radar and d a t a van (Figure 2 ) were used t o obtain position time histories o f
. - -
f i g u r e 2 . - MPS-19 radar and d a t a van a r r i v i n g and departing a i r c r a f t a t s i x a i r p o r t s . T r a f f i c measure- ments were taken during approximate three-week periods a t each a i r - p o r t f r o m October 1971 through March 1972. The a i r p o r t s i t e s selected (Figure 3 ) were a l l w i t h i n 150 NM o f Wallops S t a t i o n t o assure good l o g i s t i c s support t o t h e radar system.
Figure 3. . A i r p o r t locations The uncontrolled a i r p o r t s selected were the Sal isbury-Wicomico A i r - p o r t , Sal isbury, Maryland; Montgomery County A i r p o r t , Gaithersburg, Maryland; and Hyde F i e l d a t Clinton, Maryland. The Salisbury- Wicomico A i r p o r t had three 5,000-foot runways; has an FAA F l i g h t Service S t a t i o n a t the a i r p o r t , f l i g h t school, a i r t a x i service, a i r c r a f t maintenance, VORTAC f a c i 1 i t y , commuter service t o Washing- ton-Baltimore and i s located i n a r e l a t i v e l y low a i r t r a f f i c density region. The Montgomery County A i r p o r t i s a very busy general avia- t i o n a i r p o r t having a s i n g l e runway, r e s i d e n t corporate, p r i v a t e and sales a i r c r a f t , r e p a i r and maintenance f a c i l i t i e s , f l i g h t school and a radio beacon approach. Hyde F i e l d i s located under the Washington, D. C. Terminal Control Area (TCA), has two runways, f l i g h t school and p r i v a t e a i r c r a f t , and has constrained p a t t e r n s and a l t i t u d e s because of an adjacent a i r p o r t and t h e 1,500-foot TCA f l o o r .
The c o n t r o l l e d a i r p o r t s v f s i t e d t o o b t a i n general a v i a t i o n t r a f f i c p a t t e r n data i n these environments were R. E. Byrd I n t e r n a t i o n a l (BYRD), Richmond, V i r g i n i a ; Friendship I n t e r n a t i o n a l (BLT), B a l ti- more, Maryland; and P a t r i c k Henry (PHF) A i r p o r t , Newport News, V i r g i n i a . Each of these terminals were served by c o m e r c i a l a i r c a r r i e r s and have considerable general a v i a t i o n a c t i v i t y . These a i r p o r t s were selected t o o b t a i n data on t h e tower o n l y environ- ment (PHF), Stage I 1 service (BYRD) and Stage XI1 s e r v i c e (BLT).
A summary o f t h e tracks obtained a t each a i r p o r t i s shown i n Table I, below.
T&E 1. - Fw[y\R TRAQCS OBTAlKD PATRICK S P L ISBURY- W T G W E R Y HY DE R.E. BYRD FR1EN)SHIP tENRY TOTALS CATEMRY HI CCMl CO CCUiTY 4 1 8 549 485 No. O F TRACK5 406 5 54 4 4 9 289 368 2095 W I N G 2 7 0 494 3 76 298 DEPWTURE 0 10 36 70 200 70 60 43 258 R Y - 0 Y 23 45 3 7 50 0 20 69 163 IMTRW€NT 2 5 0 49 SlNGLT ENGlK 255 315 139 91 350 1298 111 117 125 54 1 THIN ENGINE 120:: 45 23 120 80 529 CLPWERCIbL 329 ::INCLUDES C C M W T E R SERVICE For each track, t h e radar range, azimuth, and e l e v a t i o n were r e - corded on magnetic tape a t one-second i n t e r v a l s . The reference coordinate system developed (Figure 4) normalizes a l l t r a f f i c data
ty f RUNWAY
f NORTH\ hHEADING
RADAR
-Y i
Figure 4. -Reference coordinate system t o t h e runway threshold and d i r e c t i o n . This system enables a l l t r a f f i c p a t t e r n data obtained t o be d i r e c t l y comparable regardless of the runway used f o r landing.
Radar data reduction, p a r a l l a x , and r o t a t i o n are performed by a GE-625 computer system and the reduced data i s stored i n a computer f i l e s management system i l l u s t r a t e d by Figure 5 c a l l e d Integrated Data Store (IDS)7.
GRAPHIC DISPLAY x D ' D Z D FWD. & VERT.
VELOCITIES , HEADING, BANK ANGLE, GLIDE PATH ANGLE F I L E ..nrnrr LUKKLL TIONS I DATE, TIME, LOG., TRACKING LOGS AIRCRAFT TYPE, RWY, WINDS, CEILINGS.
VISIBILI~Y,
7 I BAR. PRESSURE
WEATHE IR AND RADAR & OPERATION MISC. INPUTS LOGS , U I N D PROFILE DATA (ALT.. VEL.,
>-
C
D I R . ) , TRAFFIC DENSITY Figure 5.-Air t r a f f i c p a t t e r n data system Other data recorded f o r each t r a c k were a i r c r a f t manufacturer and model, runway used, wind speed and d i r e c t i o n , cloud c e i l i n q s , v i s i -
b i 11 t y , barometric pressure, approach type i f IFR o r unusual , and
other operator comnents. A s i t e plan was obtalned f o r each a i r - p o r t and a radar p o s i t i o n survey r e l a t i v e t o each runway was made.
T r a f f i c count data was taken by radar operators when it was n o t otherwise avai 1 ab1e a t t h e uncontrol 1 ed a i r p o r t s .
The I D S program enables r a p i d access o f a l l data from a remote graphics terminal This remote terminal w i l l be used t o e d i t , update and perf013 s t a t i s t i c a l analyses on the data base i n I D S storage. v i t h t h i s system, t h e a i r t r a f f i c s t a t i s t i c a l properties f o r any given s e t o f parametric conditions can be obtained.
lrNALY 'ICAL C J - EPT
1h.2 - i t * trafi':r: dcta obtained w i l l be u t i l i z e d t o generate math - , \ o C j e r ~ If the Jutcontrolled t r a f f i c environment. To determine t h e 5 &.at c a l pr. w i t i e s o f various t r a f f i c paramet?-s, data can be C;+*.- ., l d i n i l r s p a c e b l o c k s - - t y p i c a l l y 500 ft. 500 ft. X 100 ft.
' -as shcm by Figure 6. Each airspace blocK can be charac- I - *
0 ' '
I
' I
S
"Y' I
\ ' .
I \ 40' . .
1.. .. _...
Figure 6.-Airspace block t e r i z e d by t pe o f a i r c r a f t , speed, heading, bank angle, descent (ascent r a t e J , time of day, weather conditions (winds, v i s i b i l i t y , clouds, etc.), runway, a i r p o r t , type o f approach, and o t h e r condi- t i o n s , such as touch-and-go t r a f f i c . From t h i s airspace catalogue, t h e affect of various parametric conditions can be evaluated and For example, t h e u t i l i z a t i o n of s t a t i s t i c a l algorithms developed.
a given airspace b l x k may vary as a f u n c t i o n o f a i r c r a f t type, v i s i b i l i t y , runway length, cloud c e i l i n g , wind v e l o c i t y / d i r e c t i o n , day of week o r the standard t r a f f i c p a t t e r n i n e f f e c t a t t h e a i r - p o r t .
Based on the airspace block data, a t r a f f i c p a t t e r n math model capable o f simulating various a i r t r a f f i c s i t u a t i o n s i s possible.
This model w i l l u t i l i z e Monte Carlo o r actual a i r c r a f t f l i g h t s t o simulate m i d - a i r c o l l i s i o n s i t u a t i o n s t h a t occur i n t h e uncontrolled tei-minal airspace. A weighted percentage of time t h a t each p i l o t could have seen the other a i r c r a f t through n i s v i s i o n envelope (Figure 7) w i l l be computed f o r cach m i d - a i r c o l l i s i o n simulation.
UP -- 3 0 0 ( a ) l ' l ; + ' F 5 \ ' l A 1 7 2 -- 60° 1 x o 1 2 0 - 6 0 " 00 60° 1 2 0 0 1 I 1
L I 1 1 I 1 ' 90° ' I 1
L E F T D ~ W N R I G H T Figure 7.-Aircraft v i s i o n envelopes* B y simulation o f a l l p o t e n t i a l a r r i v a l combinations, a baseline measure of p i l o t procedure and p a t t e r n influence can be established f o r the present environment. This b a s e l i n 3 imeasure can then be u t i l i z e d t o measure the r e l a t i v e improvement i n the see-and-avoid environment f o r changes i n the uncontrolled t r a f f i c p a t t e r n concept i n f l y i n g the p a t t e r n concept.
o r f o r changes i n p i l o t procedure For example, would there be a s i g n i f i c a n t improvement i n the see- and-avoid geometry and time i f the standard p a t t e r n was a r i g h t c i r c u l a r p a t t e r n w i t h bank angles l i m i t e d t o l e s s than 15 degrees a t an a l t i t u d e o f 1,000 +200 f e e t ? Would there be a s i g n i f i c a n t improvement i n the present p a t t e r n concept if bank angles were l i m i t e d , p a t t e r n a l t i t u d e was 400 feet, o r i f p a t t e r n a l t i t u d e was maintained u n t i l t u r n i n g f i n a l ?
UNCONTROLLED TRAFFIC PATTERN CHARACTERISTICS Pattern entry--To determine the i n i t i a l t r a f f i c p a t t e r n characteris- t i c s f o r the development o f f i n a l data reduction and a n a l y t i c a l programs, the tracks obtained a t t h e Salisbury-Wicomico A i r p o r t were processed w i t h e x i s t i n g programs. From t h i s data, we were able t o i d e n t i f y some ,if the t r a f f i c p a t t e r n c h a r a c t e r i s t i c s which e x i s t f o r t h i s a i r p o r t . Mid-air c o l l i s i o n reports have c i t e d the l a c k o f adhermce t o p a t t e r n procedures as a cause i n some o f the m i d - a i r c o l l i s i o n s ' 3. A t the Salisbury-Wicomico A i r p o r t , the l o c a l l y established p a t t e r n a l t i t u d e i s 800 f e e t w i t h e n t r y t o a downwind l e f t - h a n d pattern. NPRM 71-20, "Operations a t A i r p o r t s Without Control Towers," had also been issued and established the p a t t e r n shown by Figure 8. Local FAA F l i g h t Service S t a t i o n personnel had B A S € D I R E C T 1 O N
. .-
OF L A N D I N G T A N G E N T I A L E N T R Y E N T R Y S T - R A I G H T - I N T O U P W I N D D P Y C t L P A T T E R N A L T I T U D E : 1 0 0 0 F T . A . G . L . T A N G E N T I A L E N T R Y Figure 8.-Proposed uncontrolled a i r t r a f f i c p a t t e r n encouraged l o c a l p i l o t s t o t r y out t h i s new pattern. Therefore, e i t h e r p a t t e r n procedure would have been proper a t the time our measurements were made. Entry locations were analyzed f o r 175 a i r - c r a f t tracked p r i o r t o p a t t e r n entry. The percentage o f these tracks entering each l e g i s shown on Figure 9. (Those percentages SBVSTANDARD CROSSWIND PATTERN RKLHT BASE LEFT BASE ENTRIES LNTRlES XX 61.7% xx 3.4% ~ XX = COMMUTER SERVICE ONLV Figure 9.-Sal isbury t r a f f i c p a t t e r n e n t r y d i s t r i b u t i o n designated X X r e f l e c t only the d i s t r i b u t i o n o f t h e comnuter service e n t r i e s . ) From t h i s f i g u r e , we note t h a t 33% o f a l l e n t r i e s d i d n o t adhere t o e i t h e r o f the standards and were made t o base ( l e f t o r r i g h t ) or f i n a l . I n terms o f commuter service only, 62% o f the e n t r i e s observed were made d i r e c t t o base ( l e f t o r r i g h t ) and f i n a l .
I n summary, a high percentage o f t h e general a v i a t i o n and comnuter t r a f f i c d i d n o t adhere t o established p a t t e r n e n t r y rules. It i s our opinion t h a t the Sal isbury percentages are considerably higher than o t h e r uncontrolled a i r p o r t s v i s i t e d . The FAA F l i g h t Service S t a t i o n reports of ( o r the l a c k o f ) t r a f f i c t o a l l a r r i v a l a i r c r a f t may be the f a c t o r which s i g n i f i c a n t l y influences these percentages.
Pattern l e q characteristics--To determine t h e d i s t r i b u t i o n o f a i r t r a f f i c a t various points i n the t r a f f i c pattern, s i x v e r t i c a l planes were established on the t r a f f i c p a t t e r n legs. The l o c a t i o n s o f F o r each t r a c k obtained, the these planes are shown i n Figure 10.
-5000' 0 5000' 1 1 1 1 1 1 1 1 1 1 1 1 b CROSSWIND (CW) II ..
I I
=' 5000 '
-
D W 1 UPWIND (UW) LANDING
-
DOWNWIND RUNWAY
-
D W 2 THRESHOLD - 0 I
I
BASE II FINAL
-
-5000 '
Figure 10.-Location o f v e r t i c a l planes distance ( X o r Y ) and a l t i t u d e ( Z ) were tabulated f o r computation of s t a t i s t i c a l properties. A summary o f these computations i s shown i n Table I 1 f o r a l l a i r c r a f t and f o r t h e single-engine high-wing
(SEHW) , single-engine low-wing (SELW) , and twin-engine (TE) a i r c r a f t
which produced the t o t a l t r a f f i c d i s t r i b u t i o n observed. A compari- son of the mean distances and mean a l t i t u d e s observed a t each plane i s shown i n Figures l l a and l l b , respectively.
From Table 11 and Figure 11 , we note t h a t the mean p a t t e r n distance
o f the SEHW a i r c r a f t i s approximately 0.2 NM less than S E L W a i r c r a f t
and approximately 0.3 - 0.4 NM less than TE a i r c r a f t . The TE a i r -
c r a f t mean a l t i t u d e exceeds SEHW and S E L W a i r c r a f t a l t i t u d e s on a l l legs except base and f i n a l where TE a i r c r a f t t r a n s i t i o n s t o the lowest mean a l t i t u d e . The convergence o f mean distance occurring on f i n a l i s i l l u s t r a t e d by these figures and supports mid-air c o l l i s i o n data i n t h i s area. The standard d e v i a t i o n of distance
about the mean f o r t h e t r a f f i c cases above i s t y p i c a l l y 0.3 - 0.4
NM except f i n a l where i t has converged t o approximately 200 feet.
The standard d e v i a t i o n o f a l t i t u d e t y p i c a l l y decreases a t each Table I 1 . - S t a t i s t i c a l praperties Enqine S i n g l i Sinal1 ALL @* I QW.
i nq Hihh A l t . D i s . -$‘d’ D i s . I A l t . D i s . A l t .
-
-
Number 14 915 5309 Mean ( F t . ) 5186 943 4067 903 118 3294 S t d . Dev. ( F t . ) 2674 210 2251 223 -0.77 1.47 Skewness 1-16 0.478 0.045 0.082 0.655 0.55
1.95 3.95 Klrrtos i s 4 . 5 3 2.83 1.37 1.54 11.5 I 1.5
-
0.36 SDearman Rank 0.29 -- l 4 0.4
-
13 Number 15
I - -
7591 Mean ( F t . ) 1011 949 6583 287 2613 ~ 258 2650 1.63 0.462 0.509 -0.027 6.9 2.32 Kurtosis 3.93 2.03
- -
-0.47 -0.10 0.08
-
- - - -
45 Number 54 39 844 844 4491 6539 876 8 1 6 212 1828 197 2 1 58 2806 194 1.51 3.13 -0.11 1 . 9 7 0.57 1.65 0.82 !.46 K u r t o i i s 7.05 3.25 9.06 3.63 8.17 16.96 4.62 11.1 -
- - -
0.39 -0.10 0.21 I
- - -
Number 159 - ’ 64
50 45 Mean ( F t . ) 5600 780 4730 773 5577 6860 825 749 Std. Dev. ( F t . ) 2391 193 2441 209 1736 2434 187 \ 174 Skewness 1.23 3.3 1.64 0.72 0.27 0.54 1.17
K u r t o s i s ; : ! : ’ I 6.14
7.8 4.02 3.45 3.9 16.7 5.3
-
- -
0.3? 0.42 Spearman Rank 0.11 - 0.19
~~ ~~
- -
Number 225 77 71 Mean ( F t . ) 5535 552 3995 577 5331 Std. Dev. ( F t . 2936 164 2543 2326 Skewness 1.34 1.37 2.81 0.998 1.7 K u r t o s i s 6.48 6.7 16.7 9.1 4.68
- -
-
Spearman hank 0.01 0.16 -0.03
- - 42 -
5 9 Number 159 Mean ( F t . ) -41.7 252 -77 299 -29 261 Std. Dev. ( F t . ) 196 99.6 248 93.2 187 97.9 Skewness 2.87 0.37 3.64 1.13 0.47 1.35 K u r t o s i s 27.14 2.96 26.8 4.8 6.i7 5.8
- Spearman Rank 0.17 - 0.08 -
0.21 0.02 I I 1
I
I TRAFFIC PATTERN PLANE
FINAL
. . .. ****'
\ BASE
* . . . * * * 0.
9 .
. . . * * v, I . e ' UPWIND . . * * UPWIND CROSSWIND DOWNWIND BASE FINAL
- 50
M E A N OF ALL AIRCRAFT P . .
t
....
* t
i
Figure 11 b. -Mean a1 ti tudes Figur? 1 1 . -Comparison o f mean distances and a1 ti tudes subsequent p a t t e r n l e g plane and corresponds somewhat t o the decrease i n the mean a l t i t u d e s observed. The skewness of the d i s t r i b u t i o n s i n distance and a l t i t u d e show .n Table I 1 i n d i c a t e s t h a t t h e d i s - t r i b u t i o n s i n general are n o t normal and are skewed t o the s i d e of the mean having greater distances o r a l t i t u d e s . (Skewness = 0 f o r normal d i s t r i b u t i o n . ) The kurtosis--normal d i s t r i b u t i o n i s %-of a higher value than f o r a normal the data obtained i s generally d i s t r i b u t i o n which i n d i c a t e s a more peaked d i s t r i b u t i o n shape than normal. The d i s t a n c e - a l t i tude Spearman rank-correlation c o e f f i c i e n t was computed f o r each plane and t h e values i n d i c a t e l i t t l e cor- r e l a t i o n e x i s t s between a l ti tude and distance d i s t r i b u t i o n s .
S t a t i s t i c a l analysis o f t h e d i s t r i b u t i s n s observed i n d i c a t e s t h a t Log Normal o r Extreme Value (Fisher-Tippett Type d i s t r i b u t i o n s may be used t o model t h e a i r t r s f f i c p a t t e r n legs f o r t h e Salisbury- Wicomico A i r p o r t . The t h e o r e t i c a l Log-Normal d i s t r i b u t i o n s and t h e t r a f f i c percentiles observed a t each p a t t e r n plane are shown i n Figures 12a through 12f. From these figures, we see t h a t t h e d i s - t r i b u t i o n o f uncontrolled a i r t r a f f i c i s f a r d i f f e r e n t from what one would expect f r o m t h e p i c t o r i a l p a t t e r n o f Figure 8. Tile pat- t e r n legs extend f r o m approximately 1/4 NM o u t t o 3 N M i n distance from the runway and f r o m 400 f e e t t o 1800 f e e t i n a l t i t u d e .
il ., 2; I I t
-- ---
r, . '1, L J f i g u r e 12a.-Upwind plane d i s t r i b u t i o n s
F I N A L Z . FlLL -
F I N R L X . FILL -
LOS NORMFIL. L O 5 NORMRL.
I I I c 7 ' 6 .no 12 .oo 1 2 . 6 5 0 .Gel PE R C C NTFlGE Figure 12f .-Final plane d i s t r i b u t i o n s Since the Spearman rank-correlation c o e f f i c i e n t t e s t indicates l i t t l e correlation between distance and a l t i t u d e d i s t r i b u t i o n s , the combined Log Normal d i s t r i b u t i o n s can be represented i n b i v a r i a t e formlo as shown i n Figure 13. This f i g u r e i l l u s t r a t e s the airspace n 7 900 !Z 600 I- Po PROBABILITY DENSITY
zt 300 D- - DRnnABILITY ENVELOPE
1 2 3
DISTANCE (NM)
Figure 13.-Probability density & envelopes f o r crosswind l e g t h a t t h e t h e o r e t i c a l cross-section o f t h e crosswind l e g occupies, the associated probabil i t y density and envelopes, and exemrl i f i e s the l a r g e area o f airspace a p i l o t must search t o prevent a mid-air c o l l i s i o n w i t h another a i r c r a f t .
The d i s t r i b u t i o n s above represent a l l t r a f f i c observed a t Salisbury, Maryland. This t r a f f i c was p r i m a r i l y single-engine ( h i g h and low wing) and twin-engine a i r c r a f t . A n example o f the c o n t r i b u t i o n made by each type o f a i r c r a f t f o r the OW2 plane a t Salisbury i s shown i n Figure 14. I f t h e t r a f f i c d i s t r i b u t i o n s f o r these general a i r c r a f t c l a s s i f i c a t i o n s are consistent between a i r p o r t s , t h e un- c o n t r o l l e d t r a f f i c environment a t any a i r p o r t may be modeled when the a r r i v a l rates and population r a t i o s are known.
L- o c rJ m -1 Figure 14.-Contribution by type t o t o t a l d i s t r i b u t i o n a t D W 2 MI D-AI R COLL I S I ON SIMULATION To i l l u s t r a t e a p i l o t ' s see-and-avoid problem and the method w e plan t o use f o r t h i s study, two actual tracks a t the Salisbury- Wicomico A i r p o r t were time normalized such t h a t c o l l i s i o n would occur a t t h e runway threshold.
The p o s i t i o n (X, Y ) and a l t i t u d e (Z) time h i s t o r i e s o f these a i r c r a f t are shown on Figure 15. Both o f these a i r c r a f t ( A & B) were Cessna 172's t h a t f l e w standard approaches a t a1 ti tudes near t h e pub1 i shed p a t t e r n a1 ti tude.
The view angle from one a i r c r a f t t o the other was computed f o r both a i r c r a f t depending on t h e i r heading, bank angle, and a l t i t u d e and 0 1 I I I 7 L 1 7 0 0 I50 1 0 0 T I M E T O T m E S n o L F (aecondal - 3 . 0 -2.0 -1.0 0.0 RUWI x (m) Figure 15.-Position & a l t i t u d e time h i s t o r y distance separation. A time h i s t o r y o f t h i s data was p l o t t e d on each a i r c r a f t ' s view envelope as shown i n Figure 16. From t h i s figure, it i s obvious t h a t there a r e considerable periods of time t h a t t h e p i l o t s cannot see each other.
UP UP I t Hack m r k s arc a t A I R C R A F T "8' l B O o 170° 60' W V N 6 0 ° 170" L B O " 0 ' LEFT I6HT L E F T R I G H T Figure 1 6 . - A i r c r a f t view envelopes The time h i s t o r y o f range between these a i r c r a f t and t h e periods each p i l o t could n o t see t h e other a i r c r a f t are shown 3n Figure 17.
The p i l o t o f a i r c r a f t A was able t o see a i r c r a f t 6 approximately 2.0 o A cannot see B x B cannot see A 1 . 5 h 1 .o v w 0 . 5 240 220 200 180 160 140 120 100 80 60 40 20 0 SECONDS TO THRESHOLD Figure 17. -Time h i s t o r y o f range between a i r c r a f t one-third o f t h e time during t h e l a s t 2 NM o f closure w i t h a i r c r a f t B. Other f a c t o r s t h a t would have reduced t h e chance o f seeing a i r - c r a f t B are t h a t : (1) t h e p i l o t o f A would have t o detect B against an e a r t h background; ( 2 ) B would have presented n e a r l y a head-on p r o f i l e during t h e closure from 2 t o 3/4 NM and provided l i t t l e r e l a t i v e movement i n t h e A p i l o t ' s view f i e l d a t t h a t
(3) t h e A p i l o t ' s a t t e n t i o n during t h e 120 second -
c r i t i c a l time; 90 second time period would probably be d i r e c t e d toward t h e runway i n preparation f o r the base turn.
The p i l o t i n B could have seen A only about one-tenth o f t h e time during the l a s t 2 NM o f closure. His best opportunity t o see A occurred during t h e t u r n t o t h e downwind l e g a t 190 seconds. A t t h i s time, h i s a t t e n t i o n could have been on downwind alignment r a t h e r than airsearch. Since B was below and ahead of A, the B p i l o t ' s detection o f A a f t e r h i s t u r n downwind i s very u n l i k e l y .
This example i l l u s t r a t e s the l i m i t e d amount o f time a p i l o t f l y i n g a near normal p a t t e r n may have f o r detecting other a i r c r a f t . These tracks were taken on d i f f e r e n t days; however, by chance could accurately represent a m i d - a i r c o l l i s i o n s i t u a t i o n .
CONCLUSIONS The i n i t i a l data analyzed f r o m t h e Salisbury-Wicomico A i r p o r t v e r i - f i e s t h a t the u n c o n t r o l l e d a i r t r a f f i c patterns flown are h i g h l y variable. It can be demonstrated t h a t normal p a t t e r n v a r i a t i o n s create m i d - a i r c o l l i s i o n s i t u a t i o n s i n which one o r both p i l o t s involved may be unable t o see one another a t c r i t i c a l times d u r i n g t h e i r approach. The high percentage o f non-standard e n t r i e s observed tends t o v e r i f y NTSB conclusions t h a t t h i s c o n d i t i o n may be a f a c t o r f o r concern. The sample t r a f f i c d i s t r i b u t i o n s obtained i n d i c a t e t h a t , i n general, a i r t r a f f i c i s n o t normally d i s t r i b u t e d about the mean paths i n e i t h e r distance o r a l t i t u d e . Most o f t h e t r a f f i c p a t t e r n data observed, however, can be modeled using d i s c r e t e d i s t r i b u t i o n s . A i r t r a f f i c s i m u l a t i o n u t i l i z i n g these d i s t r i b u t i o n s should provide new i n s i g h t s t o p i l o t i n g procedures and t r a f f i c p a t t e r n concepts which enhance a p i l o t ' s see-and-avoid p o t e n t i a l i n the uncontrolled environment.
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