FREQUENCY OF ANTI-COLLISION OBSERVING 

RESPONSES BY SOLO PILOTS AS A FUNCTION 

TRAFFIC DENSITY, ATC TRAFFIC WARNINGS, 

AND COMPETING BEHAVIOR 

Mark F. Lewis, Ph.D. 

FAA Civil Aeromedical Institute 

P .0. Box 25082 

Oklahoma City, Oklahoma 73125 

April 1973 

Availability is unlimited. Document may be released 

to the National Technical Information Service, 

Springfield, Virginia 22151, for sale to the public. 

Prepared for 

DEPARTMENT OF TRANSPORTATION 

FEDERAL AVIATION ADMINISTRATION 

Office of Aviation Medicine 

Washington, D.C. 20591 
Federal Aviation Administration, Office of Aviation 

Medicine, Civil Aeromedical Institute, Oklahoma City, 

Oklahoma. FREQUENCY OF ANTI-COLLISION 

OBSERVING RESPONSES BY SOLO PILOTS AS 

A FUNCTION OF TRAFFIC DENSITY, ATC TRAFFIC WARNINGS, AND COMPETING BEHAVIOR 

by Mark F. Lewis, April 1973, 4 pp. Report No. 

FAA-AM-73-6. 

Eighteen instrument-rated pilots were flown in two-hour simulated solo missions during which the frequency of traffic, 

ATC warnings, and ATC clearances were varied, while the 

visibility of the target was held constant at 100

%. In order 

to observe the target, the pilot was required to make a simple, 

overt observing response; i.e., the pilot had to press a button 

on his control wheel. If traffic was present, a burst of flashes 

became visible through the windshield. Button presses in the 

absence of traffic produced no stimulus. Each pilot was advised that his IFR mission occurred under VFR conditions and that it was his primary responsibility to maintain visual 

vigilance, although ATC would endeavor to warn him of 

possible conflicting traffic. Two values of traffic frequency 

were programmed independently of two values of ATC traffic 

warning frequency. The frequency of competing behavior 

was varied by independent scheduling of two values of ATC clearance frequency. The data revealed main effects from 

ATC clearances and from traffic warnings. Significant interactions were obtained for clearances by traffic warnings and for traffic by traffic warnings. 

I. Lewis, M. F. 

Descriptors 

Behavior 

Aviation Safety Collision 

Avoidance 

Attention 

Federal Aviation Adm

Medicine, Civil Aeromed

Oklahoma. FREQUEN

OBSERVING RESPON

A FUNCTION OF TRA

FIC WARNINGS, AN

by Mark F. Lewis, A

FAA-AM-73-6. 

Eighteen instrument-ra

ulated solo missions · d

ATC warnings, and Avisibility of the target to observe the target, t

overt observing respon

on his control wheel. 

became visible through 

absence of traffic produ

vised that his IFR m

and that it was his pr

vigilance, although AT

possible conflicting traf

were programmed inde

warning frequency. T

was varied by indepen

clearance frequency. 

ATC clearances and fr

actions were obtained 

for traffic by traffic wa
FAA-AM-73-6 

4. Title and Subtitle 5 . Report Date 

FREQUENCY OF ANTI-COLLISION OBSERVING RESPONSES BY 

SOLO PILOTS AS A FUNCTION OF TRAFFIC DENSITY, ATC 

TRAFFIC WARNINGS , AND COMPETING BEHAVIOR 

April 1973 

6. Performing Organization Code 

7. Author{s) 

Mark F. Lewis, Ph.D. 

9 . Performing Organization Nome and Address 

FAA Civil Aeromedical Institute 

P. 0 . Box 25082 

Oklahoma City, Oklahoma 73125 

12. Sponsoring_ Agency Name and Add res• 

Office of Aviation Medicine 

Federal Aviation Administration 

800 Independence Avenue, S.W. 

Washington, D. C. 20591 

15. Supplementary Nates 

8 . Performing Organization Report No. 

10 . Work Unit No. 

11 . Contract or Grant No . 

13. Type of Report and Period Covered 

OAM Report 

14 . Sponsoring Ag.;ncy Code 

This research was performed under Tasks AM-A-72-PSY-26 and AM-A-73-PSY-26. 

16. Abstract 

Eighteen instrument-rated pilots were flown in .two-hour simulated solo missions 

during which the frequency of traffic, ATC warnings, and ATC clearances were varie

while the visibility of the target was held constant at 100%. In order to observe 

the target, the pilot was required to make a simple, overt observing response; i. e 

the pilot had to press a button on his control wheel. If traffic was present, a 

burst of flashes became visible through the windshield. Button presses in the 

absence of traffic produced no stimulus. Each pilot was advised that his IFR 

mission occurred under VFR conditions and that it was his primary responsibility 

to maintain visual vigilance, although ATC would endeavor to warn him of possible 

conflicting traffic. Two values of traffic frequency were progratmned independE1.ntl

of two values of ATC traffic warning frequency. The frequency of competing behavi

was varied by independent scheduling of two values of ATC clearance frequency. 

The data revealed main effects from ATC clearances and from traffic warnings. Sig

nificant interactions were obtained for clearances by traffic warnings and for 

traffic by traffic warnings. 

17. Kl'Y Wards 

Behavior 

Aviation Safety 

Collision Avoidance 

Attention 

19. Security Classif. {of this report) 

Unclassified 

Form DOT F 1100;7 (e-s9l 

18. Distribution Statement 

Availability is unlimited. Document may 

be released by the National Technical 

Information Service, Springfield, Virgin

22151, for sale to the public. 

20. Security Classif. {of this page) 21. No. of Pages 22. Price 

Unclassified 4 $3.00 
ACKNOWLEDGMENTS 

I would like to express my appreciation to Paul E. Young, Noble S. 

Daniels, and Clifford L. Dodson who contributed much time and effort in 

implementing the task on the C-340 simulator; to Jack G. Myers and Robert 

J. Wargo, who patiently served as air traffic controllers during the study, to 

Earl D. Folk, who helped construct the counterbalanced statistical procedure; 

to Larry E. Toothaker, who suggested and reviewed the statistical procedures; 

and to Karen N. Jones and Nancy M. Bourdet, who devised and implemented 

the statistical analyses employed. 
FREQUENCY OF ANTI-COLLISION OBSERVING RESPONSES BY 

SOLO PILOTS AS A FUNCTION OF TRAFFIC DENSITY, ATC 

TRAFFIC WARNINGS, AND COMPETING BEHAVIOR 

I. Introduction. 

Topics related to collision avoidance that have 

been researched in the past include the conspicuity of the target lights1 2 3 4 and the interaction of target resolution and visibility with 

visual scanning. 5 6 Studies of human vigilance 

are numerous, but the techniques available have 

not yet been applied to collision avoidance. The 

current study applies the operant technique developed by Holland for the study of observing 

responses7 in an attempt to isolate the relevant 

variables that affect pilot vigilance in collision 

a voidance. Since one well-researched factor is 

target visibility (or conspicuity), it was decided 

at the outset to exclude visibility as a factor in 

the current study and to concentrate instead on 

those factors other than conspicuity which might 

influence pilot vigilance. Target frequency was 

selected as a variable for investigation because 

it seems obvious from Holland's work8 that observing responses, when considered as simple 

operants, are reinforced by the appearance of 

the target. The second variable, the frequency 

of Air Traffic Control (ATC) traffic warnings, 

was selected because, in the operant model, ATC 

traffic warnings may be considered as positive 

discriminative stimuli (SD) for the performance 

of the observing response, the degree of stimulus 

control exerted by these warnings being a function of the frequency of their validity; i.e., if 

observable traffic were always preceded by traffic 

warnings, traffic warnings would represent a 

perfectly reliable sn. When traffic appears in 

the absence of an ATC traffic warning or when 

the warning precedes traffic that cannot be seen, 

traffic warnings should become less effective as 

an S0 for the observing response. Finally, because pilots do not spend all their time searching 

for possible conflicting traffic, the frequency of 

behaviors that would compete with the observing 

1 

response was systematically varied. This com

petitive behavior was generated by includi

ATC clearance procedures as a variable. Ea

variable had two values (high and . low) in t

current study. 

II. Method. 

A. Subjects. The subjects were 18 instrumen

and multi-engine-rated male and female pilo

who were either FAA personnel selected fro

the available ·pilot population at the FAA Aer

nautical Center, or volunteer members of t

Oklahoma City aviation community. Those su

jects who were FAA personnel received th

regular salary for participation. Non-FAA p

sonnel were paid an hourly wage for particip

tion. 

B. Apparatus. The equipment used consist

of a fixed-base Convair-340 simulator (Curt

Wright) in which a microswitch was install

on the right side of the left control wheel. T

switch was connected to an input module of 

panel of solid-state logic modules (Lehigh Vall

Electronics) used for session timing, respon

recording, and control of experimental cont

gencies. A pulse from one output module w

used to gate a General Radio Model 1538

Strobotac to produce a 0.6-sec burst of flash

to serve as the target. The Strobotac w

mounted directly in front of the simulator a

directed towards the windshield. When p

sented, flashes from the Strobotac were alwa

visible through the translucent windshield of t

simulator, no matter where the pilot was lookin

C. Procedure. Each subject was assigned 

random to one of two groups of subjects, ea

group receiving stimulus protocols that w

counterbalanced over time and that were co

plements of the protocols used by the ot

group; i.e., since there were three 2-valued va
gqy), 

were divided into eight periods with one value 

of each variable assigned to periods in counterbalanced order in such a way that each unique 

combination of values occurred once during a 

session, but the orders of unique combinations 

were reversed for subjects assigned to the second 

group. 

After being briefed about the aircraft, each 

subject was given a flight plan (Figure 1) and 

told to maintain as closely as possible the indicated headings and airspeeds. Each subject was 

told to report passing over each checkpoint 

(ALPHA through NOVEMBER in Figure 1). 

After the briefing, the following instructions 

were read to the subject: 

72 NM 

20 MIN 

J 

L 

K 

40 NM 

II MIN 

F 

yg 

IFR mission under VFR conditions. Yo

flight plan will take you through regions 

varying traffic density. Although Air Traf

Control will endeavor to warn you of possi

traffic, it is still your primary responsibility 

look for possible traffic. 

"Every time you wish to look for traffic, pr

the button on the control wheel; if traffic is n

visible, nothing will happen. However, if traf

is present, you will see it when you press t

button. Press it now and you will obser

traffic." (At this point the experimenter wait

until the subject had pressed the switch a

observed the flashes at least once.) "If traffic 

present, it will always appear as you have j

eooo' 

@) 155 KIA

Start 

FIGURE 1. Flight plan used by participating pilots. The experimental protocol began when each pilot r epor

over point ALPHA and ended two hours and 40 minutes later, llefore each pilot had reached point NOVEMBE

2 
wish to look for traffic. Pressing it and holding 

it down will not work. If you report all traffic 

you encounter to ATC, you need not take evasive 

action and you will enjoy a safe flight. 

"You will not encounter any traffic until after 

you report over point ALPHA. Before reaching 

point ALPHA, you may press the button for 

practice, but you will not see any traffic. Please 

contact the Oklahoma City Tower on 118.3 when 

you are ready to take off." 

As indicated in Figure 1, the subject had approximately eight miri available for practice 

(with no feedback) after take-off. During these 

eight min, the pilot was expected to reach his 

assigned altitude of 8000 feet. As soon as the 

subject reported passing over point ALPHA, the 

automated experimental protocol was started. 

The session consisted of eight 20-min segments, 

during each of which the predetermined order 

of frequencies for each variable was programmed. 

Traffic warnings and traffic were both generated by feeding the same time base (25 sec) into 

two independent sets of probability gates. For 

high frequency traffic warnings, the probability 

gate selected had a value of 0.85, while the low 

frequency gate was set at 0.15. High frequency 

traffic was determined with an independent 

probability gate of 0.85, while the low traffic 

gate was set at a probability of 0.15. High frequency clearances were determined by a timebase 

of 60 sec feeding through a probability gate of 

0.75, while low frequency clearances depended 

on a 360-sec timebase via a 0.5 probability gate. 

Clearances consisted of instructions from ATC 

to change altitude, to change radio frequencies, 

to alter transponder settings, and in those cases 

where the pilot was proceeding too quickly 

through the flight plan, to reduce airspeed. 

A response produced traffic (flash burst) if the 

response occurred within seven sec of a scheduled traffic event; i.e., each event (clearances, 

warnings, and traffic) may be described as occurring on Random Interval (RI) schedules of 

reinforcement, where only traffic required a re

traffic timebase incorporated a delay of three 

from the warning timebase, so that in the ev

of simultaneous warning and traffic, the warni

could be delivered before onset of the traf

limited hold. 

III. Results. 

A repeated measures analysis of variance w

one between-subjects factor and three with

subjects factors (the equivalent of a split-p

factorial-2.222 analysis of variance9 ) indicat

that the effect of order of presentation of tre

ments (the between-subjects factor) was no

significant ( F < 1.00). Therefore, the number 

observing responses for each treatment combin

tion was combined across order of presentati

and a 23 repeated measures design,. where 

factors were within-subjects factors, was used 

complete the analysis. The cell means for 

treatment combinations are presented in Table 

The analysis revealed significant main effe

from frequency of clearances (F=4.85; df=1,1

p<.05) and from frequency of traffic warnin

(F=7.95; df= 1,17; p< .05). Significant inte

actions were. obtained for clearances x traf

warnings (F=7.17; df=1,17; p<.05) and traf

x traffic warnings (F=7.83; df=1,17; p<.05

To qualify the statements concerning the signi

cant main effects of clearances and traffic war

ings, tests of simple main effects were compute

The following main effects were significa

clearances at the low frequency level of traf

warning (F=11.18; df=1,34; p<.005), traf

warnings at the high frequency level of cle

ances (F=11.83; df=1,34; p<.005), and traf

warnings at the low frequency level of traf

(F=14.10; df=1,34; p<.001). 

Individual comparisons with Tukey's HS

test indicated that there was a significant diff

ence (p<.01) between the mean number of o

serving responses in each of the followi

situations: ( 1) 'iVhen clearances occurred at 

high frequency, more ob~erving responses we

made when traffic warning frequency was hi

TABLE 1. Mean number of observing responses. 

Clearances ( L:-o-w---:)----------;:C::-l e-a-ra_nc_ e_s - (;-;; Hco-:i-gh=-):---

Traffic (Low) Traffic (High) Traffic (Low) Traffic (High) 

=T,.... ra-:ffi=c--=w=a- r-n7in- g----,(-=- L-ow----:-) -_-_-__ - _-_-_-_ _-_-_-__ -_-_-__ -_-_-_-----:-c:158.00 165. 78------::-99=-. ..,., 28==------,--14-=-=7=-=.3=-=9-

Traffic Warning (High) - -- --------------- 247.78 173.83 206.72 209.06 

3 
(2) When traffic warning frequency was low, 

more observing responses occurred when clearance frequency was low than when clearance 

frequency was high ; ( 3) When traffic frequency 

was low, more observing responses were made 

under conditions of high frequency of traffic 

warnings than when the frequency of traffic 

warnings was low. 

IV. Discussion. 

Under ideal circumstances, a solo pilot flying 

under VFR conditions makes a number of observing responses sufficient to detect all visible 

and possibly conflicting traffic. The current 

study indicates that, in fact, the observing response rate is affected by the frequency with 

which the pilot receives traffic warnings and the 

amount of competing behavior the flying task 

imposes on his work load. That these variables 

interact with each other and with traffic density 

surely is no surprise; the results suggest that 

when competing behavior is prepotent and traffic 

density is low, the co:p.sequent decrease in traffic 

warnings yields a diminution in vigilance that 

might well result in disaster. The current study 

indicates a simple method for evaluating these 

factors and additional research is clearly indicatt~d. Because only two values were assigned to 

each variable, and because the exposure of each 

subject to each combination of variables was restricted to a relatively short duration, the reader 

sions from the data presented. 

What is definitely indicated is the need for 

full parametric study of the variables select

for study in this paper, in addition to studi

introducing variation in crew size and in th

placement and visibility -of the target (by me

tioning visibility, it is implied also that targ

size and hue are factors to be introduced in th

course of future research). 

That traffic density was not in itself a signif

cant main effect may, at first, seem surprisin

However, the subjects worked on what may b

described as a mixed RI RI schedule of rei

forcement with relatively short periods for eac

component schedule. Consequently, it is qui

reasonable for their behavior to reflect a sing

RI schedule, the mean interval of which is a

average of the component schedules. Thus, th

proposed parametric study should include vari

tion in exposure time to each combination 

variables, a procedure that might well indica

if fatigue (a factor excluded from the curre

study) affects vigilance. 

Finally, mention should be made of the com

ments made by a number of our subjects to th

effect that pressing the button was not at all lik

visually scanning for traffic. Obviously the r

sponses are not the same. However, there 

clear laboratory evidence that the rates for 

simple operant observing response are comparab

with eye movement data, when similar contin

gencies are in effect.10 

REFERENCES 

1. Gerathewohl, S. J.: Conspicuity of Steady and 

Flashing Light Signals: Variation of Contrast, 

. TOURNAL OF THE OPTICAL SOCIETY OF 

AMERICA, 43 :567, 1953. 

2. Connors, M. M.: Luminance Requirements for Hue 

Perception in Small Targets, JOURNAL OF THE 

OPTICAL SOCIETY OF AMERICA, 58:258, 1968. 

3. Connors, M. M.: Luminance Requirements for Hue 

Perception and Identification, for a Range of Exposure Durations, JOURNAL OF THE OPTICAL 

SOCIETY OF AMERICA, 60 :958, 1970. 

4. Mertens, H. W., and M. F. Lewis: Temporal Discrimination of Two-Pulse Stimuli as a Function of 

Psychophysical Method and Luminance, JOURNAL 

OF :g_IE OPTICAL SOCIETY OF AMERICA, 

62 :823, 1972. 

4 

5. Edwards, G. D., and J. L. Harris, Sr.: Visual A

pects of Air Collision Avoidance. Scripts Institu

of Oceanography Report 72-3, 1972 . 

6. Calvert, E. S. : Some Operational Aspects of the U

of Aircrew-Interpreted Devices for Preventing Co

lisions in the Air. Royal Aircraft Establishmen

lCAO Working Paper, AIR C-WP/49, 1948. 

7. Holland, J. G.: Technique for Behavioral Analys

of Hurrian Observing, SCIENCE, 125 :348, 1957. 

8. Holland, J. G. : Human Vigilance, SCIENCE, 128 :6

1958. 

9. Kirk, R. E. : Experimental Design: Procedure f

the B ehavioral Sciences, Belmont, California, Brook

Cole, 1968. 

10. Schroeder, S. R., and J. G. Holland: Operant Co

trol of Eye Movements During Human Vigilanc

SCIENCE, 161 :292, 1968.