78-1

-. pJMPARISON OF THE VISUAL ZERCEPTION OF A ,W--N-WAY '40D)J l'I

~PILOTS AND 'ý'NFILOTS 6JRING XMULATED NIGHT E]ANDING, ArPP.OA/-:,1'

w/C Henry w'ertens

Federal Aviation Administration

Oklahoma City, Oklahoma

CM.:) Document is available to the public through t1he

National Technical Information Service

Springfield, Virginia 22161

Prepared for

U. S. DEPARTiT-NT OF TRAXNSIPORTA'1ION

Federal. Aviation Adminis tration

Office of Aviation Miedicine

Washington, D.C. 205911

C,.. -.L1 

Reproduced FromD

Best Available Copy
NOTICE

This document is disseminated under the sponsorship of

the Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for its contents or use thereof.

Reproduced Best Available From Copy
Technical Report Documentation Page

I Report No. 2. Govenment Accession No. 3. Recipient's Catalog No.

/( FAA-AM+478- 15

- - Titl. and 7 -t-tIe - 5. Report Date

COMPARISON OF THE VISUAL PERCEPTION OF A RUNWAY MODEL March 1978 IN PILOTS AND NONPILOTS DURING SIMULATED NIGHT LANDING 6. Performing Organization ¢mdo

APPROACHES

8. Performing Organizatior Report No.

Au thor'sa)

Henry W. Mertens 9. Performing 0rganzation Name and Address 10. Work Unit No. (TRAIS)

FAA Civil Aeromedical Institute-/

P.O. Box 25082 11, Contract or Grant No.

Oklahoma City, Oklahoma 73125

13. Type of Report and Period Covered

12. Sponsoring Agency Name and Address

Office of Aviation Medicine

Federal Aviation Administration

800 Independence Avenue, S.W. 14. Sponsorin Agency Code

Washington, D.C. 20591

15. Supplementary Notes

This research was conducted under Tasks AM- Y-38 and AM-D-7 -PSY-67.

16. Abstract

At night, reduced visual cues may promote illusions and a dangerous tendency foi

"piiots to fly low during approaches to landing. Relative motion parallax (a difference in rate of apparent movement of objects in the visual field), a cue that can contribute to visual judgments of glide path angle, was studied for its effect on the nighttime approach .problem in two experiment..

In the first experiment, 16 pilots and 16 nonpilots adjusted the slant of a model

runway to make it appear horizontal under nighttime conditions on dynamic trials as

the model approached them, and on static trials with the model stationary. In the

second experiment, 12 pilots and 12 nonpilots performed the same task in dynamic

trials while viewing the model in a dark field as before, and while viewing the model

within a window which provided a stable visual frame of reference. Pilots &I" made

supplementary judgments in which they verbally estimated magnitude of simulated

approach angle in degrees or adjusted the model to produce a 30 approach angle.

>Neither flying experience nor a visual frame of reference enhanced sensitivity to

relative motion parallax. However, errors in horizontal adjustments were smaller in

pilots, indicating that flying experience enhances other cues in the runway image.

Direct judgments of approach angle magnitude indicated/overestimation by an approximate factor of 2. These findings give further evidence of large visual illusions in the nighttime situation and indicate that the ineffectiveness of relative motion parallax may be an important part of the night approach problem. J 17. Key Words 18. Die.rlbutlen Statement

Aircraft Landing Document is available t the public Visual Illusions through the National Tec nical Information

Motion Parallax Service, Springfield, Vi inia 22151

19. Security Classif. (of this report) 20. Security Clesslf. (,f this page) 21. No. of gee 22. Pricei

Unclassified Unclassified 22

Form DOT F 1700.7 (-72) Reproduction of completed page authorized

•, .,',' .,-,;. .:..,,•,,: ::...... •u .-•:, • ,.•:wt,•./",,• •4 l..tli'•,m m .,,•- 6. • •,.',.- '-,.. ..-. ........ 
COMPARISON OF THE VISUAL PERCEPTION OF A RUNWAY MODEL IN

PILOTS AND NONPILOTS DURING SIMULATED NIGHT LANDING APPROACHES

Introduction.

The dangerous tendency of pilots to fly too low during night visual

approaches to landing has long been recognized as an important problem (21),

but little research has been concerned with identifying the important visual

cues for judgment of approach angle (17,30). In this context, relative motion

parallax is one of the visual cues most frequently suggested as contributing

to visual judgments of angle of approach (7,11,13,14,15,24,28,29). Relative

motion parallax is defined as a difference in rate of apparent movement of

objects in the visual field (4,9). In approaches to landing, all objects in

the image of the ground plane appear to move directly away from the aim point

in a complex pattern of apparent velocities which is a function of glide path

angle and approach speed (7).

Several laboratory studies suggest that relative motion parallax can

enhance the perception of slant or shape of a surface when other cues to

orientation are present (2,3,22,26). These studies do not, however, provide

data that might be used to predict whether relative motion parallax would be

effective during approaches to landing, or if so, at what approach speeds and

distances from the runway it would be an effective cue.

Kraft (17) has shown that the presence of low values of relative motion

parallax that occur at distances greater than 4 miles from runway threshold

did not prevent experienced pilots from flying below the proper glide path

when upsloped terrain was behind the runway during simulated approaches.

Mertens (21) studied the effectiveness of relative motioui parallax due to

both radial and vertical motion on the perceived orientation of a runway model

by nonpilots in the segment of simulated approaches between 1.33 and 4.33

nautical miles from threshold. Again, no effect of relative motion parallax

of practical significance was observed. He suggested that the effectiveness

of relative motion parallax in judgments of the model runway orientation might

be enhanced by the cockpit window and by flying experience. These variables

were examined in the experiments described below regarding their relation to

the effectiveness of relative motion parallax in the perception of the

horizontal orientation of a moving runway model. Several other variables were

studied in these experiments including direction of the model in the visual

field and the psychophysical method by which perceived orientation was

measured. Judgments of a stationary model were obtained in addition to those

involving radial motion in order to assess the effect of relative motion

parallax.. - .. Ioo Im
Judgments of horizontal orientation in the experiments described below

concerned the geographical slant of the runway as distinguished from its

optical slant (6). Optical slant is defined as the slant of a surface A

relative to the line-of-sight to the surface; geographical slant is defined

as the slant of a surface relative to gravity. The perception of geographical slant involves both perception of optical slant and perception of angular

position (or height) in the visual field relative to the st raight-ahead

direction. Although judgment of optical slant is most similar to the pilot's

visual task in judging approach angle, the accuracy of judgments of

geographical slant should reflect the effects of stimulus variables on that

component involving perceived optical slant. The task involving perception

of geographical slant was first used for study of slant cues in the runway

image by Wulfeck et al. (30). It has the very useful characteristic of having

meaning to both pilots and nonpilots. That feature permits the use of nonpilots

as subjects and allows direct comparison of responses of pilots and nonpilots.

EXPERIMENT I

This experiment compared the judgments of horizontal orientation of a

runway model in pilots and nonpilots in order to evaluate the effect of

flying experience on the effectiveness of relative motion parallax. Relative

motion parallax was varied by presenting the runway model in both static and

radial motion (dynamic) conditions. The psychophysical method of adjustment

was used instead of the modified method of limits used previously (21). The

method of adjustment simulated more stable approach conditions. Observations

were made with the model approaching from two different angles below the

straight-ahead direction in the visual field in order to study the effect of

perceived direction in the visual field on adjustments of the model to the

apparent horizontal orientation. Since relative motion parallax increases

with optical slant when approaching a surface at a given approach speed, it

might be more effective at the 120 viewing angle than at the 30 viewing

angle.

Pilots also made verbal judgments of runway characteristics on a final

static trial. Judgments of the magnitude of approach angles (8) that were

produced on this final trial were obtained to aid in interpretation of errors

in horizontal orientation adjustments.

Method.

Subjects. Sixteen male pilots and sixteen male nonpilots served as

subjects. All nonpilots and 10 pilots were paid an hourly wage. The other

six pilots were volunteers who served without pay. The pilots were divided

into two groups that were matched with respect to average total hours of

flying time. The total flying hours of Group A ranged from 750 to 6,500 with

a mean of 2,963 and a standard deviation of 2,197 hours. The total hours of

pilots in the second group ranged from 800 to 5,800 with a mean of 2,831 and

a standard deviation of 1,847 hours. The ages of pilots varied from 24 to 44

2
years. The ages of nonpilots varied from 18 to 29 years. Nonpilots were

randomly divided into two groups. Nonpilots did not have experience as either

student pilots or pilots. All subjects had at least 20/20 acuity, with

correction if necessary.

Apparatus. The apparatus has been described in detail previously (20)

and is shown schematically in Figure 1. The runway model (R) was the same as

that used in the previous experiment. The model simulated the lighting of a

170-ft by 6,000-ft runway with an ALSF-2 approach lighting system without

sequenced strobe lights. The center of the model (F) moved toward the

observation point (0) along an apparent path (Q) such that the center of the

model was always at a constant visual angle (0) below the straight-ahead

direction (H) in the visual field. That angle 0 will hereafter be called the

viewing angle and was 30 on half the trials and 120 on the rest. An optical

system composed of two mirrors (Ml and M2) was used to accomplish the change

in viewing angle. The slant of the model (6) was varied by rotation in the

vertical plane and was measured accurately to the nearest 0.1°. Only runway

and approach lights were visible in the scene. The intensity of these lights

was adjusted to appear subjectively realistic to two experienced pilots who

did not otherwise participate in the experiment. Viewing was monocular to

eliminate binocular disparity, which is not normally an effective cue during

approaches to landing (24). The viewing aperture was 12 mm in diameter.

02

M2

IMP

FIGURE 1. Schematic diagram of apparatus (Al and A2,

removable targets for aligning optical systetn; El, baffle;

B2, baffle in Experiment I, light box in Experiment II; C,

cart; F, rotation axis; H, horizontal line of Bight; Ml and

M2, mirrors; 0, eye position; P1, P2, P3, segments of optical

axis; Q, apparent axis of radial motion; R, runway model; T,

track; •, viewing angle; 0, model slant).

P3
A chin rest and head rest were used to position and steady the subject's head

during observations.

Procedure. The subjects adjusted the model to appear horizontal using

the method of adjustment. When the subject pushed a toggle away from

himself, the model rotated away from him (far end downward), and when he

pulled the toggle toward himself, the model rotated toward him (near end

downward). The model rotated as long as the toggle was actuated and stopped

rotating immediately when the toggle was released. Eight of the pilots and

eight of the nonpilots adjusted the model to appear horizontal with the 30

*! viewing angle. The other half of the pilot and nonpilot groups adjusted the

model to appear horizontal with a 120 viewing angle. All subjects made

adjustments in both static and dynamic observation conditions. In the

dynamic condition, the model was visible as it moved toward the observer at a

constant simulated speed of 140 knots over a range of simulated distances

from 4.33 to 1.33 nautical miles from runway threshold. During slant

adjustments in both static and dynamic conditions, the model rotated in the

vertical plane at a constant rate of 100 per minute. At the beginning of

both dynamic and static trials, the orientation of the model was slanted with

respect to the observer's line-of-sight to the model's center (and axis of

motion P) by either 0.250 or 30. The center of the model corresponded to a

simulated distance of 1,500 ft from threshold and was located at the middle

of the simulated touchdown zone lighting system. It was determined prior to

the experiment that for both viewing angles most observers would see the

model as horizontal within this range of slants. Following practice in the

dynamic condition, four test trials were given with starting angles

alternating between 30 and 0.250. The order of starting angles was reversed

for half the subjects. Following practice in the static condition, two

trials were given with each of the two starting slants at each of the three

distances. Twelve static test trials were given each subject in blocks of

three trials with the distances given in descending order in each block. The

same two starting slants were used in static trials and were again alternated

from trial to trial. Half the subjects in each viewing-angle condition had

all dynamic adjustments before beginning static adjustments. The order was

"reversed for the remaining subjects.

When pilots had finished all adjustments to the horizontal orientation

in static and dynamic conditions. the model was set at the 1.33-nautical-mile

distance and at the 30 starting angle. The lights of the model were then

turned on and the pilots were asked to adjust the model to appear horizontal.

Next, they were asked to make several verbal judgments concerning

characteristics of the simulated runway. These included (i) the simulated

approach angle, i.e., the magnitude in degrees of the angle between the

line-of-sight to the runway threshold and the plane of the runway, (ii) the

simulated distance to runway threshold in feet or miles, (iii) the altitude

above the ground plane in feet, and (iv) the simulated runway length in feet.

These instructions may be characterized as "objective" instructions, as

discussed by Carlson (1). Nonpilots did not make verbal judgments.

4
Results.

Adjustments to Horizontal Orientation. The adjusted slant of the model

with respect to the axis of radial motion (angle e in Figure 1) when the

model appeared to be horizontal was the dependent variable and will be

referred to as generated approach angle (30). For purposes of analysis,

generated approach angle measurements were obtained from chart records of

dynamic trials for the specific simulated distances of 1.33 and 2.33

nautical miles from threshold for comparison with static judgments at the

same distances. Data for the 3.33-nautical-mile distance position were not

analyzed since on many trials of the dynamic condition, the model had not yet

been adjusted to the apparent horizontal orientation by the time it had

reached that distance.

NONPILOTS PILOTS

12.0 12.0 VIEWING ANGLE

11.5 ... "-o 11.5

11.0 11.0 12"

LiJ Error at start

L&J WI0.5 - 9.0* 10.5

• 0.0 0.0' o DYNAMIC STATIC DYNAMIC STATIC

NONPILOTS PILOTS

z 3.0- 3.0-

0.-=

2.5 2.5

Error at start

2.0 - O 2.0

0.0 L 0.0 

DYNAMIC STATIC DYNAMIC STATIC

PRESENTATION METHOD

FIGURE 2. Errors in judgments of horizontal

orientation as a function of flying experience,

presentation method, viewing angle, and

starting angle.

"At the 30 and 120 viewing angles, generated approach angles of 30 and

120, respectively, would have indicated accurate perception. Error scores

were calculated for generated approach angle responses by subtracting

5
response values from the values that would have indicated accurate perception.

Values of 0 used as starting angles (0.250 and 30) are identified in

Figure 2 in terms of the error magnitudes they represented. At the 30

viewing ;,ngle, the 0.250 and 30 starting angles represent errors of 2.750 and

00, respectively. At the 120 viewing angle, the corresponding errors are

11.750 and 90. A split-plot factorial analysis of variance was used to

assess the effects of flying experience, viewing angle, distance, presentation method (dynamic vs. static), and starting angle. The effects of these

variables excluding distance, which had little effect, are illustrated in

Figure 2.

The tests of greatest interest with regard to the possible effectiveiiess

of relative motion parallax were the main effect of presentation method,

comparing the responses in static and dynamic conditions, and the interaction

of flying experience with presentation method. The main effect of

presentation method was significant (p < .05) and indicated that errors in

static adjustments were significantly smaller by 0.150 on the average than

those in the dynamic condition. That finding is contrary to the expectation

that if ielative motion parallax in the dynamic condition were an effective

cue it wou!4- reduce errors. This effect is on the same order of magnitude as

the error of measurement inherent in responses (0.10) and therefore may not

be of practical significance. Furthermore, the interaction of flying

experience with presentation method was not significant, suggesting that

pilots did not have greater sensitivity to relative motion parallax than did

nonpilots.

The main effect of flying experience was significant (p < .01) with

pilots adjusting the model to appear horizontal with errors 0.30 less than

nonpilots on the average. The main-effect viewing angle was also significant

(2 < .01). The increase in errors at the 120 viewing angle was approximately

equal to the 90 difference in viewing angle between the 30 and 120 conditions.

Only a small difference (0.330) in the generated approach angle which was seen

as horizontal resulted from a relatively large 90 change in viewing angle.

The main effect of starting angle was also significant (k < .01) with errors

averaging 0.260 less with the 30 starting angle than with the 0.250 starting

angle. All but one of the significant interactions in the present analysis

involved this variable. Starting slant had a significantly greater effect in

pilots than in nonpilots (p < .05) and in static judgments (R < .01) than in

dynamic judgments. The second order interaction of starting angle, viewing

angle, and presentation method was also significant (k < .05) and reflects a

greater interaction of presentation method and starting anal- at the 120

viewing angle than at the 30 angle.

The only significant (p < .05) effect involving distance was the small interaction of viewing angle with distance. At the 30 viewing angle, mean

errors increased by 0.070 as distance decreased from 2.33 to 1.33 nautical

miles from threshold, and at the 120 viewing angle mean errors decreased by

0.030 over the same distance interval. The magnitude of this interaction,

although statistically significant, does not seem to be of practical

6

~ .... .... ..
significance. The main effect of distance was small and insignificant.

Generated approach angle errors were 0.020 less at 2.33 nautical miles than at

1.33 nautical miles from threshold.

Verbal Judgments of Runway Characteristics. On one final trial given

only to pilots, verbal judgments of the runway were made after they adjusted

it to appear horizontal. Means, medians, and standard deviations are given

in Table 1 for the approach angles to threshold that pilots generated in the

horizontal adjustments, for verbal judgments of the magnitude of those

generated approach angles, and for error scores obtained by subtracting the

magnitude of the generated approach angle produced by each pilot from his

verbal judgment of magnitude of that angle.

TABLE 1. Generated Approach Angles and Corresponding Verbal

Judgments of Angle of Approach to Runway Threshold

as a Function of Viewing Angle

Viewing Generated Verbal

Angle Approach Angle (A) Judgment (B) Error (B-A)

30 Mean 1.170 2.720 1.550

0.740 1.030

Median 0.710 2.500 1.790

120 Mean 1.990 3.650 1.660

a 0.690 2.760

Median 1.960 2.750 0.790

In horizontal orientation adjustments, it was shown above that errors

were much greater at the 120 viewing angle. Mean judgments of approach angle

as indicated by verbal responses show no significant increase in errors at the

120 viewing angle, and median values show a decrease. This finding

illustrates the independence of errors in adjustments to horizontal

orientation and judgments of approach angle. Verbal judgments also

consistently overestimated the physical magnitude of generated approach angles.

Judgments of simulated altitude, distance, and runway length are

summarized in Table 2 and are presented along with a summary of the actual

simulated values of altitude that were produced when observers adjusted the

model to appear horizontal on the same trial. In agreement with judgments of

approach angle, judgments of altitude overestimated the simulated values that

were produced when pilots adjusted the model to appear horizontal. Errors in

judgments of altitude were greater at the 30 viewing angle, as was the case

with medians of verbal judgments of Approach angle. Variability of responses

7
TABLE 2. Verbal Judgments of Distance From Threshold, Altitude Above

Ground, and Runway Length as a Function of Simulated Values

Altitude(feet) Distance(feet) Length(feet)

Viewing Generated Verbal Simulated Verbal Simulated Verbal

Angle Altitude Judgment Distance Judgment Length Judgment

30 Mean 163 275 8,000 3,660 6,000 6,250

a 103 85 1,320 1,773

Median 153 300 3,000 6,000

120 Mean 279 325 8,000 7,560 6,000 6,500

a 96 210 7,320 1,414

Median 272 325 5,340 6,000

was also greater at the 120 viewing angle in both altitude judgments and

judgments of approach angle. Judged distance of the runway threshold from

the pilot was greatly underestimated at the 30 viewing angle. Although

judgments if distance were considerably closer to the simulated value at the

120 viewing angle, responses were more variable. Judgments of runway length

appear to be more accuate and less variable than other verbal judgments, and

variability is not increased at the higher viewing angle.

Discussion.

The results of the present experiment confirm previous findings in this

laboratory (21) that relative motion parallax in the runway image has little

or no effect on the perception of orientation of the runway model at simulated distances as near as 1.33 miles from runway threshold and at simulated

approach speeds up to 140 knots. It was further determined that the flying

experience of pilots does not enhance their sensitivity to relative motion

parallax. Since pilots did, however, have significantly fewer errors than

nonpilots in both the static and dynamic conditions, it must be concluded

that their flying experience did have an effect on use of some other cue in

the runway scene, Other cues that might be involved are size cues, including

image shape and linear perspective, and the intensity gradient in the image

of the runway and approach lighting. Runway image shape has been shown to be

important in a study by Wulfeck et al. (30). The importance of intensity

cues in the runway image has been demonstrated by Kraft (18).

The relatively small effect of variation in viewing angle from 30 to 120

on generated approach angles and consequent large increase in errors at 120

suggest that direction of the runway in the visual field is not perceived

i ,8
accurately in the present viewing conditions in which no extra runway

Sgeographic references, such as the horizon, were simulated. The fact that

verbal judgments of approach angle on the final trial show no increase in

errors with increasing viewing angle also suggests that the increase in error

of horizontal orientation adjustments was due mainly to increased error in

perceived position of the model in the visual field. Just how variation in

viewing angle and optical slant of the stimulus affect the perceived

direction of the horizon and whether variation in perceived direction of the

horizon can influence judgments of approach angle should be examined further.

In the present experiment, error in judgments of approach angle was

independent of viewing angle.

The relatively constant magnitude of perceived approach angle errors,

which indicated overestimation of approach angle by an approximate factor of

2, is of a very serious magnitude in the context of the aircraft landing

approach situation. Overestimation of optical slant of the runway might occur

as a consequence of a perceptual organizing process called the equidistance

tendency (8,9). The equidistance tendency has been shown to make objects

appear at the same distance to the extent that effective visual cues

indicating a difference in distance are absent. Gogel (8) has cited several

examples of reduction in apparent slant of stimuli with respect to a vertical

reference plane as a function of cue reduction. Such effects are in the same

direction as overestimation of slant with respect to a horizontal reference

plane. Future research should measure the perceived direction of the runway

relative to the apparent direction of the horizon and apparent magnitude of

the generated approach angle in order to discriminate between overestimntion

of approach angle due to the equidistance tendency and errors due to misjudgment of visual direction. E. R. Wist (personal communication) has suggested

that the hypothesized operation of the equidistance tendency in the night

approach situation would be supported if errors in approach angle judgments

increased with viewing time as has been found to be the case in errors due to

the equidistance tendency in the perception of depth in a slanted line (23)

or between objects (8).

The significant effect of starting angle on generated approach angle was

present in the responses of both pilots and nonpilots, but was greater in

pilots and with the 120 approach angle. Starting angle had no significant

effect in a previous experiment (21) in which the perceived horizontal

orientation of the runway was measured by a modified method of limits rather

than the method of adjustment. A possible relation of the effect of starting

angle to perceptual events in the situation is suggested by the comments of

some subjects during debriefing that the apparent height of the model in the

visual field appeared to decrease as they decreased its optical slant. It is

possible that the optical slant of the model perceived initially on a given

trial determined the perceived direction of the model in the visual field.

Since the model would be seen as lower in the visual field with the 30

starting angle, it would be adjusted to a greater value of optical slant by

the observer in order to make it appear horizontal. This possibility could

be checked by measuring both the perceived position of the model in the visual

9
field and direct judgments of approach angle in the two viewing angle

conditions. Why the effect of starting angle should be greater in pilots is

unexplained. Perhaps the errors of pilots in perceiving direction in the

visual field were smaller than those of nonpilots.

An alternative possible "explanation" of the effect of starting angle is

in terms of adaptation-level theory. The effect of starting angle seems

related to the stimulus series order effect in the perception of a slantpd

disc found by Smith (27). He observed that apparent slant with respect to

the frontal (vertical) plane was significantly greater when the stimulus

series was presented in order of increasing slant with respect to the

frontal plane than when the stimulus series was presented in order of

decreasing slant. Smith interpreted this finding in terms of the repellant

effect of anchoring at extremes of the stimulus series. Anchoring effect

refers to the shift in adaptation level caused by extreme stimuli, which in

the present experiment would correspond to the starting angle (optical slants

of 30 and 0.250). The greater effect of starting angle in pilots as

compared with nonpilots might be explained by adaptation-level theory through

the residual effects of past flying experience on adaptation level. Although

pilots have a different task when trying to generate the ideal approach

angle, their visual experience with runways, especially in night approaches,

might have an effect on adaptation level that would generalize to the present

experimental task by virtue of the similarity of stimuli produced in real and

simulated runways. The pilot's past experience would most probably have been

with generated approach angles close to 30 on the average due to greater

information, including sources other than natural cues in the visual scene in

the real approach situation. This would increase their adaptation level,

thereby causing their generated approach angles to be greater than those of

the nonpilots who had no such prior experience. Furthermore, the effect would

be greatest at the 30 starting angle because of its closer proximity to the

adaptation level.

A third possible "explanation" of the starting angle effect might be in

terms of the equidistance tendency, mentioned above, operating in conjunction

with another perceptual organizing process called the adjacency principle (8).

According to the adjacency principle, the equidistance tendency operating

between objects such ac near and far ends of the runway would vary inversely

with their directional separation in the visual field. The result in the

present situation would be an increase in apparent (optical) slant of the

model with respect to the line-of-sight due to the equidistance tendency

which would be greaLer with the 0.250 than with the 30 starting angle. This

interpretation assumes that strength of relative tize cues would not change

significantly with change in starting angle.

The greater accuracy and lower variability in verbal judgments of

runway length suggests that these judgments were relational judgments

involving known size as well as relative size cues in the image. The known

standard size of approach zone and touchdown zone lighting could have been [ .used in conjunction with the relative image size of the runway edge lighting

V- 10
to enable greater accuracy in length judgments. In contrast, there were no

reference objects in the scene that had known altitude, distance, or

orientation that would have permitted those judgments to be made on a similar

relational basis, The importance of "known size" in length judgments might be

determined in judgments of length by two groups of nonpilots, one group given

information about the length of the touchdown zone lighting system and the

other group not given that information. Judgments of runway length would be

expected to be much more accurate and less variable in the group given size

information.

As mentioned above, Kraft (18) found that highly experienced pilots flew

below the proper glide path when upsloped terrain was behind the runway during

simulated approaches. Kraft interpreted that finding as the result of pilots'

attempting to maintain a constant visual angle height in the retinal image of

the city lights behind the runway. The present findings suggest another

contributing factor. In the present study it was shown that the geographic

orientation of runways is not perceived accurately. Pilots may learn to

produce the proper approach angle in nighttime approaches, therefore, by

judging the optical slant of the runway rather than geographic slant. Since

most runways are level, that would not cause a problem until a geographically

slanted runway is encountered. Then a pilot unfamiliar with sloped runways

might attempt to use the same optical-slant criterion used previously with

level runways since he would not perceive geographical slant accurately. That

would cause a low approach with an upsloped runway or a high approach with a

downsloped runway in agreement with anecedotal reports in the literature (24).

The most important finding of the present experiment was that verbal

judgments of approach angle and altitude as well as horizontal orientation

adjustments indicated perceptual overestimation of position relative to the

generated approach path. That finding reinforces the suggestion that there

is a direct perceptual basis for the documented tendency of pilots to fly

lower approaches at night. Additional responses involving adjustment of the

model to produce a 30 approach angle also indicated overestimation of approach

angle in the following experiment.

EXPERIMENT II

The second experiment examined the effects of a visual frame, simulating

the cockpit window, on the effectiveness of relative motion parallax in the

task of adjusting the runway model to appear horizontal as it approached the

observer. It was thought that relative motions between lights of the runway

and the stable window might make relative motions between points within the

runway image more salient and thereby enhance the motion parallax cue, since

thresholds for perception of relative motion are smaller than thresholds for

absolute motion (10,12). In addition, the effect of flying experience was

reexamined with regard to its possible interaction with the presence of a

visual frame. Two psychophysical methods were also compared. One was the

modified method of limits used in an earlier experiment (21) and the other was

the method of adjustment that was used in Experiment I. This variable was

II
added to the present experiment to permit further study of the significant

interaction of starting angle with psychophysical method that occurred in the

above experiments.

In Experiment I, pilots' verbal judgments of approach angle greatly

overestimated the angle of approach. An additional task was added in the

present experiment for pilots in order to reexamine this finding in a task

that would be more "natural." Pilots were asked to adjust the model to

appear "like a runway does when on a 30 glide path during an approach to

landing." Since pilots learn to identify acceptable angles of approach and

30 is a common glide path setting for Visual Approach Slope Indicatore, it

was reasoned that this instruction would have similar meaning to different

pilots and would assess errors at that point on the continuum of perceived

approach angle where judgments should be most accurate. Any large illusions

observed with this instruction would reinforce the previous suggestions that

perceptual errors may cause low approaches at night when vertical guidance

is entirely under visual control.

Method.

Subjects. Twelve male pilots and twelve male nonpilots served as

subjects and had at least 20/20 acuity, with correction if necessary. The

nonpilots were paid an hourly wage. The pilots were all professional pilots

in the employ of the FAA and participated during duty hours. Their total

hours of flying experience ranged from 1,350 to 13,750 hours of flying time,

with a mean of 9,360 hours and a standard deviation of 3,416 hours. The

ages of nonpilots ranged from 19 to 29 years. The ages of pilots ranged from

39 years to an estimated maximum of about 60 years.

Apparatus. The apparatus was identical to that used in Experiment I with

the exceptions that mirror Ml was replaced with a mirror having 70-percent

reflectance and 10-percent transmittance, and a light box was placed at the

position of baffle Bl to produce a luminous square frame with a dark area in

the center of which the model was seen during experimental trials. The

center of the runway model appeared 30 below the center of the square. The

center of the square window was in the straight-ahead direction from the

observation point. The height and width of the dark area in the center of

the frame was 180. The distance of the frame from the observer's eye was

61 cm.

Procedure.

a. Judgments of Horizontal Orientation. In both psychophysical

methods, subjects controlled the model to make it always appear horizontal as

the model approached the observation point. The method of adjustment was

identical to that of Experiment I. The procedure for the method of limits

was identical to the procedure used by Mertens (21). In the latter procedure,

the model was constantly rotating in the vertical plane as it approached the observation point. The subject's task was to control the direction of

12

I
rotation to make it appear horizontal by reversing the direction of rotation

with a pushbutton every time the model appeared to be rotating away from the

horizontal orientation in either direction. The model was again visible as

it approached from a simulated distance of 4.33 to 1.33 nautical miles from

threshold at the same approach speed and rate of rotation that were used in

Experiment I. All trials with a particular psychophysical method were given

before trials with the other method were begun. The order in which the two

methods were given was reversed for half the subjects. With both psychophysical methods, two practice trials were given and followed by four test

trials. Either the first two test trials or the last two test trials were

given with the visual frame of reference present. The order of trials in

which the frame was present was the same for both psychophysical methods in a

given subject. Half the subjects had the frame present on the first two test

trials with each psychophysical method and the other half of the subjects had

the frame present on the last two test trials. For the two trials in a given

condition of psychophysical method and visual frame, one trial was given with

a 30 starting angle and one trial was given with a 0.250 starting angle.

These two starting angles were alternated from trial to trial throughout the

experiment. Viewing was monocular with the right eye.

b. Judgments of 30 Approach Angle. After all judgments of horizontal

orientation had been completed, the pilots were asked to adjust the model as

it approached them on two trials so that it looked "like a runway does when

on a 30 glide path during an approach to landing." This instruction was

designed to elicit the learned perceptual criterion that a pilot uses in

flying night approaches. The 30 value seemed appropriate for this group of

pilots since the Visual Approach Slope Indicator and the Instrument Landing

System at the base airport for these pilots were set to define 30 and 2.90

approach paths, respectively.

Results.

As in Experiment I, the adjusted slant of the model with respect to the

approach path, angle 6, was the dependent variable and was measured

continuously as a function of distance over the range of simulated distances

from 4.33 to 1.33 nautical miles. Since no static condition was involved in

this experiment, rather than measuring generated approach angle at two

specific distances, the mean generated approach angle values were averaged

over both I-nautical-mile segments between 3.33 and 1.33 nautical miles from

threshold to increase the reliability of measurements. The method of scoring

chart records has been described previously (21). The mean of the two values

for an individual in each experimental condition was subtracted from 30 to

obtain an error score for each distance segment. A response of 30 would have

indicated accurate perception in both judgment of horizontal orientation and

judgment of the 30 approach angle.

Adjustments to Horizontal Orientation. A split-plot factorial analysis

of variance was used to assess the effects of flying experience (pilots vs.

nonpilots), visual reference condition (frame vs. no frame), psychophysical

13

I..
method (limits vs. adjustment), model slant at the start of a trial (starting

slant errors of 00 or 2.750), and distance interval (1.33-2.33 nautical miles

and 2.33-3.33 nautical miles). All generated approach angles were less than

30 when the model was adjusted to appear horizontal. The visual frame had no

significant effect and did not interact with other variables as can be seen

in Figure 3. The mean generated approach angle errors for pilots and

NONPILOTS PILOTS

PSYCHOPHYSICAL

3.0 3.0 METHOD

2.5 2.5

Error at orft LIMITS

, 2.0 -o" 2.0

?.n --- 2.?50

U.J

4 0.01- 0.01

W Frome No Frome Frame No Frome

W NONPILOTS PILOTS

uJ 3.0 3.0

2.5 2.5 .

o oADJUSTMENT ..

2.0 2.0

0.01 0.0

Frame No From@ Frame No From#

VISUAL REFERENCE CONDITION

FIGURE 3. Errors in judgments of horizontal orientation as a function of flying experience, visual

reference condition, psychophysical method, and

starting angle.

nonpilots were 2.060 and 2.660, respectively. This 0.600 difference in

generated approach angle errors between pilots and nonpilots was statistically

significant IC < .01). The only other statistically significant (p < .01)

main effect was due to starting angle. Generated approach angle errors were

on the average 0.140 greater when the starting angle was 0.250 rather than

30. Several interactions of starting slant with other variables were

significant. These included the first order interactions of starting slant

with flying experience (p < .05), with psychophysical method (p < .01), and

with distance (.< .01). Second order interactions involving starting slant

were its interaction with flying experience and psychophysical method

< .05) and its interaction with flying experience and distance (2 < .05).

"I14
The above-mentioned effects with the exception of the two involving distance

are also illustrated in Figure 3. The first order interactions of starting

slant and psychophysical method with flying experience reflect the much

greater effect of starting angle in pilots and the greater effect of starting angle in the method of adjustment. The significant second order interaction

of starting slant with psychophysical method and flying experience reflects

the fact that by far the greatest effect of starting angle occurred in pilots

who were responding with the method of adjustment. There was no significant

main effect of distance, but the three significant interactions involving

distance are illustrated in Figure 4. U

NONPILOTS PILOTS

3.0- 3.0 PSYCHOPHYSICAL

METHOD

2.5 - 2.5

-0

2.0 2.0 IZ- LIMITS

1. Pro of Stiot

1.-015

•0O0 0.0

1.1-11.3 1 1.31-8.U LU35-21 L6-3-11S

3.0 NO•dPILOTS PILOTS

3.0- 3.0- 2,.f 2.5- ._-.

2.0 2.0 ADJUSTMENT

1.5 1.5

0.0 S lA0M33. IJS-LSS LIS-11A . 0 L13-.811 3..5 L838.13..J .351

DISTANCE INTERVAL (NAUTICAL MILES)

FIGURE 4. Errors in judgments of horizontal

orientation as a function of flying experience,

distance, psychophysical method, and starting angle.

The interaction of psychophysical method with distance is statistically

significant (p < .05) although small. The significant (p < .01) interaction

of distance with starting angle reflects a greater effect of starting angle at

the farther distance. The significant (p) < .05) interaction of distance and

starting angle with flying experience reflects the fact that the interaction

of distance with starting angle is larger in pilots. As can be seen in

Figure 4, the significant interactions involving distance are of small

magnitude and probably not of practical significance.

15
Production of 30 of Approach Angles. Error scores of pilots for the 30

instruction were compared with error scores for the last two horizontal

adjustment trials given under the same conditions. The variable concerning

the visual frame was ignored for this analysis since preliminary analysis

revealed that it had no effect. A split-plot analysis of variance was used

with psychophysical method as a between-groups variable and three withingroups variables: type of response (30 approach angle adjustments vs.

horizontal orientation adjustments), distance interval, and starting slant

error. These data are illustrated in Figure 5.

Se H@@IeTAL

JUOGNINTS JMUINTS

2.5- 2.5 PSYCHOPHYSICAL

METHOD

210 2.0

1.5 - 1.5 LIMITS

Errw of owt

1.0 1.0 -

0.01 060.0 gc , g 

.100111INTS JMWNtTl

g, 2.5 2.5

2.0- 2.0-

1. •- 15, ADJUSTMENT

1.0 1.0 .

•1 ~DISTANCE INTERVAL. (NAUTICAL. MIL.ES)

SFIGURE 5. Errors in judgments of pilots of both 30

approach angle and horizontal orientation as a

function of psychophysical method, distance, and

starting angle.

Errors in 30 approach angle adjustments were significantly (R < .05) less

than errors in horizontal adjustments. The mean error in the 30 task was

1.500 and in horizontal adjustments was 2.090. The only other significant

effect was the interaction of starting angle with distance. The effect of

starting angle was significant in the distance interval between 3.33 and 2.33

nautical miles, but not in the nearer interval.

16
U

Discussion.

The most important finding of Experiment I1 concerning the relative

motion parallax in the runway image was that the presence of a stable visual

frame did not enhance relative motion parallax as a cue for perception of

runway orientation. That result adds to the evidence of previous experiments

in this laboratory, which strongly suggests that relative motion parallax

probably has no effect on perception of vertical position relative to the

ideal approach path in night approaches when only runway lights are visible.

The latter visual situation is commonly referred to as the "black hole" in

aviation literature, and has long been considered the most dangerous

approach-to-landing situation.

Another important finding of Experiment II was that pilots attempting to

adjust the model to produce 30 approach angles under "objective" instructions

(1) consistently overestimated approach angles by a factor of 2. That

finding strongly suggests that the tendency toward low approaches at night is

directly caused by overestimation at the perceptual level in judgments of

approach angle. This interpretation assumes that the simulated approach scene

in this laboratory contains essentially the same visual information that

exists over the same distance range in the "real world" when only runway and

approach lights are visible. This assumption should be tested by direct

comparison of pilot judgments of approach angle under the same instructions

in both simulated and real situations with similar lighting configurations.

Errors in adjusting the model to appear horizontal were lower in pilots

than in nonpilots in Experiment II, corroborating a similar finding in

Experiment I. The difference between responses of pilots and nonpilots,

however, was 0.60 in Experiment II as compared to 0.30 in Experiment 1. This

greater effect of flying experience in Experiment II is possibly associated

with a higher average total hours of flying experience in the pilots who

served in Experiment II. The average total number of hours was approximately

9,000 in Experiment II as compared with 2,000 in Experiment I. This finding

suggebts that the effects of flying experience on judgments of approach angle

should be studied more systematically with regard to the possible different

use of visual information as a function of experience.

The effect of starting angle and the interactions of this variable with

flying experience, psychophysical method, and distance were generally similar

to previous results. The greatest effect of starting angle was again in

pilots with the method of adjustment. The speculation given above concerning

interpretation of starting angle effects in terms of adaptation-level theory

is in agreement with the finding in Experiment II of a smaller effect of

starting angle in the method of limits than in the method of adjustment. In

the method of limits, the task required the subject to alternately adjust the

perceived orientation to be above and below that orientation which appeared

to be horizontal. The method of adjustment, in contrast, required only that

the model be adjusted to the orientation which appeared to be horizontal.

17
The subject would, therefore, be more likely to see orientations farther from

the starting position when using the method of limits and the starting

position would have less influence on the adaptation level as a result.

The fact that starting angle effect was smaller in the 30 adjustments

(0.120) than in horizontal adjustments (0.270) in Experiment II further

supports the interpretation of starting angle effects (at least partially) in

terms of effects on perceived direction, since perception of direction in the

visual field would not be expected to affect judgments of optical slant (30

judgments) as much as judgments of geographical slant (horizontal judgments).

* Conclusions.

The present findings corroborate earlier findings that.relative motion

parallax has little effect on the perception of the orientation of the

runway model at simulated distances as near as 1.33 nautical miles from

runway threshold, at simulated speeds of approach up to 140 knots, and with a

visual frame of reference present which oorresponds to the cockpit window.

Furthermore, it was shown that the flying experience of pilots does not 

4

enhance sensitivity to motion parallax in this situation. Further research

concerning relative motion parallax should direct attention to the

possibility that higher values of relative motion parallax than achieved in

these experiments may exceed thresholds for effectiveness of this cue.

Higher values would occur at distances of less than 1.33 nautical miles from

threshold or with the presence of extra lights in the nighttime scene

outside the runway.

The present findings do not reflect on the value of the relative motion

parallax cue for the judgment of aim point, or direction of flight path,

during approaches. Hasbrook (14), Langewische (19), and others have suggested

interesting techniques for use of relative motion parallax for this purpose.

The limited existing experimental evidence that was obtained in simulated day

and night aircraft carrier approaches suggests that judgment of aim point is

variable and perceived direction of the aim point is displaced upward in the

visual field at night (16). Errors in this direction would cause low i

approaches. More research on this topic is needed, especially on judgments of aim point in the reduced visual information conditions of night approaches.

The most important finding of this series of experiments is that pilots

misperceived both the geographical and the optical slant of the model runway.

Direct judgments of approach angle were overestimated by a factor of 2 on the

average. This demonstrates the presence of large visual illusions in night

approach conditions that do not involve reduced visibility due to

atmospheric conditions. Reduced visibility could be expected to affect

judgment of approach angle even more seriously. The inability to judge the

geographical orientation of a runway corroborates warnings in the literature

concerning the tendency to fly low on approaches to upsloped runways and high

on approaches to downsloped runways. Installation of Visual Approach Slope

Indicators at runways with significant geographic slope would seem especially

•= • 18
desirable. As mentioned above, future research should attempt a direct

determination of the relation between judgments of approach angle magnitude

in real and simulated night approach situations, to corroborate the present

findings which suggest a specific cause of problems in night approaches and

to validate the present visual simulation technique as a basis for future

experiments concerning relative motion parallax and other cues involved in

judgments of approach angles. Pilots obviously can and usually do

successfully correct for visual illusions because most VFR approaches are

performed safely. We should continue to study the method by which this

correction occurs to understand why this critical process occasionally but

tragically fails. A series of experiments in this laboratory has indicated

that the ineffectiveness of relative motion parallax as a cue when only

runway lights are visible may be an important part of this problem.

ACKNOWLE DGMENT

The author wishes to thank A. Howard Hasbrook and Professor Eugene R.

Wist for their critical reading of this paper and valuable suggestions.

A, 19
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