Technical ~eport Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

FAA-AM-77-12 

4. Title and Subtitle 5. Report Date 

PERCEIVED ORIENTATION OF A RUNWAY MODEL IN NONPILOTS JULY 1977 DURING SIMULATED NIGHT APPROACHES TO LANDING 6. Performing Organization Code 

7. Authorl s) 8. Performing Orgoni zation Report No. 

Henry W. Mertens 

9. Performing Organization Nome 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. Sponsoring Agency Code 

Washington, D.C. 20591 FAA 15. Supplementary Notes 

This research was conducted under Task AM-D-77-PSY-38. 

16. Abstract 

Illusions due to reduced visual cues at night have long been cited as contributing 

to the dangerous tendency of pilots to fly too low during night landing approaches. 

The cue of motion parallax (a difference in rate of apparent movement of objects in 

the visual field) is frequently suggested as contributing to visual judgments of 

glide path but has not been systematically studied in relation to the night approach 

problem. Thus, the present experiment examined the effect of varying levels of 

motion parallax from both radial and vertical motion on perception of the orientation 

of a runway relative to the ground. Under simulated nighttime conditions (only 

runway and approach lighting were visible), 16 nonpilots adjusted the apparent slant 

of a model runway to make it appear horizontal as the model moved toward them along 

a 3° approach path from a simulated distance of 4.33 to 1.33 nautical miles. 

Simulated approach speeds of 62 and 125 knots were used. The rate at which the 

model rotated during slant adjustments varied between 5° and 30° per minute. The 

adjusted slant of the runway model with respect to the approach path (generated 

approach angle) was the dependent variable. The average generated agproach angle 

for 256 trials was 0.5°. This consistent and large deviation from 3 (which would 

represent accurate perception) indicates the presence of strong illusions, is in 

agreement with the documented tendency of pilots to fly low approaches at night, 

and is explained in terms of the equidistance tendency and/or errors in perceiving 

the direction of the model in the visual field. The data also suggest that motion 

parallax in the runway image is neither a reliable nor an effective cue for the 

safe judgment of glide path at distances greater than l. 33 miles. 

17. Key Wards 18. Distribution Statement 

Aircraft Landing Document is available to the public Visual Illusions through the National Technical Information Motion Parallax Service, Springfield, Virginia 22151 

19. Security Classil. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 12 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

PERCEIVED ORIENTATION OF A RUNWAY MODEL IN NONPILOTS 

DURING SIMULATED NIGHT APPROACHES TO LANDING 

I. Introduction. 

Visual illusions due to reduction of available visual information at 

night have long been blamed for the dangerous tendency of pilots to fly too 

low during night landing approaches (1,10,12,13,16,17,18,20,21). Studies 

of aircraft accidents emphasize the importance of this problem with the 

finding of a high proportion of accidents in night approaches and landings 

that are not associated with adverse weather conditions (13,16). 

One of the visual cues most frequently suggested as contributing to 

visual judgments of glide slope, or angle approach, is relative motion 

parallax. This cue has not previously received parametric study in the 

context of the night approach problem. Relative motion parallax is defined 

as a difference in rate of apparent movement of objects in the visual field 

(5). 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 slope angle and approach 

speed (7). 

Threshold values of relative motion parallax for perception of depth 

between two objects have been foun~ to be about 1 minute of arc per second 

of time (9,11). The threshold value increases with separation of objects 

up to the limiting case of the absolute threshold for motion of a single 

object in an otherwise homogeneous visual field. The absolute threshold 

for motion has been found to be about 10 seconds of arc per second (3,9). 

Although equivalent thresholds for effectiveness of relative motion parallax 

in perception of slant have not been determined, several studies suggest that 

motion parallax can enhance,the perception of slant or shape of a surface 

when other cues to orientation or shape are present (2,4,19). 

The present experiment was conducted to examine the effect of varying 

levels of relative motion parallax from both radial and vertical motion on 

perception of the orientation of a runway model with respect to the 

horizontal ground plane. 

II. Method. 

Subjects. Sixteen paid volunteer male nonpilots between 18 and 29 years 

of age served as subjects. All had at least 20/20 acuity in the right eye 

The author wishes to thank A. Howard Hasbrook and Professor Walter C. 

Gogel for valuable discussions during the preparation of this paper. 

1 
as measured with the Bausch and Lomb Orthorater. All experimental judgments 

were monocular (right eye) in this experiment. 

Apparatus. A scale model was used to simulate the nighttime view of 

the lighting of a 170-ft by 6,000-ft runway with centerline and touchdown 

zone lights and a 3,000-ft-long ALSF-2 approach light system without 

sequenced strobe lights. This apparatus is described in the Appendix. The 

runway moved directly toward the subject's observation point from a position 

3° below the straight-ahead direction in the field of view. The lights of 

the model were visible over a range of simulated distances from 4.3 to 1.3 

nautical miles. Slant of the model was varied by rotation in the vertical 

plane. Only the simulated runway and approach lighting were visible in the 

scene, and their intensity was adjusted by experienced pilots to a 

subjectively realistic level. Viewing was monocular, with the right eye, 

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

approaches to landing (18). 

Procedure. The model was constantly rotating in the vertical plane as 

it approached the subject during experimental trials. The subject's task 

was to control the direction of rotation to make the model appear horizontal 

by reversing the direction of rotation with a switch every time the model 

appeared to be rotating away from the horizontal orientation. The 

independent variables were simulated approach speed, which was 62 o0 125 

knots0 and rate of rotation in the vertical plane, which was 5°, 10 , 20°, 

or 30 per minute. After practice, all rotation rates were presented twice 

in random order at one approach speed before trials at the other speed were 

given. The order of presentation of the two approach-speed conditions was 

reversed for half the subjects. 

III. Results. 

The adjusted slant of the model with respect to the approach path, 

angle e (as defined in the Appendix) 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 from threshold. The model was 

visible only in this distance range. The mean generated approach angle was 

obtained for the two !-nautical-mile segments of each approach between 

simulated distances of 3.33 and 1.33 nautical miles. The mean values were 

subtracted from 3° to obtain an error score for this segment. Scores were 

averaged over the two repetitions of a given combination of rotation rate 

and approach speed. A response of 3° would have indicated accurate perception. All generated approach angles were much less than 3° 

The mean generated approach angle for individual subjects ranged from 

1.0° to 0.2° with a standard deviation of 0.25°. The grand mean for all 

subjects was 0.5°. This value represents an error of 2.5°. Analyses of 

variance revealed that generated-approach-angle errors decreased by a 

2 
2.6 -f/) 

Q) 

Q) 

.... 

Cl 2.5 

Q) 

0 

-.... 2.4 

0 

.... 

.... 

w 

c 

0 

Q) 

~ 0 

5 10 20 30 

Rotation Rate (Degrees per min) 

Figure 1. The effect of rotation rate on generated glide slope errors. 

small statistically significant (£ < .01) amount, about 0.14°, as rotation 

rate increased from 5° to 30° per minute, as shown in Figure 1. Approach 

speed had no significant effect on generated approach angle. Errors 

averaged over all subjects were only 0.02° higher at the 62-knot speed than 

at the 125-knot ap~oach speed. There was, however, a significant (£ < .OS) 

second order interaction of approach speed with rotation rate and order of 

presentation of approach speeds. Figure 2 shows the interaction of approach 

speed with order at each of the four rotation rates. This interaction is 

due to generally greater errors among those subjects receiving the 125-knot 

speed first and a consistent increase in the magnitude of errors over the 

course of experimental trials in both order groups. 

The difference between the highest and lowest generated-approach-angle 

values in each !-nautical-mile segment was also measured for every trial. 

These range data are presented as a measure of the intrasubject variability 

of responses. Response variability was slightly but significantly (£ < .01) 

greater in the farther distance segment, as shown in Figure 3a. The effect 

of rotation rate on response variability, as shown in Figure 3b, was 

significant (£ < .01) and was about twice as great as the effect of rotation 

rate on the mean generated approach angle. A significant (£ < .OS) but 

small interaction of order by approach speed is shown in Figure 3c. This 

interaction indicates that variability decreased in the second half of the 

experiment. 

3 
~ 

5° I min ,.... I 0° I min 

~ ~ \ .... .... ..... 

Ul 

G) o--o 125 Knots First G) 

~ 

Cl 62 Knots First G) 

0 0 ........ 

~ 

0 

... 

~ 20°1 min 30° I min 

26~ w 

> r 

c l 0 

Cl) 2.5 

> 

:::!!: 

2.4 ~ 

ol I I l I I 

62 125 62 125 

Approach Speed (Knots) 

Figure 2. Generated glide slope errors as 

a function of the order by 

approa~h speed interaction at 

each rotation rate. 

04~ a 

~ c 

0.3 / ~ 0.2 

... 

., 

., 

... 1.3-2.3 2.3-3.3 62 125 co 

., Distance Approach Speed 0 

~ (Nautical Miles) (Knots) G) 

0 

c 

c o.5r b 

a:: 

0.4 

0.3 

0.2 

0.1 

0 

5 10 20 30 

Rotation Rate (Degrees per min) 

Figure 3. Variability (range) of generated 

glide slope responses as a function 

of (a) distance, (b) rotation 

rate, and (c) the order by approach 

speed interaction. 
The above-mentioned effects, although statistically significant, were 

small relative to individual differences and, with the exception of the 

effect of rotation rate on response variability, were of the same order of 

magnitude as the error of measurement inherent in responses, about 0.1°. 

IV. Discussion. 

The present findings show that relative motion parallax has little 

effect on the perception of the orientation of the runway model by nonpilots 

at simulated distances as near as 1.33 nautical miles from runway threshold. 

The slight but statistically significant decrease in errors with increasing 

rotation rate might indicate a small effect of motion parallax that was due 

to motion in the vertical plane. It is also possible, however, that this 

small effect was caused by the increase in variability of responses with 

rotation rate. Regardless of the cause, the magnitude of the effect of 

rotation rate is probably not of practical significance. The conclusion 

that relative motion parallax is not an effective cue in this situation is 

subject to the following qualifications: (i) the possibility exists that 

the visual experience of pilots in actual approaches, where feedback does 

occur, may enhance sensitivity to relative motion parallax; (ii) in actual 

approaches, the image of the runway appears inside a visual frame provided 

by the visible parts of the cockpit window, and relative motions in the 

visual field between the runway image and frame may enhance the relative 

motion parallax cue. These two possibilities are presently being examined. 

Preliminary analyses suggest that flying experience and presence of a frame 

do not make relative motion parallax more effective. A third possibility 

that should receive attention is that values of relative motion parallax 

higher than those achieved in the present experiment might be more effective. 

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 most important finding of the present experiment was that all 

observers in every stimulus ~ondition consistently, systematically, and 

grossly misperceived the physical orientation of the runway model. Wulfeck, 

Queen, and Kitz (21) studied judgments of the horizontal orientation of an 

aircraft-carrier-deck lighting system that rotated in the vertical plane but 

did not move radially. The perceptual errors observed in their study were in 

the same direction as those of the present experiment but of lesser magnitude, 

since their subjects used binocular vision. As mentioned above, monocular 

vision was used in the present experiment because it is not normally effective in the approach-to-landing situation. The illusions observed in the 

present experiment occurred despite the presence of size cues and linear 

perspective in the runway image and a range of relative motion parallax 

values that is equivalent to or greater than that occurring naturally in 

landing approaches where only runway lights are visible. Judgments of this 

experiment concerned the geographical slant of the runway as distinguished 

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

5 
relative to the line-of-sight to the surface; geographical slant is defined 

as 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 straight-ahead 

direction. 

Perceptual errors found in the present experiment are interpreted as 

indicating that the generated approach angles (i.e., optical slants of the 

runway when it was seen as horizontal) were perceptually overestimated and/ 

or the direction of the model in the visual field was misperceived. Downward displacement of the judged direction of the horizon or corresponding 

errors in judged position of objects in a dark visual field is well 

documented (15}. Overestimation of optical slant of the runway might occur 

as a consequence of a perceptual organizing process called the equidistance 

tendency (8). 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 overestimation of approach angle due to the equidistance 

tendency and errors due to misjudgment of visual direction. The role of a 

visible horizon should also be studied systematically in this context. 

The present findings suggest that visual perception of the approach 

angle may be inaccurate during night approaches when only runway lights are 

visible. Pilots obviously can and usually do successfully correct for these 

errors because most night VFR approaches are performed safely. We should 

continue to study the method by which this correction occurs in order to 

understand why this critical process occasionally but tragically fails. The 

ineffectiveness of relative motion parallax as a cue when only runway lights 

are visible may be an important part of this problem. 

6 
References 

1. Cocquyt, P.: Sensory Illusions, SHELL AVIATION NEWS, 178:19-24, 1953. 

2. Cibis, P. A., S. J. Gerathewohl, and D. Rubenstein: Depth Perception 

in Monocular and Binocular Vision. Special Report. USAF School of 

Aviation Medicine, Randolph Field, Texas, January 1953. 

3. Fox, J. N.: Perception of Real and Apparent Motion of Two Objects in 

a Structureless Visual Field, Ann Arbor, Michigan, University 

Microfilms, 1967, 124 p. 

4. Gibson, E. J., J. J. Gibson, 0. W. Smith, and H. Flock: Motion Parallax 

as a Determinate of Perceived Depth, JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 

58:40-51, 1959. 

5. Gibson, J. J.: The Perception of the Visual World, Cambridge, 

Massachusetts, Riverside Press, 1950, pp. 117-136. 

6. Gibson, J. J., and J. Cornsweet: The Perceived Slant of Visual 

Surfaces-Optical and Geographical, JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 

44:11-15, 1952. 

7. Gibson, J. J., P. Olum, and F. Rosenblatt: Parallax and Perspective 

During Aircraft Landings, AMERICAN JOURNAL OF PSYCHOLOGY, 68:372-385, 

1955. 

8. Gogel, W. C.: Equidistance Tendency and Its Consequences, PSYCHOLOGICAL 

BULLETIN, 64:153-163, 1965. 

9. Graham, C. H.: Vision and Visual Perception, New York, Wiley, 1965, 

pp. 511-516. 

10. Hartman, B. 0., and G. K. Cantrell: Psychological Factors in "Landing 

Short" Accidents, FLIGHT SAFETY, 2:26-32, 1968. 

11. Harvey, L. 0., Jr., and J. A. Michon: Detectability of Relative Motion 

as a Function of Exposure Duration, Angular Separation, and Background, 

JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 103:317-325, 1974. 

12. Hasbrook, A. H.: Anatomy of a Landing Cue by Cue, BUSINESS AND 

COMMERCIAL AVIATION, 29:54-60, 1971. 

13. Hasbrook, A. H.: The Approach and Landing: Cues and Clues to a Safe 

Touchdown, BUSINESS AND COMMERCIAL AVIATION, 32:39-43, 1975. 

14. Hochberg, J., and 0. Smith: Landing Strip Markings and the "Expansion 

Pattern": I. Program, Preliminary Analyses, and Apparatus, PERCEPTUAL 

AND MOTOR SKILLS, 5:81-92, 1955. 

7 
15. Howard, I. P., and W. B. Templeton: Human Spatial Orientation, New York, 

Wiley, 1966, pp. 280-289. 

16. Kraft, C. L.: Measurement of Height and Distance Information Provided 

Pilots by the Extra-Cockpit Visual Scene. In: Visual Factors in 

Transportation Systems: Proceedings of Spring Meeting, 1969. Committee 

on Vision, National Academy of Sciences--National Research Council, 

Washington, D.C., 1969. 

17. Kuhlman, R. L.: Approach With Care, FLYING, 78:88-90, 1966. 

18. Pitts, D. G.: Visual Illusions and Aircraft Accidents. USAF School 

of Aerospace Medicine Division (AFSC). Brooks Air Force Base, Texas, 

Report No. SAM-TR-67-28, 1967. 

19. Slocum, R. V., and W. A. Hershberger: Perceived Orientation in Depth 

from Line-of-Sight Movement, PERCEPTION AND PSYCHOPHYSICS, 19:176-182, 

1976. 

20. Vinacke, W. E.: Illusions Experienced by Aircraft Pilots While Flying, 

JOURNAL OF AVIATION MEDICINE, 18:308-325, 1947. 

21. Wulfeck, J., J. E. Queen, and W. M. Kitz: The Effect of Lighted Decks 

Shape on Night Carrier Landing. Technical Report No. 196-115. Dunlap 

and Associates, Inc., Inglewood, California, October 1974. 

8 
APPENDIX 

Description of Apparatus 

The basic concept of the apparatus was a moving runway model of 

variable slant as suggested by Hochberg and Smith (14). Added to their 

concept was a technique for modeling night runway lighting and an optical 

system for varying position of the model in the visual field, both of which 

were developed by the author. A technique for precise control and measurement of model slant, developed by Wulfeck, Queen, and Kitz (21), was also 

adapted for this apparatus. 

Runway Model. The runway model was based on a 243.8-cm-long by 

15.2-cm-wide light box. Its removable Formica top was penetrated by short 

fiber optic strands to simulate runway lights. The fiber optic strands 

were 0.508 mm in diameter, 6 mm in length, and cut off with a 45° angle on 

one end. The angled surface of each fiber was adjusted to protrude just 

above the Formica surface and to point toward the direction of the 

observation position. Red and green simulated lights were produced by 

gluing transparent plastic over the appropriate fibers on the underside of 

the light box top. The top surface of the light box was painted flat black. 

The sources were two parallel 243.84-cm instant-start fluorescent tubes 

(General Electric F96Tl2 - CWX, Deluxe Cool White) mounted 2.54 em below 

the top of the light box and separated from each other by 2.54 em laterally. 

One side of each fluorescent tube was covered with tape and painted black 

to make it opaque. As the tubes were mounted with a single pin on each end, 

they could be rotated to expose varying amounts of the unpainted sides to 

vary the amount of light reaching the fiber optic strands and, hence, the 

brightness of the simulated runway and approach lights. The brightness of 

lights was adjusted by experienced pilots to appear realistic. In the 

prototype model, fibers were glued in holes drilled in the pattern of a 

runway 6,000 ft (1828.8 m) long by 170 ft (51.8 m) wide with centerline and 

touchdown zone lighting and an ALSF-2 approach light system 3,000 ft 

(914.4 m) long. The model scale was 1,200 to 1. 

Vertical Motion System. The runway model was mounted on a cart, C in 

Figure A-1, so that a transverse horizontal axis of rotation F, which was 

perpendicular to the longitudinal axis of the runway, passed through the 

plane of the simulated runway. The model could be rotated 20° from a 

physically horizontal orientation in either direction but was limited to 

rotation from the horizontal position to 20° toward the observation point 

when viewing involved the mirror system to be described below. The rotation 

of the model was controlled by a chain-drive mechanism. The chain was 

connected to both ends of the light box and was driven at a constant rate 

by a Boston Gear Works 1/12-hp ratiomotor and model Rl2 speed control that 

gave almost instant starting and stopping. Rates of rotation in the 

vertical plane of from 0° to more than 30° per minute could be produced. 

9 
Bl 

B2 

LEGEND 

A "' rernovable targets for al1!71ing 

opt! cal sys te111 

8 = baffles 

C = cart 

F = rotation axis 

H = horizontal line of sight 

H = mirrors 

0 = eye position 

P = segments of optical axis 

Q = apparent axiS of radial 

motion 

R = runway model 

T = track 

S = approach angle 

Figure A-1. Schematic diagram of apparatus. 

Guide sprockets for the chain were located directly below the ends of the 

light box to prevent bouncing of the light box when the direction of 

rotation was reversed at high-rotation rates. A center-tapped Beckman model 

SA2880 10-turn potentiometer with 0 to 20,000 ohms of resistance and 0.05 

percent linearity was mounted on the same shaft as the sprocket that drove 

the chain. This potentiometer was used in conjunction with a regulated power 

supply to give a remote indication of model slant as a function of time as 

the model moved toward the observation position at a constant rate. Both 

a digital voltmeter and a recorder could be used to indicate model slant to 

the nearest 0.1°. A dial indicator and vernier graduated in 0.1° units 

(PIC Design Corp., catalog number AX4) was mounted on the shaft on which 

the model rotated. This instrument was used in conjunction with a 

machinist's level for calibration of electrical slant indicators. 

Radial Motion System. The runway model with its rotation system was 

mounted on a cart that moved along a level track T toward the observation 

position. A chain driven by a Graham variable speed transmission,model KFS, 

with a 1/4-hp motor pulled the cart along at a constant scale speed that was 

adjustable (e.g., 0 to 250 knots for the 1,200 to 1 scale model described 

above). The cart rode on ball-bearing wheels on a graduated two-rail 

Gaertner Scientific Co. optical bench track 8.0 meters long. This allowed 

the 1,200 to 1 scale model to move over a range of simulated distances from 

approximately 4.33 nautical miles to 1.33 nautical miles. Larger models 

10 
could be used to simulate nearer distances. A third rail located 0.9 meters 

to the side was used with an outrigger on the cart to give lateral stability 

during radial motion of the cart. Microswitches were placed along the track 

to turn the model lights on and off at the appropriate points in order to 

make the model visible or remove it from view. A solid-state logic system 

was used in conjunction with various microswitches on the track and the 

rotation system to control lights, radial- motion, and rotation of the model 

during experiments. 

Optical System. To have the axis of rotation of the model (F in 

Figure 1) move radially along an apparent line-of-sight Q at a constant 

selectable angle 8 with respect to a horizontal line-of-sight H, an optical 

system consisting of mirrors Ml and M2 was constructed. Mirror Ml was 

oriented at 45° with respect to the horizontal axis of radial motion of the 

model. Mirror M2 could be varied systematically in height and slant with 

respect to Ml by a simple system of removable pegs in the wall of the mirror 

holder so that the segment of the optical axis Pl could be reflected to the 

eye at a number of discrete viewing angles 8 measured with respect to H 

while maintaining coincidence of P3 and the axis of radial motion of the 

model. For this alignment, sighting targets Tl and T2 were temporarily 

installed at end points on the track at the exact height of the radial 

motion axis to allow fine adjustments of mirrors. With targets Tl and T2 

aligned visually from the observation point O, the value of 8 was checked 

with a theodolite. Fine adjustments of mirrors were made with shims. When 

the model was physically horizontal, it was parallel to the line of sight 

and was visible only as a thin horizontal line. When the model was moving 

toward the observation point and slanted physically 3° toward the observation 

position with its near end down (8 was 3°), the apparatus produced an image 

identical to that of viewing a physically level runway during a constant 

3° approach to landing. Simulated runway size and distance were related 

to the corresponding physical measurements of the apparatus by the scale 

factor of the model. The angle of the model's surface with respect to the 

observer's line-of-sight and the angular position of the model in the visual 

field were identical to the corresponding angles in the simulated scene. 

Equations for determining motion parallax between components of the scene 

are given by Gibson, Olum, and Rosenblatt (7) and Hochberg and Smith (14). 

The observation position was an enclosed ventilated booth. A head and 

chin rest was used to steady the subject's head in front of the viewing 

aperture Bl. Viewing was monocular to eliminate binocular disparity, which 

is not an effective cue during approaches to landing (18). Baffle B2 was 

used for control of extraneous light. A pushbutton or toggle switch was 

used by the subject for control of model rotation during experimental trials. 

A manual shutter operated by the experimenter was used to occlude the 

viewing aperture when desired. 

Three important advantages of this visual simulation technique are 

(i) excellent optical resolution, (ii) the preservation of the natural 

relation of distance to apparent brightness of light sources that are 

11 
effectively point sources (i.e., they subtend less than 1.25 minute of arc 

at the eye), and (iii) the ability to vary visual direction of the radial 

motion axis without a complex computer for synchronizing simulated attitude 

and distance changes. These characteristics are essential for a display 

intended for study of size cues (including relative size and linear 

perspective), relative motion parallax, and brightness gradient as cues 

to space perception in the night approach situation. In addition to study 

of these cues, this apparatus can also be used for studying of the effect 

of training on the judged or5entation of the model and on the memory 

processes involved in these judgments. The apparatus described above is 

thought to be a useful device for study of space perception in general and 

may be used for study of judgments of attitude, distance, and runway 

characteristics as well as perception of runway slant and approach angle. 

I 

12