lute heart rate values indicated in Figure 11 is
not statistically significant and the differences
between the reference “resting” values and those
during the approaches are relatively small. In-
terestingly, the largest differences between heart
rate reference values (established during level
flight) and the mean rates during approaches
was evidenced by Group D which used the com-
bined visual and aural/voice display. The mean
heart rates for the other groups tended to be at
or very near the reference values—with the pos-
sible exception of the combined visual and aural/
code group (I) whose mean rates declined below
the reference rate on the last three of the five
experimental approaches.

Both the increase in heart rate during the ap-
proaches and the decline in absolute heart rate
on successive approaches were highly significant
(P=<.001). The heart rate data for the five
groups also confirm the data in an earlier report*
which discusses the stress implications of in-
creased heart rate during ILS approaches. It
also suggests that regardless of the type of dis-
play cue, or the sensory channels used, the ILS
tracking task—as presently structured—may ap-
proach, and sometimes exceed the desired re-
sponse capability of the “average” instrument
pilot.

As discussed earlier, because of the dynamics
of the ILS system, the pilot must constantly
increase his response rate—with concomitant de-
creases in amplitude of control input—to keep
the aircraft on the centerline of the glide slope/
localizer “beam” as he approaches the middle
marker. In developing this study, it was thought
that the use of aural glide slope cues—repeated
at frequent intervals—might enhance the pilot’s
processing capacity, and thereby improve his
glide slope tracking performance. It was also
thought that the aural cues would lessen the
pilot’s visual work load, giving him more time
to reference other instruments and thus improve
his overall flight performance. Although these
expectations were not fulfilled, several facts and
suggestive indications emerged which may have
important bearing on future studies as well as
on display design.

First, the “A” and “N” Morse code signals can
be misinterpreted, resulting in a delay in finally
applying correct control inputs.

Second, little or no specific practice was re-
quired for effective use of the voice cue display.

16

Tracking performance on the first approach,
using the voice cues, was usually comparable to
the performance levels on those approaches in
which the visual display was utilized—despite
many hours of previous experience with the con-
ventional (visual) system.

Third, in combination with the conventional
visual display, the voice display can serve as an
effective aural warning of eacessive deviation
below glide slope—regardless of where the pilot
may be looking.

Fourth, combined with the flight director sys-
tem in more sophisticated aircraft, and deriving
its input signals from the “raw data” stage of
the aircraft glide slope receiver, the voice display
can serve as a warning of excessive deviation
should an undetected malfunction occur “down
stream” in the computer section of the aircraft
flight director display system.

Fifth, though not statistically verified because
of limitations in the statistical design of this
study, there is a rather strong indication that
aural cues may serve to reduce the variability in
glide slope tracking performance from one ap-
proach to the next by the same subject. This
effect was particularly apparent among those
subjects who did not exhibit a high degree of
initial proficiency. Such an increase in pre-
dictability of performance could have important
safety implications, even in the absence of any
overall improvement in mean performance level.

To determine if ATC communications would
interfere with effective use of the aural glide
slope cues, and vice versa, approaches were made,
using other pilots. Based on their comments and
our subjective evaluation of their performance,
it appears that pilots should have no difficulty
in effectively carrying on two-way communica-
tions while using aural (voice) glide slope cues.
This may be because of the nature of the cues;
only recognition of the words “low” or “high” is
important to performance of the tracking task.

Finally, in reviewing the results of this study,
it is apparent that although the subjects’ per-
formance was related to what they saw on the
crosspointer indicator and/or heard from the
audio display, an equally important performance
factor was the pacing, or speed, of presentation
of cues—governed by the speed of the aircraft
and by the dynamics of the glide slope/localizer
transmitting system.


The weighted Jocalézer deviation scores did not
yield statistically significant differences among
the five groups. However, there was a significant
(P=<.025) improvement in performance as a
function of the number of approaches flown.
Again, this improvement tended to be concen-
trated in the groups using some form of auditory
glide slope display but the interaction between
groups and approaches was not significant.

The results for maximum localizer deviation
within successive segments are presented in
Figure 7 and the analysis of variance is sum-
marized in Table 4. No significant differences
appear among groups but there is a significant

GLIDE SLOPE DISPLAY

VISUAL ONLY
“ VOICE ONLY

. -4 CODE ONLY
6 @—-—- VISUAL AND VOICE
bow &—-—-& VISUAL AND CODE
az
wz
zz 2001S- 4
ee | aa
5 a :
3 Fd =28—.
eo
& y 1 00T ° oo"
zu
on
2 3 1 1 1 1 j
2°
ya 4 5 6 7 8

APPROACH

Ficure 7. Means of maximum deviation right or left of
localizer centerline, for all segments combined within
groups.

(P=<.05) improvement in localizer perform-
ance as a function of the number of approaches
flown. As with glide slope performance, how-
ever, there was a significant (P=<.01) increase
in maximum deviation from localizer centerline
as the middle marker was approached.
Examination of magnitude of deviation from
glide slope and localizer centerlines at time of
aircraft passage over the middle marker shows
no significant differences among groups or among
approaches. However, if both magnitude and
direction of glide slope deviation at the middle
marker are analyzed we get the results shown in
Figure 8. Analysis of variance of these data is
summarized in Table 5. Significant differences
(P=<.05) appear among groups. The algebraic
means of the deviations from glide slope center-
line indicate that the group using only visual
glide slope cues tended to cross the middle marker
slightly below glide slope. On the other hand,
the group using the conventional display in con-
junction with voice cues averaged one to two
dots above glide slope on all five approaches.

Because the tracks flown by the aircraft fre-
quently exhibited pronounced vertical oscillations
near the middle marker, it was decided to com-
pare the maximum limits of the total range of
the oscillations during the last thirty seconds of
the approach. The results are illustrated in
Figure 9 in terms of an equivalent range of dots
on the crosspointer instrument. Analysis of
variance showed no significant differences among
groups or among approaches.

TasuE 4.—Maximum Deviations From Localizer Within Successive Segments of Approach.

(Analysis of Variance)

Source of Variation

Between Subjects.
Groups.
Subjects/Group

Within Subjects
Segments._-
Segments X Groups.
Segments X Subjects/Groups.
Approaches. --_----
Approaches X Groups.
Approaches X Subjects/Group:
‘Approaches X Segments...
Approaches X Segments X Groups.
‘Approaches X Segments X Subjects/Groups.

df MS F P
- (39)
7 4 1,907.91 2.36
35 806. 81
(1,960)
9 1,409.97 28.44 <.01
2 36 63.91 1.29
- 315 49.58
- 4 345.37 2,82 <.05
- 16 164.79 1,34
140 122, 59
36 47.63 1,13
144 58.77 1.28
1,260 42.17

1,999



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20


AURAL GLIDE SLOPE CUES: THEIR EFFECT ON PILOT PERFORMANCE
DURING IN-FLIGHT SIMULATED ILS INSTRUMENT APPROACHES

I. Introduction.

Many aircraft accidents involving fatal and
serious injuries have occurred during instrument
approaches through low clouds and precipitation
when pilots have inadvertently flown their air-
craft too far below the centerline of the electronic
glide path, striking terrain or obstructions short
of the runway.** In such accidents, “pilot
error” has often been listed as the primary cause.
Unfortunately, the predisposing factors that may
have been responsible for many of these so-called
pilot error accidents frequently escape detection
because of the subtle and intricate human factors
relationships associated with the complex man-
machine system.

Obviously, the basic cause of such accidents is
the pilot’s lack of awareness of his unusually
low altitude. This may be due to his inability
to see the approaching ground because of unex-
pected visibility restrictions, malfunction of
pertinent instrument displays, his failure to scan
appropriate instruments frequently enough to
obtain necessary altitude-position information, or
to his inherent human limitations to acquire,
process, and translate necessary information
within the constraints and demands of the total
system task.

A 1949 study® of Air Force pilots with a wide
range of flying and instrument experience showed
that 41% of the pilot’s visual fixation time was
devoted to the crosspointer indicator during
“raw data” (Non Flight Director) instrument
approaches in a C-45 aircraft. The directional
gyro, attitude indicator, and airspeed indicator
accounted for an additional 50%.

Surprisingly, only 2% of visual time was de-
voted to the altimeter, and, the frequency of ref-
erence to this vital instrument averaged only
three per minute. The remaining 7% was de-
voted to other instruments and to related visual
tasks. A more recent study® on airline pilots

making “raw data” instrument approaches in a
DC-8 simulator (with six degrees of freedom
and a primary flight display arranged in a con-
ventional “T” configuration) shows that the atti-
tude indicator and ILS crosspointer instrument
may account for as much as 70-80% of visual
fixation time. And, even these large percentages
may be conservative under certain circumstances.
For example, it has been observed by experienced
check pilots that a pilot who is fatigued or under
emotional stress tends to fixate on some one indi-
cator, leaving the other instruments virtually
unattended for long periods of time.

If these findings are representative of the
visual scan-time pattern of the instrument rated
pilot population as a whole, it appears that pilots
allocate most of their attention to those instru-
ments—the attitude indicator and crosspointer
indicator—whose individual cues are considered
to be most: critical to the total task. It is not
clear, however, whether this disproportionate
concentration of time is a function of a limited
visual information intake capacity or if it is the
result of the pilots selectively limiting their in-
formation intake to a level compatible with their
total data processing capacity. Also, this time
allocation may be a function of the display inter-
face. Human factors deficiencies in this area
can range from legibility problems to incom-
patible response requirements. The effects of
these shortcomings may combine to seriously
limit the pilots’ ability and capacity to acquire
and process needed visual information as rapidly
as required during the approach.

As a search of the literature indicates, there is
an abundance of data available on aircraft cock-
pit displays but frequently much of it has been,
and continues to be, inadequate for practical de-
sign applications And, even if basic design
deficiencies were remedied, it is possible that
pilots would continue to concentrate their visual
attention on a very limited number of displays.



Not to scale
(Glideslope angle exoggerated)

Middle Marker

RUNWAY

Outer Marker

‘Converging Cone
of ILS Glidestope

\ ne CHANGES TO TUBE

1
Touchdown Area

Figure 13. Man’s response characteristics might be more compatible with display interface if rate and magnitude
of needle movement were a function of a tubular, rather than a conical, ILS glide slope “beam”.

Perhaps the total system would be more com-
patible with human response characteristics if
the rate and magnitude of aural cue and/or of
needle movement were a function of a tubular
rather than a conical, ILS glide slope “beam”
(Figure 13). The tubular concept would pro-
vide a constant, rather than a varying, ratio be-
tween needle movement and linear deviation
from the glide slope and localizer centerline.
Although such a design concept would provide
more leeway for deviation during later stages of
the approach, it is possible that over-response of
the pilot would be minimized, with a concurrent
reduction in tracking errors. This, in turn, might
produce more accurate and more uniform per-
formance.

V. Summary and Conclusions.

This study has demonstrated that instrument
rated pilots can effectively utilize aural glide
slope cues in lieu of, or in combination with, a
conventional visual display for tracking the
glide slope path during an ILS approach. No
significant differences were found in accuracy of
glide slope tracking among groups using the five
different display modes consisting of visual and
aural cues (voice or code) or a combination of
both. Some small differences were found rela-
tive to the direction of glide slope deviation at
the middle marker but they were of little prac-
tical significance.

Initial use of aural glide slope cues resulted
in minor deterioration of localizer performance
but performance improved with trials, Also,
there was some indication that additional trials,
beyond the ones flown in this study, might have
resulted in localizer performance comparable to
that with the conventional display.

Effective use of aural-voice-glide slope cues
required little training or practice; their use in
combination with conventional and flight director
visual displays could warn a pilot of excessive
deviation below the glide slope. The data, by
interpretation, also suggest that any tracking
difficulties experienced by pilots attempting to
make a precise ILS approach are primarily re-
lated to the information processing limitation
of the human, which is in turn governed by the
dynamics of the glide slope and localizer system.
Also, loss of precise tracking capability—at the
middle marker and under the ideal conditions
existing in this study—suggests that if opera-
tional variables requiring additional mental at-
tention are added to the pilot’s tracking task, his
ability to cope effectively with the total task may
be reduced.

In essence, the results of this study suggest the
need for a fundamental analysis of the total
man-machine approach system, for without this
first step it will be difficult, if not impossible, to
design a system which is basically compatible
with the cognitive, intellectual and reactive char-
acteristics of man.



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APPENDIX

Preface to Appendix Tables

The following tables summarize the glide slope data for each experimental
group during each approach, including the pre- and post-experimental ap-
proaches, Values are the percent times spent in the respective deviation
ranges by each group on each approach between the outer and middle marker.

19



Even under ideal conditions, such as in this
study, tracking the glide slope centerline is a
demanding task with a constantly changing
control-display relationship and an ever increas-
ing requirement for accuracy as the middle
marker is approached. As shown in Figure 12,
the horizontal glide slope needle moves vertically
in reference to a “bull’s-eye” and to a series of
“dots” spaced vertically above and below the
center of the instrument. The span of the bull’s-
eye is equal to a subtended angle of approxi-
mately 0.28 degrees, relative to the vertex of the
angle at the point of intended touchdown (dis-
tance between the periphery of the bull’s-eye and
its center, and between each dot, is equal to a
subtended angle of about 0.14 degrees). Since
the distance between the top and bottom radii
comprising this angle diminishes as the vertex
is approached, it becomes evident that the verti-
cal distance equivalent to a given “dot” displace-
ment is a function of the distance between the
aircraft and the vertex of the angle.

"BULLS -EYE" J
= 2 DOTS
= 0.28° t

For example, the vertical envelope within
which an aircraft must remain to keep the glide
slope needle within the confines of the bull’s-eye
(2 dots) is about 167 feet at a point four miles
from the middle marker (33,460 feet from the
vertex), while at the middle marker (4,300 feet
from the vertex) the same envelope comprises
about 2114 feet. From this it can be seen that
the task of keeping the glide slope needle within
the center of the bull’s-eye necessitates flying the
aircraft within the constantly narrowing confines
of an already narrow isosceles triangle. There-
fore, the human performance requirements in-
herent in this tracking task constantly increase,
in relation to (a) location of the aircraft at any
given moment along the approach path; (b) re-
sponse characteristics of the total aircraft-
instrument system; and (c) velocity of the air-
craft.

From this, it is also evident that for a given
vertical displacement of the aircraft at a given
forward velocity, both the distance and rate of

Simplified Glide Slope Display

(needles and localizer dots
not shown }

a

_GROUND

RUNWAY c 4,300'

(Not to scale)

33,460'

Ficure 12. A two dot deviation, equal to a vertical displacement of the glide slope needle across the span of the
crosspointer bull’s eye, is equal to about 0.28 of subtended angle, resulting in a narrowing of the glide slope

“beam” as the middle marker is approached.

14


glide slope and the Morse code “N” (m ©) when
the aircraft was below glide slope. As with the
voice mode display, the volume of the signal
increased with increasing deviation and decreased
as the aircraft approached the glide slope path.
An idealized profile of the change in volume of
the auditory cues in relation to deviation is shown
in Figure 4. These changes in volume also pro-
vided qualitative information on the rate at
which the aircraft deviated away from, or ap-
proached, the glide slope path.

The subjects flew their approaches according
to the protocol presented in Table 1. All groups
flew the first two approaches with only the con-
ventional visual glide slope display. Approach
#1 provided orientation to the mechanics of the
task. Performance in approach #2 established
a baseline proficiency level as well as a basis for
evaluating the adequacy of group matching of
initial proficiency.

(Voice or

e

VOLUME
Increase—>

Cross Pointer

eo
|

(Not to scale)

Below Glideslope
Deviation Cue

Mn

The subjects in group A, using only the con-
ventional (visual) display during the ten ap-
proaches, served as the control group.

Groups B, C, D, and E flew approach #3 with
a variation of the experimental display that
would be used in approaches #4 through #8.
Thus, the subjects in groups D and E were forced
to rely solely on auditory cues for glide slope
tracking in approach #3. It was hoped this
would encourage use of the auditory cues during
approaches #4 through #8 when the combined
displays were available. There was no way, of
course, to: determine which display or what ratio
of cues—audio and visual—were used when a
combination of both was available.

I.

The basic glide slope data for all approaches
by the five groups are summarized and tabulated
in the appendix. For the purpose of statistical

Results

Above Glideslope
Deviation Cue

Code) (Voice or Code)

1020 Hz
“On-Glideslope”
Tone

Frovee 4. Idealized illustration shows relationships of volume of aural cues to aircraft deviation above or below
glide slope centerline. A 1020 Hz tone provided “on-course” guidance.

6


in the study. The instrument panel, typical of
many such installations in aircraft equipped for
precision (ILS) instrument approaches, utilized
a modified “T” configuration of the primary
flight instruments (Fig. 2). This modification
involved the relocation of the vertical speed in-
dicator (Arrow 1) to the right of its normal
position so as to provide a more central location
for the glide slope/localizer indicator (Arrow
2). Quick removal “slats” (Arrow 3), installed
above and to the left of the instrument panel
prevented the pilot-subjects from looking out-
side, effectively simulating an instrument en-
vironment without interfering with the safety
pilot’s outside vision.

The electronic equipment generating the aural
glide slope cues and the battery powered seven
channel Model 417 Lockheed recorder were
mounted behind the pilot’s seat. A research
engineer monitored the equipment from an ad-
joining seat and supplied indexing and reference
data to the recorder.

The A/N cue generating device consisted of
an electronic code generator whose signal and
ratio of volume output was controlled by the
output of the glide slope radio receiver. The
on-course tone in the A/N system was produced
when the signal ratio was equal. The voice cue
device consisted of a two track stereo tape player
and an on-course tone (1020 Hz) generator.

Ficure 2. Primary flight instruments in research aircraft were arranged in “I” configuration with exception of
vertical speed indicator (Arrow #1), which was moved to the right to permit a more centrally located glide

slope/localizer indicator (Arrow #2).

Removable “slats” (Arrow #3) simulated instrument weather environ-

ment for subjects without interfering with safety pilot’s outside vision.



If this were found to be the case, then considera-
tion should be given to using alternate sensory
channels for acquisition of certain applicable
flight information.

The feasibility of using alternate sensory
channels as a substitute for, or to augment, the
visual mode has been investigated by many re-
searchers. The auditory channel has received
considerable attention over the years, beginning
as early as 1936.14 Auditory signals, when used
for presentation of flight information, as opposed
to warning information (as in an annuciator)
have been most successful when used to present
only a single flight parameter such as heading
or bank angle. Attempts to develop multi-
dimensional auditory flight displays, on the other
hand, have proven less successful.

Considering the promising results of single
auditory displays, as well as experience with such
systems as Ground Controlled Approach (GCA),
Precision Approach Radar (PAR), and the
Adcock Low Frequency Radio Ranges, it was
decided to adapt some of the features of such
systems to provide an automated airborne audi-
tory glide slope display. By providing auditory
cues as a source of glide slope tracking informa-
tion, it was hypothesized that such cues (in ad-

dition to, or in lieu of, the conventional glide
slope display) might reduce or substantially
eliminate a pilot’s dependence on visual glide
slope cues—giving him more visual time to devote
to other instruments, thereby possibly improving
his overall performance. On the other hand, if
there was little or no improvement (despite
comparable ability in using aural cues) it would
appear future research attention should be di-
rected toward a more extensive and detailed
study of the pilot’s total performance capacity
in relation to the task requirements and char-
acteristics of the ILS system, the dynamics of
the aircraft, and the aircraft control-display
relationship.

In the work reported here, the research was
necessarily limited to studying the performance
effect of using auditory cues for the single pa-
rameter of glide slope information. Unfortu-
nately, due to equipment design and other
limitations, data could not be obtained on any
changes that may have occurred in the pilots’
visual scan pattern as a result of using the audi-
tory cues.

II. Equipment and Methodology.

Research Devices: A four place, single engine,
general aviation type airplane (Fig. 1) was used

Figure 1. Aircraft used in study of effects of aural cues on pilot performance during simulated instrument ap-
proaches.


movement of the glide slope needle across the
face of the instrument will increase as the air-
craft travels toward the middle marker. This
means the pilot is faced not only with the neces-
sity of constantly increasing the speed of his
cognitive, intellectual, and reactive processes, but
he must also cope with a constantly changing
control-display ratio that requires diminishing
amplitudes of control inputs to achieve a desired
degree of corrective action. It is possible, of
course, that this pacing stress may, under certain
adverse circumstances, impair not only the qual-
ity of the pilot’s tracking performance but also
his performance of vital concomitant task ele-
ments.

The fact that the pilot’s ability is taxed se-
verely, even in the idealized approaches used in
this study, is indicated by the comparatively
large deviations from the glide slope path—
often with rapid oscillations—during the last 20
to 60 seconds of the approach prior to reaching
the middle marker. Apparently, if the pilot
continues to utilize the display for tracking
rather than as a trend indicator during this latter
part of the approach, he tends to overcontrol the
aircraft, intensifying the oscillatory motion.
This, in turn, results in increased amounts of
time being spent on correcting the ever enlarging
deviation errors, causing the rapid development
of what might be called a critical “out-of-phase”
situation. Thus, the additive demands placed on
the pilot’s attention can result in selective de-
traction of time from and/or frequency of refer-
ence to, other vital instruments, as well as to
other tasks associated with making a safe ap-
proach,

The results of this study also suggest that it
may not be simply a matter of demand on visual
time alone that leads to minimizing of attention
to other instruments. The comparable perform-
ances of the groups using only auditory glide
slope cues suggests that it may be a more funda-
mental problem of total information handling
capability or perhaps, in the case of glide slope
information inputs, it may be that the combina-
tion of changing magnitude and rate of cue input
may exceed human ability to process the input
information rapidly enough to assure a high de-
gree of tracking accuracy during the last few
critical seconds of the approach. Unfortunately,
the present study could not determine how much
visual time was “saved” by presentation of audi-

15

tory glide slope cues since it was necessary for
the subjects to continue to visually obtain local-
izer cues from the crosspointer instrument. It
appears, however, that less visual time was de-
voted to this instrument during the aural cues
approaches—since localizer performance initially
deteriorated to some degree as attention was ap-
parently shifted from visual glide slope infor-
mation to that of auditory inputs. Although not
statistically verifiable, there is also some indica-
tion that localizer information may be accepted
by the pilot secondarily to glide slope informa-
tion. If this is so, it would be helpful to know
the ratio of deviation magnitudes required before
localizer cues take precedence over glide slope
cnes, The fact that a gradual improvement in
localizer performance took place as a function of
the number of approaches flown (except for
Group A using the conventional visual display)
seems to indicate that the pilots “retrained” their
scanning habits to acquire localizer information
more for its own significance than as an inci-
dental acquisition during the time they were
using aural cues for glide slope information.

The small, but significant, difference found
among the five groups relative to direction of
glide slope deviation at the middle marker may
be a spurious finding, since there was no compar-
able indication when the means for the approach
segments were analyzed. A trend seems to ap-
pear in Figure 5 but analysis of the data proved
it to be not significant. One possible explanation
is that the subjects failed to allow for parallax
effect between the glide slope needle and the in-
strument index markings. Parallax, in this in-
stance, could easily range between one half and
one “dot” depending on the position of the pilot’s
eyes in relation to the instrument. Lack of com-
pensation for such parallax would, in effect, cause
the pilot to fly his aircraft slightly below glide
slope. On the other hand, absence of parallax
with the auditory display would result in the
aircraft being flown relatively higher. The
parallax effect, if indeed present, is not so pro-
nounced during the early part of the approach
and is too inconsistent to be proven statistically
significant by the measures employed in this
study. :

Absence of significant differences among the
five groups with regard to heart rate and heart
rate changes suggests there was little difference
in stress among the groups. The spread of abso-



Miscte Marker. Ceenter ne of glide slope
‘pprosimately 200 feet above terrain

Geogrophice! Fir {Wheorerica! outer marker).
Gitte slope centerine approximorely 1960 feet

Sorety Pier poces aicrotr on
ceaterine of glide slope /tcatiter,

ree sea ara Om
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aa
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Somat \ cremate
IN .

Ficure 3. Flight path used during simulated instrument approaches.

Each circuit required approximately ten

minutes,

localizer “beam” at a point above a geographical
ground fix. Speed, power, landing gear and
flaps were set in approach configuration before
handing over control of the aircraft to the sub-
ject. Thus, all subjects commenced their ap-
proaches in similar approach geometry from the
same point in space without the stress and re-
sponsibility of navigating to that point. Upon
completion of each approach (past the middle
marker) the safety pilot raised the landing gear
and flaps and instructed the subject to “go-
around.”

After go-around, the aircraft was flown back
to the initial approach altitude by the subject.
He was then given a one-minute rest period
while the safety pilot made a 180 degree turn
inbound to the glide slope/localizer centerline,
where the procedure was repeated.

All air traffic control transmissions were ex-
cluded from the subject’s headset during the
approaches. This, along with relieving the sub-
ject from chart reading, communication and
other activities normally associated with an ac-
tual approach (as well as prior instructions to
concentrate primarily on the tracking task rather
than monitoring engine gauges and other such
ancillary duties) was done to minimize uncon-
trolled variables that might influence the sub-
ject’s ability to track the glide slope/localizer
centerline.

The flight was terminated after the tenth ap-
proach—with the safety pilot landing the air-
eraft.

Glide Slope Display Modes: Each group of
subjects used only one of the five different glide
slope displays during the experimental ap-
proaches (#4 through #8). These were:

Subject Group
and
Display Mode
A Visual Display—Glide slope infor-
mation provided solely by conven-
tional crosspointer instrument.
B Aural/Voice Display—Glide slope
information provided by taped
voice phrases and on-course tone.
Cc Aural/Code Display—Glide slope
information provided by Morse
code signal and on-course tone.
D Visual and Aural/Voice Display—
Displays A and B combined.
E Visual and Aural/Code Display—
Displays A and C combined.

The auditory glide slope displays included a
1020 Hz tone as an “on-course” cue when the
aircraft was on the approximate centerline of
the glide slope. This continuous tone was present
over a deviation range equivalent to the glide
slope needle being within the “bull’s-eye” portion
of the crosspointer instrument. Larger devia-
tions from glide slope resulted in cessation of
the 1020 Hz tone and a concomitant initiation
of an “off-course” aural cue. In the voice (fe-
male) mode presentations (individually and in
combination with the conventional display) the
aural cues consisted of the taped phrases “you
are high” or “you are low” when the aircraft
deviated more than one dot above or below the
glide slope centerline. The phrase, repeated at
tsvo-second intervals, increased in volume as the
magnitude of deviation increased. The code cues
consisted of a continuous series of the Morse code
“A” (@m) when the aircraft deviated above the



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TECHNICAL REPOPT STANDARD TITLE PAGE

T. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
FAA~AM~71~24

4. Title and Subtitle 5. Report Date

AURAL GLIDE SLOPE CUES: THEIR EFFECT ON PILOT May 1971

PERFORMANCE DURING IN-FLIGHT SIMULATED ILS INSTRUMENT [Performing Organization Code
APPROACHES

7. Author!s) 8. Performing Organization Report No.
A. Howard Hasbrook
Paul G. Rasmussen

9. Performing Organizetion Nome and Address 10. Work Unit No.
FAA Civil Aeromedical Institute
P. 0. Box 25082 11. Contract or Grant No.

Oklahoma City, Oklahoma 73125

13. Type of Report and Period Covered

12. Sponsoring Agency Name ond Address
Office of Aviation Medicine

Federal Aviation Administration
800 Independence Avenue, S.W. 14. Sponsoring Agency Code
Washington, D. C. 20590

OAM Report

15. Supplementary Notes

This work was performed under Task AM-B-70-PHY-16.

16. Abstract
Forty instrument rated Commercial and ATR pilots with 250 to 12,271 flight hours eac'

flew ten simulated ILS approaches in a single engine, general aviation aircraft.
Divided into five groups, each group used a different glide slope cue display in
combination with a modified "T' instrument panel configuration. Two types of aural
glide slope cue displays were utilized; (1) voice, and (2) Morse code signals.

No significant differences were found among the five groups relative to accuracy in
glide slope tracking. There was no apparent improvement with practice. The presenc
of glide slope cues resulted in the aircraft being flown slightly higher across the
middle marker than when only the conventional visual display was utilized. Localize
performance showed a slight but significant initial decrease in the presence of
glide slope cues with respect to only one performance measure. This difference was
minimized as a function of the number of approaches flown. No significant differenc
appeared among groups with regard to stress levels as measured by heart rate and
heart rate changes. Mean heart rates declined over successive approaches but in-
creased during each approach. Transition from the conventional visual crosspointer
display to the aural (voice) glide slope cues was achieved with a minimum of
familiarization and with no apparent difficulty.

17. Key Words 18. Distribution Statement
Aviation safety, instrument flight, Availability is unlimited. Document may be
auditory displays released to the National Technical

Information Service, Springfield,
Virginia 22151, for sale to the public.

| 2: No. of Pages [ 22. Price

24 $3.00

19. Security Clossif, (of this report) 30. Security Clossif. (of this page)

Unclassified Unclassified

Form DOT F 1700.7 (s-69)



GLIDE SLOPE DISPLAY

VISUAL ONLY
O VOICE ONLY
5 DOTS - -& CODE ONLY
-—TM VISUAL AND VOICE
—-& VISUAL AND CODE
=
Ow 4 DOTS
Zz
SE 3 vors
DOTS;
$38
Su
8
Za 2 DOTSE
a3
Frac)
a 1 o00oT -
1 1 1 4 4 4 1 41 1 j
1 2 3 4 5 6 7 8 9 10
OUTER FAP M:DDLE
MARKER SEGMENTS O PROACHES MARKER
AREA AREA
Fieure 5. Maximum deviations in either direction from glide slope centerline during approach segments, The up-

per group of curves indicate the largest single deviation during any experimental approach (#4 through #8)
by any subject in the respective groups. The lower group of curves represents the combined means for all ex-

perimental approaches for all five groups.

TaBLE 2—Maximum Deviations From Glide Slope Within Successive Segments of Approach.

(Analysis of Variance)

Source of Variation df MS F P
Between Subjects (39)
Groups...-- 4 232,29 <1
Subjects/Groups. 35 553.78
Within Subjects_ (1,960)
Segments ___ 9 3,889.74 76.99 <.01
Segments X Groups. 36 «68.40 1.35
Segments X Subjects/Groups 315-50, 52
Approaches 4 59.81 <1
Approaches X Groups. - 16 64.67 <1
Approaches X Subjects/Groups...-.-.-------- 140 80.98
Approaches X Segments - 36.25.60 <1
Approaches X Segments X Groups. - 144 26,17 <i
Approaches X Segments X Subjects/Groups - 1,260 27.99
TOTAL 1,999



SUBJECT APPROACH SEQUENCE

GROUP
(8 PILOTS
EACH) Tae 3 Lempertey 8, 8 10

A ®

®

@ w/t

@ w/T

Oy}; OO }| WB

V/T

®
®
® @ cst
7)
®

©
®
@ C/T ®
®
C/T ®

V/T
E C/T
LEGEND:
B=:
V/T =
C/T =

VISUAL CUE (CROSS-POINTER INDICATOR).
AURAL CUES (VOICE AND ON-COURSE TONE).

AURAL CUES (CODE SIGNAL AND ON-COURSE TONE).

€)V/T = VISUAL AND AURAL VOICE CUES (WITH ON- COURSE TONE).
€) C/T = VISUAL AND AURAL CODE CUES (WITH ON- COURSE TONE).

Taste 1. Sequence of display presentations used by the five groups during ten instrument approaches.

analysis, the accuracy of glide slope tracking
was evaluated on the basis of three separate
criteria; these were:

1. Percent time on glide slope between the
outer and middle markers. The aircraft was
defined as being on glide slope when deviation
from glide slope centerline did not exceed a
magnitude comparable to keeping the glide
slope needle within the periphery of the “bull’s-
eye” on the crosspointer instrument.

2. A composite performance score expressed
as a percentage. Scores were derived by
multiplying weighted ranges of deviation from

glide slope centerline by the duration of the
deviation.

3. The largest single deviation from glide
slope centerline during each of ten successive
time segments of the approach. Length of
these segments was a function of total ap-
proach time between the markers, (Note: All
measures of deviation discussed in this report
refer to deviations expressed in terms of com-
parable needle/dot indications, regardless of
whether such visual cues were available to the
subjects, during the experimental approaches
#4 through #8.)



Switching between the continuously operating
voice tracks was accomplished by inversely vary-
ing the volume of the two tracks according to
the signal ratio output of the glide slope receiver.

The control wheel was wrapped with a non-
conductive thin film plastic material to prevent
grounding of the subject’s EKG (heart rate)
signal output, which was obtained through ap-
propriately attached electrodes on the subject’s
chest.

Data Recorded: Data recorded during the
flights consisted of vertical and lateral deviations
from the centerline of the glide slope/localizer
beams, aircraft bank angle, vertical acceleration
of the aircraft, subjects’ heart rate, aural cue
inputs, and event information such as time of
passage over geographical “fixes,” calibration
checks, and other observer comments.

Subjects: Forty pilots with current FAA air-
men medical certificates and FAA instrument
ratings served as subjects. The subjects were
divided into five matched groups based on their
reported number of hours of instrument flying
experience. Some arbitrary weight was given to
recency of experience, as well as to total flying
hours of those subjects who reported the same
approximate level of instrument experience.
This method was employed to assure relatively
matched groups with regard to experience and
initial ability while retaining a statistically valid
sampling of the subject population. Subsequent
statistical tests on selected measures showed no
significant differences among groups with regard
to initial performance ability.

Flying experience of the subjects ranged from
250 to 12,271 hours, with a mean of 4,732 hours;
age ranged from 26 to 61 years with a mean of
43.5 years. Nineteen subjects had commercial
pilot ratings, twenty had airline transport rat-
ings, and one was a private pilot. Eighteen
possessed instructor ratings.

ILS Facilities: Two instrument landing sys-
tem (ILS) facilities were used for the study.
One facility served runway 35L at Will Rogers
World Airport, Oklahoma City, Oklahoma. The
other, a training facility, served runway 17 at
the Chickasha, Oklahoma airport. Approximate
elevation angle of both glide slope centerlines
was 2.5 degrees. The choice of facility for any

given flight was dictated by the prevailing wind
at the time of the flight. Subsequent examina-
tion of the data showed no apparent systematic
bias as a result of the use of either facility.

Experimental Procedure: Prior to flight each
subject was briefed by use of standard instruc-
tions on the manner in which the flight would
be conducted, the nature of the task, and the
required performance. In the aircraft, the aural
cue system was demonstrated briefly by use of a
simulation mode of operation until the subject
was familiar with the type of glide slope display
he would be using during the experimental ap-
proaches. He was also given a lightweight head-
set to wear to exclude all external radio com-
munications, minimize extraneous cockpit noises,
and permit the safety pilot to communicate with
the subject via the interphone. The subject ad-
justed the volume of the auditory glide slope
signal to the level he preferred.

The aircraft was taxied to the active runway
by the safety pilot and after “run-up,” the slats
were put in place. The take-off and initial climb-
out were made by the safety pilot. About 144
minutes after take-off, with trim set in climb
configuration, control of the airplane was given
to the subject with instructions to climb to, and
maintain, a specified altitude. After retrimming
the aircraft at the designated altitude, the sub-
ject executed a series of left and right medium
banked turns. Following these maneuvers, the
safety pilot extended the landing gear and set
the flaps, power and RPM in landing approach
configuration. The subject then practiced a par-
tial power descent to approach altitude, varying
power as needed to maintain a specified rate of
descent.

Regardless of his degree of familiarity with
the aircraft, each subject received a total of only
20 minutes in-flight practice involving the above
mentioned identical maneuvers. Limiting this
familiarization period to an “under-the-hood”
environment appears to have been effective in
reducing performance variations which might
have been expected from lack of experience with
the type aircraft used in the study.

Ten straight-in ILS approaches were per-
formed at approximate 10 minute intervals (Fig.
3). Prior to each approach, the safety pilot flew
the aircraft onto the centerline of the glide slope/


. Hodgson, D, A.:

REFERENCES

. Forbes, T. W., W. R. Garner, and J. G. Howard:

Flying by Auditory Reference OSRD

Report No. 5123, 1 June 1945.

(“Flybar”),

. Fox, J. G., I. Ferguson, and I. D. C. Andrews: An

Annotated Bibliography of Presentation of Informa-
tion in Aircraft Cockpits, S & T Memo 3/66, p. 77-78.
Technical Information and Library Services, Ministry
of Aviation, May 1966, (Available as AD 634575).

. Hasbrook, A. H., and P. G. Rasmussen: Pilot Heart

Rate During In-Flight Simulated Instrument Ap-
proaches in a General Aviation Aircraft. FAA Office
of Aviation Medicine Report No. AM 70-7, 1970.

Preliminary Studies of Auditory
Flight Control Displays, The Boeing Company Air-
plane Division Report No. D6-57104, June 1966.

18

A.

°

w

Kirchner, O. E.: Critical Factors in Approach and
Landing Accidents, (Part 1—Statistics). Project Re-
port, Flight Safety Foundation, Inc., December 31,
1960.

. Milton, J. L., R. BE. Jones, and P, M. Fitts: Eye Fixa-

tions of Aircraft Pilots: Frequency, Duration, and
Sequence Fixations When Flying the U.S.A. In-
strument Low Approach System (ILAS), USAF Tech.
Report No. 5839, October 1949.

. Taylor, V. A.: Critical Factors in Approach to Land-

ing Accidents 1959-1968. Interim Report, SAE-ASME
Report No. 854C, April 1964.

. Weir, D, H. and R, H, Klein: The Measurement and

Analysis of Pilot Scanning and Control Behavior
During Simulated Instrument Approaches, NASA
Contrator Report NASA CR-1535, June 1970.



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Tazie 5.—Glide Slope Deviation at Middle Marker (Algebraic).

(Analysis of Variance)

Source of Variation

Between Subjects_

Within Subjects __
Approaches.
Groups X Approaches.
Approaches X Subjects/Groups-

TOTAL.

df MS F P

- (39)
4 1, 614. 57 2,97 <.05
35 543.35.
(160)

- 4 102. 23 <1
- 16 234.05 <i
- 140 241.20
- 199

GLIDE SLOPE DISPLAY

VISUAL ONLY
VOICE ONLY
CODE ONLY
VISUAL AND VOICE
= VISUAL AND CODE

_ 2 007s
2,3
228
®e ~ 1 oT
Se
we
gg
Se 2
ee ER CENTERLINE
3a —* ——,
58 coor
aes
aor

g

= 2 vor:

2 00TS 5 6 7 8

APPROACH

Ficure 8. Mean glide slope deviation (algebraic) at
middle marker.

It was originally thought that changes in bank
angle might be useful as a performance measure.
Unfortunately, the data as a whole did not lend
itself to meaningful statistical treatment; large
bank angle corrections were infrequent, while
the almost constant but smaller turbulence-
induced deviations made unreliable any effort to
identify small pilot-induced bank angle correc-
tions. It was also impossible to define or relate
the effects of vertical acceleration on glide slope
tracking performance and to variations in bank
angle.

Heart rate data sampled from five 15-second
segments spaced evenly along the approach
(Figure 10) provided the results presented in

Figure 11. The analysis of variance summarized
in Table 6 shows that there were no significant
differences among the five groups relative to
heart rate and heart rate changes. However,
there was a highly significant (P=<.001)
inerease in mean heart rate, as a function of
progress from the outer marker to the middle
marker, and a significant (P=<.001) decrease
in mean heart rate across successive approaches.

GLIDE SLOPE DISPLAY

e @ VISUAL ONLY

o- 1 VOICE ONLY

4 CODE ONLY
@®—-—-B VISUAL AND VOICE
A—-~-—A VISUAL AND CODE

é

2 4 pots
Fad OTS
<

&

$

= 3 dots
é

w

2

3 2 v0Ts-+
cS

o

°o

uw

3 1pot +
<

=

z

e 1 i il 1 =)
S

F 4 5 6 ? 8

APPROACH

Figure 9. Means of total glide path variation during
last 30 seconds of approach.

il



‘Tavis 6.—Heart Rate During Five 15-Second Time Zones From Outer to Middle Marker.
(Analysis of Variance)

Source of Variation

Between Subjects.

Within Subjects
Approaches... -
Approaches X Groups.
Approaches X Subjects/Group:

Zones X Subjects/Groups.
Approaches X Zones-.-
Groups X Approaches X. Zone:
Approaches X Zones X Subjects/Groups..

df MS F P
(39)
4 2,326.80 <1
35 4, 545.05
(960)
4 247.23 8.28 <.001
16 29.51 <1
140 29.84
4 506.97 28.96 <.001
16 8.24 <1
140 17.50
16 3.82 <1
64 5.86
_ 560 5.67
999

IV. Discussion.

This study has shown that auditory cues can
be used as effectively as the conventional, visual,
method for tracking the glide slope during an
ILS approach. This is indicated by the fact
that the tracking performance of all five groups
was comparable, regardless of whether visual
cues, auditory cues, or a combination of both
were utilized,

Interestingly, although the conventional visual
display and the experimental auditory displays
proved equally effective by themselves, the avail-
ability of a combination of both types of displays
did not result in improved performance. Of the
several possible explanations for this lack of im-
provement, the most obvious possibilities are that
the subjects relied primarily on only one of the
two available displays, or that perhaps the audio
was used only as an alerting cue while the con-
ventional display was used for the tracking task.
Another possibility is that the combination of
two displays does not have an additive effect
because the tracking information, although re-
ceived through two different sensory channels,
is essentially identical—providing no additional
information to the pilot. This result would have
‘been predicted by the “single channel” theory of
attention. The present results suggest that the
useful limits of the information are dictated more
by the type, amount, and quality of the informa-
tion, than by the sensory method of presentation.

It is also possible that the subjects were al-
ready performing at or near their individual

13

performance capacities for the particular task of
tracking the glide slope and localizer. This pos-
sibility is supported by several considerations.
First, ancillary demands on the subjects were
minimized in comparison to those required dur-
ing an actual ILS approach under operational
conditions; all subjects commenced their ap-
proaches from a “no-error baseline” on the cen-
terline of the localizer and glide slope, and
normal, but time consuming activities such as air
traffic communications, navigation, and engine
performance monitoring were completely elim-
inated—with primary emphasis placed solely on
glide slope and localizer tracking. Second, the
performance by all groups showed relatively few
large tracking errors, and those that did occur
were well distributed throughout the five groups.

If the performance of the five groups in this
study is accepted as being representative of the
best that can be obtained under idealized “raw
data” instrument approach conditions, it appears
that the difficulties encountered in “real life”
situations (sometimes resulting in accidents) may
be the result of a combination of complex task
interactions that operate in such a way as to
exceed the normal response capability of the
pilot. A task that can be performed adequately,
though perhaps not ideally, even under very
favorable conditions, may well become too de-
manding when multiple task elements or emer-
gency situations are added to a tracking task
which already requires much of the pilot’s total
handling capacity.



TABLE OF CONTENTS

Abstract
Acknowledgments
Introduction
Equipment and Methodology
Results __
Discussion
Summary and Conclusions

LIST OF FIGURES

Aircraft used in study -
Panel display of primary flight instruments ~--~~~_-.
Graphic depiction of flight path flown during approaches
Relationship of volume of aural cues to magnitude of glide slope deviation --
Maximum deviations from glide slope centerline during successive segments
of approach _~_.
Percent time on localizer —
Maximum deviations from localizer centerline
Glide slope deviations at middle marker
Glide path variations during last 30 seconds of approach -.
10. Heart rates recorded during approaches between outer and middle markers —
11. Heart rate during successive approaches -_-.
12. Graphic depiction of relationship between linear distances, angular deviations,

and displacement of glide slope needle —__.
18. Graphic depiction of alternative glide slope beam configuration —_

LIST OF TABLES

1. Sequence of display presentations
Summary of analysis of variance for maximum deviations from glide slope
within segments of approach

3. Summary of analysis of variance of percent time on localizer

oR eye

PRA

4, Summary of analysis of variance of maximum deviations from localizer
within segments of approach
5. Summary of analysis of variance of glide slope deviations at middle marker
6. Summary of analysis of variance of heart rates
APPENDIX TABLES
I. Data for control group using only the crosspointer indicator for glide slope
information during experimental approaches
II. Data for group using voice cues and omi-course tone for glide slope informa-
tion during experimental approaches _
Ill. Data for group using code signal and on-course tone for glide slope informa-
tion during experimental approaches —_.
Iv. Data for group using voice cues and on-course tone in combination with
crosspointer instrument during experimental approaches —_-..

V. Data for group using code signal and on-course tone in combination with
crosspointer instrument during experimental approaches

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ACKNOWLEDGMENT

The authors wish to take this opportunity to thank all the individuals
and organizations whose cooperation and assistance made this study possible.
Our special gratitude is extended to the personnel of the FAA Flight Stand-
ards Service, Air Traffic Service, FAA Academy, and the Cardiorespiratory
Unit of CAMI’s Physiology Laboratory whose professional contributions
and personal interest deserve particular recognition.


TaBLE 3.—Percent Time Deviation to Right or Left of Localizer Centerline Did Not Exceed One Dot (Comparable
to Needle Remaining Within Instrument “Bulls-eye”).

(Analysis of Variance)

Source of Variation af MS F P
Between Subjects...._._---------------- (39)
4 3,335.45 2.71 <.05
35 1,231.59
(160)
4 508. 33 2.82 <.05
16 240. 69 1,33
140 180. 49
199

Analyses of variance showed no significant
differences among the groups with regard to the
accuracy of glide slope tracking on the basis of
any of the three criteria outlined above. Figure
5 shows the means of the maximum deviations
and the single largest deviation by any subject
within the respective groups during the ten suc-
cessive segments of the approach. In the absence
of significant differences among approaches, data
for approaches #4 through #8 have been com-
bined. Table 2 summarizes the analysis of
variance for the same data. Not unexpectedly,
there were significant increases (P=<.01) in
maximum deviations as the middle marker was
approached. This increase only becomes appar-
ent during the last half of the approach. Es-
sentially the same results are obtained if the
direction as well as the magnitude of the devia-
tions are included in the analysis. The algebraic
means tend to average slightly below glide slope
for most of the approach but above centerline
during the last two segments preceding the
middle marker.

Accuracy of Jocalizer tracking performance
was examined on the basis of the same three
criteria used to evaluate glide slope tracking
performance. The results for percent time on
localizer are presented in Figure 6 and Table 3
which summarize the analysis of variance. Sig-
nificant differences appear among the five groups,
with the one using the conventional (visual only)
display exhibiting consistently better perform-
ance than the other four groups. Interestingly,
the groups utilizing the visual display in com-
bination with the voice or code cues exhibited
somewhat better localizer performance than those
using only auditory glide slope information.
Also, there was a statistically significant
(P=<.05) improvement in performance as a
function of the number of approaches flown.

Although there was no significant interaction be-
tween groups and approaches, the improvement
tended to occur more predominantly in the
groups employing auditory cues as their sole
source of glide slope information. Differences
among the five groups were therefore less pro-
nounced at the end of the experimental sequence
than at the beginning.

GLIDE SLOPE DISPLAY

@ VISUAL ONLY
‘O VOICE ONLY

loo -4 CODE ONLY
VISUAL AND VOICE
VISUAL AND CODE
90h e
2.
a ae \
e
Bob
- a AR ee OB
5 7
ig 70F a Os
Fa Aa
= , .
£ /
~ aN
uw 6oF &---
= yos
Fs an
. a
s0- g”
40r
N
1 1 1 1 J
4 5 6 7 8
APPROACH

Ficvure 6. Time (percent) in which localizer deviation
did not exceed one dot.



“OUTER MARKER"

15 SEC.
ZONE

HEART RATE - BPM

i
i
'
‘ 1 i MIDDLE
1 MARKER
1
i

I ] I 1 F 1 I 1 Runway
18 SEC. 15 SEC. 15 SEC. 15 SEC,
ZONE ZONE ZONE ZONE

Ficure 10. Heart rate data was collected continuously between outer and middle markers during approaches.
Data used in this study were taken from five 15-second time zones spaced progressively to the middle marker.

GLIDE SLOPE DISPLAY

@ VISUAL ONLY
OQ VOICE ONLY

or 4 CODE ONLY
—'—-- VISUAL AND VOICE
a—-—-& VISUAL AND CODE
L OM= OUTER MARKER
MM = MIDDLE MARKER
z 100 ;
2 a sy!
u & Sand
&
cs
& a
?
w
x
z
go
Fd 4
fs]
+
om MM
80 L 1 1
REF. 6 7 8
H.R. 4 .

APPROACH

Fieure 11, Mean heart rate during five 15-second time zones spaced along approach path from outer to middle

marker.

12