Comparison of a Typical 

Electronic Attitude-Direction 

Indicator With Terrain-Depicting 

Primary Flight Displays for 

Performing Recoveries From 

Unknown Attitudes: Using 

Difference and Equivalence Tests

Dennis B. Beringer 

Jerry D. Ball

FAA Civil Aerospace Medical Institute

Oklahoma City, OK 73125

Kelly Brennan

Sitafa Taite

School of Industrial Engineering

University of Oklahoma

Norman, OK 73019

December 2005

Final Report

DOT/FAA/AM-05/23

Offi ce of Aerospace Medicine

Washington, DC 20591
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

Technical Report Documentation Page 

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

DOT/FAA/AM-05/23 

4. Title and Subtitle 5. Report Date 

Comparison of a Typical Electronic Attitude-Direction Indicator With December 2005 

Terrain-Depicting Primary Flight Displays for Performing Recoveries From 

Unknown Attitudes: Using Difference and Equivalence Tests 

6. Performing Organization Code 

 

7. Author(s) 8. Performing Organization Report No. 

Beringer DB1

, Ball JD1, Brennan K2, Taite S2 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

 

11. Contract or Grant No. 

1

FAA Civil Aerospace Medical Institute 

P.O. Box 25082 

Oklahoma City, OK 73125 

2

School of Industrial Engineering 

University of Oklahoma 

Norman, OK 73019 

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered 

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, DC 20591 14. Sponsoring Agency Code 

15. Supplemental Notes 

Work was accomplished under approved task AHRR521 

16. Abstract 

A study was conducted to determine if primary flight displays (PFDs) depicting terrain could be used with a 

level of safety equivalent to electronic attitude-direction indicators (EADIs) without terrain. Five groups of 8 

pilots each flew scenarios in a flight simulator using one of three PFDs (EADI, full-color terrain, uniformly 

brown terrain) with or without guidance cues. Performances of recoveries from unknown attitudes using the 

EADI were measured first as a baseline, followed by trials with one of the experimental formats. Performance 

measures included initial response time, total recovery time, and both initial and secondary control reversals. 

Traditional “difference” analyses found no significant performance differences between groups. Analyses using 

confidence intervals to assess equivalence of distributions showed that group performances were practically 

equivalent. Pilot preferences were examined and are reported. It was concluded that the specific terrain 

representations examined provided for performance at least equal to if not better than the conventional EADI.

This comparative technique is recommended for situations in which one wishes to demonstrate that a 

proposed device or system is no worse than or roughly equivalent to something already in use. 

 

17. Key Words 18. Distribution Statement 

 

 

Primary Flight Display, Terrain Display, Unknown 

Attitudes, EFIS, Equivalence Tests

Document is available to the public through the 

Defense Technical Information Center, Ft. Belvior, 

VA 22060; and the National Technical Information 

Service, Springfield, VA 22161 

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

Unclassified Unclassified 11 

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

iii

ACKNOWLEDGMENTS

The authors thank Barry Runnels for his assistance with the operation of the AGARS 

during the study and Kevin Williams for his programming of the calculations and 

graphical depictions outlined in Rogers et al. (1993). 

1

COMPARISON OF A TYPICAL ELECTRONIC ATTITUDE-DIRECTION INDICATOR WITH

TERRAIN-DEPICTING PRIMARY FLIGHT DISPLAYS FOR PERFORMING RECOVERIES

FROM UNKNOWN ATTITUDES: USING DIFFERENCE AND EQUIVALENCE TESTS

BACKGROUND

It is frequently necessary to determine whether a system 

submitted for certifi cation provides for a level of safety 

equivalent to the system(s) that it is proposed to replace. 

In most cases, practicality limits the number of individuals who can participate in airborne testing of the system, 

and data collection is sometimes, of necessity, limited to 

the use of checklists with evaluation criteria. Even when 

quantitative objective laboratory evaluations in fl ight 

simulators are employed, the efforts often culminate in 

an attempt to demonstrate that something is better than 

something else, not that something is equivalent to something else. However, we often want to say that something

is equivalent to something else, particularly when we are 

attempting to retain a standard level of system performance. It is not, obviously, suffi cient to fail to reject the 

null hypothesis (no difference) and then claim to have 

proven it. On the other side of the coin, it is also not 

necessary to demonstrate that a proposed system produces 

signifi cantly better performance than the standard. Rather, 

it should be suffi cient to be unable to detect a difference 

while also making some statement about how closely 

performance approximates a standard (equivalence). An 

approach that could be used for “equivalence” testing 

involves confi dence intervals (Rogers, Howard, & Vessey, 

1993; Reising, Liggett, Kustra, & Snow, 1998; Seaman 

& Serlin, 1998), and its implementation is shown for 

the following specifi c example.

One major component of Electronic Flight Instrumentation Systems (EFIS) is the Primary Flight Display 

(PFD). Although PFDs initially depicted attitude and 

fl ight-guidance information, they now can include forward-looking perspective views of both guidance information (Beringer, 2000) and of the outside world (Alter, 

Barrows, Jennings, & Powell, 2000). Data relevant to GA 

are available that may be useful for determining what the 

allowable range of variation in design parameters can be 

that still supports effective pilot performance. A series 

of studies was performed at the NASA Langley Research 

Center examining various terrain representations, and 

assessing pilot preferences for fi eld of view and style of 

depiction (Arthur, Prinzel, Kramer, Parrish, & Bailey, 

2004). One issue not specifi cally addressed was recovery 

from unknown or unusual attitudes. This specifi c concern 

was addressed in one certifi cation process by requiring 

that the terrain depiction be removed from the PFD when 

the aircraft exceeded certain pitch or roll criteria because 

of concern that the depicted terrain might cause confusion or interfere with recovery. However, there were no 

empirical data to indicate what role, positive or negative, 

the terrain depiction might play in the recoveries. 

A study was conducted to determine (1) if pilots would 

recover to the terrain horizon rather than the zero-pitch 

line if the two were different, as would be seen in mountainous terrain, (2) if positive guidance cues (Gershzohn, 

2001) could ameliorate any potential negative effects 

of the terrain background and (3) if the coloration of 

the terrain presentation would affect performance. A 

second step was added to the fi nal analyses to determine 

if a statistically based comparison could be used here, 

as was by Reising et al. (1998), to assess equivalence of 

performances.

METHOD

Experimental Display Formats

The baseline display was a conventional Electronic 

Attitude-Direction Indicator (EADI; blue sky, brown 

ground) with airspeed, altitude and vertical speed shown 

in tape format along the left and right edges of the display 

with a compass card at the bottom of the display (Figure 1). Two formats were added that depicted a terrain 

background, one in full color and one with blue sky and 

uniformly brown ground (simulating a PFD that has already been certifi ed). Additionally, guidance arrows were 

added to the baseline and full-color formats, producing 

fi ve combinations of ADI format and guidance information. Pitch arrows were linear (Figure 2), appearing when 

the aircraft attitude was greater than 15 degrees up or 

13 degrees down and disappearing when the aircraft was 

within 5 degrees of zero pitch, pointing from the aircraft 

symbol to the horizon. Roll arrows (Figure 1) were of arc 

form, appearing when the aircraft exceeded 25 degrees 

of bank and disappearing when the aircraft was within 

10 degrees of zero bank, pointing from the plane of the 

wings to the horizon line. The roll-command arrow took 

precedence over the pitch-command arrow when the 

aircraft was pitched down, and the priority was reversed 

when the aircraft was pitched up.
2

Terrain was based on variable-sized polygons anchored 

to elevation posts with photo-realistic texture or uniform 

brown texture applied to the polygons. The following illustrations show the PFD in full-color terrain mode with 

pitch-guidance arrow (Figure 2) and in EADI mode with 

roll-guidance arrows (Figure 1).

Horizon line. The horizon line was constructed such 

that it would have high contrast against the vast majority 

of possible backgrounds. This is not normally an issue 

with traditional head-down attitude direction indicators 

(ADIs), as the horizon on these displays is represented 

as the boundary between differently colored fi lled areas, 

often with a line of a different color between them. 

It is also possible to use a single-color line (as long as 

it conforms to MIL-STD-1787C, 5.1.2.1; Horizon 

reference; the standard does not deal specifi cally with 

terrain-depicting PFDs, nor does the SAE Aerospace 

Recommended Practice document on perspective displays deal specifi cally with this horizon-line issue) in 

terrain-depicting displays where the ground and sky 

representations are of known uniform colors (i.e., the 

Chelton display uses a uniformly brown ground and 

blue sky). Our horizon line consisted of three two-pixel 

bands alternating black-white-black on a 640 by 480 

display fi eld, consistent both with horizon lines used 

in other full-color terrain display experiments and with 

general guidelines. The PFD image was enlarged to 800 

by 600 pixels in the simulator and was approximately 

7.5 inches wide (16.4 degrees) by 5.6 inches tall (12.3 

degrees) (a 9.38 inch diagonal). The display portrayed 

about 30 degrees laterally of the outside world.

Experimental Design

A three-factor mixed design was employed, with terrain background (full-color or none) and guidance arrows 

(present or absent) as the between factors and trial block 

as the within factor. A supplemental condition, brownonly terrain, was added after contribution of guidance 

arrows had been assessed (Figure 3). Dependent variables 

included initial response time (IRT; time to fi rst control 

input), total recovery time (TRT), primary controlinput reversals (fi rst response in wrong direction), and 

secondary control-input reversals (subsequent response 

in wrong direction). 

Figure 1. EADI with roll-recovery arrows shown. Figure 2. Terrain-depicting PFD (full color) with pitchrecovery arrow.

Figure 4. Full-color terrain PFD showing Sandia 

Peak in fi eld of view.

Figure 3. Supplemental brown-terrain condition. 
3

Two sampling variables, terrain depiction at roll-out and 

attitude at recovery onset, were used to vary conditions; half 

of the trials ended facing the mountains and half facing 

a lower-elevation plain; attitudes used variation in pitch 

(+20, 0, and –15 degrees) and bank (60 degrees left, 0, 60 

degrees right) (zero-zero excepted). Three supplemental 

trials were added for approximately the last 7 pilots in each 

group, including a near-mountains trial (peak extending 

to +5 degree pitch reference), an inverted trial (between 

150- and 160-degree bank), and a 40-degree displayed 

fi eld-of-view trial (to determine if pilot preference for 

a wider displayed fi eld of view was associated with any 

performance advantage). A view showing Sandia Peak in 

the displayed fi eld of view appears in Figure 4, but the 

peak is not depicted reaching the 5-degree pitch-up line 

as was the criterion for setting up a recovery trial. 

Equipment and Participants

Data were collected using a fi xed-base simulator confi gured to represent a Piper Malibu. Given that (1) the 

usual practice in training for unknown attitude recoveries 

is to “confuse” the participant by providing misleading 

vestibular cues and that (2) real-world unusual attitudes 

are often entered slowly, without awareness, and without 

overt acceleration cues until well into a maneuver, the 

absence of motion appeared an equally acceptable condition to impoverish pilot perception as to the state of 

the aircraft. The PFD was presented on a LCD on the 

left side of the instrument panel directly in front of the 

participant, and the experimenter-pilot (EP) fl ew from 

the right seat with a repeater display. The out-the-window 

view at altitude represented a hard-IFR situation with no 

visible environmental cues (gray). Performance data were 

recorded digitally, with supplemental audio and video 

data recorded to DVD.

Participants were 40 GA pilots (38 male, 2 female) 

recruited from the local community, 8 assigned to each 

of the 5 display conditions. Age and overall fl ight hours 

were balanced across groups as participants entered the 

experiment. Ages ranged from 19 to 57 years. All were 

certifi ed minimally as Private Pilot, while many were 

instrument rated and some were fl ight instructors. Total 

fl ight times ranged from 50 to 13,000 hours.

Procedures/Tasks

Pilots completed an experience questionnaire, were 

briefed about the experimental display, and instructed 

that they would recover from unknown attitudes. They 

were told to recover to a zero-pitch, zero-bank attitude, 

regardless of altitude or airspeed, as the EP would confi gure

the aircraft such that performance was usually within the 

operating envelope (primary interest was in participant 

ability to interpret the display and determine when a 

level attitude had been restored). They then entered the 

simulator, where they were further familiarized with the 

display and the simulator. A visor was used so that direct 

vision of the display would be obscured when in the headdown preparatory position for the recovery.

Each pilot then took off from Albuquerque, climbed 

into IFR conditions, and performed 8 warm-up (baseline) 

recovery maneuvers using the basic EADI on the PFD. 

Recall that the practical test standards for the Private 

Pilot Certifi cate require performance of recoveries from 

unknown attitudes solely by reference to instrumentation, 

so that all participants had received training in this task 

prior to participation in the experiment (recency of last 

recoveries was assessed by post-test questionnaire, and 

most reported it at their check ride or biennial fl ight 

review). Each trial began with the participant in the 

head-down position and hands off the controls. The EP 

(same for all participants) then placed the simulator into 

the required attitude and heading for that trial, using 

predetermined airspeed, altitude, and heading criteria 

that had been rehearsed, and told the participant to recover. Upon completing these trials, the participant fl ew 

the simulator back to the airport for a full-stop landing. 

At this time, the display format was changed and the 

procedure was repeated. 

Experimental trials consisted of 16 recovery maneuvers (defi ned by combinations of the sampling variables 

described earlier) using the assigned PFD format. Two 

groups repeated the EADI in the experimental trials, one 

as a control and the other with guidance cues added. 

Two counterbalanced orders of the combinations of 

the sampling variables were used within the groups to 

attempt to distribute any possible learning more evenly 

across conditions. Pilot IRTs and TRTs were recorded for 

each trial. A recovery was considered complete when the 

aircraft reached ±2.5 degrees of pitch and ±5.0 degrees of 

bank and the pilot was able to maintain those values for 3 

seconds. The supplemental trials described earlier in the 

Methods section were added to the end of the session. 

Participants completed a posttest set of questionnaires 

regarding their subjective assessment of the displays, went 

through a posttest interview, and provided both solicited 

and unsolicited responses/opinions.

RESULTS

Group Parity in Baseline

The distributions of hours of experience, licensing/rating categories and age were similar enough between groups 

that any differences found between group performances 

were unlikely to be a result of those variables. Analysis of 

recovery times for the baseline trials showed that although 

the groups initially differed in their performances, they 
4

were performing comparably (no signifi cant differences) 

by the last two trials, suggesting that all groups had attained a similar level of performance prior to entering 

the experimental trials. Means by groups and trials are 

depicted in Figure 5.

Comparative Analyses Approach

It is recommend that both a test for a difference and a 

test for equivalence be performed (Rogers, et al., 1993). 

The difference-test results will be presented fi rst, followed 

by equivalence-test results. The ideal combination of 

outcomes would be that (a) performance distributions 

are not statistically different and (b) that they are statistically “equivalent.”

Performance Variables – Difference Tests

Recovery times. Multivariate Analysis of Variance 

indicated there were no signifi cant differences between 

the display confi gurations for either of the response-time 

variables. Pitch-roll TRTs averaged around 10 seconds, 

whereas roll-only recoveries averaged about 8.5 seconds. 

Pitch-only recoveries averaged approximately 8.6 to 9.0 

seconds. Figure 6 depicts mean TRTs by maneuver and 

display format. Univariate analyses were conducted to 

determine if type of maneuver was associated with any 

signifi cant differences between group performances. 

Again, no signifi cant differences were found between 

displays and type of maneuver for either of the responsetime measures.

Control reversals. There were only three clearly identifi able primary control reversals in the nearly 800 trials. 

There were no secondary reversals (initial response in 

correct direction; subsequent control movement opposite 

to input required). Recovery times for the three reversals 

were not notably different from those of other trials. Thus, 

reversals did not appear to be a factor.

Supplemental trials. Analyses were conducted for performance variables on each of the three supplemental trials, 

and no differences in performance were found between 

the display confi gurations.

Equivalence Tests

The practice of equivalence testing has been used 

widely in the fi eld of medicine to assess the effectiveness 

of new drugs or procedures as compared with existing 

medications or practices, particularly when a “generic” 

version of a medication is being introduced as a potential 

alternate to the established name brand. The central issue 

is determining if the proposed treatment or intervention is at least as good as (“equivalent to”) the existing 

one. Both Rogers et al. (1993) and Seaman and Serlin 

(1998) discuss this problem and several possible statistical approaches. The conclusion drawn is that the use of 

confi dence intervals requires fewer arbitrary decisions 

than would the use of either of two other approaches. 

In addition, a precedent for using this approach for 

performance assessment in aviation systems was set by 

Reising, et al. (1998) in evaluating pilot performance 

using pathway-display guidance for curved approaches. 

Also, this type of evaluation is presently being used to 

compare the effectiveness of fl ight-training devices and 

PC-based aviation training devices (Taylor, et al., 2004) 

for training and/or evaluating instrument skills (Taylor, 

et al., 2005; Taylor, Talleur, Rantanen, Emanuel, & 

Beringer, manuscript in preparation).

The procedure used by Rogers et al., was followed 

here. One fi rst constructs the “equivalence interval,” a 

reference mean bounded by positive and negative limits 

about that mean representing “practical” limits within 

which variations in performance are not considered to be 

of practical signifi cance. These can be based on baseline 

data or practical experience with the specifi c task. This 

can be followed by either two simultaneous one-sided 

hypothesis tests or the construction of confi dence intervals. Using the hypothesis tests, the aim of the fi rst test 

is to attempt to reject H0

: that the difference between 

the two means is less than or equal to the smaller delta 

(lower criterion) and the second to reject an H0

: that 

the difference is greater than or equal to the larger delta 

(upper criterion). If both null hypotheses are rejected, 

the comparison distribution falls within the upper and 

lower criterion limits.

Alternately, one can construct confi dence limits for 

each of the comparison conditions. If the 90% confi dence 

interval for a condition falls completely within the criterion 

interval, then equivalence is assumed. Additionally, if the 

reference mean in contained within the 95% confi dence 

interval, then we fail to reject the null hypothesis for 

the traditional difference test. Thus, the target for the 

equivalence test is to have the 90% confi dence interval 

within the equivalence boundaries and the reference mean 

within the 95% confi dence interval.

Following the defi ned procedures, means and standard 

deviations for total recovery time (TRT) were generated 

for each participant (1) across all baseline trials and (2) 

across all of the experimental trials. The baseline trials 

were used as the basis for determining a mean reference 

point and for generating criterion bounds to be used in 

comparing confi dence intervals generated for each of the 

display groups (the “experimental” conditions) with the 

reference or standard (baseline: EADI). A practical effect 

size was selected based upon the standard deviation of 

the baseline data (sd = 1.98; criterion chosen as 0.2 of 

reference mean, 1.994). Table 1 shows the means and 

other associated statistics used for this comparison.
5

Figure 5. Mean total recovery times for baseline trials by group and serial trial number.

EADI +

Arrows Color

Terrain +

Arrows

EADI

(Baseline) Color

Terrain Brown

Terrain

Roll

Pitch Down

Pitch Up

Pitch Down & Roll

Pitch Up & Roll

0

2

4

6

8

10

12Total Recovery Time (secs)

Display Format

Maneuver

Figure 6. Mean TRT by display type and maneuver.
6

The philosophy for “equivalent safety” regarding this response-time measure would be that recovery times should 

be equal to or less than those obtained with the standard. 

As such, the criterion that the entire confi dence interval 

should fall within the criterion limits for “equivalence” 

can be modifi ed to the practical consideration that only 

the upper limit of the confi dence interval is important 

and needs to be within the criterion limits. Correspondingly, only the z scores for the upper tails are of interest 

in determining that the results are “no worse than” those 

of the reference condition. 

Examination of the p values in Table 1 associated with the 

upper tails of the distributions shows that all of these were 

within the criterion limits (we reject H0

: m1-m2> or = delta1 

and accept Ha

: delta1<m1-m2<delta2). In fact, reference to 

Figure 7 shows that the 90% confi dence intervals for AA and 

AO fall completely within the criterion range, and we can 

accept those as functionally equivalent to our baseline. 

That AO is evaluated as equivalent is particularly important in that it is identical to the baseline condition (EADI 

Only), suggesting that any gain in performance from baseline 

to experimental trials was not suffi cient to be detectable as a 

difference, either by traditional difference testing (note that 

Figure 7. Experimental-condition 90% and 95% confidence intervals relative to baseline mean and upper 

and lower criteria graphed as seconds.

Table 1. Reference and comparison values for the equivalence evaluation. (For Table 1 & Figure 7;

AA=EADI w/arrows, AL = all features (arrows, full-color terrain), AO = EADI only, AT = EADI + terrain, 

BT = EADI w/brown terrain.) 

 Mean +0.2 -0.2 

Baseline (EADI) 9.87 11.96 7.98 

90% 

upper

90% 

lower

95% 

upper

95% 

lower

upper z lower z 

AO (EADI only; same as baseline) 9.31 10.50 8.11 10.84 8.03 3.48#

 -1.94+

AA (EADI plus arrows) 9.67 10.84 8.49 11.17 8.21 3.04x -2.47*

AT (full-color terrain) 9.01 10.22 7.81 10.55 7.68 3.87# -1.53 

AL (full-color terrain plus arrows) 9.1 10.36 7.84 10.70 7.70 3.59o

 -1.57 

BT (brown terrain) 8.78 9.91 7.66 10.23 7.54 4.47# -1.30 

+ p<.05, * p<.01, x p<.005, o p<.0005, # p<.0001
7

the reference mean is within the 95% confi dence interval for 

each group) or by equivalence testing. Interestingly, AL, AT 

and BT, each having a terrain-background component, are well 

within the upper limit but are all just out of the lower limit 

of the criterion range (meaning a tendency towards shorter 

recovery times). Thus, they would be judged as no worse than 

the baseline and potentially (not signifi cantly) better.

Questionnaires and Posttest Interviews

Pilots indicated that they were focusing their attention 

on the relatively prominent zero-pitch line and did not 

regard the terrain depictions as signifi cant contributors to 

their recovery task. Neither preference for the 40-degree 

fi eld of view nor for guidance arrows was paired with performance differences. Participants also expressed a relatively 

uniform preference for the terrain-depicting displays.

SUMMARY AND CONCLUSIONS

It appears that a zero-pitch line of the contrasting components and of the thickness and extent specifi ed allows 

pilots to discern the zero-pitch reference from other display 

features and to perform recoveries from unknown attitudes 

regardless of the terrain format used. It also appears that 

the directional-guidance arrows did not produce the effect 

found in a previous experiment (Gershzohn, 2001) despite a 

positive pilot response, and the frequency of reversals was too 

low to draw any conclusion from them. Although the mean 

total recovery times tended to be shorter for those display 

conditions that depicted background terrain, the differences 

were neither statistically nor practically signifi cant.

Given the previous fi ndings (indicating enhanced terrain 

awareness attributable to terrain depictions), combined with 

the lack of detrimental effects found in this study relative 

to recoveries from unknown attitudes, there would appear 

to be few signifi cant obstacles to the implementation of 

this type of PFD for general aviation use. Caveats to be 

observed, however, would be that (1) similarly constructed 

terrain depictions are used, (2) the zero-pitch line is clearly 

differentiable from the terrain and sky depictions regardless 

of the type of background and (3) that the direction of offdisplay pitch-line locations are clearly indicated.

Further, it appears that the combination of difference 

testing and “equivalence” testing, using a practical difference based upon baseline data for the chosen application 

and task-performance variables, can provide a reasonable 

means by which to determine practically equivalent levels 

of pilot performance and, thus, equivalent levels of safety. 

Using this approach in the fi eld and/or in a very applied 

environment will, however, always face the limitations 

imposed by restricted sample sizes, high performance variability, and the challenge of selecting meaningful practical 

effect sizes.

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R.V., and Bailey, R.E. (2004). Flight simulator 

evaluation of synthetic vision display concepts 

to prevent controlled fl ight into terrain (CFIT). 

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