# Simulator Fidelity -- The Effect of Platform Motion

Dr J Bürki-Cohen, US Department of Transportation, E Boothe and Dr N
Soja, Consultants, Dr R DiSario, Bryant College, Dr T Go, Massachusetts
Institute of Technology and Dr T Longridge, Federal Aviation
Administration, USA

CORRECTION

Due to a cell reference error, the number of subjects in Figures 5 and 6
on page 23.5 were reported incorrectly. The correct *n*'s for V~1~ cut
Transfer are 16 for the Motion group and 18 for the No-Motion group.

Simulator Fidelity -- The Effect of Platform Motion

Judith Bürki-Cohen

Engineering Psychologist

Volpe Center, U.S. Department of

Transportation, Cambridge, Massachusetts

Edward M. Boothe

Consultant, Flight Simulation and Training, Atlanta, Georgia

Nancy N. Soja

Consultant, Experimental Psychology

Brookline, Massachusetts

Robert DiSario

Assistant Professor

Bryant College, Smithfield, Rhode Island

Tiauw Go

Postdoctoral Associate

Massachusetts Institute of Technology, Cambridge, Massachusetts

Thomas Longridge

Manager, Advanced Qualification Program Federal Aviation Administration,
Washington, D.C.

# 

# [Abstract]{.underline}

This research is part of the Federal Aviation Administration\'s (FAA)
initiative towards promoting affordable flight simulators for U.S.
commuter airline training. This initiative becomes even more important
as the FAA is considering regulatory action that will mandate the use of
simulators for all air carrier flight-crew training and qualification.
Consequently, sound scientific data on the relationship between certain
simulator features such as platform motion and their effect on the
transfer of pilot performance and behavior to and from the respective
airplane become very important. The present study examined the effect of
platform motion (i.e., FAA qualified Level C six-degree-of-freedom
synergistic motion) in the presence of a high-quality wide-angle visual
system on 1) pilot performance and behavior for *evaluation* prior to
any repeated practice or training, 2) *the course of training* in the
simulator, and 3) the *transfer of skills* acquired during training in
the simulator with or without motion to the simulator with motion as a
stand-in for the airplane (quasi-transfer design). Every effort was made
to avoid deficiencies in the research design identified in a review of
prior studies, by measuring pilot stimulation *and* response, testing
both maneuvers and pilots that are *diagnostic* of a need of motion,
avoiding pilot and instructor *bias*, and ensuring sufficient
statistical *power* to capture operationally relevant effects. Results
of the analyses and their implications are presented in this paper.

## Nomenclature 

### FAA Federal Aviation Administration

PTS Practical Test Standards

### RTO Rejected Take-Off

V~1~ Take-off decision speed; the minimum speed in the take-off,
following a failure of the critical engine, at which the pilot can
continue the take-off and achieve the required height above the take-off
surface within the take-off distance.

V~1~ cut Engine failure at or above V~1~ with continued take-off

V~2~ Take-off safety speed; a speed that will provide at least the
gradient of climb required by the airplane certification rules with the
critical engine inoperative.

PF Pilot Flying

PNF Pilot Not Flying

I/E Instructor/Evaluator

n Sample size

*p* Probability of null hypothesis (i.e., no effect of motion)

*r* Pearson correlation coefficient

STD Standard Deviation

[Introduction]{.underline}

This research effort is part of the Federal Aviation Administration\'s
(FAA) initiative towards promoting the availability and affordability of
flight simulators for U.S. commuter airline training. This initiative
(Ref. 1) becomes even more important as the FAA is proposing a rule that
would mandate the use of simulators for all air carrier training and
qualification, limiting the use of the aircraft itself as a training
option even for small regional airlines. However, there is a lack of
sound scientific data on the relationship between certain key training
device features, such as platform motion cuing, and their effect on the
transfer of performance to and from the airplane. This project will
develop a scientific basis to assure that FAA requirements promote full
transfer of performance between simulator and airplane---anything less
would compromise safety. The data will also help the FAA to evaluate air
carrier proposals for the alternative use of other training equipment in
lieu of full flight simulators. The first stage of this multi-year
project was a state-of-the-art review of key aspects of flight
simulation, involving both FAA and Industry subject matter expert
workshops (Refs. 2 and 3) and an extensive literature review (Refs. 4
and 5). Based on this review, an empirical investigation of flight
simulator requirements which seeks to correct deficiencies in the
research design of prior studies has been initiated.

The present study empirically examined the effect of platform motion
(i.e., FAA qualified Level C six-degree-of-freedom synergistic motion)
in the presence of a high-level visual system (i.e., wide-angle
collimated cross-cockpit) on pilot training and pilot evaluation. It
addressed the questions of whether the motion provided by an FAA
qualified Level C simulator affects 1) First Look evaluation of pilot
performance and behavior prior to any simulator practice, 2) the course
of Training in the simulator, and 3) the Transfer of training acquired
during training in the simulator with or without motion to the simulator
with motion as a stand-in for the airplane. The analysis also examined
whether the grading criteria used by the instructors/evaluators (I/Es)
were affected by the presence or absence of motion. The statistical
power of the experiment was also considered.

[Method]{.underline}

An FAA qualified Level C flight simulator was used in the experiment. It
represents a 30 passenger, three crew, turboprop airplane with
wing-mounted twin engines and counter-rotating propellers. The six
degree-of-freedom synergistic motion system with hydraulically actuated
legs is capable of a 60 inch stroke. The high quality visual system
provides wide angle collimated cross-cockpit viewing with a 150 degrees
horizontal and 40 degrees vertical field of view available to each
pilot.

The research was conducted using regional airline pilots in recurrent
training. Data were collected from 42 crews. Two experiments were
combined into one session to minimally disrupt the host airline's
training and evaluation program, as well as to reduce pilot adaptation
to a simulator configuration. "First Look evaluation" was designed to
assess the effect of motion on the effectiveness of the simulator as a
tool for evaluating the crew's aviating skills. In other words, it
assessed the degree to which a pilot's existing skills transferred
*from* the airplane *to* the simulator for each simulator configuration.
This assessment needed to occur during the very initial exposure of the
crew to the simulator, so that pilots' behavior and performance would
reflect their actual skills in the airplane with as little contamination
as possible from potential adaptation to a particular simulator
configuration. The second experiment was designed to assess the effect
of each simulator configuration on skill acquisition in the simulator,
and, most importantly, on subsequent transfer of these skills to the
airplane. This experiment was called "Training and Transfer testing."
Training transfer was measured by comparing the effect of training
received in the simulator, with and without motion, on performance and
behavior in the simulator with motion (as a stand-in for the airplane,
\"quasi-transfer\" design).

Two test maneuvers (i.e., pilot tasks) were chosen to maximize
satisfaction of criteria described in the literature as diagnostic for
the detection of a motion requirement, given the constraint that the
experiment was conducted in the context of an FAA approved training
program. These criteria included 1) closed loop, to allow for motion to
be part of the control feedback loop to the pilot; 2) unpredictable and
asymmetric disturbance, to highlight an early alerting function of
motion (Ref. 6); 3) high gain and high thrust, to magnify any motion
effects; 4) high workload with crosswind and low visibility, to increase
the need for redundant cues such as provided by motion, out-the-window
view, instruments and sound; and 5) short duration, to prevent pilots
from adjusting to a lack of cues. Engine failures on take-off with
either rejected take-off (RTO) or continued take-off (V~1~ cut) were
deemed as fulfilling most of these criteria, while requiring minimum
disruption to the host airline\'s existing training program. To prevent
bias, the state of the motion system was kept concealed from all
participants.

A laptop computer was programmed to control the simulator and record
events with minimal I/E intervention, eliminating the need for the
presence of an experimenter that might have contaminated the regular
training/evaluation environment and enabling the I/E to focus on
behavior and performance of the crew. Even more importantly, this also
eliminated any need to inform the I/E (or the crew) of the interest in
motion and the motion state of the simulator for each maneuver, thus
minimizing any bias.

The stimulation of the pilot by the simulator and the pilots\' responses
were measured by recording 78 simulator state and control input
variables at a high sampling rate, resulting in a vast amount of
objective data on simulator performance and pilot performance, behavior,
and workload. Two forms of subjective data were also collected. First,
at the conclusion of each maneuver the I/E provided a grade for the
just-completed maneuver. Second, at the end of the training period and
again at the end of the transfer period all participants were queried on
PF performance and workload as well as simulator comfort and
acceptability.

[Motion Stimulation Provided by the Test Simulator]{.underline}

For the test simulator, the actually measured roll and longitudinal
accelerations followed the airplane model fairly well given the
limitations inherent to all simulators. For vertical acceleration,
however, the motion system of the test simulator did not respond much to
the command provided by the equations of motion. This is especially true
for V~1~ cut maneuvers. However, because the engine failures used in our
experiment do not produce much vertical acceleration, the lack of
vertical acceleration cuing may not be very important.

More important, however, is the finding that failure-induced lateral
acceleration was not well represented by the motion system of the test
simulator. Not only was it greatly attenuated, but visual inspection of
the measured response does not lead to an easy distinction of
failure-induced lateral acceleration, unlike the response derived from
the equations of motion (relatively high peak shortly after engine
failure). This may represent a significant deficiency in pilot
stimulation, because lateral acceleration may act as a useful cue for
proper failure recognition and for delivery of appropriate action. To
the best of our knowledge, however, the importance of lateral versus
other cues in failure recognition has not been systematically examined
in the literature.

[Analysis of I/E Grades]{.underline}

Figure 1 shows the grade distribution obtained by the two groups at
First Look evaluation and Transfer. The possible grades were 1
(unsatisfactory), 2 (FAA Practical Test Standards, Ref. 7), 3 (company
standards), and 4 (excellent). The experimental session appeared to have
been effective in simulating a real training session in that the crews\'
performance improved across the session. Specifically, combining the two
motion groups (or looking at them individually), the grades for RTOs and
V~1~ cuts improved across the training trials. This was even stronger
for the V~1~ cuts, which elicited lower grades than the RTOs during
First Look, but caught up by Transfer.

Turning to the effect of motion, the presence or absence of motion had
no effect on the grades for the RTOs at either First Look or Transfer.
There was also no effect of motion for the V~1~ cuts at First Look.
Whether crews were trained with motion or without had also no effect on
Transfer to the simulator with motion or improvement from First Look (or
last training) to Transfer, at least not when comparing group means or
number of grades of one and two vs. three and four. However, the
motion-trained crews did receive more grades of two than the crews who
had not previously had motion, and fewer grades of one (none actually).
Additionally, there was no effect of motion on the course of Training or
on the amount of training required before reaching the criterion needed
to move onto Transfer for either of the maneuvers.

![](media/image1.wmf){width="3.2in" height="2.1in"}

Fig. 1 Grade Distribution

[Analysis Of Objective Data]{.underline}

From the 78 variables recorded in the experiment, a set of criterion
measures was derived for determining whether or not motion had an effect
on training and evaluation of the tested pilot task. These were
categorized into performance and workload/behavior measures. Performance
measures reflect a pilot\'s control precision and efficiency in handling
the airplane by measurements such as flight path deviations and reaction
time. Workload/behavior measures describe how a pilot uses the controls
by measurement of control inputs. A guide to the determination of the
measures was provided by the PTS and by the company standards of the
host airline itself. An additional goal was to capture performance and
workload immediately after the engine failure, because disturbance
motion was expected to act as an alerting cue to the pilots that would
enhance early performance. The list of the measures can be seen in Ref.
5. Most of the measures were computed over the 15 second time period
following an engine failure. Exceptions include measures of reaction
times and time to reach 400 ft altitude. In general, lower numerical
values of the measures indicate better performance or lower workload.

The effect of motion on First Look evaluation, Transfer of training to
the simulator, Training Progress, and improvement from last training
trial to Transfer testing was examined. An attempt was also made to
capture the criteria used by the I/Es when they were grading the pilots
on the respective maneuvers by performing regression analyses between
I/E grades and objective measures. These analyses showed whether the
presence or absence of motion affected which measures I/Es considered
for grading.

The objective measures that are discussed in this paper are the ones
that are either listed in the PTS, were used by the instructors for
grading, or showed an effect of motion. For each measure, the
statistical power was determined (i.e., the smallest effect that could
be detected given the idiosyncratic variability between crews with a
probability of .80). The power of the experiment was found to be
sufficient to capture any operationally relevant effects.

[I/Es\' Grading Criteria]{.underline} Linear and logistic regression
analyses on the relationship between the grades and the objective
measures were used to infer the I/Es\' grading criteria and whether the
platform motion had an affect on these criteria. Although the logistic
regression was considered to be more appropriate for cases involving
ordinal data (like the grading system used here), the results of both
regression analyses were quite similar. The regression models obtained
were not meant to model I/E\'s decision process in determining the
grades, which is actually very complex. They were only used to examine
whether any available measures contributed to the I/E\'s grading
criteria.

For RTOs, regardless of whether the platform motion was on or off, the
measures of lateral and heading deviations played an important role in
predicting I/E grades. For V~1~ cuts, the results of the regression
analyses suggest that the platform motion status may affect grading. In
both motion-on and motion-off conditions, some (but not the same)
lateral measures seemed to affect I/E grades. However, the level of
importance of other types of measures in the I/Es\' grading criteria
depended on the status of the platform motion. Notably, longitudinal
measures appeared to matter mainly when the platform motion was on.

####### ![](media/image2.wmf){width="2.5in" height="2.05in"} 

*p* = .033

![](media/image3.wmf){width="2.6in" height="2.2020833333333334in"}

*p* = .906

Fig. 2 RTO First Look: Directional Performance

####### First Look Evaluation, RTOs Motion did not affect performance in heading deviation, lateral deviation, power lever reaction time, nor did it affect any workload measures. It did, however, improve Integrated Yaw Activity (integral of absolute yaw rate for 15 seconds after engine failure, see Fig. 2 \[i\]), a measure which was not found to be important in the I/Es\' grading criteria. This suggests that for First Look evaluation, the presence of motion may improve performance, but not to the extent of affecting grades.

![](media/image4.wmf){width="2.5in" height="2.0236111111111112in"}[First
Look Evaluation, V~1~ Cuts]{.underline} Motion did not affect bank angle
or heading control variables (and these, especially bank angle, are
important for grades) or reaction time. Interestingly, there is a
marginal chance that motion may have improved the pitch angle standard
deviation (*p*\<.10) (Fig. 3). This effect was, however, physically
small and not accompanied by any other performance or workload effects.
This, together with the fact that there was practically no simple
correlation between STD Pitch Angle of Motion pilots and grades (*r*^2^
= .01), and even the stepwise regression model selecting three more
longitudinal measures accounts for no more than 30 percent of the
variance in the grades, suggests that the platform motion would not
affect pilot grades during First Look evaluation. This result also
validates the subjective grade results presented earlier.

*p* = .096

Fig. 3 V~1~ Cut First Look: STD Pitch Angle

[Training Transfer, RTOs]{.underline} Training Transfer was tested for
all crews on the simulator with motion activated as a stand-in for the
airplane. Despite the fact that the Motion crews were trained and tested
on the same simulator configuration, they did not do any better than the
No-Motion crews with any RTO performance and workload measure.
Additionally, the power of the experiment was generally higher after
training, and still no effects of prior motion were found. One *caveat*
is that for heading control, although there was no difference between
the two groups, more No-Motion crews improved than Motion crews between
the last training and the Transfer testing (see Fig. 4). This may
indicate that the addition of motion was beneficial, although during
Transfer testing the two groups performed at the same level (as just
described).

\[i\] In this and subsequent figures, numbers next to data points refer
to sample size

![](media/image5.wmf){width="2.7in" height="1.8993055555555556in"}

![](media/image6.wmf){width="0.7in" height="0.475in"}

![](media/image7.wmf){width="2.7in" height="1.8472222222222223in"}

Fig. 4 RTO Last Training vs. Transfer: Directional Control Performance

[Training Transfer, V~1~ Cuts]{.underline} Having been trained with
motion did improve speed control (*p*=.006), i.e., Integrated Airspeed
Exceedance (integral of absolute airspeed deviation outside (0,+5 knots)
band from the recommended V~2~) following V~1~ cuts during Transfer
(Fig. 5). It came at the price of increasing pitch angle standard
deviation (*p*=.025), but was still advantageous because of the critical
role speed plays in aircraft control and safety, e.g. for clearing
obstacles and maintaining a margin above stall speed. However, there was
also an increase in Integrated Yaw Activity by the motion-trained group,
although it did not appear to affect heading.

With regard to workload during V~1~ cuts, the Motion group had fewer
wheel reversals than the No-Motion group (*p*=.059), whereas the
No-Motion group had fewer pedal reversals than the Motion group
(*p*=.008) (see Fig. 6). The increased number of Wheel Reversals of the
No-Motion group was not accompanied by any lateral performance
differences. The increased number of pedal reversals of the Motion
group, however, was accompanied by an increase in Integrated Yaw
Activity, as was discussed earlier. The difference was not apparent at
First Look, nor did a combined Analysis of Variance (ANOVA) of
Motion/No-Motion by First Look vs. Transfer find a significant
interaction, probably due to the variability in number of pedal
reversals for the Motion group during First Look. The questionnaire data
indicated that the Motion group felt the pedal was less like the
airplane than the No-Motion group did.

![](media/image8.wmf){width="2.6in" height="2.1618055555555555in"}

*p* = .006

![](media/image9.wmf){width="2.6in" height="2.1770833333333335in"}

*p* = .025

Fig. 5 V~1~ Cut Transfer: Longitudinal Performance

![](media/image10.wmf){width="2.6in" height="2.1347222222222224in"}

![](media/image6.wmf){width="0.7in" height="0.475in"}

*p* = .059

![](media/image11.wmf){width="2.6in" height="2.140972222222222in"}

*p* = .008

Fig. 6 V~1~ Cut Transfer: Wheel and Pedal Reversals

[Training Progress, RTOs]{.underline} No statistically significant
differences in improvement from first to last training trial were found
between groups for any of the measures (all *p*\>.2). This suggests that
the platform motion did not affect the training progress of the pilots.

Also, the overall number of crews (Motion and No-Motion) improving in
lateral performance and workload measures was significant for most
measures, with the exception of Integrated Yaw Activity with no overall
improvement and pedal reversals, which actually increased after
training. When looking at the groups separately for these two measures,
neither of the groups shows any improvement or deterioration. This
confirms that the pilots generally did improve during training
regardless of the motion status of the simulator.

![](media/image12.wmf){width="2.7in" height="1.8458333333333334in"}

![](media/image6.wmf){width="0.7in" height="0.475in"}

![](media/image13.wmf){width="2.7in" height="1.8472222222222223in"}

Fig. 7 V~1~ Cut First vs. Last Training: Longitudinal Performance

[Training Progress, V~1~ Cuts]{.underline} The course of training for
V~1~ cuts reflected the Transfer results. Motion did improve training
progress for Integrated Airspeed Exceedance, but hindered training for
STD Pitch Angle (the price for reduced Integrated Airspeed Exceedance)
(Fig. 7) and also for the heading variables. For workload variables,
there were no differences between the two motion groups.

The data indicate that the No-Motion group improved on more measures
than the Motion group. While Motion crews improved in Integrated
Airspeed Exceedance and STD Column Position only, the No-Motion crews
improved in Integrated Bank Angle Exceedance, Heading Deviation, Time to
Reach 400 ft Altitude, and STD Pitch Angle. During Transfer, however,
the No-Motion group surpassed the Motion group only with steadier pitch
angle and yaw activity; and the actual size of these differences was
very small.

The above discussion indicates that the training without motion was at
least as effective as the training with motion, and the earlier results
on Transfer show that although some differences were found in training
progress between the two groups, they did not translate into
operationally relevant differences during Transfer.

[Analysis Of Questionnaire Data]{.underline}

Each of the PFs and PNFs was given two questionnaires (i.e., one after
Training and one after Transfer) that each had six questions (i.e.,
control precision, control strategy and technique, workload, gaining
proficiency, simulator comfort and acceptability). Each I/E was also
given two questionnaires, each with five questions (i.e., the same
questions as above, but without acceptability). PFs responded always
with reference to themselves. PNFs and I/Es referred to the PFs, with
the exception of comfort and, for the PNF, acceptability.

Regarding motion, given all of these questions, only four differences
were found between the Motion and No-Motion crews. 1) After Training,
the PNFs from the No-Motion crews rated the control precision of the PFs
better than the PNFs from the Motion crews did. 2) The PFs from the
No-Motion crews, once transferred to the simulator with motion, rated
their control precision higher than their motion-trained counterparts.
This is possibly because of the contrast between the added motion and
the lack of motion they had been experiencing. 3) In contrast, after
Transfer, the I/Es gave higher ratings for performance to the PFs from
the Motion group than to the PFs from the No-Motion group. 4) Looking
across both questionnaires, the PFs from the No-Motion crews gave better
ratings to the simulator for training (\"gaining proficiency\") than the
PFs from the Motion crews.

[Conclusions and Recommendations]{.underline}

The results of this study indicate that the motion provided by the test
simulator, which may or may not be typical of other FAA qualified Level
C flight simulators, does not, in an operationally significant way for
the tasks tested, affect either First Look evaluation, Training
Progress, or Transfer of training acquired in the simulator with or
without motion to the simulator with motion. It also doesn't
consistently affect the PFs', PNFs', and I/Es' subjective perception of
the PFs' performance, workload, and training, or of their own comfort in
the simulator. Neither does it affect the acceptability of the simulator
to the PF and the PNF.

Two *caveats* have to be kept in mind, however. First, the simulator
used in this study may not have provided sufficient motion to be
effective. The measurements indicate that the simulator may have failed
to provide lateral acceleration cuing representative of the aircraft for
the test maneuvers (RTO and V~1~ cut).

A second *caveat* is that the current study used the simulator with
motion as a stand-in for the airplane. Although some may believe that
this quasi-transfer design needs to be validated, others may say that
high-level simulators have been validated as a stand-in for the airplane
by many years of use of the simulator for total flight training. Also,
given that the motion-trained group transferred to the same simulator
configuration that they had been trained in, whereas the No-Motion group
transferred to a configuration that was new to them (i.e., the motion
configuration), the Motion group should have had an advantage. Based on
the quasi-transfer results, it is unlikely that it would have had a
greater advantage transferring to an airplane.

Clearly additional steps must be taken to determine the extent to which
it may or may not be appropriate to draw generalizations from these
results. These should include a comparison of the objective measures
from the motion system used in this experiment with such measures taken
from other FAA qualified Level C simulators to determine whether or not
the motion used in the present study is representative. This should be
followed by an investigation on whether operational relevant effects of
motion would be found with a simulator where the motion is manipulated
to assure that it is representative of the airplane for the maneuvers
selected. Additional maneuvers that may be diagnostic and a different
pilot population should be tested as well. Ideally, some validation of
the quasi-transfer design with a real airplane would also be undertaken.

[Acknowledgments]{.underline}

This work was funded by the Office of the Chief Scientist for Human
Factors of the Federal Aviation Administration, AAR-100. The authors
would like to thank the FAA Program Manager Dr. Eleana Edens for her
guidance throughout the project. Many thanks also to Mr. Paul Ray,
Manager of the National Simulator Program Office AFS-210, and Dr. Ed
Cook of the same office, for their continued interest and advice.

The opinions expressed are those of the authors and not necessarily
those of the Department of Transportation, the Federal Aviation
Administration, or the U.S. Government.

[References]{.underline}

1.  Longridge, T., Ray, P., Boothe, E.M., and Bürki-Cohen, J.,
    [Initiative towards more affordable flight simulators for U.S.
    commuter airline training]{.underline}, paper presented at the Royal
    Aeronautical Society Conference on Training---Lowering the Cost,
    Maintaining the Fidelity, London, May 1996.

2.  Transcript \[ii\] of the Joint FAA/Industry Symposium on Level B
    Airplane Simulator Aeromodel Validation Requirements, Washington
    Dulles Airport Hilton, March 13-14, 1996.

3.  Transcript \[ii\] of the Joint FAA/Industry Symposium on Level B
    Airplane Simulator Motion Requirements, Washington Dulles Airport
    Hilton, June 19-20, 1996.

4.  Bürki-Cohen, J., Soja, N., & Longridge, T., [Simulator Platform
    Motion --- The Need Revisited]{.underline}, The International
    Journal of Aviation Psychology, Vol. 8, No. 3, 1998, pp. 293-317.

5.  Bürki-Cohen, J., Soja, N.N., Go, T.H., Boothe, E.M., DiSario, R.,
    and Jo, Y.J., [Simulator Fidelity: The Effect of Platform
    Motion]{.underline}, Report No. DOT/FAA/RD-00/XX, in prep aration.

6.  Gundry, J., [Man and Motion Cues]{.underline}, paper presented at
    the Third Flight Simulation Symposium, London, April, 1976.

7.  Federal Aviation Administration, [Airline Transport Pilot and Type
    Rating Practical Test Standards,]{.underline} FAA-S-8081-5B, U.S.
    Government Printing Office, Washington, D.C., July 1995.

> \[ii\] Available in electronic format from Dr. Thomas Longridge,
> Advanced Qualification Program Manager, AFS-230, tel. (703) 661-0275