Training Value of a Fixed-Base Flight Simulator with a Dynamic Seat

Judith Bürki-Cohen[^1] and Andrea L. Sparko[^2]

USDOT/RITA/Volpe National Transportation Systems Center, Cambridge, MA
02142

Tiauw H. Go[^3]

Nanyang Technological University, Singapore 639798

In this paper, we first explain that pilots experience airplane motion
via multiple perceptual systems, which makes motion a candidate for
simulation via stimulation of only a subset of these systems. Next, we
discuss the relative merit of vestibular cues when piloting an airplane.
This is followed by a comparison of the vestibular cues received in the
airplane and those possible, or practicable, in an airline-pilot
training simulator, considering also the history of flight-simulator
motion and alternative technologies. We conclude that a vast body of
research has shown that accurate cues are not achievable at present, and
that those available have not been shown to improve transfer between
airplane and simulator. We then examine the cost of motion, and posit
that it may prohibit some airline pilots from reaping the benefits of
simulator training, with a concomitant loss in passenger safety. This
consideration is especially pertinent given the world-wide training
needs. Moreover, the equipment, facility, and maintenance costs
associated with hexapod-platform motion systems may serve to discourage
operators from upgrading the simulator's fidelity in other important
areas, such as assuring that the simulator cockpit does in fact match
the equipment in the target aircraft, and that the simulation includes
realistic operational representation of the national air space,
including the air-traffic-control environment. We describe current and
planned research on the training effectiveness of an alternative
approach, which provides trainees with visual motion and heave-onset
cues in what otherwise corresponds to an FAA Level D Full Flight
Simulator in terms of data fidelity. This includes the results of a
"proof-of-concept" phase that culminated in the successful type-rating
of six pilots on a twin-engine turboprop airplane.

# Nomenclature {#nomenclature .Heading-Not-Numbered}

*\$* = United States dollars

*ft* = foot/feet

*g* = acceleration of gravity

*˚H* = degrees horizontal

*kg* = kilogram

*m* = meters

*s* = seconds

*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* = degrees vertical

# Motion Cue Sensation and Perception

TRANSPORT AIRPLANES move through the air. Some of this movement is
commanded by the pilots in their effort to follow a predetermined flight
path. Some of it is generated by outside disturbances, such as weather
or airplane systems.^1-3^ According to many authors (see, e.g., Ref. 4),
the first type of motion, which provides pilots with feedback on their
control actions, matters only when controlling inherently unstable
vehicles, such as helicopters. Because the scope of this paper is the
training of pilots of stable air-transport category airplanes, we will
focus on the second type of motion, which alerts pilots of events that
require corrective action from the pilot. Before discussing how and
whether to simulate motion stimulation during airline-pilot training, we
need to understand how pilots use motion cues. How do they sense ownship
movements? What sensations or combination of sensations do they use to
perceive the status of their airplane and fine-tune their control
actions?

These questions are critical to determine the requirements for tools
that efficiently and effectively train pilots who are trusted with the
safety of the flying public. It has been long established that
ground-based simulator training is both safer^5^ and more effective than
in-air training because pilots can fly, with the possibility of
corrective repetitions, carefully designed scenarios simulating the most
critical in-air eventualities (Line Oriented Flight Training, LOFT).
This includes extreme weather and system failures requiring advanced
control skills, but also situations requiring excellent cognitive,
managerial, and interpersonal skills. Examples for the application of
the latter are situations requiring rapid decision making based on
complex and often incomplete information, managing of insufficient,
overlapping, or unpredictable crew and automation resources, and
resolution of disagreements with crew members or air traffic control on
how to resolve a situation.

## Multimodal Perception of Motion Cues

The most controversial issue in determining flight simulator
requirements is how and whether to simulate airplane motion. Unlike the
representations of the out-the-window view, the instruments, the
movement and resistance of the control-input devices, and the sound,
which are predominantly perceived by one single human sensory system,
airplane motion is sensed by multiple receptors (see, e.g., Refs. 6 and
7):

1\) The photosensitive receptors in the retina of the eye are responding
to changing size, shape, and texture as well as successive changes in
position of targets moving across the out-the-window view as a function
of airplane motion. From this information, the brain indirectly derives
velocity and acceleration. The same receptors also register the changes
in the instruments indicating airplane position directly or indirectly,
such as altitude, airspeed, vertical speed, and attitude (roll, pitch,
heading). Unlike the mechanoreceptors described below, photoreceptors do
not habituate to repeat stimulation.

2\) The vestibular apparatus in the inner ear contains mechanoreceptors
that send direct signals to the brain when they are deflected as a
function of changes in head accelerations in the six degrees of freedom
(DOF). The receptors for head rotations are located in three roughly
orthogonal semicircular ducts reminiscent of a rate gyro. When the head
is slightly tilted forward, the lateral duct lies in the horizontal
plane, its receptors registering yaw accelerations. The planes of the
anterior and posterior ducts are approximately vertical and offset by 45
degrees from the sagittal and coronal planes. Their sensors signal pitch
and sway accelerations. Linear head movements, including those resulting
in a change of the head's orientation towards gravity, are transduced
via the two otolith organs, the utricle and the saccule. The sensory
area (macula) of the utricle is approximately parallel to the lateral
semicircular duct in the horizontal plane. The macula of the saccule is
roughly parallel to the sagittal plane of the head.

3\) Pilots also perceive airplane motion via mechanoreceptors
distributed over the body (tactile and proprioceptive sensors). If
pilots use the perceptions resulting from such stimulation to control an
airplane, they are sometimes somewhat pejoratively referred to as
"flying by the seats of their pants," i.e., without the use of the
instruments.^8^ For transport airplanes, which are inherently stable,
the direct or mechanical sensations of motion may indeed primarily occur
via the mechanoreceptors in the skin and the supporting tissues of the
parts of the body touching the seat and the back of the pilot's chair
(tactile receptors). Occasionally, particularly strong accelerations or
decelerations jerk pilots forward or push them into their seats, which
would additionally stimulate proprioceptive sensors registering the
state of muscles and joints.

4\) Finally, the inner ear registers changes in pitch and intensity of
engine noise, which also may contribute to an indirect perception of
motion (see, e.g., Ref. 9). The sound waves are collected by the outer
ear and sent, via the eardrum, to the ossicles of the middle ear, where
the signal is amplified to apply pressure on fluid in the cochlea of the
inner ear, which in turn stimulates pressure-sensitive hair cells.

## Vection: The Illusion of Motion from Vision

It is this multimodality of ownship-motion perception that sets the
simulation of airplane motion apart from the simulation of the
perceptions experienced when flying an airplane. While the
out-the-window view, the instruments, and the feel and force-feedback
received from the control-input devices must be directly simulated to
result in a percept that is comparable to the one experienced in the
airplane (e.g., there is no perception of an out-the-window view without
an image generator simulating it), simulator motion can be perceived
without actual physical motion stimulating the vestibular and the
proprioceptive systems, as long as the eye perceives the changes that
would be expected from motion in the out-the-window view. This illusion
of motion is called vection.

Most of us have experienced vection: while waiting at a traffic light,
we might catch the movement of a neighboring car in our peripheral
field-of-view, and step on our brake to avoid violating a signal. The
perceived changes in our position relative to the one of the neighboring
car tricks us into believing that it is our car that is moving,
presuming that it is the other car that remains stationary.

The phenomenon of vection is the first of four reasons why the need for
vestibular motion simulation in the presence of a wide-field-of-view
visual system is increasingly a subject of debate, while the need for an
out-the-window view, force feedback, and sound has been taken for
granted despite the fact that their effect on transfer of skills and
behavior between the airplane and the simulator has also not been fully
documented. The remaining reasons for this debate are, second, the
potential dangers of relying on vestibular motion cues to determine
aircraft attitude and required control actions; third, the limitations
of how sustained accelerations and decelerations can be realistically
simulated and the effectiveness of these attenuated cues; and fourth,
the cost of providing pilots with effective motion cues. These
questionable reasons will be discussed below, before describing an
effort by a turboprop airplane manufacturer to provide its world-wide
customers with high-quality training via an alternative to a six-DOF
motion Full Flight Simulator (FFS).

# Vestibular Motion Cues---Friend or Foe? {#vestibular-motion-cuesfriend-or-foe .Style-Heading-1-+-Left:--0\"-First-line:--0\"}

The short answer to the question of whether vestibular motion cues help
pilots while flying an airplane is yes and no. Under normal flight
conditions for a stable transport airplane, where motion cues result
from pilots' control inputs, it is generally agreed that pilots can
easily dispense with vestibular feedback on their control actions, so in
this case, vestibular cues are neither friend nor foe. In the case of
outside disturbances from weather or malfunctions, however, vestibular
cues may be processed faster than visual cues.^1-3^ This has been
documented theoretically^10,11^ and in our research, where pilots
responding to V~1~ cuts in a fixed-base simulator compared to pilots in
a motion simulator showed a pedal-response time delay even when they
knew that an engine failure would occur, as well as which engine would
fail.^12,13^ However, the response delay was less than half a second,
much smaller than what would be expected from looking at the literature,
which mentions vection effect-latencies in the order of seconds
(referring to the more complex computation process of deriving
accelerations from position and time). This may be due to the fact that
pilots were already immersed in and actively controlling the illusionary
motion before the failure occurred.^14^ Other factors that may have
shortened pilots' response times include a wider field of view, lower
visual accelerations, a difference in axes of the perceived motion
compared to those tested in the laboratory in earlier studies,^15^ or
the naturalistic scene instead of the more abstract geometrical patterns
of earlier studies.^16^ Also, even if stimulation of the vestibular
system should result in somewhat faster response times, a potentially
equally important advantage of the visual system is its lack of
habituation, which in the vestibular system leads to a loss of
sensitivity with repeat stimulation.^6^ All this indicates that in some
situations encountered in airline operations, physical motion cues may
be a friend, but neither an important, or, for that matter, a reliable
one.

Finally, due to the limited reliability of the vestibular system, there
are situations where it is recommended that pilots ignore any but the
visual cues gathered from the instruments. The vestibular system fairly
accurately perceives motion only as long as the motion sensations are
experienced within normally occurring limits.^17^ In terms of piloting,
this means that sensations in flight that are outside of the normal
flight envelope may not be accurately interpreted by the vestibular
system. Unusual attitudes and airplane upsets, for example, are uncommon
flight experiences, and therefore often result in spatial disorientation
during flight. Spatial disorientation has been defined as "\[a failure\]
to sense correctly the position, motion or attitude of the aircraft or
of him/herself within the fixed coordinate system provided by the
surface of the earth and the gravitational vertical" (from Benson, 1988,
as cited in Ref. 18). Examples of disorientation experienced in flight
include misperceptions of the magnitude or direction of actual rotation
resulting in false sensations of rotation (angular, or somatogyral,
illusions such as the "graveyard spin" or the Coriolis effect) or
misidentification of a nonvertical force as a vertical force resulting
in a false sensation of body tilt (linear, or somatogravic, illusions
such as nose-high pitch experienced during forward acceleration or an
inverted sensation that sometimes occurs when the airplane is leveled
off suddenly after a climb in weather). Disorientation can also take the
form of a combination of somatogyral and somatogravic illusions, such as
the "leans," which can result in the often fatal "graveyard spiral."^19^
These types of spatial disorientation have led to several
loss-of-control accidents; in fact, they are the "only type of accident
that is not reducing in world-wide statistics."^20^ The urgency of
effective training in the prevention, recognition, and recovery from
unusual airplane attitudes is therefore increasingly recognized. Due to
a lack of flight test data and the limitations of the conventional
simulator motion platform, however, the use of conventional motion-base
training simulators for upset recovery training is controversial.
Without the accurate visual, instrument, and especially motion cues to
simulate an upset, there is a possibility of negative training. False
motion cues, for instance, could prime pilots to inaccurately identify
actual in-air upsets. Moreover, they may distract trainees from
acquiring a habit of focusing on their instruments, their only reliable
reference during an in-air upset. When pilots are tricked by their
vestibular system into disorientation, they must read their instruments
to recognize the airplane's actual orientation in the air. Only then
will they be able to take the appropriate control actions for recovery.

# Comparison of Simulator and Airplane Motion {#comparison-of-simulator-and-airplane-motion .Style-Heading-1-+-Left:--0\"-First-line:--0\"}

One could argue that if pilots are to ignore misleading motion cues for
unusual attitudes in the airplane, they need to be trained to do so also
in the simulator. However, it has been documented that such maneuvers
lie outside the validated data envelope of today's training simulators,
and even if the data were available, the motions required would lie
outside the capabilities of the standard Stewart hexapod-motion platform
used in training.^21^ Airplane motion has relatively complex and rich
dynamics even during normal flight. The complexity arises from various
nonlinearities affecting the overall motion, while the richness is due
to various sources of dynamics that may be excited during flight through
the atmosphere. The motion that a pilot experiences inside the cockpit
is not only the three dimensional nonlinear rigid-body motion of the
airplane, but also the motion due to flexible structural dynamics. The
latter may interact with the rigid-body dynamics. Various kinds of
internal or external inputs, such as pilot commands or atmospheric
disturbances that the airplane encounters during its flight, may excite
all these dynamics at once.

If such complex and rich airplane-motion dynamics are to be completely
simulated to achieve high physical fidelity, the dynamics model used
must include and capture all aspects of the dynamics involved that
contribute to the resulting dynamics. Even in the context of the
transport-airplane category, which is the scope of the discussion here
and the dynamics of which are considered quite benign in the whole
aircraft spectrum, it would be very difficult and costly to build such a
complete dynamics model. Moreover, in the context of creating a
sufficient level of realism in the simulator cockpit for airline-pilot
training, one can question whether such a complete dynamics model is
necessary. Fortunately, in many cases a relatively high degree of
subjective realism can be achieved by using a mostly rigid-body aircraft
dynamics model with some special effects to simulate the other dynamics.
The simplification of the flight dynamics model used in simulation
removes certain parts of the airplane-motion dynamics and reduces the
physical fidelity in the translation of real airplane motion to the
presentation of motion in a motion-base flight simulator.

Another simplification or modification in the simulation of airplane
motion is driven by the motion-system hardware limitations. It involves
the generation of motion cues within the constraints of the
motion-hardware capabilities. These hardware constraints have an even
stronger impact on the physical fidelity of the resulting motion than
the dynamics-model simplification just described. Most improvements of
the motion platform since the inception of motion-base flight simulation
have been in overcoming some of these hardware limitations, which
translates into increased motion-platform capability, be it in the
maximum displacement that it can produce or in the frequency bandwidth
that it can dynamically cover. Most of the effort in improving the
motion-cue generation is driven by the unsubstantiated opinion that
better training can be achieved by immersing the pilots in higher motion
realism. Also some regulatory bodies, such as the U.S. Federal Aviation
Administration (FAA) or European Joint Aviation Authorities (JAA), still
require pilot training and evaluation in motion-base flight simulators.
Before discussing the fidelity level of the motion cues used in today's
airline-pilot training and evaluation, various motion systems are
reviewed below.

## Motion Simulation Technology

### Early Systems

The earliest flight simulator designs did employ some type of elementary
motion, because "it was mistakenly believed during this period that the
vestibular apparatus enabled a person to sense orientation in the air as
well as on the ground (Robinson 1973). It was later realized that
orientation depends largely on vision."^22^ One example from this period
was the Sanders Teacher of 1910, a modified airplane on a universal
joint which relied on natural gusts of wind to rotate and move in all
three rotational DOF.^22^ Other examples moved in response to
instructors' manually produced disturbances, such as the Antoinette
trainer which could move in pitch and roll.^22,23^ The Ruggles
Orientator (1917) could move in pitch, roll, yaw, and heave via electric
motors responding to the control inputs of the pilot and the
instructor.^22^ The motions provided by these early simulators, however,
were far from accurately simulating the forces experienced in an actual
aircraft.

The Link Trainer was perhaps the first system to impart some perception
of realism (although its motion system came soon under scrutiny too, see
below). It was developed by Edwin Link in 1930 and used compressed air
bellows as actuators. These allowed for a more controlled actuation in
the three rotational DOFs. Motors produced attitude disturbances while
an electric suction pump controlled the stick and rudder responses. The
Link trainer, however, had no instruments and no visual system.^22^

Around this time, instrument-flight training was introduced and many
fixed-base devices were developed to teach instrument flying. Link,
however, continued to employ motion in his newly developed instrument
trainer, the so-called "Blue Box." The instruments of the Blue Box were
controlled either pneumatically or mechanically and the fuselage could
yaw 360 degrees.^22^ Pitch and roll movements, however, were very
limited.^24^

As instrument flying became more common, the training value of these
early motion systems came into question. The Link Trainer, specifically,
was criticized as unable to accurately simulate aircraft motions. Soon
fixed-base simulators were preferred over motion simulators. This
preference was bolstered by the argument that pilots should fly by
instruments rather than by the "seat of the pants."^22^ Although Ed Link
continued to insist that "trainer motion was needed even if incorrect,
since motion was present in flying," he eventually developed a
fixed-base trainer based on electronic analogue technology that sold
very well.^22^

Because of increased flight-test data requiring increased computing
power, simulation almost completely shifted to digital computers, which
allowed flight simulation to really take off. Most of these new
simulators had no motion systems until the late 1950s,^22^ when the
Stewart platform came into use.

### Stewart Platform Motion 

Current state-of-the-art motion systems commonly used in airline-pilot
training simulators employ six-DOF Stewart platform (hexapod) motion
with six synergistically actuated legs. Also with modern computational
power, the motion transport delay can be minimized and the generation of
the motion cues can be much better synchronized with the other cues than
in the simulators described in the previous section. That said, however,
the fundamental limitation of such motion platforms is still there,
i.e., they are not capable of generating the sustained acceleration cues
found in flight. Because of the limited linear displacement capability
of the actuators (typically 3 to 6ft), the motion cues are constrained
by limits in duration and amplitude, regardless of what the
flight-dynamics model commands. Various washout algorithms have been
developed in order to generate "realistic" motion cues within these
constraints. The general approach is to generate physical motion-onset
cues intended to serve an alerting function and to rely on other cues,
notably visual, for sustained motions.

Because of the fundamental limitations above, other approaches that are
capable of generating better sustained motion cues have been developed.
Four such approaches are described below. As we will see, however, they
may not be practical for application to today's airline-pilot training
regardless of their success in improving the simulation of motion cues.

### Vertical Motion Simulator (VMS)

The VMS is located at the National Aeronautics and Space Administration
(NASA) Ames Research Center. It consists of a four-DOF platform mounted
on a two-DOF large-amplitude platform. It is capable of providing 35ft
sway and 50ft heave displacements.^25^ As its name implies, the largest
amount of motion it can provide is in the vertical direction (heave)
with up to 0.75g acceleration. With such a large vertical excursion, the
VMS is capable of providing faithful g-loading of normal aircraft
maneuvers, including those experienced by large transport aircraft in
the critical approach and landing phases.^26^ However, even the maximum
loading produced by the VMS in its maximum acceleration is still only a
fraction of the maximum design loads of a typical civilian (4g) or
military (9g) aircraft.^27^

### Centrifuge Motion Simulator

A centrifuge-type flight simulator, such as Wyle's Dynamic Flight
Simulator (DFS), can produce a sustained high level of acceleration. The
DFS consists of a 30ft balanced cantilever arm with a dual gimbal
three-DOF gondola. It is capable of generating up to 15g acceleration.
The primary axis of the gondola can be varied according to the gimbal
orientation. Although the capability of generating sustained
acceleration is desirable, such a centrifuge system may generate
disorientation from pilots' head movements resulting in a vestibular
Coriolis effect.^28^

### Desdemona

The Desdemona simulator concept has been developed by the Dutch applied
scientific research organization TNO and the Austrian simulator company
AMST Systemtechnik. It combines the six-DOF platform motion concept with
the centrifuge design. It consists of a fully gimbaled cockpit, capable
of unlimited rotation in all directions, mounted on 8m horizontal and 2m
vertical tracks. Desdemona can produce 0.5g horizontal and vertical
accelerations along the tracks, and in centrifuge mode it can generate
loads up to 3g.^29^

### In-flight Simulator

As the name implies, in-flight simulation uses an aircraft as a
simulator, and thus it does not have the same motion limitations as
ground-based simulators. NASA and Calspan are among the pioneers in this
area.^30,31^ For use as an in-flight simulator, the aircraft-dynamic
responses need to be modified to closely mimic the dynamics of the
simulated aircraft. Hence, the simulation is only as good as the
understanding of the characteristics of the simulated aircraft.
Moreover, the limitations of the in-flight-simulator aircraft also
restrict the flight envelope of the simulation. In-flight simulation is
more difficult, more time-consuming, and usually more expensive than
ground-based simulation and hence, it is reserved for those portions of
the flight-training regime that cannot be adequately evaluated on the
ground.^30^

Even though the four approaches described above are capable of
overcoming some of the limitations of the Stewart motion platform, the
costs to procure and maintain such systems are prohibitive for pilot
training and evaluation. Therefore, for training and evaluation of
airline pilots requiring "full" motion simulation, the Stewart platform
is still the most common choice. But do such platforms provide effective
motion cues, i.e., do they affect transfer of skills between simulator
and airplane for training and evaluation of airline pilots?

## Motion Fidelity in Airline-Pilot Training

When considering motion-fidelity requirements for airline pilot
training, we have to differentiate between physical and perceptual
motion fidelity. Physical motion fidelity is defined as the match
between motion cues in the simulator and in the airplane. As indicated
in the previous section and as we will describe in more detail below,
there are limits to such physical fidelity imposed by the constraints on
the Stewart motion platform. But what about the perceptual fidelity of
motion simulation, achieved by stimulating all senses available to
perceive motion? It is defined as a match between both pilots'
subjective perception of the simulator and the airplane, and also
between pilots' performance and control strategy or behavior in the
simulator and the airplane. To determine this more quantitative
definition of perceptual fidelity requires carefully controlled
experiments involving many pilots. To avoid this obstacle, it has been
suggested that perceptual fidelity can be deduced from the knowledge of
how a human pilot senses motion, and that motion-cueing algorithms
should be designed based on pilot-motion-perception models (see, e.g.,
Ref. 32). However, such models are far from complete, and will still
have to be validated via the more cumbersome human-in-the-loop
experimental process.

Several studies report on the degree of physical fidelity of the type of
motion widely used in today's airline-pilot training (e.g., Refs. 12,
33, and 34). Refs. 33 and 34 compared the response of the motion
platform with the actual aircraft response, or the predicted response
from the aircraft dynamics model, using engine failures on take-off.
Some results reported by Ref. 33 indicated that the simulator motion
response using a typical six-DOF Stewart platform was much suppressed
compared to the actual aircraft's response. They also indicated the
presence of above-threshold opposite-direction motions in some DOFs due
to the washout system (for engine failure at take-off, these appeared in
roll, sway, pitch, and heave). Ref. 34 reported that the failure-induced
lateral acceleration, arguably the most important motion cue to
recognize engine failures at take-off, was not well represented by the
motion system of the FAA Level C simulator of an airplane with
wing-mounted engines used in that study. Not only was the response
greatly attenuated, but visual inspection of the measured response did
not lead to an easy distinction of failure-induced lateral acceleration,
unlike the response derived from the flight-dynamics model (relatively
high peak shortly after engine failure). If such motion cues are assumed
to be the primary alerting cues, then all these data lead to the
legitimate question of whether the motion cues provided by the Stewart
platform are sufficient to provide the pilots with adequate levels of
motion cueing. Due to concerns raised by the relatively low level of
failure-induced lateral acceleration produced by the test simulator,
data from other simulators representing aircraft with wing-mounted
engines were obtained through the FAA National Simulator Program Office
(NSPO). The NSPO manager sent letters to 30 operators of FAA
AC120-40B^35^ Level C/D simulators, with a surprisingly low response
rate of fewer than 30 percent. Initial analysis was performed on eight
data sets, concluding that "\[t\]he data pretty much confirm what we saw
with the \[Refs. 36 and 34\] airplane; quite small \[lateral
accelerations\]."[^4] Unfortunately, only four data sets were available
for further examination. Comparison of the failure-induced lateral
accelerations produced by these simulators further suggested that the
motion-system performance of the test simulator was not atypical.^37^

In a follow-up study^12^ an FAA Level D research simulator at NASA was
used. Without adjustments to the motion system, it fell in the low
fidelity region for translational and rotational motions in all axes
according to the Sinacori-Schroeder motion fidelity criteria for gain
and phase error distortion.^38,39^ With some adjustments, the
translational-lateral axis, which was considered critical for the
maneuvers tested, could be brought into the medium fidelity range (and
the translational-vertical axis slightly improved), but not without
trading off some of the fidelity of the rotational axes, especially yaw
(see Fig. 1).

![](media/image1.png){width="5.497916666666667in"
height="2.6555555555555554in"}

The above discussion suggests that the physical fidelity of the motion
used in today's commercial pilot training is far from reflecting the
real world. However, airline pilots have been successfully trained with
these low levels of motion fidelity for many years. Also, virtually no
scientific evidence supports the notion that flight-simulator
platform-motion bases contribute to transfer of training across a range
of aircraft types, missions, maneuvers, and measures (see, e.g., Refs.
12, 34, 40, and 41; for a review of older studies, see Ref. 42). This
indicates that even if the physical fidelity of motion used in
airline-pilot training is poor, when such motion is combined with other
cues, it may deliver a sufficient level of perceptual fidelity. Whether
this perceptual fidelity depended on the presence of physical motion
cues was specifically tested in the studies reported in Refs. 12, 13,
34, 37, and 40. These studies systematically compared opinions,
pilot-control inputs, and flight precision of pilots being tested and
trained in a simulator with and without motion. Later, pilots were all
tested again in a simulator with motion to examine whether there were
differences attributable to whether or not they had received physical
motion cues during training. High transfer of the skills acquired in the
simulator was found regardless of the motion condition during training.
With this in mind and with the current cost of motion-base training
simulators that is beyond the budget of many small regional carriers, it
is logical to shift the focus to the training effectiveness of the
device rather than its physical motion fidelity.

# Cost of Simulating Motion Cues Via A Stewart Platform {#cost-of-simulating-motion-cues-via-a-stewart-platform .Style-Heading-1-+-Left:--0\"-First-line:--0\"}

For lack of empirical data supporting the need for vestibular motion
cues for transfer of performance and behavior between the simulator and
the airplane, supporters of the need for a Stewart platform-motion
system often mention the success of over 25 years of using FAA Level B
and C simulators for recurrent and Level D simulators for initial
zero-flight-time training and evaluation, the former equipped with a
three-DOF platform, the latter two with a six-DOF platform.

In reply to this argument, two issues need to be raised. One is the lack
of definition of the performance of such systems. With regard to the
Level B simulator, which is sufficient for complete recurrent
zero-flight-time training and evaluation, the participants of the
FAA/Industry Symposium tasked "to make updated recommendations
concerning the limits, tolerance and dynamics of motion cuing...to
reduce costs without compromising that essential motion cuing" ended up
recommending, admittedly without empirical evidence but based on expert
intuition, "a motion system having a minimum of four degrees of freedom
which must be at least pitch, roll, sway and heave."^43^ Regardless of
this recommendation, it seems likely that experienced airline-pilots in
the United States will continue to be trained and evaluated, even with
future regulatory changes, in Level B simulators requiring no more than
three-DOF motion systems. Moreover, even if pilots should be trained
with six-DOF motion, the motion system will be tested only against the
manufacturer's specifications and against its initial performance, which
is not much of an improvement over the current predominantly subjective
assessment. It seems that despite the strong sentiment in some circles
that hexapod-platform motion is essential for pilot training and
checking, that opinion is not matched by a willingness to require
rigorous qualification standards for such systems.^44^

The second issue to be raised in response to proponents of the "if it
ain't broke, don't fix it" stance is the cost of such an argument, and
the resulting potential of depriving many passengers world-wide of the
safety benefit gained from simulator training. In 2004, the 20-year
life-cycle cost including procurement, maintenance, and electricity for
a hydraulic motion system was estimated at approximately 1.32 million
dollars per "training simulator." (The estimate for an electric system
was 0.93 million dollars, mainly due to a reduction in operating
costs.)^45^ It is important to note that this does not include the
construction and maintenance of a facility that can accommodate the size
and weight of a motion-equipped simulator, nor does it include the labor
of the highly trained technicians required to maintain the system. It
also excludes the cost of the increased complexity of initial and
recurrent qualification of the simulator. Travel costs of trainees to
distant facilities (especially for simulators of which there is a
shortage) and of time lost due to scheduling issues and/or motion-system
malfunctions are also not included. This indicates that the real cost of
motion is much higher than the number cited in Ref. 45.

For airlines that have to rent simulator time, estimates in rental
increase due to motion range between 25 and 50 percent. Ref. 46 cites
Level D simulator costs between \$550 and \$1,100 per hour in 2005.
Rentals of FFSs simulating small airplanes may even be higher because of
their scarcity, so commuter airlines are hit the hardest. For recurrent
pilots training 10 hours per year mostly in Level D simulators (although
a Level B would be sufficient), the yearly cost can be \$11,000 per year
or more per pilot. For initial pilots, who spend about 32 hours in a
Level D simulator, this cost can rise to \$35,200 or more per pilot.
According to the Bureau of Labor Statistics, there were about 70,000
airline pilots in 2006 in the United States.[^5] This means that for
recurrent training in a rented simulator, U.S. airlines alone could
spend an additional 385 million dollars per year for motion. Accounting
for the increasing number of pilots overall and the new-hires due to
attrition of existing pilots, the cumulative cost of motion amounts to
billions of dollars in a few years just for the United States. The
question is whether motion is really the best place to invest all these
resources, when its practical transfer benefit, albeit theoretically
plausible, remains elusive despite many decades of research attempting
to prove otherwise.

# World-Wide Training Needs {#world-wide-training-needs .Style-Heading-1-+-Left:--0\"-First-line:--0\"}

The original motivation for the FAA/Volpe Center investigation of
air-carrier-pilot flight simulator requirements (see Refs. , 13, 34, 37,
40, 42, 43, and 47) had been the FAA's so-called "Commuter Rule," which
was at the heart of the FAA's "One Level of Safety" initiative for all
airlines, large or small.^47,48^ The Commuter Rule was published in
December 1995 and required all commuter airlines operating airplanes
with a two-person crew (i.e., those with ten or more passengers) to
train and qualify these crews under the same rules as the major
airlines. Included in this rule was a requirement to extend the training
and testing emphasis on predominantly stick and rudder skills, to the
integration of manual skills with cognitive skills such as crew
resource, automation, and task management. As mentioned in the
introduction, such an increase in training and testing complexity can
only be achieved by exposing pilots to carefully designed line-oriented
scenarios in a flight simulator. The problem arising from this
requirement was that for the smallest commuter airplanes, the cost of
acquiring/renting an FFS, if it is even available, may exceed the cost
of acquiring/operating the real airplane. Similarly, the cost and loss
in revenue resulting from using the real airplane for training is much
smaller than for large airplanes, so that in some cases, the use of the
airplane as a training tool is a cost-effective alternative to renting
time on an off-site simulator, which incurs additional costs of
transporting and housing pilots to and at the training site. So, the
ultimate goal of the FAA/Volpe Center flight simulator requirements
program was to ensure that all cueing requirements applied to flight
simulators do contribute to improving transfer of performance between
simulator and airplane for effective training and evaluation. This is
the best means to provide all airlines with access to effective
simulator training and evaluation, while maintaining or improving
quality of training and accuracy of evaluation.

This motivation applies at least as urgently to international training
needs. There is a world-wide pilot shortage.[^6] Pilots come from
increasingly different backgrounds and may have very little prior
experience, whereas the cognitive demands of operating an airplane
continue to increase with increasing automation, congestion, and
congestion-relief measures. The need for effective training tools is
especially urgent in remote locations of the world, where aviation is
the primary means of transportation and where it is especially important
to bring quality training tools to the pilots.

# An Alternative Approach: Visual Motion With Heave Onset Cues {#an-alternative-approach-visual-motion-with-heave-onset-cues .Style-Heading-1-+-Left:--0\"-First-line:--0\"}

The considerations discussed in the previous sections motivated a
leading turboprop manufacturer and world-wide provider of type-rating
training to consider the question of whether it would be possible, by
availing themselves of the latest research results and training-tool
technologies, to stem the potential for a reduction in training
opportunities due to the pressing need for pilots together with the high
cost of qualified training tools. In concert with their National
Aviation Authorities (NAA), a member of the JAA, they developed the
concept of a high fidelity, fixed-base training device with a wide
field-of-view visual system, referred to as a Full Flight Trainer (FFT),
that presumably will be within the financial reach of many turboprop
customers. The perception of motion in this device was generated mainly
by the visual cues, assisted, in the FFT-X, by a dynamic seat providing
some vestibular, proprioceptive, and tactile motion cueing.

An experimental type-rating program is in progress. Under close
supervision by the NAA, a set of pilots with multipilot-airplane
licenses has been fully type-rated on the simulated airplane using a
protocol including ground school, simulator training on the FFT-1 and
FFT-X (as a replacement for the FFS sessions in the provider's
traditional training program), plus base flight training.

Phase 1, resulting in complete type-ratings of six pilots without any
use of an FFS, proved the concept that a fixed-base training device,
followed by standard training flights in the airplane, can prepare
pilots for a successful type-rating. In addition to the type-rating
qualification by the examiners, questionnaires were administered to the
pilots and instructors after flying the FFT and the airplane, rating, as
appropriate, the pilots or the FFT. A final debriefing session was also
conducted, with some of the pilots having informally experienced the FFS
before this debriefing. This phase will be described in detail below.

Phase 2 resulted in eight type-ratings. It followed the procedures
developed for the first phase, without formal data analysis. This phase
served as a test bed for collection of objective data for Phase 3 and
will not be described.

Phase 3, which will represent a more formal evaluation of the training
value of the FFT, is in the planning stages. In addition to
questionnaire data, data on pilot-vehicle performance and behavior will
be collected directly from the simulator. Moreover, two groups of pilots
will be trained: one following the protocol of the first type-ratings,
with pilots being trained in the FFT-X, the other following a
traditional protocol where the training phase is done in the Level D
FFS. Data will be collected by querying pilots and instructors and
recording data on pilot-vehicle performance from the training tools.
Pilots will be evaluated on the FFS in a quasi-transfer design before
transferring to the airplane. Finally, for comparison purposes with the
earlier studies conducted in the framework of the FAA/Volpe Center
flight simulator requirements program,^12,13,34,37^ some of the most
diagnostic maneuvers from those studies will be added to the training
and testing regime.

## Phase 1 Procedures

### Participants

The first set of type-rating candidates consisted of six employees of
the NAA. Pilots were divided into two groups based on their experience,
and the training regimes were adapted accordingly. Experience was
defined in terms of multipilot-crew and airline experience. The two
"Experienced" pilots held a multipilot type-rating license and had
airline experience. They had flown a total of 14,000 and 11,000 hours
respectively, of which 8,500 and 6,000 hours were as members of a
multipilot crew.

The four "Nonexperienced" pilots held single-pilot type-rating licenses
only and had no airline experience. Their total flight hours ranged
between 6,000 hours (2,600 of which were in a military turboprop
airplane) and 563 hours. One of them, with 4,500 hours, was a flight
instructor.

The three simulator instructors and two airplane instructors were all
employed by the training provider. The type-rating examiners were
deployed by the NAA.

### Equipment

After ground school, pilots underwent training in two different training
tools, the FFT-1 and the FFT-X. These FFTs simulate a 48-passenger
airplane with two wing-mounted turboprop engines and 18,600kg maximum
take-off weight capacity. Both FFTs feature 3-channel projection visual
systems with a 180^o^H×40^o^V field-of-view. Their display systems are
different, however. The FFT-1 uses an uncollimated cylindrical screen,
while the FFT-X uses a collimated spherical screen. The FFTs are
intended to elicit the same perceptions and thus provide the same
training value as a Level D simulator, via Level D aerodynamics, engines
and ground handling models, systems representation, sounds and visual
system (the latter only for the FFT-X, because the FFT-1 image is not
collimated).

In addition to providing visual motion cues, the FFT-X generates
physical heave motion cues via three electric jacks in the seat
pedestal. They follow a relatively simple model of touch-down, taxiing,
and turbulence motion. Vibrations representing different propeller
speeds are simulated via loud-speakers fitted under the pilots' seats.
The purpose of these mainly proprioceptive/tactile motion cues is to
provide a disturbance motion onset cue that can be directly perceived,
instead of having to be differentiated using changes in position and
shape over time.

The FFT-1 has been qualified by the NAA for Category (CAT) I operations,
type-rating, and recurrent training. The FFT-X has been given training,
testing, and checking credits equivalent to the ones usually granted to
a Level B FFS, including low-visibility operations (CAT I, CAT II, low
visibility take-off), captain upgrade, recency check (three take-offs
and landings in the past 90 days), induction course, and Operator
Proficiency Check (OPC).

### Protocol

The protocol depended on the level of experience of the trainees,
namely, whether they had airline experience or not. Experienced trainees
completed a four-week training course and Nonexperienced trainees
completed a six-week training course. Table 1 shows the training
sequence and tools for the two levels of experience.

Before transferring to hands-on training, trainees acquired the
theoretical knowledge required to fly the airplane by attending ground
school. Training occurred via a course software instruction system
developed by the training center, cockpit trainers, and classroom
instruction by certified instructors using copyrighted materials
developed by the training center. The curriculum included information on
systems, operating procedures, limitations, and performances. It also
included a safety course with cockpit-evacuation mockup sessions and a
Crew-Resource-Management (CRM) course. Nonexperienced trainees
additionally received theoretical instruction in specific Multi-Crew
Coordination (MCC). Trainees had to demonstrate their knowledge in
systems, safety, and performances by completing closed-book multiple
choice tests before proceeding to the FFT-1.

After satisfactory completion of ground school, trainees proceeded to
hands-on training in the FFTs. Each four-hour FFT session was introduced
by a one-hour briefing and followed by a half-hour debriefing. Trainees
spent half of each session as Pilot Flying (PF) and Pilot Not Flying
(PNF) in their usual position (e.g., captains flew from the left seat).
In the FFT-1, Experienced and Nonexperienced trainees were trained in
normal, abnormal, and emergency operations and CRM in five sessions and
eight sessions, respectively. Experienced trainees additionally
experienced LOFT. For both groups, this was followed by four FFT-X
training sessions preparing them for all operations and more complex
CRM. The last session assessed their progress in an eventful LOFT (see
Table 1). This was followed by the final Type-Rating Skill Test
administered by NAA personnel.

Training in the airplane was conducted during the last day of the four
or six-week course for Experienced and Nonexperienced trainees,
respectively. In the airplane, each trainee acted as PF in his usual
seat, with the instructor serving as PNF. Experienced pilots flew four
take-off- and landing patterns, Nonexperienced pilots flew six patterns
and additional training maneuvers including a simulated engine
flame-out. The type-rating certificate was awarded after the FFT-X skill
test and final reports from the airplane instructors to the NAA.

### Maneuvers Flown In the FFT

Table 2 shows the maneuvers trained in the FFT-X and evaluated during
the check ride in the FFT-X (as required in Appendix 2 to JAR-FCL 1.240
and 1.295^49^).

To be considered mastered, maneuvers had to be flown smoothly and
accurately within the airplane's limitations. The successful outcome of
the maneuver must never be in doubt. The general limits to be observed
were +/--100ft for altitude, +50ft/--0ft for starting a go-around at
decision height, +50ft/--0ft for minimum descent height/altitude
(MDH/A), +/--5 degrees for tracking on radio aids and half scale
deflection for a precision approach, +/--5 degrees heading with all
engines operating, +/--10 degrees with simulated engine failure, and
+/--5 knots for speed with all engines operating, +10 knots/--5 knots
with simulated engine failure.

### Data Collection

Participants' opinions were collected at various levels of formality.
Most important of all was the type-rating certificate awarded by the
NAA. Next were detailed questionnaires modified by the training center
from those used in the FAA/Volpe Center studies. Additional forms gave
an opportunity to express general opinions. Finally, participants were
invited to a final round-table debriefing in the presence of the NAA
decision makers. At that point, some of the trainees had experienced the
FFS informally. No data were recorded from the simulator or the
airplane. Table 3 shows the types of data collected. We are reporting
the questionnaire data and a summary of the final debriefing.

## Results of Phase 1

The most significant result, of course, was the successful type rating
of six pilots in a two-crew turboprop transport airplane, four of them
without prior experience in a multicrew airline environment. This shows
that the final performance of trainees in the FFT-X was, at a minimum,
satisfactory. In the next sections, we will look at the opinions of the
instructors and trainees, first on trainees' performance and how they
achieved it, then on the quality of the FFT-X as a simulator of the
airplane. Before going into these details, however, it must be noted
that these ratings were collected during actual training, where some
pilots may rate a maneuver more than once and others may never rate the
same maneuver. This is especially true for the ratings collected before
the type-rating in the FFT-X. The maneuvers in the airplane were either
rated by all six pilots or by all four Nonexperienced pilots.

### Ratings of Pilot Performance and Behavior

**In the airplane:** The successful type-rating was wholeheartedly
confirmed by the instructors rating the pilots in the airplane after
their successful type-rating skill test. Not one pilot was rated as
performing worse than a typical pilot for any of the eight maneuvers.
For single-engine go-arounds and for the take-offs with engine
flame-outs at 400ft above ground level (AGL), all pilots were rated as
performing the same as a typical pilot in an FFS. For the two-engine
take-offs and landings, the non-precision approaches, and the
single-engine Instrument Landing System (ILS) approach and landing, two
of the trainees performed "moderately better" than a typical pilot,
whereas for zero-degrees landing flaps two trainees performed "much
better."

Moreover, pilots' control strategy and technique was rated as "similar"
or the "same as typical pilot's" for all of these maneuvers. The
trainees themselves compared their control strategy and technique in the
airplane to the one in the FFT-X, and rated it at least the same or
similar for all but two of the eight maneuvers: two of the six pilots
found their strategy and technique somewhat and very different,
respectively, for the twin-engine take-off, and one found it somewhat
different for the twin-engine landing.

Instructors rated trainees' workload as identical to experienced pilots
in the same airplane for the same eight maneuvers. Again, the trainees
themselves compared their airplane workload with their FFT-X workload
and found the FFT-X workload the same or only "somewhat" higher or lower
for all maneuvers, with the exception of one pilot who found the
airplane workload much lower for the twin-engine take-off.

The instructors rated gaining proficiency as neither harder nor easier
than for a typical pilot for five of the eight maneuvers. Two pilots
each, however, obtained "somewhat easier" ratings for zero-flaps
landings, single-engine ILS, and take-off with engine flame-out at 400ft
AGL. Most trainees, comparing the ease of gaining proficiency in the
airplane and the FFT-X, gave at least "neither harder nor easier"
ratings to the FFT-X, with every maneuver also receiving one "much
easier" rating. Two (landing with single and twin engines) of the eight
maneuvers received one "somewhat harder" rating (but the combined
ratings still averaged out to "neither harder nor easier"), and the
twin-engine take-off received two "somewhat harder" ratings (but the
ratings still averaged to "neither harder nor easier").

**In the FFT-X:** Performance was already remarkably good after
transitioning from the FFT-1 to the FFT-X. Looking at the instructor
performance ratings for all pilots across all maneuvers, 25 maneuvers
(71 percent of the total 35 maneuvers) were consistently rated as being
flown equally or better than by typical pilots in an FFS. Only three
maneuvers (crosswind take-off, instrument take-off, and take-off with
engine failure) were ever rated as having been flown "much worse than
\[a\] typical pilot" in an FFS, and the average ratings for each of
these was nearer to "same as typical pilot."

The instructors perceived the trainees' control strategy and technique
consistently as at least similar, if not identical, to a typical pilot
in an FFS in 71 percent of maneuvers trained in the FFT-X. Only two
maneuvers (take-off at maximum weight and take-off with engine failure)
obtained any ratings of being flown very differently. Trainees, who were
asked to anticipate their control strategy in the airplane, consistently
estimated that their control strategy in the FFT-X was no more than
somewhat different than anticipated in the airplane for 89 percent of
the maneuvers. Three maneuvers (crosswind landing and take-off and
twin-engine landing) received one rating of "very different" from the
airplane, but still averaged closer to "somewhat similar." Take-off at
maximum weight was estimated as "very different" by two of the six
trainees, but all others rated it as "somewhat similar."

Instructors consistently perceived trainee workload in the FFT-X as
typical in 69 percent of the maneuvers. Two maneuvers were
inconsistently rated as sometimes causing higher, then again as causing
lower workload. Only five maneuvers were ever perceived as causing "much
higher" workload than the workload typically experienced in an FFS, but
all of these maneuvers also received ratings of the same or even lower
workload (instrument, rejected, maximum-weight, engine-failure, and
crosswind take-offs). Trainees were asked to compare their workload in
the FFT-X to the workload that they would expect in the airplane (which
they had never flown), and 37 percent of the maneuvers received ratings
of higher workload, but all of these also received ratings of the same
workload from at least as many trainees.

For gaining proficiency in the FFT-X, the lowest rating from any of the
instructors was "somewhat harder" than for a typical pilot in an FFS for
20 percent of the maneuvers. In each case, however, this was balanced by
"neither harder nor easier" or, in two of these maneuvers, "somewhat
easier" ratings. The other 80 percent of the maneuvers were judged to be
taught as easily, and in two ratings for go-arounds, as more easily than
for to a typical pilot in an FFS. This was confirmed by the pilots, who
found many maneuvers "somewhat easy" or "very easy" to learn. Fewer than
30 percent of the maneuvers had ratings of "somewhat hard," in most
cases amply balanced by "easy" ratings. The only two maneuvers that
broke even between "somewhat hard" and "somewhat easy" were crosswind
take-offs and landings.

### Ratings of FFT-X Overall Acceptability, Handling Qualities, and Control Feel

One very important aspect of the acceptability of a simulator is the
comfort, in terms of absence of simulator-induced nausea or
disorientation, experienced by instructors and especially pilots. The
reason this is so important is that some research results have shown
that discrepancies between visual and vestibular motion stimulation may
induce discomfort (the so-called "sensory conflict" theory, described,
e.g., in Ref. 50). Conflicts can occur even in a motion-base simulator,
when the visual and vestibular cues do not match up.^51^ It is all the
more remarkable, then, that there was no evidence of discomfort in the
FFT-X, where there is hardly any vestibular stimulation, causing the
visual system to sense motion while the vestibular system does not. One
reason may be that the effects of sensory conflict reportedly have been
more pronounced in those who are not actively engaged in the flying task
(See, e.g., Ref. 52, and earlier discussion of vection).

Despite a reasonable assumption that the FFT-X may produce sensory
conflict, there was not one FFT-X instructor comfort rating that was
lower than "neither lower nor higher" than in an FFS. Trainees were
asked to make the comparison anticipating their comfort in the airplane,
and rated their comfort in the FFT-X as equal or higher for 83 percent
of the maneuvers. Instrument take-offs and single-engine landings had
all equal ratings for comfort with the exception of one "somewhat lower"
rating. Twin-engine landings had three "somewhat lower" vs. five equal
ratings. Take-off at maximum weight earned one "much lower" comfort
rating, with one more "somewhat lower" and four equal ratings. Crosswind
landing and take-off each also earned one "much lower" rating, each also
with four equal ratings. Although these results already testify to the
high comfort experienced in the FFT-X, trainees' ratings of their
comfort in the FFT-X after the actual experience of the airplane were
even better; all FFT-X comfort ratings were marked as equal to the
comfort experienced in the airplane, with the exception of one
twin-engine take-off rating, for which one of the six pilots rated his
comfort "somewhat lower" in the FFT-X than in the airplane.

Only the six trainees were asked to rate the overall acceptability of
the FFT-X, both before and after they had flown the airplane. Before
trainees had experienced the airplane, 77 percent of maneuvers in the
FFT-X received one rating of "needs minor improvements" or better from
all pilots. Seven of those maneuvers that received a "needs minor
improvements" rating also received "excellent" ratings. Only four of the
35 maneuvers received more than one "needs major improvements" rating,
and each of these was perceived as "satisfactory as is" by at least one
of the other pilots. There was not a single rating of "uncontrollable."
After having experienced the airplane, the FFT-X was rated excellent by
at least one of the pilots for every single maneuver, and only one
single maneuver got one "needs major improvements" rating.

Again only the trainees were asked to rate the FFT-X's handling
qualities, the feel and response of the controls, and the visual, sound,
and overall cues, again before and after experiencing the real airplane.
When trainees compared the handling qualities of the FFT-X to the ones
they imagined the airplane to have, they all rated the handling
qualities during 25 of the maneuvers at least equal or in a few cases
even better than the anticipated handling qualities of the airplane. For
five of the remaining maneuvers, only one of the pilots rated the
handling qualities as "somewhat less satisfactory." For the rest of the
maneuvers, the take-off with engine-failure handling quality ratings
averaged close to 0 (equal to airplane), but the handling quality
ratings for the twin-engine and crosswind landing and the take-offs with
crosswind and at maximum weight were all rated, on average, closer to
-1, which meant somewhat less satisfactory than expected in the airplane
(the worst single rating from any pilot was -2). But once trainees had
experienced the airplane, only three of the eight maneuvers received any
ratings of "somewhat less satisfactory," and only take-off with engine
flame-out at 400ft AGL received more than one such rating.

The feel and response of controls was considered identical to the
"imagined" airplane for 71 percent or 25 of the 35 maneuvers (this
includes one maneuver where one "slightly less responsive" rating was
canceled out by one "somewhat more responsive" rating). For seven more
maneuvers, the controls were rated "slightly/somewhat" different by only
one pilot. Of the remaining three maneuvers, none were farther than half
a point from a rating of zero, which meant equal to the anticipated feel
of the airplane's controls. Only during take-off at maximum weight and
with crosswind were the controls of the FFT-X perceived as much less
responsive by one pilot, and the low take-off with crosswind rating was
somewhat counterbalanced by another pilot that found the controls
somewhat more responsive. The controls during twin-engine landings were
rated three times as slightly less responsive. After the experience of
the airplane, however, practically all of these imagined differences
disappeared. For twin-engine take-offs, one out of the six ratings
described the FFT-X controls as "somewhat more responsive" than the
airplane. The take-off with engine flame-out at 400ft AGL had one rating
each of "slightly less/somewhat more" responsive that canceled each
other out. During the seven remaining maneuvers, the controls were
perceived as exactly like the airplane.

Before experiencing the airplane, trainees consistently rated the
overall cues of the FFT-X as "standard-fair" or better for all maneuvers
but landings and take-offs with crosswind or windshear, where the cues
were rated "slightly deficient" by one of the pilots for each maneuver
(but each of these maneuvers also received two ratings of "good"), and
for twin engine landings, where two pilots rated the cues as "slightly
deficient" (but these ratings were also balanced out by two "good"
ratings). In all five of these maneuvers, the motion cues appear to have
been the main reason for the "slightly deficient" ratings, although
vision may have contributed for take-offs with windshear and sound may
have contributed for twin-engine and crosswind landings: for take-off
with windshear and crosswind landings, vision and sound, respectively,
were given one "slightly deficient" rating each. The sound cues for
twin-engine landings received three "slightly deficient" ratings.
Windshear at landing motion cues appear to be the worst, with four
"slightly deficient" ratings and an average rating between "standard"
and "slightly deficient"; followed by landing with twin engines, with
one "very deficient" and three "slightly deficient" ratings and an
average rating closer to "standard." Third worst rated were the motion
cues for windshear at take-off, with three "slightly deficient" ratings
and an average close to "standard." These three maneuvers additionally
received one, one, and two "good" motion ratings, respectively. After
having experienced the airplane, however, there was not one overall
rating that was lower than "slightly different from airplane," and with
the exception of the twin-engine take-off with four "slightly different"
vs. two "same as airplane" ratings, half or more of the ratings were
"same as airplane." There were, however, two "very different from
airplane" individual ratings for motion, one for the landing with the
flaps at zero degrees and the other for the twin-engine landings. Each
of these ratings was balanced by one "same as airplane" rating, however.

### Final Debriefing

The final debriefing included all trainees, all but one instructor,
officials from the NAA, representatives of the airplane manufacturer and
training provider, and the first author. It took about seven hours,
including a brief presentation by the first author of the FAA/Volpe
Center research results and a preliminary discussion of the
questionnaire results. Trainees and instructors took turns relating
their experiences. Some of the trainees had informally experienced the
FFS the evening before. The discussion took place in the native language
of the trainees and comments were translated by the first author.

**Trainee comments:** The first two trainees to express their opinions
were both employed by the NAA's accident investigation office, had only
single-pilot experience, and had flown the FFS the night before. They
both related that they had no problem adapting to the airplane after
training in the FFT. Both mentioned that the sound in the FFT was better
than in the FFS; the second trainee to speak extended this comment to
the vibrations as well. The second and more experienced (in terms of
total flight hours) speaker mentioned a problem with turbulence, and
that the instruments were too bright. He felt that the engine failures
were easier to handle in the airplane, and that the go-around was more
abrupt in the airplane. He nevertheless concluded that the FFT was an
excellent tool, elaborating that "few accidents are occurring because of
a difference in reaction time \[presumably he was relating to the 0.4s
faster pedal reaction time in the simulator attributed to the alerting
function of motion in Refs. 13 and 40\]. Most accidents happen because
of a lack of training."

The next two speakers were highly experienced, with 6,000 and 8,500
hours in an airline environment. They worked for the NAA's flight
inspection and training and schools office, respectively. Both reported
that they had been successfully trained to fly the airplane in the
FFT-X, that the training progressed steadily, and that there were no
transition problems. They both suggested improvements with regard to
nose-wheel steering and flight director (too sensitive). One of them
asked that the intercom be improved. He also said that the dynamic seat
did not add anything and that he did not perceive turbulence from the
instruments (the first two speakers interjected that they disagreed). He
added that the visuals of the FFT-X were excellent. The other speaker
mentioned that there were no mechanical noises in the FFT (presumably,
he was referring to the landing gear).

The final two trainees were employed by the NAA's office of training and
schools and by the NAA's flight training department, with 4,500 and
6,000 hours in a single-pilot environment only. They both said that they
had no problem transitioning to the airplane and that the airplane was
easier to fly than the FFTs. They mentioned a few additional problems,
mostly tuning or "growing pains," such as with an airport visual model,
signage, axis control, slow torque, rotor trim, turn coordinator ball,
faster response to engine failure, and lack of lateral acceleration. As
with the previous two less experienced pilots, the discussion digressed
into the merit of increased access to simulators with increased
affordability, and that this would help compensate for the steadily
decreasing flight hours in new recruits.

**Simulator instructor comments:** A first instructor pointed out that
although he had trained the experienced pilots, they had not been in a
turboprop airplane for 25 years. He agreed with the need for improvement
of the nose-wheel steering. Otherwise, he confirmed that there were no
transition problems, mentioning that the improvement for the second
failure was better in the FFT than in the FFS. He also experienced the
job of the instructor to be easier in the FFT than in the FFS. The
second simulator instructor agreed with the need for improvement of the
intercom and of the earphones. Otherwise, he reported normal adaptation
and standard progress similar to that of the FFS.

**Airplane instructor comments:** The airplane instructors reported that
the experienced pilots needed two take-off and landing patterns each,
whereas the less experienced pilots needed between three and five
patterns. They added that "there doesn't appear to be a difference
between FFS and FFT training." Pilots handled crosswind fine even after
training without motion; they seemed very adaptable.

**Conclusion:** The NAA decision maker concluded that there were no
training problems. However, a few technical adjustments were needed,
especially for the flight phases near the ground. He proposed to
continue the experimental type-rating phase without restrictions,
provided, for international training, that the NAA of the trainees
agreed. The representatives of the training provider pointed out that
this was the world's first type rating on a fixed-base simulator. The
NAA decision official reiterated that the focus in determining flight
simulation standards should be on effective stimulation of the pilot,
rather than emphasizing rote simulation of the airplane.

## Plan for Phase 3

The first set of type ratings showed agreement between the type-rating
examiners of the NAA, the training-center simulator instructors,
airplane-type-rating instructors, and trainees employed by the NAA. They
concluded that the FFTs effectively prepared trainees with and without
multicrew-piloting experience for flying the airplane. The ability to
use FFTs, in combination with a set of four to six structured
familiarization patterns flown in the actual airplane instead of a Level
D FFS, would greatly improve training opportunities world-wide. This
would increase pilot proficiency especially at smaller airlines by
greatly facilitating access to an appropriate training tool. Including
motion in a simulation of an airplane that does indeed move, however,
appears to have an almost irresistible appeal, despite the documented
limitations of a hexapod and despite years of research unable to
document an operationally relevant benefit of motion on transfer of
training to the airplane. We are therefore planning to further examine
the effectiveness of training in the FFT by a) comparing it directly
with the corresponding FFS and b) collecting objective data in addition
to participants' opinions.

### Design

As mentioned above, the basic design of this study will be to compare
two groups of pilots, one trained in the FFS, the other in the FFT. Data
will be collected during training and evaluation in the respective
device and, for all pilots, during transfer to the FFS as a stand-in for
the airplane. Data will be recorded directly from the training tool on
pilot-vehicle performance (pilot control inputs and flight precision).
All participants (instructors, trainees, examiners) will also complete
questionnaires tapping their opinions on the training tools and the
effect of the tools on flying strategies, performance, workload, and
comfort.

### Hypotheses

The working hypothesis is that when all pilots are tested in the FFS
(and subjectively rated in the airplane), the pilots also trained in the
FFS will perform better, control the airplane more steadily and with
lower workload, and will acquire skills faster and feel more comfortable
than pilots trained in the FFT-X. This will not only be evident from the
data recorded directly from the FFT-X and the FFS, but also from the
type-rating results and the questionnaires administered to the
type-rating examiners, simulator and airplane instructors, and trainees.
The null hypothesis is that the two groups' performance, control
behavior, and comfort when tested in the FFS will be similar, regardless
of whether they were trained in the FFS or FFT-X. This also implies that
the type-rating results and the opinion data collected will be similar
for and from the two groups.

### Design Challenges

The main challenge in this study will be to prevent the individual
differences between the pilots trained in the two devices from masking a
potential device effect. Similarly, any other differences within and
between the two groups and their treatment can affect the statistical
power to find a device effect, or even masquerade as such an effect.
Sources for such differences can be found in all the participants (as
applicable, type and level of experience, aptitude, attitude including
bias towards one or the other device, stamina, technique, etc.) as well
as in the two training regimes (four vs. six weeks) and any other,
unintended differences in procedures and training regimes. It is
therefore critical that all possible measures are taken to strengthen
the power of the experiment, namely:

-   Compare carefully matched crews based on training regime and
    experience

-   Strictly regiment procedures and training administered

-   Avoid instructor/examiner bias by concealing prior training tools

-   Counterbalance all other participants across groups on relevant
    variables

-   Record as much data as possible, at a high sampling rate

### Independent Variables

The main independent variable is the training tool, i.e., training in
the FFS or the FFT-X. A secondary independent variable will be the
training regime, i.e., four weeks or six weeks. A third independent
variable will be when the data will be collected, i.e., during training
in the FFS or FFT-X, during type rating in the FFS or FFT-X, or during
transfer testing in the FFS. The Training Tool (TT) is a between-groups
variable with two levels, FFS and FFT-X. The Training Regime (TR) is
also between-groups (and counterbalanced over the TT variable) with two
levels, four vs. six weeks. The Phase (P) variable is a within-groups
variable with three levels, training vs. type rating vs. testing. The
design is therefore a 2 (TT between) by 2 (TR between) by 3 (within)
factorial design.

### Dependent Variables

Two types of dependent variables will be collected: opinions of
participants (evaluators, instructors, and trainees) and recordings from
the training. Opinions will be collected after training and type rating
in the respective tools, transfer testing in the FFS, and airplane
training. Recordings will be taken on pilots' flight precision and
control inputs during training and type rating in the respective tools.
To provide a means of comparison with earlier FAA/Volpe Center studies,
a set of maneuvers that were trained and tested in these earlier studies
will be added to the curriculum.

# Conclusion {#conclusion .Style-Heading-1-+-Left:--0\"-First-line:--0\"}

In this paper, we have shown that pilots experience airplane motion via
multiple perceptual systems, which makes motion a candidate for
simulation via stimulation of a subset of these systems. Next, we
discussed the relative merit of vestibular cues when piloting an
airplane, and argue that a combination of visual information from the
out-the-window view and the instruments combined with force feedback
from the controls provide sufficient cues most of the time. Pilots are
taught that under certain flight conditions ignoring the visual
information from the instruments and relying on physical motion cues may
lead them to misunderstand the attitude of the airplane and enter, or
fail to recover from, unusual attitudes. Our research focused on
determining the effectiveness of simulator hexapod-platform motion on a
realistic sample of those flight maneuvers where motion cueing might be
expected to be necessary for training effectiveness. We compared the
veracity of the vestibular cues received in the simulator with those
experienced in an airplane, considering not only current airline-pilot
training simulators, but also those used in the past and those equipped
with advanced technologies. We found that the motion practicable today,
as in the past, falls far short of accurately simulating the motion cues
experienced in the airplane and that pilots have been trained totally
and successfully in simulators without adequate motion for longer than a
quarter century. It could be argued, then, that this alone shows that
full platform motion is not needed for a successful training outcome.
Moreover, our studies have repeatedly shown that the type of motion
available on an airline-pilot training and evaluation simulator does not
improve transfer between airplane and simulator for a variety of
airplanes and critical maneuvers where motion should serve an alerting
function. We then found that the cost of motion may prohibit some
airline pilots from reaping the benefits of quality simulator training,
with a concomitant loss in passenger safety. This conclusion is
especially pertinent given the world-wide need for effective simulators,
driven by a shortage of pilots, a reduction in prior experience among
new-hires, and an increase in the complexity of the flying task due to
increased congestion and automation. The approach described in the final
sections of the paper, with the FFT-X providing trainees with visual
motion and heave-onset cues with an FAA Level D FFS data package,
appears to hold promise in alleviating this situation and increasing
flight safety world-wide. It is to be feared, however, that a confluence
of such factors as the face-value of simulating airplane motion, the
resources that have been poured into developing motion systems, the
profits that can be gained from selling such systems, and the all too
human reluctance of changing an apparently effective system of training,
will nip such efforts in the bud. However, if the null hypothesis of the
evaluation study for the FFT-X is confirmed, i.e., if once more training
without a hexapod-motion platform is shown to be equivalent to training
with motion, it would be a disfavor to world-wide safety to continue to
require that sparse training resources be poured into acquiring and
maintaining motion systems. Any savings on platform-motion hardware and
facility costs should be applied to upgrade the fidelity in other
important areas, such as assuring that the simulator cockpit does in
fact match the equipment in the target aircraft, and that the simulation
includes realistic operational representation of the national air space,
including the air-traffic-control environment.^53^

# Acknowledgments {#acknowledgments .Heading-Not-Numbered}

The authors thank Dr. Eleana Edens from the FAA Air Traffic Organization
Human Factors Research and Engineering Division and Dr. Thomas Longridge
from the FAA Flight Standards Service Voluntary Safety programs, for
helpful comments and support of this work. We also thank Dr. Stephen
Popkin, the Chief of the Volpe Center's Human Factors Division, and Dr.
Michael Zuschlag, a senior researcher in the division, for their
pertinent comments.

# References {#references .Heading-Not-Numbered}

[^1]: Engineering Psychologist, Human Factors Division RTV-4, 55
    Broadway. Member AIAA.

[^2]: Engineering Psychologist, Human Factors Division RTV-4, 55
    Broadway.

[^3]: Assistant Professor, School of Mechanical and Aerospace
    Engineering, Member AIAA.

[^4]: ^Email\ from\ E.M.\ Boothe\ to\ first\ and\ third\ author,\ October\ 26,\ 2000,\ 15:39^

[^5]: ^U.S.\ Department\ of\ Labor.\ Occupational\ Employment\ and\ Wages,\ May\ 2006.\ Bureau\ of\ Labor\ Statistics\ 2006\ \[cited\ 24\ July\ 2007\];\ Available\ from:\ http://www.bls.gov/oes/current/oes530000.htm^

[^6]: ^Slobodan\ Lekic.\ Worldwide\ Pilot\ Shortage\ Affecting\ Safety,\ Analysts\ Say.\ 17\ July\ 2007.\ Associated\ Press\ \[cited\ 9\ August\ 2007\];\ Available\ from:\ http://www.aviation.com/ap_070717_worldwidepilotshortage.html^