testing tunnel-in-the-sky displays and flight control systems with and without flight simulator...

8/7/2019 Testing Tunnel-in-the-Sky Displays and Flight Control Systems With and Without Flight Simulator Motion ISAP_035

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TESTING TUNNEL-IN-THE-SKY DISPLAYS AND FLIGHT CONTROL SYSTEMS

WITH AND WITHOUT FLIGHT SIMULATOR MOTION

Max Mulder1, Jerome Chiecchio2, Amy R. Pritchett2, M. M. (Ren) van Paassen1

1 Delft University of Technology, Faculty of Aerospace Engineering, Control and Simulation division

Delft, the Netherlands2 Georgia Tech, Schools of Industrial and Systems Engineering and Aerospace Engineering,

Atlanta, USA

Tunnel-in-the-sky displays have shown great potential for reduced pilot workload and for reduced navigation error.

However, further improvements are s till worthy of investigation. One challenging consideration is the relationshipbetween modern flight control systems and the tunnel display. For example, flight control systems may now be

developed that allow the pilot to directly control the direction of the aircraft motion relative to the ground,

facilitating the tracking of a ground-based tunnel considerably. Another major c onsideration is the means by which

tunnel displays may be accurately tested in a cost-effective manner. Nearly-identical studies of tunnel displays in

flight tests and fixed-based simulator tests have achieved dramatically different results, suggesting that the task of

flying a tunnel may be very sensitive to pilot-perceived motion. This study tests several combinations of the tunnel

display with advanced flight control system concepts, and examines in particular the impact of flight simulatormotion on pilot workload, control behavior and ability to track the tunnel.

Introduction

Tunnel displays may enable aircraft to closely follow

intricate trajectories as a means of improving air

traffic management efficiency, meeting noise

abatement conc erns, etc. (Grunwald, 1984, Mulder,1999). While studies to date have demonstrated the

potential benefits of tunnel displays, further

improvements are still worthy of investigation. One

important consideration is the relationship between

modern flight control systems and the tunnel display.

That is, advanced fly-by-wire control algorithms may

now be implemented that allow the pilot to directly

command the direction of the aircraft motion relativeto the ground, facilitating pilot tracking of a ground-based tunnel considerably (Veldhuijzen, Mulder, Van

Paassen, & Mulder, 2003).

Another major concern is the means by

which tunnel displays may be accurately tested in a

cost-effective manner. Nearly-identical studies of

tunnel displays in flight tests and simulator tests have

led to remarkably different results. E.g., pilot path-

following performance with a tunnel display in

conducting a straight-in approach was a factor 2 to 3times worse when conducted in real flight as

compared to performance in a fixed-base simulator

(Mulder, Kraeger, & Soijer, 2002). This suggests thatthe task of flying a tunnel may be very sensitive to

pilot-perceived motion. While extensive tests in the

past have outlined the type of testing facility suitable

for many piloting tasks, tunnel displays are

sufficiently novel that appropriate methods of testinghave not been fully explored; these insights may also

help relate flying the tunnel to other piloting tasks

and further understand the relationship between pilot

perception of motion and flight control behavior.

The study presented in this paper tests thecombination of a basic tunnel-in-the-sky display with

several advanced flight control system concepts, and

focuses in particular on the impact of flight simulatormotion on pilot control behavior and ability to track

the tunnel. The paper is structured as follows. First,

the rationale behind the experiment is discussed in

detail. It will be shown that the results presented in

this paper are a sub-set of a much larger multi-

objective experiment, the results of which are

reported in other papers at this conference

(Chiecchio, Pritchett, Kalaver, Van Paassen, &

Mulder, 2003; Veldhuijzen et al. 2003). Then, the

experiment design and setup are described, followedby a discussion of the experimental results.

A Multi-Objective Experiment:

Testing Tunnel Displays, Flight Control Systems,

and Pilot Fault Detection Behavior

In the summer of 2002, a group of researchers from

the Georgia Institute of Technology (USA) visitedDelft University of Technology (the Netherlands).

Their mission was to study the pilot detection of

faults in the flight control system, using Delft

Universitys high-fidelity flight s imulator, SIMONA.

This investigation fitted nicely into an existing line ofresearch at Delft University on the development of

fly-by-wire control algorithms suitable for being used

in conjunction with the tunnel-in-the-sky display. Thevarious research perspectives from both parties led to

the definition of what we called the multi-objectiveexperiment. Before elaborating further on this

experiment, first the main initial research interests of

the Atlanta and Delft parties will be outlined.


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Aiding pilot in detecting faults in the flight control

loop

Georgia Tech is involved in a NASA-funded study

on the pilot detection of faults in the flight control

loop. Problems with loss of control are a major

safety issue. Any degradations in performance in the

flight control loop impact the aircraft stability and the

pilot ability for control. These faults can be caused

by many systems, such as the control mechanisms by

which the pilot enters commands, the actuation of theaircraft control surfaces, faults in the sensors and

displays providing feedback to the pilot, etcetera.

Compounding pilot difficulty in responding to these

faults, they may be masked by flight control systems

that attempt to compensate for the fault until they

reach the limit of their control authority.

Few systems currently exist for aiding pilotsin detecting these types of faults, in particular those

not caused by simple mechanical failures. Several

technologies may enable such pilot aids, includingon-board simulation and adaptive estimation of the

expected aircraft dynamics in nominal conditions for

comparison to actual conditions. However, the role

and function of these aids remains to be determined.

The design of a simple aural and visual alert facesconcerns with setting its threshold and commensurate

potential for false alarms and mis-use (Pritchett,

2001). Likewise, while a unified display of flight

control loop health can provide the pilot beneficial

information and predictive power, it may also be

difficult to comprehend, especially when a flight

control system is masking the impact of the fault, and

may not by itself enable the pilot to understand howto control the aircraft in its damaged state.

The aim of the Georgia Tech research team

was to investigate the possibilities for providing pilot

support in detecting faults in the flight control loop

through alerts and a display of flight control loop

health. To provide pilots with an environment that is

as realistic as possible, the SIMONA research

simulator was used. The pilot task targeted for this

study was a relatively high workload manualtrajectory-following control task with flight control

systems ranging from conventional direct-link to the

more advanced maneuver-demand control concepts.

During this task, pilot monitoring for faults is to beobserved, as well as pilot reactions to faults.

Task-oriented control-display systems

The ability of pilots to manually control their aircraftalong complex curved approaches using the tunnel

display is widely recognized to be one of the main

qualities of the display. By presenting the pilot the

trajectory to follow in an intuitive three-dimensional

fashion, while at the same time including the

constraints of the trajectory-following guidance, the

tunnel display is related very closely to the pilots

primary aircraft guidance and control task.

Since the early investigations of the tunnel

display, however, it has been known that a basic

tunnel does not always lead to acceptable levels ofpilot performance and workload (Grunwald, 1984).

The mainstream of tunnel research has focused on

augmenting the display with symbols that can help

pilots in controlling the aircraft through the tunnel.

Symbols like the flight-path vector, showing the

instantaneous direction of the aircraft motion, and,

most importantly, the flight-path predictor, showing

the aircraft future position a couple of seconds ahead

(usually in conjunction with a frame moving ahead inthe tunnel that acts as the reference for the predictor

symbol), showed to greatly facilitate the pilots task(Grunwald, 1984). The presentation of especially the

flight-path predictor, however, considerably changes

the task and also a pilot control behavior, namelyfrom the integrated feedback of aircraft attitude,

flight-path and position relative to the tunnel to atwo-dimensional pursuit tracking task with preview

(Mulder, 1999). That is, the pilot is constantly trying

to keep the flight-path predictor symbol in the middle

of the predictor r eference frame moving ahead of the

aircraft, rendering the tunnel geometry itself to be

merely a situation awareness aid. Also, with these

display-augmentation features, the pilot remains

operating in the innermost control loop, constantlycompensating for the path-following errors caused by

weather effects such as turbulence or wind. This can

result in considerable levels of pilot workload and asub-optimal path-following performance.

In (Veldhuijzen et al., 2003), a radically

different perspective on improving the pilot-display

interaction is presented. Here, the aim is not to

augment the display through the addition of symbols,

but rather to investigate the possibilities ofmanipulating the basic aircraft control. That is, with

the introduction of fly-by-wire (FBW) flight control

systems in modern aircraft, the relation between the

pilots control inputs and the corresponding response

of the aircraft can be designed. Veldhuijzen et al.

(2003) compared the basic, direct-link pilot manual

control with a tunnel display with two different FBWcontrol concepts: attitude-orientedcontrol and flight-

path-oriented control. The attitude-oriented FBW iscomparable to the current way Airbus aircraft are

being controlled. The pilot stick inputs act as attitude-

rate (i.e. pitch-rate and roll-rate) commands to theFBW system. When the stick input is zero, the FBW

controllers keep the selected attitude constant, even

in the presence of disturbances acting on the vehicle.In the flight-path-oriented FBW solution the pilot


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stick inputs act as flight-path vector rate (i.e. track-

angle rate and climb-angle rate) commands, or, inother words, the pilot commands the aircraft direction

of motion relative to the ground. This control solution

comes very close to the primary task of pilots when

flying with the tunnel display, i.e. the accurate

tracking of a trajectory fixed with respect to the

ground. Optimizing the display as well as the controls

from the perspective of the pilots primary guidance

task (tracking ground-based tunnels in the sky) yields

a so-called task-oriented control-display interface.

Veldhuizen et al. (2003) show that the

combination of the tunnel display with an advanced

flight-path-oriented fly-by-wire control system

significantly improves the pilot path-following

performance and reduces the pilot workload

considerably. This study was conducted in a fixed-

base flight simulator. In the same period of this study,results of a flight test campaign showed remarkable

differences in pilot path-following performance in

tracking straight tunnel trajectories as compared toearlier experiments conducted in the same fixed-base

flight simulator (Mulder et al., 2002). This raised the

concern whether the pilot behavior and the

corresponding metrics like performance, as

obtained in the fixed-base simulator are indeedrepresentative for (or can be extrapolated to) pilot

behavior in real flight. Hence, a follow-up study was

planned in the SIMONA flight simulator, to examine

the possible effects of having simulator motion on the

results of the Veldhuijzen et al. (2003) study.

Rationale of the multi-objective experiment

An experiment has been designed in which theresearch interests of the Delft and Atlanta parties

were served as good as possible. It was decided to

invite the pilots for the Delft experiment in which

the combination of tunnel displays and three flight

control systems was investigated with and without

simulator motion. When pilots arrived in the

morning, they were quickly briefed and

measurements started. Pilots were left completely

unaware of the fact that, first of all, the objective was

to study the effects of flight simulator motion, and

also, even more important, that at the end of the

morning trials, just before the lunch break, anintended and easily detectable fault occurred in the

flight control loop. This allowed us to test the pilot

agility and response to such a fault in a high

workload situation, in the case they really did not

expect a fault to happen at all.And surprised they were. During lunch, after

the pilots were calmed down a little, they were

briefed about the complete intentions of the

experiment, that is, the Atlanta experiment. It was

made explicit to them that during some of the

afternoon sessions a variety of faults in the flight

control loop could occur, and they were asked to

detect when such a fault had happened and comment

on the type of f ault (e.g. sensors, flight controls, etc.)

they thought it was. They were also introduced in the

operation of an alerting system, the so-called fault-o-meter, that was designed to help them in detecting

and coping with the fault as it occurred. This alerting

system always provided pilots with the right

information at the right time. But again, pilots were

tricked in the last run, as in that run the alerting

system indicated that a fault had occurred when it did

not. This last surprise was included to test the

potential for over-reliance on an indication (and alert)

of FCS health, as indicated by accepting a false alarmfrom the system as truth. After this second surprise,

some pilots made it very clear that they would neveragain participate in these kinds of experiments.

Design of the multi-objective experiment

From the above it is clear that the whole experiment,which lasted one day for each pilot, can be regarded

as the concatenation of four smaller experiments. In

the firstexperiment, the emphasis was only on pilot

behavior in flying complex trajectories with a tunnel

display, with a var iety of fl ight control systems, with

and without simulator motion. The second

experiment is the first surprise trial, where the pilot

is confronted with an unexpected fault. In the third

experiment, the pilot fault detection capability is

investigated. Finally, the fourth experiment consisted

of the second surprise trial, where the alertingsystem presents a false alarm.

This paper presents the results of the first

experiment only. The results of the other experiments

are all discussed in detail in (Chiecchio et al., 2003).

Experiment

The goal of the experiment was to investigate the

impact of simulator motion on pilot tunnel tracking

performance with various flight control systems.

METHOD

Subjects and instructions. Twelve professional pilots

participated in the experiment. They were instructedto fly the aircraft through the tunnel as accurate as

possible. They were briefed correctly about all

objectives of the experiment, except one: that ourintention was to examine the effects of flight

simulator motion. Rather, pilots were briefed with the

statement that In this experiment we will be testingdifferent motion models (italics not included in the


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briefing). In the experimental trials, all cues with the

exception of motion were present, such as theinitialization motion procedure of the simulator, even

when motion was turned off during the run. This was

done such that pilots could not predict if motion was

going to be on or off in a particular run.

Apparatus and setup. Subjects were seated in

the cabin of the SIMONA flight simulator. The

tunnel display was shown on a 15 inch LCD display

in front of the subject. A green flight-path vectorsymbol was presented. To the side of the tunnel

display a standard navigation display and engine

indication display were shown. The aircraft was

flown through the control column; the pilots had no

rudder panel at their disposal.

Independent measures. The experiment had two

independent variables. First of all, three flight control

systems (FCS) were defined: (1) the direct-link

manual control, i.e. no control-augmentation at all;(2) the attitude-oriented fly-by-wire system; and (3)

the flight-path-oriented fly-by-wire system. These

flight control systems were identical to those tested in

(Veldhuijzen et al, 2003). For the flight-path FCS, a

second (yellow) flight-path vector symbol was

presented on the tunnel display, representing thecommanded flight-path.

The second experimental variable was the

flight simulator motion: motion was either on or off.

Experiment design. A factorial within-subjects

design was employed, consisting of six conditions (3

FCS systems x 2 motion states). All conditions wererun three times, yielding 18 measurement runs.

Aircraft model. The aircraft flown was a small

business jet, the Cessna Citation 500. A realistic non-

linear dynamic model was used. The aircraft was

equipped with a yaw-damper to improve lateral

stability, and also with an auto-throttle that kept the

velocity constant at 150 kts (TAS). This allowed

subjects to fly the a ircraft using the control column

(giving commands to the flight control system) only.

Weather model. The weather model included a

constant headwind, with magnitude 15 kts andheading 25 degrees off the runway centerline. A

patchy turbulence was simulated with an intensity

that resulted in a relatively strong disturbance.

Procedure. The pilots had to fly curvedapproaches to a fictitious airport. Two tunnel

trajectories were used, which were mirrored relative

to the runway centerline (note that the wind vector

was mirrored as well). The tunnel width was 45 [m].

The tunnel trajectories were quite complex and wereonly possible to fly with the tunnel display, at

reasonably high levels of workload.

Each pilot flew a total of 24 runs, of which 6

were training runs. The runs were blocked by flight

control system, i.e. they were run as 3 sets of (2 + 6).

During the training runs the motion was always on.

Each run lasted approximately 5 minutes. After each

run the pilot workload was assessed using the NASATask Load Index (TLX). After these 24 runs the first

surprise run was flown, with the FCS fault, see

(Chiecchio et al., 2003) for more details.

Dependent measures. The dependent measures

in this first experiment were: (1) the path-following

performance, expressed in the aircraft position and

flight-path angle errors (measured relative to the

tunnel trajec tory); (2) pilot control activity, i.e. thecontrol column deflections and their derivatives; (3)

aircraft-related variables such as the attitude-rates

and the normal acceleration; and (4) the pilotsubjective workload (the NASA TLX ratings).

Data processing. Of each run, the first 10 seconds

were disregarded to allow the pilots some time to getinto the task. The data belonging to the last 500

meters before the runway were also discarded, as

here pilots were preparing for landing.

Hypotheses. The main experimental hypothesis

was that, independent of the three flight control

systems used, the presence of motion stimuli helped

pilots to better control their aircraft and therefore

improve their performance. It was expected that thispositive effect of motion increased when the level of

flight control automation decreased. That is, the

advantage of having motion stimuli was hypothesized

to be larger for the direct link FCS than for the

advanced flight-path-oriented FCS. The reason for

this hypothesis is that the motion stimuli are

generally believed to be useful in particular for

controlling the aircraft inner loops, such as attitudeand flight-path. With the attitude-oriented and

flight-path-oriented flight control systems the

attitude control loop is automated, and the relation

between the pilot control command and the aircraft

attitude response is less direct than in the situation offull manual control (the direct link).

Experimental Results

The experimental data were analyzed using anAnalysis of Variance (ANOVA), with pilot (12

levels) as a random effect and fcs (3 levels) and

motion (2 levels) as fixed effects. The main results

of this analysis are summarized in Table 1.


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Table 1. Result s of a full-factorial ANOVA on the variables involving pilot control activity, path-following

performance, aircraft-related variables, and workload. In this table **, *, and o represent significance levels of

p 0.01, 0.01 < p 0.05, and 0.05 < p 0.10, respectively. The other symbols are explained in the text.

pilot control activity path-following performance aircraft-related variables workload

e a de/dt da/dt Xe Ve Te e e p Q Nz zTLX

main effects

FCS ** ** ** ** ** ** ** ** ** ** ** ** **Motion ** o ** ** o O o

two-way effects

FCS x motion ** o

(a) (b) (c)Figure 1. The means and 95% confidence limits of the STD of the pilot elevator control derivative de/dt (a), the

RMS of the total position errorTe (b) and the normalized pilot workload ratings zTLX (c) (data of all pilots). The

three flight control systems are shown along the horizontal axis (1=direct link, 2=attitude-oriented, 3= flight-

path oriented), with the results grouped in clusters of simulator motion (off = 0, on = 1).

Control activity. The main effects fcs and motion

are both significant for the pilot vertical stick

deflections e (fcs: F2,22=71.204, p


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link FCS, and the other FCS in between. Simulator

motion resulted in a significant increase (p

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