The program proceeded along five avenues during 1985. Guidance and control strategies for penetration of microbursts and wind shear, application of artificial intelligence in flight control and air traffic control systems, the use of voice recognition in the cockpit, the effects of control saturation on closed-loop stability and response of open-loop unstable aircraft, and computer aided control system design are among the topics briefly considered. Areas of investigation relate to guidance and control of commercial transports as well as general aviation aircraft. Interaction between the flight crew and automatic systems is the subject of principal concern

Stengel, Robert F.

English

NASA Technical Reports Server

N81- 2 1 6 0  3 
INVESTIGATION OF AIR TRANSPORTATION TECHEJOLOGY 
AT PRINCETON UNIVERSITY, 1985 
Robert F. Stengel 
Department of Mechanical and Aerospace Engineering 
Pr i nce ton Un i ve r s i t y 
Princeton, New Jersey 
SUMMARY OF RESEARCH 
The Air Transportation Technology Program at Princeton Univer- 
sity, a program emphasizing graduate and undergraduate student 
research, proceeded along five avenues during 1985. 
0 Guidance and Control Strategies for Penetration of 
Microbursts and Wind Shear 
0 Application of Artificial Intelligence in 
Flight Control and Air Traffic Control Systems 
0 Use of Voice Recognition in the Cockpit 
0 Effects of Control Saturation on Closed-Loop 
Stability and Response of Open-Loop-Unstable Aircraft 
0 Computer-Aided Control System Design 
Areas of investigation relate to guidance and control of commer- 
cial transports as well as general-aviation aircraft. Interac- 
tion between the flight crew and automatic systems is a subject 
of principal concern. 
Recently, it has become apparent that severe downdrafts and 
resulting high-velocity outflows present a significant hazard to 
aircraft on takeoff and final approach. This condition is called 
a microburst, and while it often is associated with thunderstorm 
activity, it also can occur in the vicinity of dissipating con- 
vective clouds that produce no rainfall at ground level. Micro- 
burst encounter is a rare but extremely dangerous phenomenon that 
accounts for one or two air carrier accidents and numerous gen- 
eral aviation accidents each year (on average). Conditions are 
such that an aircraft's performance envelope may be inadequate 
for safe penetration unless optimal control strategies are known 
and applied. While a number of simulation studies have been 
directed at the problem, there are varied opinions in the flying 
community regarding the best piloting procedures, and optimal 
control strategies remain to be defined. 
Graduate student Mark Psiaki has undertaken a study of gui- 
dance and control strategies for penetration of microbursts when 
55 
https://ntrs.nasa.gov/search.jsp?R=19870018170 2020-03-20T10:13:32+00:00Z
encounter is unavoidable. It has been shown that simple control 
laws could greatly reduce an aircraft's response to wind 
shear[l]. Although the response mechanism is the same, jet 
transport and general aviation aircraft behave somewhat differ- 
ently in microbursts; the larger, heavier aircraft are ~ more 
adversely affected by variations in the horizontal wind, while 
the smaller, lighter aircraft have greater difficulty with the 
downdraft. 
Our emphasis has shifted to the determination of optimal con- 
trol strategies for the microburst encounter[?]. The study has 
begun with the computation of optimal control 'histories using 
steepest-descent and second-order gradient algorithms. Once an 
envelope of safe flight has been determined for a typical jet 
transport, attention will be directed at a general aviation type, 
and optimal closed-loop control laws will be investigated. Dur- 
ing the past year, a survey paper on the subject was pub- 
lished[3]. 
Undetected system failures and/or inadequately defined recov- 
ery procedures have contributed to numerous air carrier incidents 
and accidents. The infamous DC-10 accident at Chicago's O'Hare 
Airport, in which l o s s  of an engine pod, subsequent l o s s  of sub- 
systems, and asymmetric wing stall led to disaster, provides a 
prototype for the'kind of tragedy that could be averted by intel- 
ligent flight control systems. (An intelligent control system is 
one that uses artificial intelligence concepts, e.g., an expert 
systems program, to improve performance and fault tolerance.) 
Although many methods of modern control theory are applicable, 
the scope of the problem is such that none of the existing theo- 
ries provides a complete and practical solution to the problem. 
At the same time, heuristic logic may be applicable, but it has 
yet to be stated in satisfactory format. 
Graduate student David Handelman is developing a knowledge- 
based reconfigurable flight control system that will be imple- 
mented with the Pascal programming language using parallel micro- 
processors. This expert system could be considered a prototype 
for a fault-tolerant control system that could be constructed 
using existing hardware. Although funding for this effort has 
shifted to another source, results will have direct bearing on 
air transportation systems, as illustrated by [4]. In a parallel 
effort, graduate student Chien Huang is using the LISP program- 
ming language to investigate the utility of a string-oriented, 
recursive logical system in the same role. A principal distinc- 
tion between this and the previous approach is that flight con- 
trol code will be modified in response to control system fail- 
ures. 
Senior Takashi Sensui has begun to apply artificial intelli- 
gence principles in the study of air traffic control (Sensui, 
1986). Working in the computer language PROLOG, he is designing 
optimal control logic for terminal area operations. 
Prospects for reducing pilot workload through voice inputs to 
the aircraft are extremely promising, and graduate student Par- 
vada Suntharalingam has completed a sbort study of the character- 
istics of a representative system (Suntharalingam, 1985). Using 
a commercially available system that is capable of recognizing 
continuous speech and which takes advantage of structured gram- 
mars, she demonstrated the effects of the number of training 
passes on misrecognition and rejection rates. Her results show 
that the system's errors are not greater than manual input 
errors of similar information. 
One of the virtues of highly reliable electronic flight con- 
trol systems is that an aircraft's stability and response, i.e., 
its closed-loop flying qualities, can be tailored to the pilot's 
needs. For reasons of performance and maneuverability, it may be 
desirable to design the aircraft so that its natural (unaug- 
mented) modes of motion are unstable, with the understanding that 
the flight control system will provide the necessary stability by 
deflecting control surfaces to counter potentially divergent 
motions. Because control surfaces have limitations on their dis- 
placements and rates of travel, stability can be restored only 
within a bounded region about the trim point. If the aircraft's 
motions exceed the boundaries, the available control forces and 
moments will not be sufficient to prevent divergence. 
Graduate student Prakash Shrlvastava is develeping methds fer 
determining the stability boundaries and control response for 
systems containing control saturation[ 5,6] . Analysis is carried 
out using phase-plane plots, in which saturation and stability 
boundaries are represented by straight lines, stable tralectories 
approach equilibrium points, and unstable trajectories diverge to 
infinity. The analysis pertains to systems containing unequal 
saturation boundaries, as well as those with multiple saturating 
controls. 
Future control-system engineers will benefit from design pro- 
cedures that are computer-intensive, and it is important to cre- 
ate computer programs that allow designers to describe and ana- 
lyze complex systems interactively. Senior Russell Nelson 
developed a control-system analysis program based on the LISP 
language mentioned above (Nelson, 1985) . Analysis proceeds with 
classical concepts of transfer-function analysis in a program 
that can be run on a personal computer. 
The FAA/NASA grant supporting student research in air trans- 
portation technology has inestimable value in helping educate a 
new generation of engineers for the aerospace industry, and it is 
producing research results that are  relevant to the continued 
excellence of aeronautical development in this country. 
57 
ANNOTATED REFERENCES 
1. Psiaki, M.L., and Stengel, R.F., "Analysis of Aircraft Control 
Strategies for Microburst Encounter" , AIAA Journal of Guidance, 
Control, and Dynamics, Vol. 8, No. 2, Mar-Apr 1985, pp. 553-559. 
Penetration of a microburst during takeoff or approach is 
an extreme hazard to aviation, but analysis has indicated that 
risks could be reduced by improved control strategies. Atten- 
uation of flight path response to microburst inputs by eleva- 
tor and throttle control was studied for a jet transport and a 
general aviation aircraft using longitudinal equations of 
motion, root locus analysis, Bode plots of altitude response 
to wind inputs, and nonlinear numerical simulation. Energy 
management relative to the air mass, a pitch-up response to 
decreasing airspeed, increased phugoid-mode damping, and 
decreased phugoid natural frequency were shown to improve 
microburst penetration characteristics. Aircraft stall and 
throttle saturation were found to be limiting factors in an 
aircraft's ability to maintain flight path during a microburst 
encounter. 
2. Psiaki, M.L., and Stengel, R . F . ,  "Optimal Flight Paths Through 
Microburst Wind Profiles", presented at the AIAA 12th Atmos- 
pheric Flight Mechanics Conference, Snowmass, CO, AIAA Paper 
NO. 85-1833-CP, Aug 1985. 
The problem of safe microburst wind shear encounter during 
the approach and climb-out flight phases was addressed using 
flight path optimization. The purpose was to investigate the 
physical limits of safe penetration and to determine control 
strategies that take full advantage of those limits. Optimal 
trajectories for both jet transport and general aviation air- 
craft were computed for encounters with idealized and actual 
microburst profiles. The results demonstrate that limits to 
control system design rather than to the aircraft's physical 
performance may be the deciding factor in an aircraft's capa- 
bility for safe passage through a wide class of microbursts. 
The best control strategies responded to airspeed loss in an 
unconventional manner: by raising the nose to maintain lift. 
3. Stengel, R.F., "Solving the Pilot's Wind-Shear Problem", Aero- 
space America, Vol. 23, No. 3, Mar 1985, pp. 82-85. 
Although aircraft have had to contend with wind variability 
since the beginning of aviation, the July 1982 crash of Pan 
American Flight 759 on the outskirts of New Orleans, which 
killed 153 people, riveted attention on the hazards of flying 
in wind shear. Microbursts, unusually severe downdrafts 
accompanied by intense outflows, prove especially hazardous 
during takeoff or landing. Within a minute or less the air- 
craft may be subjected to a headwind, then a downdraft, and 
then a tailwind. Just as the pilot throttles back to arrest 
ballooning above the nominal path caused by increase airspeed, 
headwind vanishes and downdraft causes the aircraft to lose 
lift. Lift is further degraded as the aircraft enters the 
outflow. A microburst compounded by heavy rain is thought to 
have caused the crash of Pan Am's Flight 759. The article 
addesses several aspects of the problem and proposes both 
short-term and long-term solutions. 
4. Handelman, D.A., and Stengel, R.F., "Combining Quantitative 
and Qualitative Reasoning in Aircraft Failure Diagnosis", pre- 
sented at the 1985 A I M  Guidance, Navigation, and Control Con- 
ference, Snowmass, CO, AIAA Paper No. 85-1905-CP, Aug 1985. 
The problem of in-flight failure-origin diagnosis is 
addressed by combining aspects of analytical redundancy and 
artificial intelligence theory. The objective is to use a 
mathematical model of the aircraft's dynamic behavior to sup- 
plement the knowledge used for diagnosis. A method is devel- 
oped whereby qualitative causal information about a dynamic 
system is drawn from its model. Based on sensitivities of the 
equations of motion to worst-case failure modes, a measure of 
the relative capacity of system elements to affect one another 
is derived. A diagnosis procedure combining problem reduction 
and backward-chaining ordered search uses this knowledge to 
reduce a list of elements capable of failure to a relatively 
small list of elements suspected of failure. Examples illus- 
trate use of the knowledge base and the problem-solving mecha- 
nism that has been developed. 
5 .  Shrivastava, P.C. , and Stengel, R.F. , "Stability Boundaries 
for Aircraft with Unstable Lateral-Directional Dynamics and 
Control Saturation", presented at the 1985 AIAA Guidance, Nav- 
igation, and Control Conference, Snowmass, Co, AIAA Paper No. 
85-1948-CP, Aug 1985. 
Aircraft that do not possess inherent (aerodynamic) stabil- 
ity must rely on closed-loop control systems for stable opera- 
tion. Because there are limits on the deflections of an air- 
craft's control surfaces, the region of stable operation also 
is bounded. These boundaries were investigated for a lateral- 
directional example, in which vertical fin size was inadequate 
to provide directional stability and where aileron and rudder 
deflections were subject to saturation. Fourth-order models 
were used in this study, with flight control logic based upon 
minimum-control-energy linear-quadratic regulator theory. It 
was found that the stability boundaries can be described by 
59 
unstable limit cycles surrounding stable equilibrium points. 
Variations in regions of stability with gain levels and com- 
mand inputs are illustrated. Current results suggest guide- 
lines for permissible limits on the open-loop instability of 
an aircraft's lateral-directional modes. 
6. Shrivastava, P.C. , and Stengel, R.F. , "Regions of Stability 
with Unequal Saturation Limits and Non-Zero Set Points" , pre- 
sented at the 24th IEEE conference on Decision and Control, 
Fort Lauderdale, FL, IEEE Paper No. S-0128, Dec 1985. 
Constraints on the magnitudes of control variables limit 
the region where open-loop unstable systems can be stabilized 
using feedback control. Variations in regions of stability 
with unequal control saturation limits and non-zero set points 
are illustrated for single-input unstable linear systems with 
one or two unstable eigenvalues. The regions of stability for 
saddle-point and unstable-node-type singularities increase in 
one of the saturation limits, but they become invariant when 
the larger control limit exceeds a certain value; the stabil- 
ity regions vanish for non-zero set points that saturate the 
controls. The unstable-focus-type singularity exhibits strik- 
ingly different characteristics. These results suggest guide- 
lines for obtaining desired stability regions for different 
types of singularities. 
ANNOTATED R IBLIOGRAPHY 
1. Nelson, R.F., "Computer Aided Control System Design", Prince- 
ton University Independent Work Report, May 1985. 
The objectives of this project were to develop an interac- 
tive computer program to serve as a tool in the design of con- 
trol systems and to evaluate the utility of the LISP program- 
ming language in this application. An environment of LISP 
functions that allows the user to define and examine a system 
characterized in the frequency domain was developed. The pro- 
gram generates a block diagram of a multiloop system, then 
computes frequency-response characteristics between arbitrary 
points in the diagram; these can be displayed as Bode or Nyq- 
uist plots. The LISP language was found to provide a flexi- 
ble, interactive tool that could aid the development of large- 
scale systems. 
60 
2 .  Sensui,  Takashi, "Automation of t h e  Terminal Area A i r  T r a f f i c  
Control System - Progress Report", Pr ince ton  Univers i ty  Inde- 
pendent Work Report, Jan 1986. 
The ob jec t ive  of the pro jec t  i s  t o  design log ic  f o r  a f u l l y  
automated terminal  area control  sys tem.  Design spec i f i ca t ions  
have been def ined,  and appl icat ion of an exper t  system i s  d i s -  
cussed. A b r i e f  comment concerning t h e  programming of simpli-  
f i e d  models i s  included. 
3. Suntharalingam, P . ,  "A Test on the  R e l i a b i l i t y  and Performance 
of t h e  Verbex Se r i e s  4000 Voice Recognizer", In t e rna l  Memoran- 
dum, Princeton University Laboratory fnr C n f i t r n l  and Autcma-  
t i o n ,  Sept 1985. 
Speech input  i s  becoming an increas ingly  important form of 
d a t a  en t ry .  The ob jec t  o f  t h i s  exe rc i se  was t o  t e s t  and 
explore  the  f ea tu res  of a voice recogni t ion u n i t  t h a t  enables 
communication between human and machine. The u n i t  i s  speaker- 
dependent, i . e . ,  it requires  t r a i n i n g  w i t h . a  given vocabulary 
a s  spoken by a s ing le  person. The speaker ' s  voice p a t t e r n s  
a r e  s to red  on a s o l i d - s t a t e  ca r t r idge  and a r e  subsequently 
used as templates f o r  recognition. Two additional f ea tu res  of 
t h e  u n i t  a r e  i t s  a b i l i t y  t o  recognize continuous speech 
( s t r i n g s  of words a s  opposed t o  s ing le  commands) and i t s  use 
of s t ruc tu red  grammars, i n  which words of spec i f i ed  c l a s ses  
( s u b j e c t ,  p red ica te ,  modifier,  e t c . )  must  be spoken i n  a f ixed  
o rde r .  Tes ts  were run t o  determine t h e  voice recognition 
u n i t ' s  e r r o r  r a t e s  a s  funct ions of machine t r a i n i n g  l eve l  and 
t o  determine t h e  r e l a t i v e  mer i t s  of s t ruc tu red  and unstruc- 
t u r e d  grammars. I t  was found t h a t  e r r o r  r a t e s  var ied  from one 
speaker t o  t h e  next;  some speakers requi red  more t r a i n i n g  
passes  than o the r s  t o  achieve a given l e v e l  of recognition 
accuracy. I t  a l so  was noted t h a t  a s t ruc tu red  grammar 
r e q u i r e s  less t r a i n i n g  than an unstructured one t o  achieve a 
given accuracy. However, t h e  l a t t e r  may be p re fe r r ed  f o r  i t s  
g r e a t e r  f l e x i b i l i t y ;  with s u f f i c i e n t  t r a i n i n g ,  t h e  error r a t e s  
may be reduced t o  acceptable l e v e l s .  
61 


Investigation of air transportation technology at Princeton University, 1985

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