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Recovery S t r a t e g i e s f o r M i c r o b u r s t E n c o u n t e r s Using R e a c t i v e and Forward-Look
Wind S h e a r D e t e c t i o n
David A . Hinton*
NASA Langley R e s e a r c h C e n t e r
Hampton, V i r g i n i a
At9-VWf3
Href
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Abstract
An e f f o r t i s i n p r o g r e s s by t h e
NASA, FAA, a n d i n d u s t r y t o r e d u c e t h e
t h r e a t of c o n v e c t i v e m i c r o b u r s t wind
s h e a r phenomena t o a i r c r a f t .
This
p a p e r d e s c r i b e s an e f f o r t t o q u a n t i f y
t h e b e n e f i t s of forward-look s e n s i n g
and t o d e v e l o p and test a c a n d i d a t e
set of s t r a t e g i e s f o r r e c o v e r y from
inadvertent microburst encounters
during
the
landing
approach.
Candidate s t r a t e g i e s w e r e developed
and
evaluated
using
a
batch
s i m u l a t i o n c o n s i s t i n g of a point-mass
p e r f o r m a n c e model o f a t r a n s p o r t c a t e g o r y a i r p l a n e f l y i n g through an
a n a l y t i c a l m i c r o b u r s t model.
The
candidate
strategies
were
then
evaluated i n piloted simulations
u s i n g a f u l l dynamic a i r p l a n e model.
The r e s u l t s of t h i s e f f o r t i n d i c a t e
t h a t t h e f a c t o r which most s t r o n g l y
a f f e c t s a microburst recovery i s t h e
t i m e a t which t h e
recovery
is
initiated.
Improving t h e a l e r t t i m e
by 5 s e c o n d s g e n e r a l l y p r o v i d e d a
g r e a t e r recovery performance i n c r e a s e
t h a n c o u l d b e a c h i e v e d by changing
t h e recovery s t r a t e g y
Forward-look
a l e r t s given 1 0 seconds p r i o r t o
microburst e n t r y permitted recoveries,
t o b e made w i t h n e g l i g i b l e a l t i t u d e
loss.
T h i s t r e n d was s u b s t a n t i a t e d
i n p i l o t e d tests.
.
Nomenclature
D
Eh
F
9
h
-
Airplane drag
Airplane energy h e i g h t
Wind s h e a r h a z a r d i n d e x
Gravitational acceleration
Airplane a l t i t u d e
*Aerospace T e c h n o l o g i s t , V e h i c l e
O p e r a t i o n s R e s e a r c h Branch
Copyright @ 1989 by the American Institute of Aeronautics
and Astronautics, tnc. No copyright is asserted in the
United States under Title 17, U.S.Code. The U.S.Govern
ment has a royalty-free license to exercise all rights under
the copyright c!aimed herein for Governmental purposes.
All other rights are reserved by the copyright owner.
T
v
W
Wh
wx
YP
Reference a l t i t u d e f o r
recovery s t r a t e g i e s
Total airplane thrust
Airplane airspeed
A i r p l a n e weight
V e r t i c a l wind component,
updraft positive
H o r i z o n t a l wind component,
a l o n g t h e a i r p l a n e ground
track
Airplane p o t e n t i a l f l i g h t path
angle
Numerous a i r c a r r i e r a c c i d e n t s
and i n c i d e n t s have r e s u l t e d from
inadvertent encounters
with
the
a t m o s p h e r i c wind s h e a r a s s o c i a t e d
w i t h " m i c r o b u r s t " phenomena, i n some
c a s e s r e s u l t i n g i n h e a v y l o s s of
life.
A microburst i s a strong,
l o c a l i z e d downdraft t h a t s t r i k e s t h e
g r o u n d , p r o d u c i n g winds t h a t d i v e r g e
r a d i a l l y from t h e impact p o i n t .
An
a i r p l a n e p e n e t r a t i n g t h e c e n t e r of a
symmetric m i c r o b u r s t w i l l i n i t i a l l y
e n c o u n t e r an i n c r e a s i n g headwind,
f o l l o w e d by a s t r o n g d o w n d r a f t a n d
rapidly increasing tailwind.
The
effects
of
the
downdraft
and
i n c r e a s i n g t a i l w i n d may e a s i l y e x c e e d
the
climb
and
acceleration
c a p a b i l i t i e s of t h e a i r p l a n e , c a u s i n g
an u n a v o i d a b l e l o s s o f a l t i t u d e and
airspeed.
These e n c o u n t e r s c o n t i n u e
t o occur
since the
ability
to
reliably
predict
or
detect
a
m i c r o b u r s t i n an a i r p l a n e ' s f l i g h t
path, i n an o p e r a t i o n a l environment,
does n o t y e t e x i s t .
The N a t i o n a l A e r o n a u t i c s a n d
S p a c e A d m i n i s t r a t i o n (NASA) a n d t h e
F e d e r a l A v i a t i o n A d m i n i s t r a t i o n (FAA)
a r e addressing t h i s hazard through
t h e I n t e g r a t e d Wind S h e a r Program.
The g o a l i s t o r e d u c e t h e h a z a r d of
low l e v e l w i n d s h e a r t o a i r c r a f t
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o p e r a t i o n s t h r o u g h improved a i r b o r n e
and ground-based wind s h e a r d e t e c t i o n
s y s t e m s , crew a l e r t i n g and f l i g h t
g u i d a n c e s y s t e m s , and t r a i n i n g and
operating procedures.
NASA i s
i n v e s t i g a t i n g t h e a i r b o r n e a s p e c t s of
the
problem
through
hazard
c h a r a c t e r i z a t i o n , sensor technology,
and f l i g h t management s y s t e m s .
P r e v i o u s r e s e a r c h h a s shown t h e
performance
available
from
an
airplane
following
an
optimal
t r a j e c t o r y ( r e f s . 1 a n d 2 ) when f u l l
knowledge o f t h e m i c r o b u r s t f l o w
f i e l d i s known.
The a p p l i c a t i o n of
these optimal recovery concepts t o
p r a c t i c a l r e c o v e r y g u i d a n c e l a w s was
s t u d i e d ( r e f . 3 ) and t h e performance
of t h o s e g u i d a n c e l a w s i n p i l o t e d
operations, i n t h e takeoff encounter
c a s e , was e v a l u a t e d i n t h e s i m u l a t o r
study described i n reference 4 .
The
s t u d i e s d e s c r i b e d i n r e f e r e n c e s 3 and
4 showed t h a t a d v a n c e d g u i d a n c e l a w s
enabled recovery t o take place a t
h i g h e r minimum a l t i t u d e s t h a n w i t h
baseline constant p i t c h techniques,
b u t t h a t r e c o v e r y a l t i t u d e was v e r y
sensitive t o small deviations i n
airplane pitch history.
In piloted
simulation tests, t h e performance
d i f f e r e n c e s between v a r i o u s r e c o v e r y
strategies
were
statistically
insignificant.
This
result
emphasizes t h e need f o r microburst
avoidance.
Two l e v e l s o f m i c r o b u r s t a v o i d a n c e
a r e possible:
(1) t o t a l l y a v o i d an
encounter
with
the
microburst
phenomena, a n d ( 2 ) a v o i d p l a c i n g an
a i r c r a f t i n a c r i t i c a l low-energy
s i t u a t i o n , by i n i t i a t i n g a r e c o v e r y
procedure p r i o r t o microburst e n t r y .
The f i r s t l e v e l of a v o i d a n c e i s t h e
u l t i m a t e g o a l o f t h e program, b u t
r e q u i r e s a h i g h e r d e g r e e of s e n s o r
development t h a n t h e second l e v e l .
Advances i n f o r w a r d - l o o k i n g wind
s h e a r s e n s o r t e c h n o l o g i e s have r a i s e d
t h e i s s u e s o f how much f o r w a r d - l o o k
distance i s necessary t o ensure
a i r p l a n e s u r v i v a l during a recovery
p r o c e d u r e a n d how r e c o v e r y g u i d a n c e
c o n c e p t s w i l l b e a f f e c t e d by f o r w a r d look data.
This paper d e s c r i b e s an
e f f o r t aimed a t q u a n t i f y i n g t h e
b e n e f i t s of r e l a t i v e l y s h o r t range
f orward-look s e n s i n g and d e v e l o p i n g
recovery guidance concepts, f o r t h e
a p p r o a c h - t o - l a n d i n g c a s e wind s h e a r
encounter, u t i l i z i n g both reactiveonly and forward-look
wind s h e a r
detection.
I n an e f f o r t t o d e t e r m i n e t h e amount
of f o r w a r d - l o o k d e t e c t i o n n e c e s s a r y
t o ensure airplane survival,
an
a n a l y s i s of a i r p l a n e energy height
d u r i n g a m i c r o b u r s t e n c o u n t e r was
conducted.
The p u r p o s e o f
the
a n a l y s i s was t o g a i n i n s i g h t of how
changes i n m i c r o b u r s t s t r e n g t h and
detection
delays
or
advances
( f o r w a r d - l o o k ) would a f f e c t a i r p l a n e
survivability,
by
examining t h e
changes i n energy h e i g h t a c r o s s t h e
events.
Airplane energy height i s
defined as:
V
is
airspeed,
g
is
where
g r a v i t a t i o n a l a c c e l e r a t i o n , and h i s
altitude.
From r e f e r e n c e 3, t h e
p o t e n t i a l f l i g h t p a t h a n g l e of an
a i r p l a n e i n t h e p r e s e n c e of w i n d
s h e a r c a n b e a p p r o x i m a t e d by:
y,=--
T-D
W
F
where T i s a i r p l a n e t h r u s t , D i s
a i r p l a n e drag, W i s a i r p l a n e weight,
and F i s t h e " F - f a c t o r " :
The two wind t e r m s d e s c r i b e t h e wind
s h e a r impact on t h e c l i m b a n g l e
c a p a b i l i t y of t h e a i r p l a n e , i n t e r m s
of t h e h o r i z o n t a l s h e a r W x
and
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v e r t i c a l wind ( W h ) . M u l t i p l y i n g t h e
p o t e n t i a l f l i g h t p a t h a n g l e by
a i r s p e e d p r o v i d e s an a p p r o x i m a t i o n t o
t h e p o t e n t i a l r a t e of change of
energy h e i g h t .
Equation 4 can be i n t e g r a t e d a c r o s s a
wind s h e a r s c e n a r i o t o d e t e r m i n e t h e
change i n energy h e i g h t .
These
e q u a t i o n s a l o n e do n o t i n c o r p o r a t e
t h e e f f e c t s of d i f f e r e n t airplane
t r a j e c t o r i e s through a shear o r
p r e d i c t t h e a c t u a l a l t i t u d e of a n
a i r p l a n e a f t e r a s h e a r , b u t can be
u s e d t o e s t i m a t e t h e change i n e n e r g y
h e i g h t given an event encounter.
rst Madel
The
microburst
model
(ref.
5)
r e p r e s e n t s an a x i s y m m e t r i c s t a g n a t i o n
point
flow t h a t
s a t i s f i e s mass
c o n t i n u i t y and i n c l u d e s boundary
l a y e r e f f e c t s n e a r t h e ground.
The
boundary l a y e r e f f e c t s and s p a t i a l
v a r i a t i o n i n o u t f l o w a n d downflow
closely
match
real-world
observations.
The m i c r o b u r s t h a s a
maximum o u t f l o w o f 37 k n o t s a t a n
a l t i t u d e of 120 f e e t and a t a r a d i u s
of 2 , 3 9 1 f e e t .
The s e v e r i t y of t h e
modeled s h e a r i s r e p r e s e n t a t i v e o f
m i c r o b u r s t s t h a t have c a u s e d a i r c r a f t
accidents.
A s a c o n s e q u e n c e of t h e
boundary l a y e r e f f e c t s , t h e a p p a r e n t
s e v e r i t y of t h e s h e a r i s d e p e n d e n t on
t h e a l t i t u d e of t h e e n c o u n t e r .
Shear D e t e c t i o n
b a t c h s i m u l a t i o n of v a r i o u s wind
s h e a r e n c o u n t e r s was c o n d u c t e d t o
f u r t h e r e v a l u a t e t h e b e n e f i t s of
forward-look d e t e c t i o n and develop
microburst recovery s t r a t e g i e s .
The
b a t c h s i m u l a t i o n c o n s i s t e d of
a
point-mass
airplane
model,
an
a n a l y t i c a l m i c r o b u r s t model, a n d a
s i m p l e wind s h e a r d e t e c t i o n scheme.
A
The a i r p l a n e model i s b a s e d on a
Boeing 737-100 f l y i n g i n a v e r t i c a l
plane.
The g r o s s w e i g h t was s e t a t
90,000 pounds and s e a l e v e l s t a n d a r d
a t m o s p h e r i c c o n d i t i o n s were assumed.
D u r i n g a p p r o a c h e s , t h e wing f l a p s
w e r e s e t t o 25 d e g r e e s , t h e g e a r was
down, a n d t h e t h r u s t was c o n t r o l l e d
by a n a u t o t h r o t t l e t o m a i n t a i n t h e
137 k n o t r e f e r e n c e s p e e d .
The g l i d e
s l o p e was m a i n t a i n e d u n t i l a c t i v a t i o n
o f t h e wind s h e a r a l e r t , a t which
t i m e t h e r e c o v e r y s t r a t e g y was
initiated.
D u r i n g e s c a p e maneuvers,
t h e wing f l a p s and l a n d i n g g e a r
p o s i t i o n s were l e f t unchanged and t h e
t h r u s t was
i n c r e a s e d t o 24,000
pounds.
This emulates t h e recovery
procedures currently being taught t o
airline crews.
Wind s h e a r d e t e c t i o n l o g i c was u s e d
t o a c t i v a t e t h e recovery control
laws.
The d e t e c t i o n was b a s e d on t h e
F - f a c t o r o f t h e wind s h e a r .
A Ff a c t o r t h r e s h o l d o f 0 . 1 5 was u s e d t o
d e t e r m i n e when t h e s h e a r h a d b e e n
entered.
The t h r e s h o l d F - f a c t o r d o e s
not
occur a t any g i v e n s p a t i a l
l o c a t i o n i n t h e m i c r o b u r s t model, b u t
depends on a i r p l a n e a l t i t u d e , f l i g h t
p a t h , and a i r s p e e d .
Variable t i m e
a d v a n c e a n d d e l a y was implemented t o
s i m u l a t e forward-look
s e n s o r s and
reactive device delays.
The advance
i s d e f i n e d a s t h e number of s e c o n d s
t h a t t h e a l e r t is given before t h e
t h r e s h o l d F - f a c t o r would h a v e b e e n
e x c e e d e d i f no a l e r t had been g i v e n .
The d e l a y i s d e f i n e d a s t h e number of
seconds t h a t t h e a l e r t i s given a f t e r
t h e t h r e s h o l d F-factor i s exceeded.
A d e l a y of 5 s e c o n d s i s c o n s i d e r e d t o
approximate
the
response
of
realizable
reactive
detection
systems.
The r e c o v e r y c o n t r o l l a w s
u s e t h e two d e t e c t i o n d i s c r e t e
signals
to
begin
the
recovery
s t r a t e g y and t o d e t e r m i n e c o n t r o l law
g a i n s t h a t a r e d e p e n d e n t on t h e a l e r t
type.
te
m
d
Shear
Recovery
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Of t h e v a r i o u s r e c o v e r y s t r a t e g i e s
t e s t e d , t h r e e a r e d i s c u s s e d below.
The c o n t r o l law f o r e a c h s t r a t e g y
limited the target pitch attitude t o
t h e v a l u e t h a t would p l a c e t h e
a i r p l a n e a t t h e s t i c k s h a k e r a n g l e of
a t t a c k and t h e p i t c h r a t e was l i m i t e d
t o 3 d e g r e e s p e r second.
Manual St-.
This s t r a t e g y
i s c u r r e n t l y i n u s e by t h e a i r l i n e
.
6)
Since t h i s
industry
(ref.
s t r a t e g y was d e s i g n e d t o b e manually
flown i n t h e absence of guidance
commands, t h e e x a c t p r o c e d u r e u s e d
w i l l v a r y s l i g h t l y from p i l o t t o
pilot.
F o r t h i s e f f o r t t h e manual
recovery
was
approximated
by
i n i t i a l l y rotating the airplane t o a
p i t c h a t t i t u d e of 1 5 d e g r e e s .
This
p i t c h a t t i t u d e was m a i n t a i n e d i f i t
produced a zero o r p o s i t i v e f l i g h t
path angle.
I f 1 5 degrees of p i t c h
was i n s u f f i c i e n t t o m a i n t a i n l e v e l
f l i g h t , h o w e v e r , t h e c o n t r o l law
would f u r t h e r i n c r e a s e p i t c h i n a n
attempt t o maintain l e v e l f l i g h t .
Path Anale S t r a t e g y .
This s t r a t e g y required t h e a i r p l a n e
t o f l y a f l i g h t p a t h a n g l e t h a t was a
f u n c t i o n o f a l t i t u d e , wind s h e a r Ffactor,
and
available
airplane
performance .
When t h e p o t e n t i a l
f l i g h t p a t h a n g l e was p o s i t i v e , t h a t
c l i m b g r a d i e n t was m a i n t a i n e d .
When
t h e p o t e n t i a l f l i g h t p a t h a n g l e was
n e g a t i v e , t h e t a r g e t climb g r a d i e n t
was a l t i t u d e d e p e n d e n t .
Below a
reference
altitude
(Href)
the
strategy
attempted
to
climb
r e g a r d l e s s o f wind s h e a r s t r e n g t h ,
under t h e assumption t h a t o b s t a c l e s
must b e c l e a r e d .
The t a r g e t f l i g h t
p a t h a n g l e was 0.03 r a d i a n s a t ground
l e v e l , reducing l i n e a r l y t o l e v e l
f l i g h t a t Href.
Above H r e f , t h e
s t r a t e g y maintained one-half
the
p o t e n t i a l f l i g h t path angle.
This
permitted a descent t o be maintained
a t t h e higher a l t i t u d e s i n order t o
r e d u c e t h e r a t e a t which a i r s p e e d was
lost.
Href was set t o 100 f e e t f o r
t
r e a c t i v e a l e r t r e c o v e r i e s a n d 400
feet
for
forward-look
alert
recoveries.
These a l t i t u d e s p r o v i d e d
t h e b e s t o v e r a l l performance i n
preliminary d a t a runs.
Both t h e
p o s i t i v e and n e g a t i v e f l i g h t p a t h
angle t a r g e t values w e r e limited t o
0.06 r a d i a n s t o prevent e x c e s s i v e
climb o r descent r a t e s during t h e
recovery.
The f l i g h t p a t h a n g l e was
f u r t h e r l i m i t e d t o prevent descent
below t h e g l i d e s l o p e .
GlideSloD
This
s t r a t e g y attempted t o emulate t h e
characteristics
of
the
optimal
approach a b o r t t r a j e c t o r i e s d e s c r i b e d
i n reference 7.
That e f f o r t showed
t h a t t h e optimal recovery t r a j e c t o r y
i n i t i a l l y produced a d e s c e n t and
later transitioned t o level flight.
The l e v e l f l i g h t p a t h was flown u n t i l
e x i t i n g t h e wind s h e a r .
For t h i s
study,
that
trajectory
was
a p p r o x i m a t e d by i n i t i a l l y t r a c k i n g
t h e g l i d e s l o p e , a t go-around t h r u s t ,
until
the
altitude
reached
a
r e f e r e n c e a l t i t u d e ( H r e f ) . The v a l u e
of Href was 100 f e e t f o r r e a c t i v e
a l e r t r e c o v e r i e s a n d 500 f e e t f o r
forward-look a l e r t c a s e s .
After
r e a c h i n g Href, t h e s t r a t e g y a t t e m p t e d
t o maintain a zero f l i g h t path angle
u n t i l e x i t i n g t h e shear.
The g l i d e
s l o p e was chosen a s t h e d e s c e n t a n g l e
t o provide obstacle clearance during
t h e recovery.
tor
The L a n g l e y s i x - d e g r e e - o f - f r e e d o m
v i s u a l motion s i m u l a t o r was u t i l i z e d
i n t h e p i l o t e d tests.
The s i m u l a t o r
provides a generic transport airplane
flight
deck
equipped
with
conventional
electromechanical
instrumentation.
The p i l o t was
provided with an Attitude-Director
Indicator (ADI) , airspeed indicator,
t u r n and s l i p i n d i c a t o r , b a r o m e t r i c
altimeter, radar altimeter, v e r t i c a l
speed i n d i c a t o r , heading i n d i c a t o r ,
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and
engine
instruments.
pressure
ratio
The recovery guidance was presented
on the AD1 by conventional dual cue
command bars. The recovery guidance
was presented to the pilot in the
same format for all three control
laws. A pitch target was calculated
to accomplish the strategy goal, and
the pitch command bar was positioned
at that point on the ADI. The roll
command-bar was driven by bank angle,
so that the command was nulled
whenever the wings were level. Full
scale deflection of the roll bar
occurred at a bank angle of 60
degrees.
A fast/slow indicator in
the AD1 display was driven by speed
error, with full scale deflection
occurring at 10 knots of error.
An out-the-window display of terrain
was provided on the cockpit forward
windows.
This visual display was
driven by a terrain model board and
permitted the execution of visual
approaches and landings. No visual
meteorological cues or effects were
shown.
Audio cues for wind and
engine noise were also provided.
Pilot control input was through a
wheel
and
control
column
hydraulically loaded in pitch and
roll, hydraulically loaded rudder
pedals, and independent throttle
levers. A stick shaker function was
implemented on the control column.
The simulator was driven with a full
nonlinear math model of a Boeing 737100 airplane with Pratt & Whitney
JT8D-7 engines. The model included
lift and drag coefficient data to 24
degrees angle of attack, and the
effects on those coefficients of
pitch rate, control deflection, and
ground effect. Variations in aileron
and elevator control forces with
airspeed and trim position were fed
back to the pilot. The aircraft was
flown in the landing configuration at
a gross weight of 90,000 pounds.
Standard sea level temperature and
air density were selected.
Models
Two microburst models were utilized
in the piloted simulation. The first
was the same analytical model used
for the batch simulation effort. The
a
numeric
second
model
was
representation of a microburst as
generated by the Terminal Area
Simulation System (TASS) program
(refs. 8 and 9) . The TASS program
implements a numeric atmospheric
simulation,
which
determines
parameters such as temperature, wind
components, pressure, and liquid
water content at grid points within a
cube of airspace during the evolution
of a microburst.
For this effort,
the TASS model was initialized with
the atmospheric conditions that
produced a microburst at the DallasFort Worth (DFW) Airport in August,
1985.
The resulting TASS output
closely resembles the microburst
involved in the DFW accident.
The two microburst models differ
primarily in the scale of the event
and in the precursors entering the
microburst.
The TASS model is a
large
scale
event,
with
the
performance-decreasing wind change
taking place over a distance of about
12,000 feet, or more than double the
corresponding distance
in the
analytical model. Prior to entering
the performance-decreasing region,
both models include a performanceincreasing region.
This region
involves only an increasing headwind
in the analytical model. In the TASS
model, the performance-increasing
region involves both an increasing
headwind and a strong updraft.
Both reactive and forward-look
detection capabilities were used.
The F-factor was used in the
implementation of both detection
types.
In the reactive detection
case, knowledge of the local
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h o r i z o n t a l and v e r t i c a l wind was u s e d
t o calculate t h e F-factor.
The
r e a c t i v e a l e r t was g i v e n when t h e
calculated
F-factor
exceeded
a
t h r e s h o l d value f o r 5 seconds.
In
t h e forward-look d e t e c t i o n c a s e , t h e
F - f a c t o r was e s t i m a t e d u s i n g wind
divergence information a t a detection
p o i n t ahead of t h e a i r p l a n e .
The
d e t e c t i o n p o i n t was l o c a t e d a l o n g t h e
i n s t a n t a n e o u s f l i g h t p a t h v e c t o r of
the
airplane,
at
a
distance
c o r r e s p o n d i n g t o 10 s e c o n d s of f l i g h t
time.
The d e t e c t i o n t h r e s h o l d v a l u e
o f t h e F - f a c t o r was s e t a t 0 . 1 2 f o r
the reactive alert.
To c o m p e n s a t e
f o r known e r r o r s o u r c e s i n t h e
f o r w a r d - l o o k F - f a c t o r e s t i m a t o r and
t o a c h i e v e a f o r w a r d a l e r t 10 t o 1 5
s e c o n d s i n a d v a n c e of t h e r e a c t i v e
a l e r t , t h e forward-look d e t e c t i o n
t h r e s h o l d was s e t a t 0 . 1 0 .
e a c h c a s e t h e m i c r o b u r s t was l o c a t e d
on t h e e x t e n d e d runway c e n t e r l i n e .
One s c e n a r i o p o s i t i o n e d t h e c o r e of
t h e a n a l y t i c a l m i c r o b u r s t 1,640 f e e t
A second s c e n a r i o
f r o m t h e runway.
p o s i t i o n e d t h e same m i c r o b u r s t a t a n
i n c r e a s e d d i s t a n c e of 3,600 f e e t from
t h e runway.
The c l o s e - i n l o c a t i o n
a p p r o x i m a t e d t h e m i c r o b u r s t geometry
of a b a t c h s i m u l a t i o n run w i t h an
i n i t i a l a l t i t u d e o f 140 f e e t .
The
more d i s t a n t l o c a t i o n a p p r o x i m a t e d a
batch s i m u l a t i o n run w i t h an i n i t i a l
a l t i t u d e o f 240 f e e t .
The t h i r d
s c e n a r i o p o s i t i o n e d t h e TASS model
1,640
feet
from
the
runway.
A d d i t i o n a l m i c r o b u r s t g e o m e t r i e s and
locations w e r e included i n t h e test
matrix t o provide v a r i a b i l i t y t o t h e
p i l o t s , a l t h o u g h t o o few r e p e t i t i o n s
were r u n t o b e i n c l u d e d i n t h e
analysis.
Alerts were
P r i o r t o each d a t a run t h e a i r c r a f t
was i n i t i a l i z e d a t a n a l t i t u d e of
1 , 2 0 0 f e e t , on t h e c e n t e r l i n e s of an
Instrument
Landing System
(ILS)
l o c a l i z e r and g l i d e s l o p e .
The
a i r p l a n e was t r i m m e d f o r a t h r e e
d e g r e e d e s c e n t a t a n a i r s p e e d of 137
knots, w i t h l a n d i n g f l a p s and g e a r
position selected.
The p i l o t t a s k
was t o t r a c k t h e ILS u n t i l r e c e i v i n g
e i t h e r a r e a c t i v e o r a forward-look
wind s h e a r a l e r t .
A t t h a t point the
t a s k was t o a p p l y maximum r a t e d
thrust
and
initiate
an
escape
maneuver, u s i n g t h e f l i g h t d i r e c t o r
guidance.
Since t h e performance
e f f e c t of t h e a l e r t t i m i n g was b e i n g
evaluated, t h e p i l o t s w e r e asked not
t o b e g i n a go a r o u n d maneuver b e f o r e
r e c e i v i n g an a l e r t .
The r u n was
t e r m i n a t e d upon g r o u n d c o n t a c t o r
s u c c e s s f u l e x i t from t h e m i c r o b u r s t .
presented both v i s u a l l y
A red annunciator l i g h t
and a u r a l l y .
d i r e c t l y above t h e A D 1 and a w a r b l i n g
tone indicated a reactive a l e r t .
The
r e d l i g h t was c a n c e l e d a f t e r t h e Ff a c t o r d e c r e a s e d t o 0.05 and t h e
a u d i o t o n e was c a n c e l e d a f t e r t h r e e
seconds.
An amber l i g h t above t h e
r i g h t c o r n e r o f t h e A D 1 and a s t e a d y
t o n e i n d i c a t e d a forward-look a l e r t .
The amber l i g h t was c a n c e l e d e i t h e r
by t h e a c t i v a t i o n o f t h e r e a c t i v e
a l e r t , o r t h e forward-look F - f a c t o r
d e c r e a s i n g t o 0.05.
The a u d i o t o n e
was c a n c e l e d a f t e r 10 s e c o n d s , a f t e r
pilot
activation
of
t h e escape
guidance,
or after
receiving a
r e a c t i v e a l e r t , whichever occurred
first.
P r i o r t o r e c e i v i n g an a l e r t ,
t h e f l i g h t d i r e c t o r was u s e d i n an
instrument landing system t r a c k i n g
mode.
Upon r e c e i v i n g a f o r w a r d - l o o k
alert,
t h e e s c a p e g u i d a n c e was
a c t i v a t e d by t h e p i l o t p r e s s i n g a
t h r o t t l e - m o u n t e d T a k e o f f - G o Around
(TOGA) s w i t c h .
Upon r e c e i v i n g a
reactive alert, the flight director
automatically switched t o t h e escape
guidance.
.
,
t a l Condltlons
Three microburst s c e n a r i o s w e r e used
for the s t a t i s t i c a l analysis.
In
A t o t a l of s e v e n r e s e a r c h and a i r l i n e
p i l o t s p a r t i c i p a t e d i n t h e study.
The a v e r a g e a m o u n t
of
turbojet
e x p e r i e n c e was o v e r 5000 h o u r s .
Five
of t h e s e v e n p i l o t s h a d c o m p l e t e d t h e
FAA wind s h e a r t r a i n i n g a i d program.
The m a t r i x u s e d f o r d a t a a n a l y s i s
c o n s i s t e d of t h r e e r e p e t i t i o n s of
t h r e e microburst scenarios, t h r e e
r e c o v e r y s t r a t e g i e s , a n d two a l e r t
t i m e s ( r e a c t i v e o n l y and forward
l o o k ) f o r a t o t a l of
54 r u n s .
Another 11 r u n s , n o t i n c l u d e d i n t h e
a n a l y s i s , w e r e i n s e r t e d throughout
the
matrix
for
the
alternate
microburst scenarios.
strong shears
altitude loss.
with
essentially
no
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Results
I n t h e r e s u l t s t o be presented, d a t a
f o r a Boeing 737-100 a i r p l a n e w e r e
used.
The c o n f i g u r a t i o n was assumed
t o b e g e a r down, f l a p s s e t a t 25
d e g r e e s , a n d a w e i g h t of 90,000
pounds.
In t h i s configuration t h e
r e f e r e n c e approach s p e e d i s 137 k n o t s
a n d t h e s t i c k s h a k e r s p e e d , a t goa r o u n d t h r u s t , was e s t i m a t e d t o b e
107 k n o t s .
The v a l u e of ( T - D ) /W
was s e t a t -0.05 f o r t h e approach and
a t 0.16 f o r t h e r e c o v e r y .
The e n e r g y h e i g h t
a n a l y s i s was
c o n d u c t e d f o r v a r i o u s wind s h e a r
s t r e n g t h s and a l e r t t i m e v a l u e s . The
F - f a c t o r of t h e s h e a r s was v a r i e d
from 0.10 t o 0.30 and t h e s h e a r w i d t h
was 5 , 0 0 0 f e e t .
A l e r t t i m e was
v a r i e d f r o m -20
s e c o n d s t o 60
A negative
alert time
seconds.
i n d i c a t e s an a l e r t received a f t e r
e n t e r i n g t h e s h e a r , which s i m u l a t e s a
r e a c t i v e system o r p i l o t recognition
o f t h e s h e a r , and a p o s i t i v e a l e r t
t i m e indicates an a l e r t received
p r i o r t o shear entry, t o simulate a
forward-look d e v i c e .
The r e s u l t s a r e
The f i g u r e
d e p i c t e d i n f i g u r e 1.
shows t h a t w i t h a 1 0 t o 1 5 s e c o n d
d e l a y i n d e t e c t i n g t h e p r e s e n c e of
t h e wind s h e a r , e v e n a r e l a t i v e l y
weak s h e a r w i l l r e s u l t i n t h e l o s s of
300 t o 500 f e e t of e n e r g y h e i g h t .
The b e n e f i t o f r e d u c i n g t h e t i m e
r e q u i r e d t o detect a wind s h e a r c a n
e a s i l y be seen.
F o r e a c h s e c o n d of
improvement i n t h e t i m e r e q u i r e d t o
g i v e a r e a c t i v e a l e r t , t h e energy
height l o s s across t h e event i s
reduced about 4 0 f e e t .
S t i l l greater
b e n e f i t s a r e a c h i e v a b l e with t h e use
of forward-look wind s h e a r d e t e c t i o n
and a l e r t i n g .
A l e r t s given only 15
t o 20 s e c o n d s p r i o r t o s h e a r e n t r y
would p e r m i t r e c o v e r y from r e l a t i v e l y
I n e a c h b a t c h r u n , t h e a i r p l a n e was
i n i t i a l i z e d on a t h r e e d e g r e e g l i d e
s l o p e , 4,000 f e e t from t h e c e n t e r of
t h e microburst.
The geometry of t h e
m i c r o b u r s t e n c o u n t e r was v a r i e d by
c h a n g i n g t h e i n i t i a l a l t i t u d e of t h e
a i r p l a n e . The i n i t i a l a l t i t u d e , wind
shear encounter a l t i t u d e , recovery
a l t i t u d e , and a l e r t t i m e f o r t h e s e
r u n s a r e shown i n t a b l e 1.
The
encounter a l t i t u d e i s t h e a l t i t u d e a t
which t h e wind s h e a r was d e t e c t e d and
t h e recovery i n i t i a t e d ,
and t h e
recovery a l t i t u d e i s t h e lowest
a l t i t u d e during t h e encounter.
The
i n i t i a l a l t i t u d e s v a r i e d f r o m 300
f e e t t o 700 f e e t above g r o u n d l e v e l
i n 100 f o o t i n c r e m e n t s and t h e a l e r t
t i m e v a r i e d from a 10 second f o r w a r d
l o o k t o a 1 0 second d e l a y i n 5 second
increments.
Of t h e f a c t o r s e x p l o r e d , t h e f a c t o r
that
produced
the
greatest
improvement i n r e c o v e r y a l t i t u d e was
the alert time.
The improvement i n
r e c o v e r y a l t i t u d e w i t h e a c h 5 seconds
improvement i n t h e a l e r t t i m e was
generally g r e a t e r than t h e difference
i n performance between
recovery
s t r a t e g i e s . Depending on t h e i n i t i a l
altitude,
t h e recovery a l t i t u d e
i n c r e a s e r a n g e d from 1 1 4 f e e t t o 498
f e e t when t h e a l e r t t i m e was improved
from -5 t o +10 s e c o n d s .
Figure 2
i l l u s t r a t e s t h e r e l a t i v e e f f e c t of
changing t h e a l e r t timing o r t h e
recovery
strategy.
The
three
s t r a t e g i e s a r e shown w i t h a 10 second
forward-look a l e r t and a 5 second
delay i n s i t u a l e r t .
The d i f f e r e n c e
i n r e c o v e r y a l t i t u d e s between t h e
recovery
strategies
are
small
compared t o t h e e f f e c t of improving
the alert.
A t h r e e f a c t o r ~ n a l y s i so f V a r i a n c e
(ANOVA) was c o n d u c t e d on t h e r e c o v e r y
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altitude data t o detect significant
d i f f e r e n c e s i n t h e t h r e e recovery
strategies,
two a l e r t t y p e s , and
three microburst scenarios.
The
average recovery a l t i t u d e s f o r t h e
v a r i o u s combinations of t h e t h r e e
f a c t o r s a r e shown i n t a b l e 2 and t h e
ANOVA r e s u l t s a r e shown i n t a b l e 3 .
The ANOVA i n d i c a t e s a d i f f e r e n c e i n
a l l t h r e e f a c t o r s , a t a 0.01 l e v e l of
significance.
Examination of t a b l e 2
shows, however, t h a t t h e d i f f e r e n c e
i n performance between t h e t h r e e
r e c o v e r y s t r a t e g i e s was a b o u t 20 f e e t
w h i l e t h e d i f f e r e n c e i n performance
between t h e a l e r t t y p e s was a b o u t 300
feet.
Hence, t h e r e c o v e r y a l t i t u d e
improvement p r o d u c e d by i m p r o v i n g t h e
a l e r t t i m e by 1 5 s e c o n d s was o v e r an
o r d e r of m a g n i t u d e g r e a t e r t h a n what
was a c h i e v e d by c h a n g i n g t h e r e c o v e r y
strategy.
The s t a t i s t i c s a l s o show i n t e r a c t i o n
between t h e a l e r t and s h e a r and
between t h e a l e r t a n d g u i d a n c e a t a
0 . 0 1 l e v e l of s i g n i f i c a n c e .
Table 2
shows t h a t t h e a l e r t t y p e p r o d u c e d a
s m a l l e r change i n recovery h e i g h t
w i t h t h e TASS model t h a n w i t h t h e
a n a l y t i c a l model.
The d a t a a l s o
shows t h a t v a r y i n g t h e g u i d a n c e
produced l a r g e r changes i n recovery
performance i n t h e r e a c t i v e a l e r t
c a s e t h a n i n t h e forward-look a l e r t
case.
F i g u r e 3 shows e x a m p l e s o f a i r c r a f t
t r a j e c t o r i e s during runs with t h e
a n a l y t i c a l model p o s i t i o n e d 1 , 6 0 0
f e e t from t h e runway.
T h i s geometry,
t h e a l e r t t i m i n g , and t h e recovery
s t r a t e g i e s a r e t h e same a s t h e b a t c h
runs
shown i n
f i g u r e 2.
The
c h a r a c t e r i s t i c s of t h e r e c o v e r i e s
a g r e e w e l l between t h e b a t c h and
real-time simulations.
I n each case,
the
manual
strategy
initially
produces a climb,
f o l l o w e d by a
d e s c e n t a t t h e s t i c k s h a k e r a n g l e of
attack.
The o t h e r s t r a t e g i e s p r o d u c e
a more l e v e l t r a j e c t o r y t h a t r e c o v e r
a t e s s e n t i a l l y t h e same a l t i t u d e .
The r e c o v e r i e s s u g g e s t t h a t r e c o v e r y
a l t i t u d e i s n o t t h e o n l y p a r a m e t e r of
interest
i n evaluating recovery
strategies.
The s t r a t e g i e s t h a t
maintained
a
lower
trajectory
produced h i g h e r minimum a i r s p e e d s and
less o r no t i m e a t t h e s t i c k s h a k e r
angle of a t t a c k , while producing
s i m i l a r recovery a l t i t u d e s a s t h e
manual s t r a t e g y .
The p i l o t s c o m p l e t e d q u e s t i o n n a i r e s
and p r o v i d e d comments on t h e a l e r t i n g
a n d g u i d a n c e . Of t h e s e v e n p i l o t s ,
s i x b e l i e v e d t h a t t h e t i m e l i n e s s of
t h e f o r w a r d - l o o k a l e r t was a b o u t
r i g h t , w h i l e one b e l i e v e d t h a t t h e
a l e r t was t o o l a t e .
No o n e b e l i e v e d
t h e a l e r t was t o o e a r l y .
When a s k e d
i f t h e 1 0 - s e c o n d a d v a n c e a l e r t was
s u f f i c i e n t t o make a normal go-around
p r o c e d u r e more a p p r o p r i a t e t h a n a
programmed e s c a p e maneuver, s i x o f
t h e p i l o t s s a i d yes.
The p i l o t s w e r e
a l s o asked i f a i r p l a n e configuration
should be h e l d c o n s t a n t d u r i n g escape
maneuvers w i t h t h e two a l e r t t y p e s .
In
the
case
of
reactive-only
a l e r t i n g , s i x of t h e p i l o t s i n d i c a t e d
t h e configuration should be constant.
I n t h e c a s e of f o r w a r d - l o o k a l e r t i n g ,
o n l y two of t h e p i l o t s b e l i e v e d i t
was a p p r o p r i a t e t o m a i n t a i n a f i x e d
configuration.
The p i l o t s w e r e a s k e d
t o r a t e t h e t h r e e recovery s t r a t e g i e s
i n o r d e r o f p r e f e r e n c e , w i t h "1"
a s s i g n e d t o t h e most
preferred
strategy.
The a v e r a g e s o f a l l t h e
p i l o t s was 1 . 3 f o r t h e f l i g h t p a t h
a n g l e s t r a t e g y , 2 . 1 f o r t h e manual
s t r a t e g y , and 2 . 6 f o r t h e g l i d e s l o p e
strategy.
Analytical
analysis,
batch
s i m u l a t i o n s , and p i l o t e d s i m u l a t i o n s
i n d i c a t e t h a t t h e f a c t o r which most
strongly
effects
a
microburst
r e c o v e r y i s t h e t i m e a t which t h e
recovery i s i n i t i a t e d .
In nearly a l l
microburst s i t u a t i o n s evaluated i n
batch simulations,
improving t h e
a l e r t t i m e by 5 s e c o n d s p r o v i d e d a
g r e a t e r recovery performance i n c r e a s e
t h a n c o u l d b e a c h i e v e d by c h a n g i n g
t h e recovery s t r a t e g y .
Forward-look
a l e r t s g i v e n 10 s e c o n d s p r i o r t o
microburst e n t r y permitted recoveries
t o b e made w i t h n e g l i g i b l e a l t i t u d e
loss.
In p i l o t e d simulations, t h e
average recovery a l t i t u d e only v a r i e d
about 20 feet between the recovery
strategies tested. In contrast, the
average recovery altitude varied
nearly 300 feet between the two alert
times tested (-5 seconds and +10
seconds .
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References
Miele, A. ; Wang, T.; and Melvin,
W.W.:
"Maximum
Survival
Capability of an Aircraft in a
Severe
Windshear,"
Rice
University, Aero-Astronautics
Report No. 213, 1986.
Miele, A. ; Wang, T.; and Melvin,
W.W. :
"Gamma Guidance Schemes
and Piloting Implications for
Flight in a Windshear," Rice
University, Aero-Ast ronautics
Report No. 212, 1986.
Hinton,
David
A.:"FlightManagement Strategies for Escape
From Microburst Encounters,"
NASA TM-4057, 1988.
"PilotedHinton, David A. :
Simulation
Evaluation
of
Recovery Guidance for Microburst
Wind Shear Encounters," NASA TP2886,~0T/F~~/Ds-89/06,
1989.
"A Simple,
Oseguera, Rosa M. :
Analytical
3-Dimensional
Downburst
Model
Based
on
Boundary Layer Stagnation Flow,"
NASA TM-100632, 1988.
Boeing Co. :
"Wind Shear
Training Aid.
Volume
1 Overview Pilot Guide, Training
Program", Contract DFTAO-1-86-C00005, Feb. 1987. (Available
from NTIS as PB88 127 196.)
Miele, A.; Wang, T.; Tzeng, C.
Y ; and Melvin, W. W. : "Optimal
Abort Landing Trajectories in
the Presence of Windshear," Rice
university, Aero-Astronautics
Report No. 215, 1987.
.
"The Terminal
Proctor, F. H. :
Area Simulation System, Volume
I:
Theoretical Formulation,"
NASA CR-4046, 1987.
9.
Proctor, F. H.:
"The ~erminal
Area Simulation System, Volume
11:
Verification Cases," NASA
Table 1
Note:
N u m b e r s i n p a r e n t h e s i s i n d i c a t e t h e w i n d shear e n c o u n t e r a l t i t u d e f o r a
p a r t i c u l a r i n i t i a l a l t i t u d e and a l e r t t i m e .
INITIAL
ALTITUDE
( f e e t)
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B a t c h Run R e c o v e r y A l t i t u d e s
STRATEGY
ALERT T I M E
-5
MANUAL
F L I G H T PATH
GLIDE SLOPE
MANUAL
F L I G H T PATH
GLIDE SLOPE
MANUAL
F L I G H T PATH
GLIDE SLOPE
MANUAL
F L I G H T PATH
GLIDE SLOPE
MANUAL
F L I G H T PATH
GLIDE SLOPE
0
(Seconds)
+5
Table 2
Average Recovery Altitudes in Piloted Runs
Recovery Strategy
-
Manual
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Alert >
Reactive
Flight Path
Angle
Forward
Reactive
Glide Slope
Forward
Forward
Shear
Scenario
TASS
Analytical
at 1640 ft
Analytical
at 3600 ft
All Shears
All Runs
This
Strategy
All Reactive Alerts:
Table 3
100 ft.
All Forward Look Alerts:
Analysis of Variance of Recovery Altitudes in Piloted Simulation
Source of
Variation
Sum of
Squares
Degrees of
Freedom
Mean
Square
Shear
Alert
Strategy
Interactions:
Shear/Alert
Shear/Strategy
Alert /Strategy
All Factors
Error
398 ft.
805824.0
360
2238.4
Computed Critical F
f
at Level of
Significance
0.05
0.01
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-20
--+FORWARD
W
40
20
0
REACTIVE-
LOOK
ALERT TIME, sec.
Fig. 1
Effect of Alert Time on Airplane Energy Height Change
During a Wind Shear Encounter
FORWARD LOOK
ALERT
,
01
30
40
50
60
70
80
90
100
TIME, sec.
Fig. 2
Effect of Forward-Look Capability and Recovery Strategy
Variation in Batch Simulation
-
MICROBURST
?
i
600
w
c3 400 3
-TRAINING
-----FLIGHT
200
-
AID
PATH ANGLE
--GLIDESLOPE
r
0
20000
15000
10000
5000
0
5000
DISTANCE FROM RUNWAY, ft.
Fig. 3
Effect of Forward-Look Capability and Recovery Strategy
Variation in Piloted Simulation
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