Atmospheric Hazards to FlightRobert Stengel,

Aircraft Flight Dynamics, MAE 331, 2010

! Microbursts

! Wind Rotors

! Wake Vortices

! Clear AirTurbulence

Copyright 2010 by Robert Stengel. All rights reserved. For educational use only.http://www.princeton.edu/~stengel/MAE331.html

http://www.princeton.edu/~stengel/FlightDynamics.html

Frames of Reference

! Inertial Frames! Earth-Relative

! Wind-Relative (Constant Wind)

! Non-Inertial Frames! Body-Relative

! Wind-Relative (Varying Wind)

Alternative Reference Framesfor Translational Dynamics

! Earth-relative velocity in earth-fixed polar coordinates:

vE=

VE

!

"

#

$

%%%

&

'

(((

vE=

VE

!E

"E

#

$

%%%

&

'

(((

! Earth-relative velocity inaircraft-fixed polar coordinates(zero wind):

! Body-frame air-mass-relativevelocity:

! Airspeed, sideslip angle, angleof attack

vA=

u ! uw( )

v ! vw( )

w ! ww( )

"

#

$$$$

%

&

''''

=

uA

vA

wA

"

#

$$$

%

&

'''

VA

!A

"A

#

$

%%%

&

'

(((=

uA

2+ v

A

2+ w

A

2

sin)1vA/V

A( )

tan)1wA/V

A( )

#

$

%%%%

&

'

((((

!rI= H

B

IvB

!vB=1

mFBvA( ) +H I

BgI! ""

BvB

!! = L

B

I"

B

!!B= I

B

"1M

BvA( ) " "! B

IB!

B#$ %&

! Rate of change ofTranslational Position

! Rate of change of AngularPosition

! Rate of change ofTranslational Velocity

! Rate of change ofAngular Velocity

Rigid-Body Equations of Motion

! Aerodynamic forces and moments depend on air-relative velocityvector, not the earth-relative velocity vector

Wind Shear Distributions Exert Momentson Aircraft Through Damping Derivatives

! 3-dimensional wind field changes in space and time

wEx,t( ) =

wxx, y, z,t( )

wyx, y, z,t( )

wzx, y, z,t( )

!

"

####

$

%

&&&&E

! Gradient of wind produces different relative airspeedsover the surface of an aircraft

! Wind gradient expressed in body axes

!Clshear" Clpwing

#w

#y$ Clpfin

#v

#x

!Cmshear" Cmqwing ,body ,stab

#w

#x

!Cnshear" Cnrfin ,body

#v

#x

WB= H

E

BW

EH

B

E

WE=

!wx!x !w

x!y !w

x!z

!wy!x !w

y!y !w

y!z

!wz!x !w

z!y !w

z!z

"

#

$$$$

%

&

''''

Aircraft Modes of Motion

! Longitudinal Motions

! Lateral-Directional Motions

!Lon(s) = s

2+ 2"#

ns+ #

n

2( )Ph

s2

+ 2"#ns+ #

n

2( )SP

!LD(s) = s" #

S( ) s" #R( ) s2 + 2$%ns+ %

n

2( )DR

! Wind inputs thatresonate with modes ofmotion are especiallyhazardous

Natural frequency : !n , rad / s

Natural Period : Tn =2"

!n

, sec

Natural Wavelength : Ln = VN Tp , m

Pitch Angle and Normal VelocityFrequency Response to Axial Wind

! !"# j$( )

"Vwind j$( )

! VN!" j#( )

!Vwind j#( )

MacRuer, Ashkenas, and Graham, 1973

! Pitch angle resonance at phugoid natural frequency

! Normal velocity (~ angle of attack) resonance at phugoid andshort period natural frequencies

Pitch Angle and Normal VelocityFrequency Response to Vertical Wind

MacRuer, Ashkenas, and Graham, 1973

! Pitch angle resonance at phugoid and short period natural frequencies

! Normal velocity (~ angle of attack) resonance at short period natural frequency

!!" j#( )

VN!$wind j#( )

=!" j#( )

!"wind j#( )

Sideslip and Roll Angle FrequencyResponse to Vortical Wind

=!" j#( )

!pwind j#( )

=!" j#( )

!pwind j#( )

MacRuer, Ashkenas, and Graham, 1973

! Sideslip angle resonance at Dutch roll natural frequency

! Roll angle is integral of vortical wind input

Sideslip and Roll AngleFrequency Response to Side Wind

MacRuer, Ashkenas, and Graham, 1973

! Sideslip and roll angle resonance at Dutch roll natural frequency

!" j#( )

!"wind j#( )

=!" j#( )

!$wind j#( )

Microbursts

1/2-3-km-wide“Jet”impinges on surface

High-speed outflowfrom jet core

Ring vortexforms inoutlow

Outflow strong enough toknock down trees

The Insidious Nature ofMicroburst Encounter

! The wavelength of the phugoid mode and the disturbanceinput are comparable

DELTA 191 (Lockheed L-1011)

http://www.youtube.com/watch?v=BxxxevZ0IbQ&NR=1

Importance of Proper Responseto Microburst Encounter

! FAA Windshear Training Aid, 1987,addresses proper operatingprocedures for suspected windshear

! Stormy evening July 2, 1994

! USAir Flight 1016, Douglas DC-9,Charlotte

! Windshear alert issued as 1016 begandescent along glideslope

! DC-9 encountered 61-kt windshear,executed missed approach

! Plane continued to descend, strikingtrees and telephone poles beforeimpact

! Go-around procedure was beguncorrectly -- aircraft's nose rotated up -- but power was not advanced

! That, together with increasingtailwind, caused the aircraft to stall

! Crew lowered nose to eliminate stall,but descent rate increased, causingground impact

Optimal Flight PathThrough Worst JAWS Profile

! Graduate research of Mark Psiaki

! Joint Aviation Weather Study (JAWS)measurements of microbursts (ColoradoHigh Plains, 1983)

! Negligible deviation from intended pathusing available controllability

! Aircraft has sufficient performancemargins to stay on the flight path

Downdraft

Headwind

Airspeed

Angle of Attack

Pitch Angle

Throttle Setting

Optimal and 15° PitchAngle Recovery during

Microburst Encounter

! Graduate Research of Sandeep Mulgund

! Airspeed vs. Time! Altitude vs. Time

! Angle of Attack vs. Time

Encounteringoutflow

Rapid arrestof descent

Nonlinear-Inverse-Dynamic Control

! Nonlinear system with additive control:

! Output vector:

! Differentiate output until control appears ineach element of the derivative output:

! Inverting control law:

!x t( ) = f x t( )!" #$ +G x t( )!" #$u t( )

y t( ) = h x t( )!" #$

y

d( )t( ) = f * x t( )!" #$ +G * x t( )!" #$u t( ) ! v t( )

u t( ) = G * x t( )!" #$ vcommand % f * x t( )!" #$!" #$

Landing Abort using Nonlinear-Inverse-Dynamic Control

! from Mulgund

400

500

600

700

800

900

1 103

-7500 -5000 -2500 0 2500 5000 7500

Umax

= 60 ft/sec

Umax

= 70 ft/sec

Umax

= 80 ft/sec

Umax

= 90 ft/sec

Umax

= 100 ft/sec

Umax

= 110 ft/sec

Altitude (

ft)

Range (ft)

100

150

200

250

300

350

-7500 -5000 -2500 0 2500 5000 7500

Umax

= 60 ft/sec

Umax

= 70 ft/sec

Umax

= 80 ft/sec

Umax

= 90 ft/sec

Umax

= 100 ft/sec

Umax

= 110 ft/sec

Airspeed (

ft/s

ec)

Range (ft)

-5

0

5

10

15

-7500 -5000 -2500 0 2500 5000 7500

Umax

= 60 ft/sec

Umax

= 70 ft/sec

Umax

= 80 ft/sec

Umax

= 90 ft/sec

Umax

= 100 ft/sec

Umax

= 110 ft/sec

Alp

ha

(d

eg

)

Range (ft)

Angle of Attack Limit

Wind Shear Safety Advisor

! Graduate research of Alexander Stratton

! LISP-based expert system

ON-BOARD DATA

Reactive sensorsWeather radarForward-lookingLightning sensorsFuture products

AIRCRAFT

SYSTEMS

CREW

Interface

ADVISORYSYSTEM LOGIC

GROUND-BASEDDATA

LLWASTDWRPIREPSForecastsWeather dataFuture products

Estimating the Probability ofHazardous Microburst Encounter

! Bayesian Belief Network! Infer probability of hazardous

encounter from

• pilot/control towerreports

• measurements

• location

• time of day

Wind Rotors

Aircraft Encounters witha Wind Rotor

Radius, ft

TangentialVelocity,

ft/s

! Tangential velocity vs. radius forLamb-Oseen Vortex

Geometry and Flight Condition of JetTransport Encounters with Wind Rotor

! Graduate researchof Darin Spilman

! Flight Condition! True Airspeed = 160 kt

! Altitude = 1000 ft AGL

! Flight Path Angle = -3˚

! Weight = 76,000 lb

! Flaps = 30˚

! Open-Loop Control

! Wind Rotor! Maximum Tangential

Velocity = 125 ft/s

! Core Radius = 200 ft

a) co-axial, ! = 0

b) ! ! 0

35

35

vortex

vortex

wind

!

35

Typical Flight Paths inWind Rotor Encounter

! from Spilman

-50

0

50

100

150

200

250

! [

deg

]

0 5 10 15 20 25 30Time [sec]

"i = 0

"i = 60˚

-60

-50

-40

-30

-20

-10

0

! [

deg

]

10

20

0 5 10 15 20Time [sec]

25 30

"i = 0˚

"i = 60˚

-300

-200

-100

0

100

200

300

-400 -300

h [

ft]

-200 -100 0 100 200 300y [ft]

400

rotor core radius

flight path

initial entry point

Linear-Quadratic/Proportional-Integral Filter (LQ/PIF) Regulator

LQ/PIF Regulation ofWind Rotor Encounter

! from Spilman

-50

0

50

100

150

200

! [

deg

]

0 2 4 6 8 10

Time [sec]

LQR-PIF controlno control input

0

200

400

600

800

1000

1200

h (

AG

L)

[ft]

0 2 4 6 8 10

LQR-PIF control

Time [sec]

no control input

Wake Vortices

C-5A Wing Tip Vortex Flight Test

http://www.dfrc.nasa.gov/gallery/movie/C-5A/480x/EM-0085-01.mov

L-1011 Wing Tip Vortex Flight Test

http://www.dfrc.nasa.gov/gallery/movie/L-1011/480x/EM-0085-01.mov

Models of Single and DualWake Vortices

Wake Vortex Descentand Downwash

Wake Vortex Descent andEffect of Crosswind

! from FAA Wake Turbulence Training Aid, 1995

Magnitude and Decay ofB-757 Wake Vortex

! from Richard Page et al, FAA Technical Center

Aircraft as Wake VortexGenerators and Receivers

! Vorticity, !, generated by lift in 1-g flight

! =KgeneratorW

"VNb

! Rolling acceleration response to vortexaligned with the aircraft's longitudinal axis

Kgenerator

!4

!

Kreceiver

!

CL!

2"VNb

!p =

Kreceiver

1

2!VN

2Sb

Ixx"

Rolling Response vs. Vortex-Generating Strength for 125 Aircraft

! Undergraduate summer project of James Nichols

FAA Wake Vortex Flight Test andNTSB Simulation of US Air 427

! B-737 behind B-727 in FAA flight test

! Control actions subsequent to wake vortexencounter may be problematical

! US427 rudder known to be hard-over from DFDR

NTSB Simulation of American 587

! Flight simulation obtained from digital flight data recorder (DFDR)tape

Digital Flight Data RecorderData for American 587

American 587 (Airbus A-300-600)Encounter with B-747 Wake

! PIO and/oraggressive use ofrudder seen aspossible cause

! Aviation Daily,5/22/02! Boeing Issues Detailed

Guidance On RudderUse For Roll Control

Causes ofClear Air Turbulence

! from Bedard

DC-10 Encounter with Vortex-Induced Clear Air Turbulence

! from Parks, Bach, Wingrove, and Mehta

DC-8 and B-52H Encounterswith Clear Air Turbulence

! DC-8: One engine and 12 ft ofwing missing after CATencounter over Rockies

! Specially instrumented for airturbulence research after someoperational B-52s were lost

! Vertical tail lost after a severeand sustained burst (+5 sec) ofclear air turbulence violentlybuffeted the aircraft

! The Boeing test crew flewaircraft to Blytheville AFB,Arkansas and landed safely

Conclusions

! Critical role of decision-making, alerting, andintelligence

! Reliance on human factors and counter-intuitive strategies

! Need to review certification procedures

! Opportunity to reduce hazard through flightcontrol system design! Disturbance rejection

! Failure Accommodation

! Importance of Eternal vigilance