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Background
When an aircraft wing generates
lift, it also produces horizontal, tornado-like
vortices that create a potential wake-vortex hazard
problem for other aircraft trailing. The powerful,
high-velocity airflows contained in the wake behind
the generating aircraft are long-lived, invisible,
and a serious threat to aircraft encountering the
system, especially small general aviation aircraft.
Immediately behind the wake-generating aircraft
is a region of wake turbulence known as the roll-up
region, where the character of the wake that is
shed from individual components (wingtips, flaps,
landing gear, etc.) is changing rapidly with distance
because of self-induced distortions. Farther away
from the generating aircraft is an area of the
wake known as the plateau region, where the vortices
have merged and/or attained a nearly constant structure.
Even farther downstream from the generating aircraft
is a wake area known as the decay region, where
substantial diffusion and decay of the vortices
occur due to viscous and turbulence effects. Depending
on the relative flight path of a trailing aircraft
in the wake-vortex system, extreme excursions in
rolling motion, rate of climb, or even structural
load factors may be experienced during an encounter
with the wake. If the encounter occurs at low altitudes,
especially during the landing approach, loss of
control and ground impact may occur. The severity
of this wake-vortex hazard is mainly dependent
on the size, geometry, and operating conditions
of the generating and trailing aircraft; the distance
between the two aircraft; the angle and altitude
of the encounter; and local atmospheric conditions
that influence the position, strength, merging,
and decay of the vortices. In general, a pair of
vortices drift downward with time behind the generating
aircraft, and the strategy recommended to the pilot
for avoiding vortex encounters is for the trailing
aircraft to fly at altitudes equal to or above
that of the flight path of the preceding aircraft.
However, on many occasions (particularly near the
ground), the vortices may persist at the generated
altitude or even rise to a slightly higher altitude
because of atmospheric conditions. If the vortices
reach the ground, they typically move outward from
the aircraft at a speed of about 2 to 3 knots in
calm-wind conditions. However, if there is an ambient
wind, then the net movement of the vortices is
the sum of the ambient wind velocity and the no-wind
motion of each vortex. A light crosswind can cause
one vortex to remain nearly stationary over the
runway, which will continue to pose a threat to
the landing aircraft. Finally, operations from
parallel runways with less than 2,500 ft separation
require alertness for crosswinds that may push
vortices onto the active runway. Because of these
complex factors, the fundamental behavior of wake
vortices and their avoidance have been especially
challenging problems to the aviation community
since the earliest days of flight.
Under visual flight rules
(VFR) operations, the responsibility for aircraft
separation distances may be given to the pilot
during the approach phase. In this situation, the
primary constraint on following distance is usually
the time interval for the leading aircraft to clear
the runway prior to the landing of the following
aircraft. However, under instrument flight rules
(IFR) conditions, air traffic control has direct
responsibility for separation according to FAA-mandated
standards that are a function of the weight classifications
of the leading and trailing aircraft. A more complete
discussion of the separation standards is included
in a later section. An analysis of aviation accidents
indicates that probable vortex-related accidents
constitute a relatively small percentage of all
single aircraft accidents and that the vortex safety
problem has been largely confined to general aviation
aircraft (including business jets) operating under
VFR conditions. In addition, the most frequent
cause of vortex-related accidents involves an aircraft
landing behind another aircraft on the same runway;
the takeoff condition has been virtually free of
vortex accidents. Perhaps the most important observation
is that no accidents under IFR conditions have
happened when full FAA separations were provided
between aircraft. Prime reasons for the extremely
small accident rate due to wake-vortex encounters
are the IFR separation standards and the increasing
awareness of the wake vortex problem on the part
of operational personnel for both VFR and IFR conditions.
Sketch of wake characteristics
behind generating aircraft.
Different vortex hazards
created by relative flight
paths of generating and trailing aircraft.
Sequence of photographs
of Boeing 747 on landing approach as industrial
smoke
dramatically defines one of trailing vortices.
Photo ©Bob Stoyles.
Wake-vortex separation requirements
under instrument flight
rules conditions aggravate airport capacity problem.
Although separations standards
have proven to be effective from a safety point
of view, they are frequently well in excess of
the spacing required (due to weather conditions
that rapidly decay or drift the wakes) and, as
a result, significantly reduce airport operating
capacities and impose costly delays affecting the
airlines and the general public. With the projected
accelerated growth in air traffic operating from
essentially the same number of airports in the
future, these penalties will become extremely large.
Some quantitative measure of the penalties imposed
by wake-vortex separation distances can be obtained
by considering an example case of a runway operating
with a 3-nmi IFR radar spacing and a full capacity
of 30 operations/hr. If the separation between
aircraft using the runway is increased to 5 nmi,
then capacity would be cut by a third. If the separation
is increased to 7 nmi, capacity would be cut in
half. Obviously, the operating costs to airlines
and passengers are severely impacted.
In 1970 NASA Marshall Space
Flight Center conducted flyby studies of vortex-wake
behavior with smoke
ejected from tower entrained into vortex to permit
visualization of character and persistence of vortex.
Thus, strong incentives exist
to remove wake-vortex turbulence as an impediment
to air traffic operations at and around airports
while retaining current levels of safety. Historically,
two approaches have been used by researchers in
attempts to mitigate the vortex problem. One approach
has been to attempt to alter the aerodynamic vortex
pattern shed by the generating aircraft so that
its effects on other aircraft would be minimal.
The second approach has been to develop and install
wake-vortex detection and avoidance systems that
would increase runway capacity by varying the separation
distance to conform to the aircraft and the meteorological
conditions present. A complete vortex separation
system of this type must consider many factors,
such as the detection or prediction of the presence
and strength of the vortices at a given time; an
evaluation of the threat on the basis of vortex
strength, location, and trailing aircraft characteristics;
and the determination of the proper hazard avoidance
action.
Langley Research and Development
Activities
The NASA Langley Research
Center has actively pursued research to mitigate
the wake-vortex hazard for over 45 years, beginning
with flight tests in 1955 led by Christopher C.
Kraft, Jr., to measure the wake-velocity characteristics
of a P-51 aircraft. In 1968, Langley participated
in a brief exploratory program to probe the vortices
of an FAA Convair 880 transport using a T-33 aircraft.
After the emergence of the first jumbo-sized transports,
the FAA requested NASA’s assistance in 1969
to determine the wake characteristics of large
aircraft; this resulted in flight tests of a B-52
and C-5 aircraft by the Dryden Flight Research
Center. Impressed with the potential wake hazard
of these large aircraft, the FAA issued (January
1970) interim IFR separation standards with a minimum
trailing distance of 10 miles for aircraft behind
the C-5 or Boeing 747. After additional NASA and
FAA flights of the C-5, Boeing flights of the 747
and 720, and flyby studies by the FAA, the FAA
revised the separation standards to 5 mi behind
aircraft with gross takeoff weights of over 300,000
lb. At the same time, Langley researchers Harry
A. Verstynen, Jr., and R. Earl Dunham, Jr., participated
in flight tests using a T-33 to measure the velocity
profile of the wake of a C-5 aircraft. Perhaps
the most interesting result of the study was the
fact that, under the atmospheric conditions of
the test site (Wright Patterson Air Force Base),
the vortices often could be found above the flight
path of the C-5. This early preview of the powerful
influence of meteorological conditions on vortex
behavior highlighted one of the most complex factors
associated with the vortex hazard.
The focus of Langley’s
research in the early 1970s was to alter and minimize
the aerodynamic wake-vortex characteristics of
generating aircraft. In the 1980s, the studies
were redirected to emphasize the fundamental character
of wake-vortex phenomena and studies of the physics
involved in the formulation of separation standards.
The most recent focus in the 1990s has been the
integration of advanced meteorological and vortex
hazard technologies and sensors to permit the development
of an integrated system for reduced separation
for increased capacity. One of the most significant
contributions by Langley researchers and their
partners to the national air transportation system
of the 1990s was joint activities with the FAA,
which led to the quantification and development
of separation standards and an emerging automated
approach to spacing requirements.
NASA Wake-Vortex-Alleviation
Program
In the summer of 1972, faced
with what was a growing concern over the potential
impact of large aircraft on the safety of small
aircraft operations and the explosive growth of
air traffic, the FAA requested NASA’s help
to develop technology and design information that
might be used to alter the aerodynamic characteristics
of the vortex pattern of generating aircraft so
that the intensity of wake-vortex encounters might
be minimized. At the same time, the FAA (with some
help from NASA) focused on the development and
installation of wake-vortex detection and avoidance
sensors and systems at airports. NASA responded
to the FAA’s request, and research activities
began immediately at the Langley Research Center,
the Ames Research Center, and the Dryden Flight
Research Center. The coordinated research program
performed at the centers investigated the effectiveness
of a myriad of aerodynamic schemes such as spoilers,
vortex generators, wingtip vortex-attenuating devices,
steady and pulsed mass injection, oscillating control
inputs, and span load variations. The effectiveness
of the schemes was assessed by measurements on
trailing aircraft configurations and visualized
with various flow visualization techniques. Tests
were completed in several unique NASA facilities
and several contractor water channels, as well
as actual aircraft flight tests. A complete discussion
of these extensive studies far exceeds the intended
scope of this publication. Thus, the included information
is restricted to only highlights of the program,
and the reader is referred to the bibliography
for sources of more detailed information.
At Langley, Joseph W. Stickle
led the Center’s Wake-Vortex-Alleviation
Program, with several teams conducting the effort
by using various ground-based facilities and aircraft
flight tests. In addition to a broad experimental
research program, analytical studies of vortex
viscous effects and interactions were conducted
by industry and academia under Langley contracts.
Noteworthy contributions in the analytical effort
were contributed by Alan J. Bilanin and Coleman
duP. Donaldson of Aeronautical Research Associates
of Princeton, Inc.
Facilities
An important initial task
of Langley’s research program on wake-vortex
alleviation was to assess the many devices and
concepts that had been proposed to aerodynamically
alter vortex formation and decay. In accomplishing
this objective, Langley had to develop facility
test capabilities that could permit studies of
the characteristics of the wake-vortex system from
the point of generation to locations far downstream,
representing in scale dimensions the downstream
distances of interest for trailing aircraft. The
testing problems, measurement techniques, and scaling
of Reynolds number and viscosity effects were predominant
issues in the program. Langley’s ground-based
facilities included the Langley 14- by 22-Foot
Tunnel (formerly the Langley V/STOL Tunnel and
the Langley 4- by 7-Meter Tunnel) and the Langley
Vortex Research Facility (VRF), which was a new
facility derived from an inactive NACA towing basin.
The 1,800-ft long, water-filled towing basin had
been modified with a new overhead carriage system
to propel models, whereas observations and measurements
of the wake characteristics of the passing model
were made at a fixed observation position. In conjunction
with complementary facility tests at Ames and the
Tracor Hydronautics Ship Model Basin at Laurel,
Maryland, these Langley facilities carried the
load of Langley’s ground tests. Researchers
at Ames and Langley agreed to use similar 0.03-scale
models of the Boeing 747 for common representation
of a generating aircraft, and trailing aircraft
models used in the studies ranged from simple wing
models to representative business jet aircraft.
Other configurations of interest including the
Lockheed L-1011 and the Douglas DC-10 were also
tested.
Sketch of Langley Vortex
Research Facility.
Model of Boeing 747 during
wake-vortex testing in Langley 14- by 22-Foot Tunnel
with traversing rig mounted downstream to permit
measurements of trailing wake.
In the 14- by 22-Foot Tunnel,
the test setup consisted of a generating transport
model mounted to a static force test sting-strut
apparatus in the tunnel test section, and the trailing
model was mounted on a special strut-traverse mechanism
that could be mounted at various distances downstream
in the tunnel test section or farther downstream
in the tunnel diffuser section. In this approach,
the model could be remotely moved about 6 ft laterally
and vertically to permitting researchers to probe
the strength of the trailing vortices shed by the
generating model. With the use of flow visualization
techniques, such as smoke and neutrally buoyant
hydrogen soap bubbles, positioning the model into
specific areas of the wake-vortex pattern behind
the generating model was possible. The magnitude
of rolling moment imposed on the trailing model
by the generating model was measured with a strain-gauge
balance and analyzed for various generator configurations.
The Langley Vortex Research
Facility was a unique approach to wake-vortex research
in which the impact of the wake shed by a moving
aircraft model was measured on a moving trailing
model and observed and measured (by laser velocimetry)
at a fixed observation point in the ambient air
of the facility as time progressed following the
passage of the model. James C. Patterson, Jr.,
conceived and led the development and operational
research for the facility. A gasoline-powered automobile
carriage was mounted on an overhead track with
the vortex-generating model blade-mounted beneath
the carriage. The trailing model was also attached
to the carriage through a series of trailers, which
resulted in the trailing model being located at
a scale distance of about 1 mile downstream of
the generating model. After the carriage was launched,
the automotive drive system accelerated to a velocity
of about 100 fps, which was held constant by cruise
control throughout the length of the covered test
area. At the test position, inside the covered
area, smoke produced by vaporized kerosene was
deployed and entrained by the wake for flow visualization.
High-speed motion-picture cameras were used to
film the motion of vortices produced by the generating
model, and the aerodynamic forces experienced by
the model were recorded.
Highlights of Wake-Vortex-Alleviation
Research
The scope of Langley’s
Wake-Vortex-Alleviation Research Program included
ground-based subscale model tests, theoretical
studies, and full-scale aircraft flight tests to
identify concepts and techniques that would reduce
the rolling motion imparted to a smaller trailing
aircraft. The investigation of a particular concept
usually began with a preliminary evaluation through
flow visualization of the wake-vortex pattern,
with and without the vortex-alleviation concept.
If the initial flow visualization indicated a change
in the vortex structure, a quantitative assessment
of the effectiveness was undertaken, either through
detailed velocity measurements or through measurements
of the vortex-induced rolling moment imposed on
a trailing-wing model. Many in the international
research community believed it would be impossible
to alter the wake, whereas others believed that
the task could be easily accomplished.
Flow visualization of wake
of Boeing 747 model with inboard wing flaps deflected
in Langley Vortex
Research Facility. Time sequence shows model entering
test area and vortices shed by the flaps and
wingtips rotating around each other. As wingtip
vortices approach the ground they are displaced
laterally.
During the course of the
program, several concepts were identified that
altered the wake and reduced the upset on a trailing
aircraft. For example, the injection of turbulence
into the vortex field was found to alter the vortex
structure and cause premature aging and dissipation
of the trailing vortices. Turbulence from jet engines
also changed the vortex structure, as did an alteration
of the span-load distribution. The combined effects
of turbulence injection and span-load alteration
through the use of wing spoilers were also found
to be effective in altering the wake. Even oscillating
inputs to the wing control surfaces proved effective.
Langley researchers were
constantly challenged by the complexity of the
wake flow field for representative transports.
Many concepts that appeared to affect the wake
properties in the immediate roll-up area behind
the generating aircraft were found to have little
impact on the magnitude of roll upset at downstream
distances representative of the location of trailing
aircraft. Furthermore, it was found that numerous
interacting vortices were shed by the typical transport
in the landing configuration. For example, in addition
to the vortices expected at the wingtips, strong
vortices were also shed at the edges of wing trailing-edge
flaps, and aft fuselage. As a result of these types
of interactive vortex effects, some wingtip vortex
control concepts that were known to provide beneficial
effects for cruise drag (such as winglets) had
little or no effect on the wake vortex hazard when
the aircraft was in the flaps-down, landing approach
configuration.
As previously mentioned,
the program was closely coordinated such that common
concepts were cross tested in different facilities
for correlation and verification of effectiveness;
however, certain researchers focused on particular
concepts in their studies. An excellent summary
of the results of both successful and unsuccessful
vortex alleviation concepts by the Langley staff
is given in NASA SP-409, Wake Vortex Minimization
(see bibliography).
Research in the VRF was led
by James C. Patterson, Jr., with assistance from
by Frank L. Jordan, Jr. Both researchers had worked
under the supervision of Richard T. Whitcomb in
the Langley 8-Foot Transonic Pressure Tunnel, and
were familiar with Whitcomb’s interest in
controlling the wingtip vortex with drag-reduction
wingtip concepts, such as winglets. Patterson and
Jordan investigated a wide range of alleviation
concepts in the VRF, which included the potential
of utilizing the high-energy wake produced by the
large jet engines incorporated on wide-body transports
for vortex alleviation. Interest in this concept
was stimulated by earlier work that indicated forcing
a mass of air forward into a vortex would interrupt
the vortex axial flow (which provides the energy
that normally sustains the vortex long after its
generation). Because the jet exhaust wake of large
engines is a source of high energy, it was hypothesized
that when this energy was directed into the vortex,
the wake hazard might be mitigated. After considerable
research on engine location effects, thrust reversers,
and differential engine thrust, it was determined
that operating the outboard engines at maximum
thrust while operating the inboard engine thrust
reversers at idle thrust resulted in a significant
reduction in the roll upset of the trailing aircraft.
Although considered not practical from an operational
viewpoint, these positive results provided additional
incentive for further research.
Laser beams used to measure
air velocities at specific points in wake for Boeing
747 model in VRF.
Langley’s early flight
test activity in the aircraft wake-vortex minimization
program was led by Joseph W. Stickle, Earl C. Hastings,
Jr., and James C. Patterson, Jr. In one Langley
flight project led by Hastings, Robert E. Shanks,
and test pilot Robert A. Champine, a vortex-attenuating
device referred to as a “spline” was
developed from ground-based testing and assessed
during flight tests of a C-54 propeller-driven
transport. Early research leading to the spline
concept had been conducted by Patterson in the
VRF with a wing panel. It was proposed that an
unfavorable or positive pressure gradient applied
just downstream of the wingtip might force the
vortex to dissipate. In the initial testing, this
mechanism was verified by the brute-force approach
of utilizing a decelerating parachute at each wingtip.
As a more practical application of the idea, the
spline concept was tested and found to produce
the same vortex-attenuating effect as the decelerating
chute. The spline configuration was envisioned
to be retracted during cruise flight and deployed
only for landing, when the vortex hazard was greatest.
After extensive parameter variations in the VRF
and in-flight assessments with the C-54 as a generating
aircraft and a Piper PA-28 general aviation aircraft
as a probe aircraft, the researchers concluded
that the spline device was effective in reducing
the strength of the trailing vortex. When the PA-28
approached behind the basic C-54 (no splines),
the PA-28 could not approach closer than about
3 nmi before full aileron deflection was required
to prevent a severe roll off. For the C-54 with
splines, however, the PA-28 could be easily flown
to a separation distance of less than 1 nmi. The
vortex attenuation achieved in flight was greater
than that obtained in the VRF, and the researchers
anticipated that the installation of stowable spline
devices on commercial jet aircraft would avoid
the obvious penalties in climb capability in the
landing phase of flight and increased approach
noise due to increased power settings.
Langley’s evaluation
of spline device on C-54. Top photographs show
VRF results for basic (left) and modified model,
showing diffuse wake with spline. Lower photographs
show installation of splines on full-scale aircraft.
R. Earl Dunham, Jr., conducted
cooperative studies with Vernon J. Rossow at the
Ames Research Center on the effect of trailing-edge-flap
settings on wake alleviation. These far-ranging
tests included wind-tunnel studies at Ames and
Langley, water tow experiments, and full-scale
flight tests of a Boeing 747 at the Dryden Flight
Research Center. The motivation for this project
was to distribute the lift on the wing so that
the interactions of wake vortices would lead to
a very diffused wake. This approach was attractive
because of the potential ease of application and
minimal retrofit costs. The results of these extensive
investigations demonstrated that variations in
span-load distribution using flap deflections could
produce significant reductions in the wake-vortex
hazard. For example, an approximately 50-percent
reduction in both the wake rolling moment imposed
on a trailing aircraft and aircraft separation
requirement was achieved in ground-based and flight
experiments by deflecting the inboard trailing-edge
flaps more than the outboard flaps.
NASA conducting flight
assessments of wake-alleviation concepts
at Dryden with NASA Boeing 747, T-37, and Learjet
aircraft.
Langley’s Delwin R.
Croom led research activities in the Langley 14-
by 22-Foot Tunnel. Yet another concept that significantly
modified the wake of representative jet transport
configurations was developed by Croom and the intercenter
NASA team. The focus of Croom’s studies was
the use of wing spoilers, commonly used by jet
transports (for speed brakes and to decrease aerodynamic
lift after landing), as a possible method of vortex
attenuation, because the deflection of spoilers
will inject turbulence into the wake as well as
alter the span-load distribution. Croom had been
inspired by earlier investigations conducted at
Ames in 1970 that combined wind-tunnel and flight
investigations of the effects of wingtip-mounted
spoilers on the wake characteristics of a Convair
990 transport. The Ames experiment, however, ended
with reports from pilots of a Learjet probe aircraft
citing no differences in wake behavior between
the modified and unmodified transport. In 1971,
Langley initiated more detailed semispan wing studies
in the VRF to determine the proper location for
a spoiler to cause the largest alteration to the
trailing vortex. Testing then shifted to the 14-
by 22-Foot Tunnel and the Tracor Hydronautics Ship
Model Basin, with emphasis on the impact of deflecting
various combinations of the wing spoilers available
on the Boeing 747 configuration. Exploratory testing
by Croom in the 14- by 22-Foot Tunnel, which began
in March 1975, defined an effective spoiler configuration,
which had a spoiler deflection of about 30∞.
The effectiveness of the spoiler concept was verified
in the VRF and the Hydronautics facility. With
these promising results, a flight program using
a NASA Boeing 747 aircraft was initiated at Dryden.
In the flight program, NASA T-37 and Learjet aircraft
were used to penetrate the trailing vortex.
Tests without the wing spoilers
of the Boeing 747 deployed produced violent roll
upset problems for the T-37 aircraft at a distance
of approximately 3 miles. In one instance, the
wake of the 747 caused the T-37 to perform two
unplanned snap rolls and develop a roll rate of
200 deg/sec, despite trailing the jetliner by more
than 3 miles. Tests showed the rotational velocity
of the 747 wake vortex could exceed 240 km/hr and
persist for a distance of 30 km. With two spoilers
on the outer wing panels deflected, the T-37 could
fly within a distance of 3 miles and not experience
the upset problem. Although initial flight results
tended to verify the effectiveness of the spoiler
concept, additional flight tests indicated that
the effectiveness was sometimes not repeatable
(probably because of atmospheric conditions) at
low altitudes. Later, flight tests of a Lockheed
L-1011 at Dryden indicated considerably less effectiveness
of the spoiler concept. Finally, additional concerns
over unacceptable buffet characteristics produced
by the spoilers shelved further research on the
concept.
At the conclusion of the
NASA Wake-Vortex-Alleviation Program, Langley and
its intercenter partners had conducted extensive
ground-based and flight research for a myriad of
alleviation concepts. These concepts included altered
span loading, turbulence ingestion, mass ingestion,
oscillating controls, and combinations of these
approaches. Actual flight evaluations indicated
that several aerodynamic attenuation concepts were
effective and these concepts would probably be
operationally practical. However, in addition to
issues such as effects on buffet and aircraft controllability
and costs of retrofit, a significant obstacle to
the implementation of this technology remained
unconquered: the tremendous impact of variations
in meteorological conditions on wake persistence
and decay characteristics. Because of these appropriate
concerns, none of the promising wake-alleviation
concepts identified by the NASA research was incorporated
into civil aircraft of the 1990s. Nonetheless,
an immense increase in knowledge of the nature
and sensitivity of aircraft aerodynamic wake characteristics
was obtained and served as the fundamental building
block for future advanced approaches to the operational
prediction, detection, and avoidance of the wake
hazard. For an excellent summary of experiences
and the outlook for wake alleviation concepts,
the reader is referred to the excellent paper by
Vernon J. Rossow in the bibliography.
Vortex Characterization
As the funding and momentum
of the NASA Wake-Vortex-Alleviation Program were
dramatically reduced in the 1980s, the focus of
wake-vortex research turned away from aerodynamic
alleviation concepts. Instead, Langley researchers
directed their efforts toward more fundamental
studies of the impact of atmospheric conditions
on wake-vortex formation and decay.
During the 1980s, George
C. Greene, Dale R. Satran, G. Thomas Holbrook,
and others from Langley began to reassess the impact
of meteorological conditions on wake characteristics.
Critical changes in vortex position, strength,
and decay were analytically and experimentally
determined as a result of atmospheric density stratification
caused by temperature gradient effects and temperature
inversions. Dramatic demonstrations of the impact
of stratification in the VRF, together with earlier
FAA-sponsored observations of vortex-wake descent
and decay characteristics, and the experiences
of the Wake-Vortex-Alleviation Program during flight
tests at Dryden, provided new insight and sensitivity
to the powerful influences of real atmospheric
effects on the wake-vortex hazard problem.
Typical meandering of trailing
vortices due to local atmospheric effects.
Aircraft Separation Standards
Before 1970, radar operating
limits and, to a lesser extent, runway occupancy
restrictions dictated aircraft separation standards—no
regulatory aircraft separations were imposed because
of wake vortices. Separation requirements for IFR
conditions were established in 1970 after NASA,
the FAA, industry, and others conducted flight
tests to determine the wake-vortex characteristics
of existing jet aircraft. Until March 1976, separation
distances of 5 nmi were required for “nonheavy”
aircraft (less than 300,000 lb) trailing heavy
aircraft (greater than or equal to 300,000 lb),
and separations of 3 nmi for all other conditions.
In 1976, the distances were increased, with the
maximum being 6 nmi for a “small” aircraft
(less than 12,500 lb) trailing a heavy aircraft.
As the 1980s closed, new
concerns arose regarding the wake-vortex characteristics
of certain new transports and the spacing requirements
for them. In particular, a series of wake-related
accidents and incidents involving the Boeing 757
during landing approaches resulted in a concern
over the wake characteristics of this particular
transport. In one accident on December 18, 1992,
a Cessna Citation crashed while on a VFR approach
at the Billings Logan International Airport, Billings,
Montana. The two crew members and six passengers
were killed. Witnesses reported that the airplane
suddenly and rapidly rolled left and then contacted
the ground while in a near-vertical dive. Recorded
ATC radar data showed that, at the point of upset,
the Citation was about 2.8 nmi behind a Boeing
757 and on a flight path that was about 300 ft
below the flight path of the 757. Then, on December
15, 1993, an Israel Aircraft Industries Westwind,
operating at night, crashed while on a VFR approach
to the John Wayne Airport, Santa Ana, California.
The two crew members and three passengers were
killed. Once again, witnesses reported that the
airplane rolled abruptly and that the onset of
the event was sudden. The Westwind was about 2.1
nmi behind a Boeing 757 and on a flight path that
was about 400 ft below the flight path of the 757.
An additional accident, involving a Cessna 182
during VFR conditions, resulted in loss of the
aircraft but no fatalities. Additionally, significant
but recoverable losses of control occurred for
a McDonnell Douglas MD-88 and a Boeing 737 (both
required immediate and aggressive flight control
deflections by their flight crews) trailing Boeing
757 aircraft.
Although all the wake accidents
had occurred during visual conditions, when pilots
are responsible for wake turbulence avoidance,
the NTSB sent an urgent recommendation to the FAA
to increase the controller-imposed IFR landing
separation distances behind the Boeing 757 and
similar weight aircraft to 4 nmi from 3 nmi for
the 737, MD-80, and DC-9; to 5 nmi from 3 nmi for
aircraft such as the Westwind or Citation; and
to 6 nmi from 4 nmi for small airplanes. By June
1994, the FAA had accepted some of the NTSB recommendations,
and separation standards were modified August 1994.
Meanwhile, the aviation community initiated several
exercises to determine if the wake of the Boeing
757 was more hazardous than other transports.
Tower flyby tests of the
Boeing 757 and 767 had been conducted at the NOAA
vortex facility in Idaho Falls, Idaho, during 1990.
The NOAA results proved to be controversial because
they showed, for a peculiar set of weather conditions
that lasted about 0.5 hour, the vortex velocity
of the 757 was approximately 50 percent higher
than that of the 767 at similar vortex ages (younger
than 60 sec) measured in less favorable weather
conditions. However, the results also showed that
overall the wake of the 757 decayed faster than
that of the 767; in fact, the wake behaved as would
be expected for an aircraft of the size and weight
of the 757. However, the single unusual measurement
was widely quoted as showing that the 757 should
be treated as a heavy category aircraft like the
Boeing 747 and 767. Another factor cited as relevant
to the 757 accidents was the approach speed of
the aircraft (125 knots) is relatively slow, in
part because of its relatively low wing sweep and
large wing area. As a result, business jets (such
as those involved in the accidents) approach at
higher speeds with inadvertently close separation.
Current FAA standards for
aircraft separation during IFR conditions. Note
2.5-nmi
separation increased to 3 nmi when airport has
>50 sec runway occupancy time.
At the request of the FAA,
Langley’s Roland L. Bowles, George C. Greene,
and others participated in the analysis and deliberations
over the Boeing 757 wake characteristics. A wake
turbulence government and industry team, composed
of representatives from the FAA, NASA, air carriers,
pilots, air traffic controllers, and manufacturers,
provided the FAA with recommendations on how to
best separate aircraft to prevent wake turbulence
incidents and accidents. A key analysis providing
support to the reclassifications of weight was
performed by Bowles and George Washington University
(GWU) graduate student Chris Tatnall. Following
hotly debated analyses and discussion among various
agencies, industry, and the airlines, the FAA implemented
new aircraft separation standards on August 17,
1996, for all aircraft operating in the United
States under IFR conditions. Separation standards
for small aircraft traveling behind a Boeing 757
increased from 4 to 5 nmi, and 57 types of aircraft,
including several business jets and some smaller
commercial aircraft, were moved from the large
to small aircraft category. Specifically, the small
category was changed to less than 41,000 lb (previously
less than 12,500 lb); the large category, to 41,000
lb to 255,000 lb (previously 12,500 lb to 300,000
lb); and the heavy category, to 255,000 lb or more
(previously 300,000 lb or more).
Controversy over the new
standards existed, however, with objections to
the increased distances. Opponents of the new regulations
pointed out that all reported wake turbulence incidents
or accidents had occurred in VFR conditions. There
had been no reports of wake turbulence upset accidents
(757 related or otherwise) from aircraft operating
in IFR conditions; therefore, controversy still
exists as to whether the modifications to IFR separation
standards will prevent VFR accidents such as the
Billings and Santa Ana crashes.
Special instrumentation
carried by Langley OV-10 research
aircraft for wake characterization research flights.
As the separation issues
spawned by the 757 controversy became a high-level
concern, Langley researchers were stimulated to
examine the more general subject of the development
of a more scientific approach to determining separation
standards. Extensive studies were required to provide
the tools and understanding necessary to provide
confidence if the separation standards were to
be reduced for improved airport capacity. Efforts
in the NASA Terminal Area Productivity Project
within the NASA Advanced Subsonic Transport Program
provided the impetus and funds for these contributions.
Key capabilities in achieving the goals set for
the program were the definition of valid wake models,
analytical tools to examine the severity of encounters,
and the development of a high-fidelity simulation
model that could be used to develop wake vortex
encounter hazard criteria. Langley’s R. Earl
Dunham, Jr., Eric C. Stewart, Dan D. Vicroy, Robert
A. Stuever, and George C. Greene contributed significant
leadership during ground- and flight-testing activities
that provided the foundation for the development
of simulations (both piloted and unpiloted) for
wake-vortex encounter analysis. In addition to
this analytical work, a successful first-ever feasibility
study was conducted by Jay M. Brandon, Frank L.
Jordan, Jr., Catherine W. Buttrill, and Robert
A. Stuever to determine if free-flying models in
the Langley 30- by 60-Foot (Full-Scale) Tunnel
could be used in the analysis of the vortex hazard.
In 1995 and 1997, Langley
conducted flight tests of a modified North American
Rockwell OV-10 research aircraft behind a C-130
at the NASA Wallops Flight Facility to generate
a quantitative, detailed set of data on wake characteristics
for use in the validation of simulators and wake
prediction methods. A unique feature of this research
was that atmospheric data were obtained along with
the wake measurements. The OV-10, which was equipped
with special instrumentation for atmospheric measurements,
first probed the atmosphere and completed a “weather
profile” run before joining with the C-130
and probing the wake of the C-130. About 230 wake
penetrations at different atmospheric conditions
were accomplished; this provided valuable data
for wake characterization research.
Analytical studies of separation
effects on upset parameters were conducted by Stewart
and Stuever, as well as further improvements in
the state of the art for piloted simulator studies
of hazard criteria. The ultimate objective of this
effort was to permit the definition of hazardous
and nonhazardous levels of vortex encounters for
various aircraft types and atmospheric conditions
and pave the way for a predictive element that
might be used in a real-time ATC system. Unfortunately,
funding for the study was eliminated before the
final objective could be accomplished.
During the 1990s, the Langley
staff continued to participate in supporting national
issues and safety investigations involving wake-vortex
phenomena. After USAir Flight 427 (a Boeing 737)
plunged from the sky near Pittsburgh on September
8, 1994, killing 127 passengers and 5 crew members,
the NTSB frantically accelerated its efforts to
determine what might have triggered the 6,000-ft
nose dive. A bump (a sudden airspeed increase detected
by the plane’s flight-data recorder) indicated
that the 737 had encountered wake turbulence created
by a Delta 727 that preceded Flight 427 into the
Pittsburgh International Airport. Flight 427 trailed
the 727 by 4.1 nmi, well within the FAA regulation
that requires two planes of such weights to maintain
a separation of 3 nmi. As part of the investigation
to determine the potential impact of such an encounter,
the NTSB requested that Langley conduct flights
of its specially instrumented OV-10 and 737 research
aircraft trailing an FAA 727 generating aircraft.
Following the longest aviation accident investigation
in safety board history (4 years), the results
of this cooperative activity helped investigators
conclude that the vortex encounter might have been
the initiating mechanism resulting in a hardover
failure of the rudder actuator, which was determined
to be the primary cause of the accident.
Aircraft Vortex Spacing System
Program
In the 1990s, the NASA Terminal
Area Productivity (TAP) Program directed its resources
toward the extremely challenging objective of providing
the same levels of airport capacity during instrument
operations that are presently experienced during
visual airport operations. Within the elements
of the TAP Program, the Langley Research Center
was tasked to perform the research and development
required to devise an automated wake vortex spacing
system, known as the Aircraft Vortex Spacing System
(AVOSS). The AVOSS concept would use available
and emerging knowledge of aircraft wake generation,
atmospheric modification of the wakes, wake encounter
dynamics, and operational factors to provide dynamic
spacing criteria for use by Air Traffic Control
(ATC). When considering ambient weather conditions,
the wake separation distances between aircraft
could possibly be relaxed during appropriate periods
of airport operations. With an appropriate interface
to planned ATC automation, spacing could be tailored
to specific generating/trailing aircraft types
rather than the existing broad weight categories
of aircraft. The fundamental architecture for the
AVOSS system was first proposed by Roland L. Bowles
of Langley, who had played a key role in the initiation
and highly successful completion of the Langley
Wind-Shear Program. The lead researcher and project
manager for AVOSS was David A. Hinton, assisted
by Leonard Credeur and an extraordinary team that
included Fred H. Proctor and others that had made
many contributions.
The development of the AVOSS
concept built on past wake-vortex research conducted
by NASA, the FAA, the Volpe National Transportation
Systems Center, and industry. Advances in computational
fluid dynamics modeling, weather sensors, ATC automation,
and aircraft vortex behavior predictions had advanced
to the point that encouraged the implementation
of a practical AVOSS concept. The most critical
single element in the development of the system
was the accurate representation of vortex behavior
in the airport area, especially the effects of
meteorological characteristics. The AVOSS differed
substantially from previous efforts to characterize
wake vortex systems in that the atmospheric conditions
from the surface to the top of the instrument approach
path were measured and used rather than only surface
winds. This analysis is significant in situations
where temperature inversions or wind gradients
along the approach path may require greater spacing
intervals than would be predicted by surface winds
alone. Because metering of aircraft to meet airport
acceptance rates occurs during the vectoring and
descent process as aircraft enter the initial approach
area, the AVOSS system was required to provide
a predictive capability of 30 to 50 min in advance
of the actual approach to take full advantage of
reduced wake constraints.
The general AVOSS structure
designed by Langley includes a meteorological subsystem
that provides current and expected atmospheric
states to a predictor subsystem. The predictor
subsystem utilizes the atmospheric data, airport
configuration, and aircraft specifications to predict
the separation time required for a matrix of aircraft.
A sensor subsystem monitors actual wake-vortex
position and strength to provide feedback to the
predictor subsystem and to provide a warning. The
AVOSS as demonstrated did not interact with ATC
but actually ran in a “shadow mode.”
David A. Hinton’s conceptual
design for AVOSS included a broad perspective of
the research required in the areas of analytical
studies, wind-tunnel testing, field evaluations,
and flight tests. In the area of numeric wake vortex
modeling, Fred H. Proctor’s Terminal Area
Simulation System (TASS), which had proven highly
effective in the successfully completed NASA-FAA
windshear program, was modified to model the effects
of various atmospheric conditions of the behavior
of aircraft vortices. Crucial to the validation
of TASS, prediction algorithm development, and
full system testing and demonstration was a field
effort sponsored by Langley and conducted by the
MIT Lincoln Laboratory. This effort provided comprehensive
field capability to gather meteorological, aircraft,
and wake data at major airports. The Lincoln effort
established a facility at the Memphis International
Airport and in 1994 provided the most comprehensive
wake-vortex and weather data obtained to date with
approximately 600 aircraft wakes studied. Data
collection was also performed in 1995. During these
field measurements, the Langley OV-10 aircraft
participated by collecting atmospheric wind, temperature,
and humidity data along the approach path to answer
questions concerning the variability of critical
atmospheric parameters.
MIT Lincoln Laboratory
continuous wave lidar at Memphis for wake measurements
in 1994.
The AVOSS concept was originally
conceived to use two factors, singly or in combination,
for reducing aircraft spacing. These factors would
be wake-vortex motion out of a predefined approach
corridor and wake decay below a strength that is
operationally significant. Initial predictions
indicated that AVOSS technology has the potential
to reduce takeoff delays as well as increase single-runway
throughput by 10 percent or more during conditions
requiring instrument approaches.
An ambitious goal of demonstrating
the AVOSS concept at the Dallas-Fort Worth International
Airport (DFW) was set for 2000, and a proof-of-concept
prototype of AVOSS was installed there in 1997.
MIT Lincoln Laboratory and Langley set up an extensive
suite of meteorological sensors using two sodar
(sound detection and ranging) systems and a Doppler
radar profiler (to measure winds aloft), an instrumented
150-ft tower, and shorter towers to estimate the
required atmospheric profiles. In addition, algorithms
were developed for using the two FAA Terminal Doppler
Weather Radars (TDWR) in Dallas-Fort Worth as high-resolution
wind profilers and to combine the wind data from
the various sensors into a single wind profile.
One key instrumentation capability
developed under the Langley leadership of Ben C.
Barker, Jr., was a pulsed coherent lidar system.
The pulsed coherent lidar system was designed by
Coherent Technologies, Inc. (CTI), under a NASA
Small Business Innovation Research (SBIR) contract
to provide the necessary confirmation that actual
wake-vortex behavior agreed with predictions. The
transceiver uses a solid-state, eye-safe laser
beam that is expanded through a telescope and directed
to a hemispherical scanner that scans the beam
across the approach path. Light reflected from
microscopic particles in the air, and shifted in
frequency due to the swirling particle motion in
the vortex, is detected by the lidar transceiver.
These return signals are then analyzed to detect,
track, and measure strength of the wake vortices.
The milestone demonstration
of AVOSS at Dallas-Fort Worth International Airport
occurred July 18–20, 2000. As implemented
for the demonstration, the system provided spacing
values to separate aircraft from wake-vortex encounters
by defining a corridor of protected airspace, predicting
wake motion and decay at numerous locations along
the approach path for all aircraft, providing a
safe separation criteria for the entire approach,
and monitoring safety with a wake-vortex sensor.
AVOSS did not render any go- or go-around decisions
nor did it actually alter real spacing. AVOSS produced
recommended reductions in spacing, measured actual
wakes to compare with the predictions, and then
developed statistics to determine the effectiveness
and safety of the reduced spacing. This system
ran in real time with automated weather observations,
data quality assessments, and automated comparison
of predicted and measured wake behavior. Although
the data were not provided to Air Traffic Control
during the demonstration, a large audience of airport,
airline, and government officials were able to
watch AVOSS predictions and confirm lidar measurements
in real time.
Locations of AVOSS components
at
Dallas-Fort Worth International Airport.
Langley pilot Philip Brown
flies Thrush Commander over
smoke injection system for visualization of wake
of aircraft.
Key members of the weather
subteam included MIT Lincoln Laboratory, North
Carolina State University, Langley, and the National
Oceanic and Atmospheric Administration (NOAA);
the prediction subsystem team was composed of Langley,
NorthWest Research Associates, Inc., and the Naval
Post-Graduate School; and the wake detection subsystem
team was Langley, the Research Triangle Institute,
MIT Lincoln Laboratory, and the Volpe National
Transportation Systems Center.
Based on results of the DFW
experiments, the increase in calculated daily throughput
averaged 6 percent and ranged from 1 to 13 percent.
At DFW, a capacity increase of 6 percent means
6 additional planes that would normally face delays
would be allowed to land each hour. The average
throughput gain translates to a 15- to 40-percent
reduction in delay when applied to realistic capacity
ratios at major airports.
In May 2001, the AVOSS project
won the Administrator’s Award at NASA’s
Turning Goals Into Reality Conference, and the
Air Transport Association named AVOSS to its top
10 list of air traffic control improvements.
Other Wake-Vortex Activities
Another Langley aircraft
wake research activity of the late 1970s and early
1980s involved the potential alteration of the
wake of agricultural aircraft used in aerial applications
for improved efficiency (more uniform distribution
patterns) and reduced drift of potentially harmful
insecticides and herbicides. As part of a larger
NASA Aerial Applications Program that began in
1976, these aerodynamic studies included ground
testing in the Langley 30- by 60-Foot Tunnel and
the Vortex Research Facility (VRF), as well as
cooperative flight tests of an Ayres Thrush Commander
agricultural aircraft. In the 30- by 60-Foot Tunnel,
Frank L. Jordan, Jr., and H. Clyde McLemore conducted
extensive aerodynamic evaluations of the full-scale
Thrush Commander aircraft with various dispersal
systems installed. In addition to identifying performance-enhancing
airframe modifications, such as wing-fuselage fillets,
the characteristics of droplets spread by liquid
spray rigs (using water spray) were also determined
in the wind tunnel. In the VRF, the ability to
simulate the aerial dispersal of materials from
small-scale models and the development of numerical
methods to predict particle trajectories were demonstrated.
Exploratory tests of various wake control concepts,
including wingtip winglets, were also conducted.
In 1984, pilot Philip W.
Brown and researchers Dana J. Morris, Cynthia C.
Croom, and Bruce J. Holmes conducted flight tests
of the Thrush Commander aircraft at the NASA Wallops
Flight Facility to collect experimental data on
the wake characteristics of the aircraft and the
impact of aircraft modifications on particle deposition
patterns during representative aerial spraying
operations. The researchers developed theoretical
methods simultaneously with the experimental efforts
to simulate the dispersal of particles, including
the complex interaction of the dispersed particles
with the aircraft wake. The results of the study
indicated good agreement between the experimental
results and the theoretical predictions, and the
ability to change the wake characteristics to produce
desirable effects on deposition and drift characteristics
were demonstrated. The success of the study provided
fundamental information for aerial application
operators, and the theoretical method known as
AGDISP has been provided to designers and other
pertinent users such as the U.S. Forestry Service.
At the same time that researchers
were attempting to modify the wake-vortex characteristics
of aircraft to provide solutions to safety and
airport capacity issues, others were attempting
to control and harness the energy expended in the
formation of wingtip vortices in an effort to improve
aircraft performance. The pioneering efforts of
Langley’s Richard T. Whitcomb and his conceptual
development and maturation of wingtip-mounted winglets
is the most outstanding example of an application
of performance-enhancing control of wingtip vortices.
Whitcomb’s approach, however, stimulated
additional efforts to reduce aircraft-induced drag
through the use of wingtip devices. James C. Patterson,
Jr., worked for Whitcomb in both wake alleviation
research as well as drag-reduction efforts. Patterson
led efforts on research on two wingtip vortex control
concepts for drag reduction. In the first concept,
Patterson and Whitcomb explored the potential of
using wingtip-mounted jet engines to modify the
formation of the wingtip vortex in a manner beneficial
to reducing induced drag. The scope of the studies
included tests of powered semispan models in the
Langley 8-Foot Transonic Pressure Tunnel and other
facilities. Although significant reductions in
drag were measured in these experimental studies,
real-world concerns involving aircraft controllability
and other issues have limited the application of
this concept to date.
Another performance-enhancing
concept explored in ground testing and limited
flight testing by Patterson was the use of wingtip
turbines for reduced cruise drag or power extraction.
In this concept, multiblade turbines mounted at
each wingtip are either fixed in the swirling wingtip
vortex for induced drag reduction or allowed to
freewheel and rotate (driven by the wingtip vortex)
for the generation of electrical power for aircraft
systems. Patterson’s research on the tip
turbines included tests in several Langley wind
tunnels and limited flight tests using Langley’s
PA-28R research aircraft.
To complete this brief survey
on Langley contributions to wake-vortex technology,
it should be pointed out that Langley researchers
have participated on numerous occasions with the
DOD on many high-priority classified activities
requiring expertise in the field.
Applications
As indicated by the foregoing
discussion, the Langley Research Center has expended
considerable effort and made valuable contributions
to the Nation’s knowledge and approach to
the wake-vortex hazard. The results of the early
Wake-Vortex-Alleviation Program, although frustrating
to the researchers who had hoped for an aerodynamic
solution to the problem, nonetheless serve as a
foundation of knowledge for potential airframe
modifications to mitigate the problem. In addition
to providing clarification and data for the civil
applications, the work resulted in activities in
support of the military, such as analysis and improvement
of C-17 paratroop capabilities. The vortex characterization
research focused the attention of the research
community on the impact of atmospheric conditions
on the prediction and control of vortices. Combined
meteorological and wake-vortex data sets from Memphis
and DFW deployments are in use internationally,
including Canada, Germany, and France. Langley’s
research on spacing requirements provided analysis
to support reclassification of weight categories.
Finally, the development of an integrated aircraft
spacing concept such as AVOSS has developed weather
profiling, wake lidars, and wake prediction to
a point where a wake system implementation is feasible
and demonstrated the potential of technology to
provide solutions to the current and impending
capacity issues at major U.S. airports.
As the new millennium begins,
the Nation faces a rapidly growing issue of airport
capacity, and the FAA must ultimately provide options
for solutions. NASA’s research has provided
fundamental technology and stimulated interest
by airport management, the Air Transport Association
(ATA), and the FAA in developing wake systems for
delay reduction. Aircraft manufacturers like Boeing
and Airbus are also beginning to design new aircraft
with wake characteristics in mind.
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