regulations require commercial airliners to be separated in flight by
up to six nautical miles (11.12 km) due to the potential hazard caused
by the swirling air left in their wakes. This 'wake vortex' is now the
subject of intense European research to understand the nature of the phenomenon
and find ways of making air travel safer while reducing congestion around
airports. With the world airline fleet expected to double in size over
the next 15 years and the giant A3XX entering service in 2005, solutions
to the wake-vortex problem cannot come too soon for the European aerospace
is a wake vortex?
as a ship leaves a wake behind it in the sea, an aircraft leaves
a wake in the air. An aircraft's wake is in the form of two counter-rotating
swirling rolls of air - the wake vortices - that trail from the
wings of the aircraft. The wake vortex pair may last for several
minutes and stretch for many kilometres behind the aircraft. The
strength of the vortices basically depends on the aircraft weight,
divided by the product of air density, flying speed and wingspan.
This property generally increases with aircraft weight.
lifetime of a vortex depends upon local meteorological conditions.
Vortices last longer in calm air and atmospheric turbulence hastens
Why do wake vortices matter? It is a question of safety. The rapidly
swirling air in a vortex can catch the wings of a following aircraft
with potentially disastrous results. Tests with experienced test
pilots have shown that even commercial airliners can be thrown out
of control if they follow too close behind a large aircraft such
as a Boeing 747.
vortices are normally invisible and pilots have no warning that
they are flying into one. For this reason, the International Civil
Aviation Organisation (ICAO) lays down strict rules about the permitted
spacing between aircraft, based on their size. In instrument flying
conditions aircraft may follow no closer than three nautical miles
(5.56 km), and a small aircraft must follow at least six nautical
miles (11.12 km) behind a heavy jet such as a Boeing 747.
separations are conservative: they do not completely avoid the effects
of wake vortices, but they are sufficient to be safe in most meteorological
all airline pilots will have had encounters with vortices, usually
on the final approach to airports. They are experienced as a buffeting
of the aircraft. While of little concern to passengers and crew
who are wearing seat belts at this stage, pilots regularly report
minor injuries to crew members standing up or moving around the
cabin. However, thanks to ICAO regulations on separations, there
have been no serious accidents reported with passenger airliners.
impetus for further study of wake vortices, now a major concern
in North America as well as in Europe, is twofold:
A new generation of heavier airliners, such as the Airbus A3XX,
is on the drawing board and, if no action is taken, they are expected
to produce even more severe vortex problems; and
Many busy airports in the USA and Europe are already working near
capacity limits, at least during peak hours. A better understanding
of the wake-vortex phenomenon would permit aircraft to fly closer
together when local weather conditions were suitable and so ease
congestion. Increasing capacity in this way would be a better
solution than building new runways.
In the longer term, engineers may be able to design aircraft that
produce less hazardous wakes. It is not possible to avoid vortices
being created - they are an inescapable consequence of aerodynamics
- but suitable design of the wing may help them decay more quickly
and so be less of a hazard.
for a multinational approach
vortex formation is affected by the aircraft's flaps, slats and
undercarriage, not to speak of exhaust from the engines. The prediction
of the wake characteristics at several kilometers downstream of
the generating aircraft is extremely complicated and the physics
is not yet fully understood. Wake vortex research is therefore an
ideal candidate for support at the European level. The EU-funded
VORTEX Thematic Network is an overarching project that brings
together work being done in many centres in industry, universities
and government establishments. Co-ordinated by DaimlerChrysler
Aerospace (DASA) in Bremen, Germany, it forms the nucleus of
a community of researchers investigating problems that range from
basic physics to aircraft engineering.
Research is focused on two main activities:
Formation of vortices and how they might be controlled; and
Safety issues surrounding their effects on aircraft.
wake vortices form and how they can be controlled
major EU-funded project on the characterisation and control of wake
vortices is called C-WAKE,
also co-ordinated by DASA.
It began in January 2000 as a follow-on to the successful EUROWAKE
project. It has four major objectives:
To identify and explore means of reducing wake-related hazards
to following aircraft;
To assure that the present ICAO separation distances will remain
future large aircraft;
To provide a database to enable manufacturers to design aircraft
featuring low-vortex characteristics; and
To develop a validated prediction model for industrial use that
also incorporates the wake-vortex database.
of the most useful tools for studying wake vortices is the classic
wind tunnel. A scale model of the aircraft is placed in a stream
of air and vortices form in a very similar way to the full-size
aircraft. The main facilities used in this work are the German-Dutch
Windtunnels (DNW) a consortium operated by aerospace laboratories
in the two countries.
can be made visible by releasing smoke into the tunnel. However,
while this is useful, it does not allow for the kind of measurements
that scientists would like to make. One of the new techniques recently
developed to study airflow is Particle Imaging Velocimetry (PIV).
involves the introduction of a mist of fine particles, such as droplets
of olive oil, into the airstream. A laser creates a sheet of light
downstream of the model aircraft and, as the particles pass though
the sheet, they are photographed by a high-speed camera. By tracking
the movement of particles in successive images, a computer can calculate
the speed of the airflow in great detail and produce maps of the
PIV technique is so promising for industry that the EU has set up
another thematic network, PivNet,
to extend the measurement technique from 2D to 3D.
vortex formation downstream
tunnel studies are fine for studying the formation of vortices and
following their development for up to a dozen wing spans behind
the aircraft. But, to see how they evolve further downstream, other
techniques are used. One such is the catapult facility run by ONERA
(Office National d'Études et de Recherches Aérospatiales)
at Lille. Here a model aircraft is released from a catapult and
glides along a 30-m flight path where its wake can be studied using
similar techniques to the wind tunnels. The wake can be followed
for around 80 wingspans behind the model; a new catapult coming
into operation soon will extend that to 200 spans.
technique quite new to the wake vortex community is the 'towing
tank', where a model is towed through a long tank of water. Initial
experiments are planned in the towing tank at Delft
University in the Netherlands to explore the potential of this
technique. If successful further tests will be made in the much
larger towing tank facility of INSEAN in Italy.
however, there is no substitute for direct observation of aircraft
in flight to study the full development and decay of the vortex.
The best developed technique is known as 'lidar' (LIght Detection
And Ranging), and is similar in principle to radar. A beam of infrared
laser light - which is not visible to the eye - is shone into the
sky and scatters back off tiny particles of dust and water droplets.
By studying the reflections received from the particles, researchers
can measure their distance and speed, and so look for disturbances
in the air caused by vortices.
centres of lidar research in C-WAKE are DERA
(Defence Evaluation and Research Agency) in the UK and DLR
(Deutsches Zentrum für Luft- und Raumfahrt) in Germany.
the lidar concept towards a practical on-board detection system
was the objective of MFLAME
(Multifunction Future Laser Atmospheric Measuring Equipment). The
object is to design a scanner that can be installed in commercial
aircraft to warn of vortices on the flight path. In tests in the
spring of 2000, the MFLAME demonstrator was set up below the approach
path at Toulouse airport in France and succeeded in obtaining images
of vortices from aircraft passing overhead.
Avionique, the system is expected to be taken up by the proposed
I-WAKE project and to lead to a marketable product by 2005 to 2008.
the implications for air safety
is the concern of the second major EU-funded project, called S-WAKE,
which follows on from the earlier WAVENC
project. Co-ordinated by the National
Aerospace Laboratory (NLR) in Amsterdam, S-WAKE has several
important objectives :
To define suitable weather categories for wake-vortex safety for
aircraft on the approach glide path;
To improve the physical understanding of wake-vortex evolution
and decay in the atmosphere;
To assess what a 'low-vortex' design means in practice for a following
To establish realistic flight simulation environments for investigating
wake-vortex encounter safety aspects and pilot response;
To establish a validated probabilistic safety assessment environment;
To analyse the safety aspects for current practice; and
To define possible new concepts which allow a safe mitigation
of current separation rules under certain conditions.
first priority is to continue the work started in WAVENC to develop
mathematical models of how vortices evolve and decay, taking account
of different weather conditions. The work is being managed in France
(Centre Européen de Recherche et de Formation Avancée
en Calcul Scientifique).
Researchers led by the DLR Institute of Flight Research in Braunschweig
will conduct a series of flight tests with aircraft from the Dutch
and German aeronautical research laboratories and an aircraft from
the Technical University of Braunschweig. In these tests aircraft
will deliberately be flown in the wake of the DLR ATTAS aircraft
and aircraft movements and accelerations will be measured in detail.
The aim of these tests is to validate models for the wake encounter
that will later be implemented in flight simulators led by DASA
in Hamburg. Flight simulators in Germany and the Netherlands will
be adapted to enable systematic simulations of realistic encounters
with vortices by light, medium and heavy aircraft over the full
range of expected conditions. Airline pilots will be involved in
group led by NLR will improve risk assessment by developing better
models for predicting the degree of risk for common operations.
The idea is to arrive at safe separations that can be justified
by local conditions rather than the rather conservative regulations
now in force. This work may lead to proposals for new regulations
on aircraft separation, especially near airports.
approach to vortex logging
encounters with decaying vortices are not unusual, there is a surprising
dearth of systematic data. The major EU study so far is ETWIRL
(European Turbulent Wake Incident Reporting Log), which ran for
two years to 1999 and was co-ordinated by RED
Scientific. It recorded about 180 incidents at European airports,
which are now available for study.
and other schemes for voluntary incident reporting suffer from incompleteness:
how many incidents are not reported? So an important task of S-WAKE,
led by researchers at National
Air Traffic Services (NATS) in London, is to devise an automated
method of extracting information about vortex encounters from flight
data recorders. Data from British Airways flights landing at London's
Heathrow airport will be analysed and is expected to yield up to
300 incidents from 80,000 flights.
information like this will help researchers assess just how often
aircraft fly into vortices and the level of risk this really involves.
will the research lead?
will all this activity ultimately benefit air passengers?
naturally take a keen interest in wake vortex research, and representatives
of the International
Federation of Airline Pilots Associations (IFALPA) regularly
attend the Wake Vortex thematic network workshops and other meetings.
They would like to see some kind of on-board system for alerting
them to the presence of vortices, not only directly ahead of their
aircraft but also off to the side - it is possible unwittingly to
fly parallel to a vortex until it drifts into the path.
controllers, too, would like to be able to operate an advisory system
to warn pilots of adverse conditions and ideally be able to 'see'
vortices around their airports. And of course designers would like
to know how to build large aircraft that do not have such dramatic
effects on the airspace in their wake.
wake vortices pose safety problems and limit capacity in the skies.
Research in this area is therefore a European priority under the
New perspectives in aeronautics key
action. There are currently three main EU-funded projects concerned
with such research:
VORTEX (BRRT 985050) - a thematic network providing a forum
for exchanging ideas on all aspects of the wake-vortex phenomenon;
- 'Wake vortex characterisation and control' (G4RD-1999-00141)
investigates the conditions under which vortices form and how
they might be controlled. This was preceded by EUROWAKE;
- 'Assessment of wake vortex safety' (G4RD-1999-00099) examines
safety issues surrounding the encounters of aircraft with vortices
and followed on from the WAVENC
Other recently completed projects include:
-'Multifunction future laser atmospheric measurement equipment'
(BRPR 960182), which demonstrated an on-board vortex detection
- 'European Turbulent Wake Incident Reporting Log' an experimental
reporting scheme for wake-vortex incidents.
- 'WAke Vortex Evolution and WAke Vortex ENCounter
related thematic network, PivNet
(BRRT975037), deals with particle image velocimetry, one of
the technologies widely used for wake vortex research. A proposed
new project I-WAKE, will concentrate on developing instrumentation
for the detection of wake vortices both from the ground and in