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Graphic element Research > Growth > Research projects > Aeronautics projects > Wake vortex: mind the draught
Graphic element Wake vortex: mind the draught

International 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 industry.

What is a wake vortex?

Just 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.

The lifetime of a vortex depends upon local meteorological conditions. Vortices last longer in calm air and atmospheric turbulence hastens their decay.
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.

Limiting airspace capacity

Wake 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.

These separations are conservative: they do not completely avoid the effects of wake vortices, but they are sufficient to be safe in most meteorological conditions.

Nearly 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.

  Further study essential

The impetus for further study of wake vortices, now a major concern in North America as well as in Europe, is twofold:

  1. 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
  2. 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.

  Need for a multinational approach

Wake 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 WAKE 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:

  1. Formation of vortices and how they might be controlled; and
  2. Safety issues surrounding their effects on aircraft.
  How wake vortices form and how they can be controlled

The 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:

  1. To identify and explore means of reducing wake-related hazards to following aircraft;
  2. To assure that the present ICAO separation distances will remain valid for future large aircraft;
  3. To provide a database to enable manufacturers to design aircraft featuring low-vortex characteristics; and
  4. To develop a validated prediction model for industrial use that also incorporates the wake-vortex database.

One 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.

Vortices 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).

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 vortex behaviour.

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.

  Tracking vortex formation downstream

Wind 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.

A 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.

Ultimately, 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.

Major 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.

Taking 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.

Co-ordinated by Sextant 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.

  Defining the implications for air safety

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 aircraft;
  • 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.

A 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 by CERFACS (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 this study.

A 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.

  Systematic approach to vortex logging

Although 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.

ETWIRL 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.

Objective information like this will help researchers assess just how often aircraft fly into vortices and the level of risk this really involves.

  Where will the research lead?

How will all this activity ultimately benefit air passengers?

Pilots 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.

Ground 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.


What is a wake vortex?
Limiting airspace capacity
Further study essential
Need for a multinational approach
How wake vortices form and how they can be controlled
Tracking vortex formation downstream
Defining the implications for air safety
Systematic approach to vortex logging
Where will the research lead?


Key EU-funded research

Aircraft 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:

WAKE VORTEX (BRRT 985050) - a thematic network providing a forum for exchanging ideas on all aspects of the wake-vortex phenomenon;
C-WAKE - '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;
S-WAKE - 'Assessment of wake vortex safety' (G4RD-1999-00099) examines safety issues surrounding the encounters of aircraft with vortices and followed on from the WAVENC project.

Other recently completed projects include:

MFLAME -'Multifunction future laser atmospheric measurement equipment' (BRPR 960182), which demonstrated an on-board vortex detection system; and
ETWIRL - 'European Turbulent Wake Incident Reporting Log' an experimental reporting scheme for wake-vortex incidents.
WAVENC - 'WAke Vortex Evolution and WAke Vortex ENCounter

A 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 the air.


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