Today, air traffic is safely controlled from the airport tower. Virtually all the risks are known, and appropriate precautions are in place.

Nevertheless, there remains one little-known hazard – wake turbulence – invisible giant air vortices that trail behind an aircraft, posing a danger for following planes, and hence of extreme importance for aviation and research. In this case, the vortices happen to be visible, thanks to the vapour trail which follows the aircraft.

This 3D animation shows how a small aircraft, following too close to a larger one, runs the risk of losing control of flight attitude when it flies into this whirlpool of air. The smaller, following aircraft experiences a high rolling moment, causing it to tip over.

The research project presented here focuses on how wake vortices come about, how they behave in free air, and what countermeasures can be taken. The question is how to control the invisible motion of the swirling air.

The phenomenon of wake turbulence becomes apparent when vapour trails form in the vortices behind an aircraft, or when the swirling of the air is made visible by artificial smoke, resulting in bizarre shapes in the sky. At the same time, they are an indication of the complex effects of the atmosphere

on the behaviour of wake vortices. Despite the seemingly endless expanse of the sky, there are already bottlenecks in air corridors, especially in the vicinity of airports, as sufficient separation has to be maintained between aircraft for the turbulent wake to dissipate. This places limits on the capacity of airports.

The standard separation is currently 6 miles. If the second aircraft is heavier than the first, this can be reduced to 4 miles. If air traffic continues to grow, the minimum separation requirements will lead to overcrowding of the airspace in the vicinity of airports. Based on current trends, air traffic is increasing at a disproportionate rate.

Demand increasingly exceeds airport capacity. This creates pressure on air traffic control services responsible for coordinating the complex operations in and around an airport. The pressure on pilots is also increasing. Flying into wake turbulence is a safety hazard for pilots and passengers.

The complex interaction of airport and air traffic needs to be looked at as a whole. This is the purpose of the DLR Wake Vortex Project, named Vrbleslepe. The DLR is collaborating with a number of research groups involving national, European and other international partners.

The aim is to find ways of influencing the stability and rolling moment of wake vortices at the point at which they are generated. Firstly, it is a matter of finding out more about the behaviour of the vortices

in a wind tunnel. Modified wing configurations can also be initially tested in this way. Further tests are then conducted in a water-towing tank and with free flight models fired by a catapult. A one-to-one trial with an Airbus A340 addresses the questions of how instabilities come about

and how the vortex can be induced to decay faster. The motion of the air, made visible by smoke generators, is recorded using a LIDAR, a highly sensitive laser-based measuring instrument.

The solution is to generate a multiple vortex system. For example, using moving flaps which, depending on their position in relation to one another, each generate a secondary vortex with a different direction of rotation or by employing a fixed but differentiated flap position. In the trial, the inner vortex rotates slowly about the main vortex.

A counter-rotating secondary vortex is formed on the inner wing by the differentiated flap position. The plan view illustrates quite clearly how this causes the main vortex to decay closer

to the aircraft. In the water tank, too, the presence of a secondary vortex induces the main vortex to decay substantially faster. The trials and the data gathered are used to produce a number of computer simulations.

Here you see a comparison of the real behaviour and the simulation. The high degree of similarity between the two suggests that the simulation is an accurate prediction tool so further trials can be performed on a virtual basis. Another measure is to install a gust sensor capable of detecting wake vortices in addition

to natural air motion on the aircraft. Aircraft controls attuned to this react faster than a pilot could ever do. As part of the Aviator project, the first trials with this laser technology are conducted sideways out of the window. The DLR testbed of VFW614ATAS takes off for a test flight.

The motion of the air mass ahead of the aircraft is scanned in a fast sequence. The results indicate high potential for further development. Another factor in the behaviour of wake vortices is the influence of the ambient air. This makes it imperative to conduct trials under realistic conditions.

In addition to detailed flight planning, the ATAS controls had to be modified. The counter movement of the inner and outer landing flap produces the second vortex system. After a thorough check, the ATAS is ready to take off. A DLR Falcon will document the trial from the air.

The coloured smoke makes the wake vortices visible already at take-off. The smoke generator ignites at high altitude. All eyes and cameras are directed to the flying testbed. The DLR Falcon measures the structure of the vortices from above.

By comparison, the success of the measure is now visible. The decay, shown above, induced by the secondary vortex, is faster. The modifications implemented on the aircraft are clearly worthwhile.

The crosswind causes vortices to drift. This can cause problems, especially at airports with close parallel runways. Here you see the vortex on the LIDAR display and the subsequent evaluation of the drift of the wake vortex.

Practical use could be made of this. Under certain meteorological conditions, the aircraft can be stacked at reduced separation. Vortices decay through turbulence and thermal stratification,

which strongly influence the behaviour of wake vortices, especially close to the ground. This, too, is tested under realistic conditions. As part of the Wirblesleppe, Seawake and Aviator projects, sophisticated trials are being conducted with the participation of the DLR,

Onera, Kinetic and Airbus. The complex system of vortices in their environment are recorded electronically and subsequently numerically analysed. The simulation shows three different patterns of decay as a function of distance from the ground. This animation shows a wake vortex in a turbulent shear flow.

Here we have the simulation of a case registered by an instrumentation at London Heathrow. After crossing a line of trees, the wake vortex behaves quite differently under the influence of a crosswind from under laboratory conditions.

From dropping above the line of trees, the wake vortex rises again and thus presents an unexpected hazard to following air traffic. To detect such phenomena in good time, you need a system that interacts well to predict and observe wake vortices.

This includes a detailed weather report, an estimate of the pattern of wake vortices, determination of danger zones and the resulting stacking of aircraft.

A good weather report must cover not only the overall weather situation but also the specific conditions in the airport vicinity, for example particular terrain features such as differences in height, water surfaces, woods or other vegetation and the properties of the ground. Individual terrain features induce complex horizontal and vertical air motion,

as illustrated here by changes in the atmospheric boundary layer in the course of a day. In this way it is possible to predict the vertical motion of the vortex and its horizontal drift with different degrees of probability.

The intensity of the vortex remains stable for a while and then drops off rapidly, as you can see from the diagram and the real footage parallel to it. Once the behaviour of a wake vortex has been captured,

the question arises as to where and under what time of decay following air traffic is still at risk. For test purposes, a two-propeller Dornier 128 owned by the Technical University of Braunschweig flew into the wake vortices of the ATAS. These are the dramatic pictures filmed from the cockpit.

As soon as the Dornier 128 flies into the wake vortices, it experiences heavy rolling moment to the right. Such data collected in the course of numerous flights make it possible to infer a danger region that corresponds to the red rectangle.

The red ovals are the real danger zones. Depending on the drift and decay of the vortex, it is also possible to calculate the danger region along a glide path.

The simulations show firstly the glide path to 13 stacked gates and then the drift of the vortex in the form of a round tail for each gate. The more precise the prediction, the faster the next aircraft can follow.

In this way, elements for observing weather and vortices are added to the weather and vortex prediction system.

Ideally, every airport would be equipped with such instrumentation, which, under favourable weather conditions, would allow a significant increase in capacity. The high point of the research project is a series of campaigns in which the systems described in this film are used.

The ATAS takes off in Ulbefaffenhofen. On the ground are the different observation systems distributed at strategically important positions around the airport. The Kinetic Lidar, the Honora Lidar and that of the DLR.

The wind and temperature profiler from Metec, the weather station with weather radar, supported by climbing radiosondes. The ATAS crosses the test field and the computers receive a whole stream of data to be evaluated.

A further trial on a larger scale was conducted in 2003 in the south of France as part of the Aviator project. Here, too, the measurement systems were placed at three different points to enable the air motion during the overflight to be accurately attributed. The test bed, an Airbus A340, was fitted with smoke generators, known as smoke pods.

The giant aircraft takes off. The measuring instrumentation has been prepared at the trial site and the overflight is eagerly awaited. Smoke pods are ignited and the A340 approaches the test field.

This trial also generates a mass of data for the computers to evaluate. The actual measurement of wind speed within the boundary layer shows by comparison that the prediction was very good. The DLR research is aimed at enabling aircraft separation

to be safely reduced to two and a half miles. This would constitute a significant step towards meeting the continuously increasing demands for safety and efficiency in air transport. For no matter what scale civil aviation assumes in the future, if air travel is to be safe, the answers must be found today.

The DLR is cooperating on this with numerous national, European and international partners.