The domain of wake turbulence is a field on which many scientists have worked for several years without being directly connected to a real operational improvement because it took time to understand how to reduce wake turbulence risk from a strictly operational point of view, according to Vincent Treve, who works in the research and development (R&D) unit at Eurocontrol. However, researchers have now developed some changes to address the issue.

Wake turbulence recurrently hits the headlines as a threat to the safety of flight operations. The fact is that wake turbulence is in the very nature of flight. Anything flying generates lift, and induced drag is a byproduct of lift. “Induced drag carries a lot of energy in the form of two airflows that rotate one against the other and remain for certain time behind an aircraft. This is wake turbulence,” said Treve.

“Wake vortices are generated by the movement of the air from the bottom of the wing toward the upper surface. The heavier the aircraft, normally the more intense the vortex,” explained Marcel Martineau, a pilot and aviation consultant.

A wake vortex becomes a problem when there is a lot of it and another aircraft is passing by. “It is quite common to pass through the wake turbulence generated by another airplane, but normally it is very low. Only occasionally pilots will encounter more severe events of wake turbulence,” said Treve.

“If you have a large aircraft behind a smaller one you may not even feel any turbulence at all. So, first, for being exposed to some turbulence you need to have in front of you an aircraft of a comparable size to yours or a larger one," said Treve. “Then you must have a set of external circumstances to experience wake turbulence. You need to be close enough, and then the wind or the turbulence must be in your route or trajectory. If you are landing on a runway and you have 15 knots crosswind, of course, you will never encounter any turbulence. Anything produced by the aircraft in front of you will be blown away by the wind.”

Coping with Wake Turbulence

Designers have so far been unsuccessful in attempts to prevent aircraft from creating wake vortices by generating a quick distortion of the vortices and hastening their decay. “There have also been attempts to design systems for inducing a decay in the vortices," said Treve, "but in reality so far we cannot see anything really improving or reducing the strength of the wakes generated by the aircraft. You cannot avoid the generation of vortices. The problem is to identify what distance you have to maintain with the preceding aircraft for being safe."

While there have not been significant advancements in design, there are good practices for pilots to implement to prevent wake turbulence. “It can help to have a flight stay above the wake turbulence if conditions permit by landing long; it can also help to stay above the turbulence to taking off before the point where the previous aircraft has rotated when conditions permit,” said Martineau.

In recent times upset recovery training has become a tool to counter wake turbulence. “Upset recovery is part of normal pilot training. There have been procedures developed and guidance materials produced by the Federal Aviation Administration (FAA), Eurocontrol, and the European Aviation Safety Agency (EASA) which explain what the best way to react is. Of course, if you are at altitude you cannot react in the same way as if you are close to the ground when indeed you are more sensitive to an upset condition,” said Treve.

Air traffic management (ATM) plays an important role in preventing wake turbulence events. “In some countries, ATM will not issue a takeoff clearance until the separation has been achieved, but in other countries, they will issue a clearance but it is always up to the pilot to take off when [they] feel it is safe to do so,” said Martineau. “ATM will normally separate the aircraft according to their wake turbulence category. For instance, a small aircraft will need greater distance behind a heavy aircraft than heavy aircraft behind a small airplane."

With regard to flight crew reaction in case of wake turbulence, Eurocontrol has run a series of full-flight simulation sessions to assess the criticality of wake encounter and designed separations as a function of the assessed criticality. “When we come up with separation design, we define the distance at which an aircraft must be behind another one as a function of the size of each of them and we verify that in the worst case the wake and the upset entailed by the wake will be manageable and acceptably safe," said Treve. “This we do through flight simulation by repeating an occurrence and asking different pilots to rate different circumstances, attitudes, and aircraft types before coming up with a level of exposure that is considered acceptably safe, with the criteria being that we want the pilot to rate the severity as still safe to land and not even think about a go-around. When we design separation we do not consider the possibility of a go-around. It should be safe to land."

Eurocontrol's research has generated an ATM improvement with regard to wake separation. “Nowadays many airports are following the recommendations we have produced for optimizing both safety and capacity in relation to wake separation. Today the separation design for remaining safe has been massively revised and implemented in London, Paris, Leipzig, Toulouse, and Vienna while other airports are also on their way to implementation. Many airports in the U.S. as well are now revising the separation minimums for better coping with the wake turbulence risk. This means that when it was not needed separation has been reduced whereas in a certain number of case separation has been increased in the interest of safety,” he concluded.