The Coanda exhausts on the Williams FW35Williams FW35United Kingdom, 2013 > 20135 photos
and Caterham CT03Caterham CT03United Kingdom, 2013 > 201313 photos
have been ruled illegal by the FIA specifically because of the vanes that are around the exhaust outlet. 

The FW35 has a two-part vane at the top of the exhaust exit, and the Caterham has a one-part vane. The FIA claims that these vanes completely enclose the exhaust and has an effect on the aerodynamic performance of the car. To ensure that this does not happen, the rule stipulates that no part of the car can enclose the Coanda channel. 

The inherent problem with the decision is where to decide Coanda exhausts are affecting aerodynamics. By their very design, these exhausts shape the way that the air leaves the exhaust. The FIA says that they may have an "incidental" effect on aerodynamics but not too much. The question is how to define incidental when it comes to the design. 

The technical directors of Williams and Caterham both claim that their designs are legal and plan to ask for clarification. 

"The team spoke with the FIA yesterday which is when they gave us their view. The team are now seeking further clarification on this and a decision as to whether this design will be carried forward will be made before the first race," said Williams F1 spokesperson Sophie Eden. 

The rule will not take effect until the first race of the season in Australia. The teams have the right to continue using the exhaust in testing, but unless their appeals are successful, they will not be allowed to use them in the race. 

Source: Autosport