To ensure this does not occur there is a pro-rata reduction in the aircraft's crosswinds limits, which in turn limits the sideways forces acting on the aircraft, thus ensuring sufficient directional control. This is the explanation for approach and landing; for takeoff the converse is true. The rudder applies a force to counter the crosswind forces as the aircraft accelerates down the runway. At the same time the tyres are accommodating these forces through sidewall torsion, and giving grip with the runway surface. As the aircraft transitions from a ground vehicle to a flying vehicle rudder input is stopped by the pilot and the aircraft will weathervane. The subsequent drift angle will allow the aircraft to fly on a straight course. Pilots may receive this data through a "Snowtam runway state decoder" which forms an appendix to the internationally recognised METAR (METeorological Actual Report). In some countries in Europe, pilots will not receive local updated/modified braking action reports directly from an Air traffic control (ATC) source unless a recent braking action test has been carried out and is being officially issued. ATC may advise other pilots that they have received a pilot report of a braking action but since these reports can be variable and subjective, without any empirical value, it should be treated as an advisory. Braking action tests are subject to many variables such as:  This reported data is used by the airport operators and authorities to determine whether the runway should be closed for de-icing or contamination removal or remain operational until the next scheduled or requested test or report. Pilots/ATC may request that an official braking action test be carried out prior to a landing. In Europe the format of braking action declarations are given using the Greek term mu which is the co-efficient of friction  Good = a mu value of 0.4 and above; measured snowtam decode is 95  Med/Good = a mu value of 0.36 to 0.39; measured snowtam decode is 94  Med = a mu value of 0.30 to 0.35; measured snowtam decode is 93  Med/Poor = a mu value of 0.26 to 0.29; measured snowtam decode is 92  Poor = a mu value of 0.25 and below; measured snowtam decode is 91  UNRELIABLE = reading unreliable; measured snowtam decode is 99 READING not measurable or not operationally significant; snowtam decode is  Snowtam Format reference is International Civil Aviation Organization (ICAO) document Annex 15 Appendix 2.