friction with them ensures that the pilot can continue to keep the aircraft aligned with the runway as the aircraft decelerates during the landing roll. If however the surface friction is diminished because of contamination then this may upset the balance of forces resulting in insufficient directional control to keep the aircraft on the runway. 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 Braking action tests are subject to many variables such as: It is an instantaneous report and its data integrity may not be valid after a short period of time in active or changing weather conditions. Caution, the data value is an average/mean value for the runway length (usually split into thirds) and does not rule out that localised areas are better or worse than reported. The scheduled time interval frequency of such tests and their reports may not be regular. In other words, one may be reading an old braking action report attached to an up-to-date METAR. Various manufacturers of friction testing equipment provide different readings (non homogenous) on the same surface. Most of these friction testing devices employ using a trailing wheel or tyre