As described by the Federal Aviation Administration (FAA), wind analysis is of fundamental importance for determining runway orientation. Ideally a runway should be aligned with the prevailing wind in order to minimise the crosswind components. In aviation, a crosswind is the component of wind that is blowing across the runway making a landing more difficult than if the wind were blowing straight down the runway. If a crosswind is strong enough it may exceed an aircraft’s crosswind limit and an attempt to land under such conditions could cause structural damage to the aircraft. Adverse wind conditions (i.e., strong crosswinds, tail winds and wind shear) are involved in 33 % of approach and-landing accidents.
Crosswinds can also occur when travelling on roads, especially on large bridges and highways, which can be dangerous for motorists because of possible lift force created as well as causing the vehicle to change direction of travel.
Generally, a crosswind is any wind that is blowing perpendicular to a direction.
Each aircraft has a uniquely stated maximum crosswind component derived from flight test experiments. For example a Boeing 727-200 has a maximum crosswind component of 35 knots (17.8 m/s), while a Cessna 172 has a maximum crosswind component of 17 knots (8.7 m/s).
Wind coverage is defined as the percentage of time that crosswinds are below an acceptable velocity. According to the FAA standards (FAA AC 150/5300-13), the minimum wind coverage considering all the observations is 95 percent. This means that for the 95% of the time, the crosswind component must be smaller than the maximum crosswind component of the aircrafts landing in a specific airport.
In designing runway orientation, the most desirable runway is one that has the largest wind coverage and minimum crosswind components.
The WindRose PRO software can be used for analysing a long series of data and calculating, for each possible runway direction, the wind coverage and the crosswind components (maximum, average and median). The software can also be used to evaluate the correct orientation of an existing runway. Moreover, since WindRose PRO allows date/time filtering of the input data, it is possible to evaluate the wind coverage and the crosswind components even for airports which work only in particular seasons (for example during summer) or only during day time. For each direction, the WindRose PRO output contains information about the wind coverage, the maximum crosswind from left and right, the average and the median (i.e. the 50th percentile) of the absolute value of the crosswind components, the maximum headwind, the maximum tailwind and the average and the median of the absolute value of the headwind components.
As an example, we considered the Cagliari Elmas airport (Sardinia, Italy) whose runway is oriented from South East to North West. We collected the METAR data of the airport for the period 2008-2011, decoded them and prepared for WindRose PRO. More than 98% of the data were valid. The derived wind rose is represented in the following figure (left). It is observed that the prevailing wind direction is aligned with the runway, as it must be. The wind rose of the maximum (red) and average (green) speed shows that the maximum wind speed observed from 2008 to 2011 is 18 m/s, and comes from North West.
The wind coverage has been calculated for a hypothetical aircraft with a maximum crosswind speed of 8 m/s. WindRose PRO has been instructed to test 72 possible runway orientations, starting from 0 degree with steps of 5 degrees. It is observed that orientation N must be intended as the orientation from N degrees to (N+180) degrees. The following figure (left) shows that the maximum wind coverage, which is equal to 100%, is obtained for the two directions 115 degrees and 120 degrees. Only directions from 25 degrees to 50 degrees have a wind coverage smaller than the 95% indicated by the FAA. Finally, an example of numerical results of WindRose PRO is also presented in the figure (right) as a table. For each runway orientation the following parameters are calculated: wind coverage, maximum crosswind from left and right, average and median of the absolute values of crosswinds, maximum headwind and tailwind, average and median of the absolute values of headwind.
Acknowledgements: the foreground image has been taken from the Crosswind landing article on Wikipedia.