Until recently the gust requirements laid down by the airworthiness authorities had not caused any serious difficulty in design. This was due to the fact that other loading conditions (manœuvring loads imposed by the pilot) had been more severe than the loads produced by gusts. Over the past few years, various factors have altered this position. Increase of aeroplane speeds, increase of operating altitudes, decrease of the manœuvring loads (in the case of large aeroplanes) and increase of size, have each contributed to raise the gust loads until they have become, in many cases, the main design criteria for the flight structure.