The speeds quoted for gust loads on an aircraft date back to the 1940s but they continue to be used. Examination of actual flight data from current aircraft show them to still be relevant.

It is a design requirement of CS25 (old JAR25) that an airframe structure must be strong enough to withstand gusts both positive and negative that would cause a change of angle of attack so increasing load factor at three specified design speeds.

Speed VB (max gust intensity speed) +/- 66 ft/sec Speed VC (design cruise speed) 50 ft/sec and VD (design dive speed) 25 ft/sec.

The speeds are EAS and the data is based on sea level flight. At altitude as TAS increases the effect of the gust which changes the direction of the relative airflow will be reduced as it is the resultant of the gust speed and aircraft speed.

VB is a low speed so the aircraft would be flying at high alpha, a gust of 66 ft/sec would cause the wing to stall, as CL reduces at the stall the airframe could not be overstressed. However the aircraft is more likely to flying nearer to VC hence the requirement for the 50 ft/sec gust. Higher speed means lower alpha so it is unlikely that a gust would cause the wing to stall, but an increase CL would increase load factor.

The design speeds are not published in the pilot’s flight manual, in practice a speed of VRA/MRA (rough air speed) is stated. This speed is between VB and VC and will give protection against over stressing the structure and stalling.