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The speed VB in gust diagram...
  • A
    equals 50 ft/s.
  • B
    equals 66 ft/s.
  • C
    equals 55 ft/s.
  • D
    equals 25 ft/s.

Refer to figure.
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
  • Speed VD (design dive speed): +/- 25 ft/sec.

VB is quite a low airspeed so the aircraft would be flying at high alpha. It is a suggested speed for penetrating turbulent air when that turbulence is particularly severe.VB is the maximum speed at which a gust will not overly stress the aircraft.

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