Difference between VA and VB Speeds | Importance of VB (Turbulence Penetration Speed)

While VA (Design Maneuvering Speed) is a familiar concept, there’s another airspeed that often takes a backseat but holds immense significance in turbulent skies: VB (Turbulence Penetration Speed).

By definition VA represents the maximum speed at which the aircraft can be safely subjected to full and abrupt control movements without exceeding its structural limits. Learning just about VA isn’t sufficient as VA doesn’t consider external loads (by turbulence) which can be added to existing load created by control inputs.

Loads experienced by an aircraft are the cumulative result of various factors, including pilot control inputs and external forces such as turbulence. These forces act simultaneously on the aircraft, and their effects are additive. This means that if, for example, an abrupt control input from the pilot results in a 2G load being applied to the aircraft, and at the same time the aircraft encounters turbulence generating an additional 2G load, the total load experienced by the aircraft becomes 4G.

In this scenario, the pilot’s control input contributes 2G to the total load (which can be safeguarded if aircraft was previously flying at or near VA speed), while the turbulence adds another 2G, resulting in a combined load of 4G.

Pilots must be mindful of the potential for additive loads. By maintaining situational awareness and applying smooth, deliberate control inputs, pilots can minimize abrupt changes in load and ensure the structural integrity and stability of the aircraft throughout the flight.

VB—also known as rough air airspeed—is slower than VA. VB is the maximum speed at which a gust will not overly stress the aircraft. VB, is a critical airspeed limitation for transport-category aircraft, designed to ensure safe operation in rough air conditions. Unlike VA, which focuses on structural integrity during aggressive maneuvers, VB is specifically aimed at minimizing stress on the aircraft caused by gusts or turbulence. Again, VB isn’t perfect and must maintain speed but introduces additional margin.

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