What is VBV on Boeing 737? (Variable Bypass Valve)

The Variable Bypass Valve (VBV), also known as the Variable Bleed Valve, is a crucial component of the Boeing 737 aircraft. As the name suggests, the VBV allows for the regulation of bypass airflow around the engine, resulting in improved engine performance and efficiency. In this article, we will delve into the workings of the Variable Bypass Valve, its role in the Boeing 737, and its impact on the overall performance of the aircraft.

Understanding the Variable Bypass Valve

The Variable Bypass Valve plays a significant role in the engine’s performance by controlling the amount of bypass airflow. The bypass airflow is the air that avoids passing through the combustion chamber and instead flows around the engine core. By adjusting the bypass airflow, the VBV helps optimize engine efficiency and maintain stable operation across a wide range of operating conditions.

Internally, the Variable Bypass Valve consists of a series of carefully designed blades that can be adjusted to control the bypass airflow. These blades are positioned within the engine’s bypass duct, allowing them to control the amount of air that bypasses the core. The angle and position of the blades can be adjusted based on the engine’s operating conditions, such as power settings and aircraft speed.

When the Variable Bypass Valve opens, it increases the amount of bypass airflow, thus reducing the air entering the combustion chamber. This results in a more efficient combustion process, as excess airflow can dilute the fuel-air mixture, leading to incomplete combustion. By controlling the bypass airflow, the VBV helps maintain the appropriate fuel-to-air ratio and optimize the engine’s performance.

The Role of Variable Bypass Valve on the Boeing 737

The Variable Bypass Valve is a critical component of the engine bleed air system on the Boeing 737. The bleed air system extracts air from the engines to power various aircraft systems, such as pressurization, air conditioning, and anti-icing. The VBV assists in regulating the airflow through the bleed air system, ensuring consistent and reliable operation of these systems.

One key aspect where the Variable Bypass Valve plays a crucial role is during the engine’s high-power operations, such as takeoff and climb. The valve adjusts the bypass airflow to support the required engine thrust while maintaining a stable combustion process. By allowing the appropriate amount of air to bypass the core, the VBV contributes to the engine’s efficiency, reducing fuel consumption and enhancing overall performance.

Moreover, the Variable Bypass Valve also aids in the prevention of compressor stalls. A compressor stall occurs when the airflow through the engine’s compressor becomes disrupted, leading to a loss of engine performance or even a complete stall. The VBV helps avoid this by regulating the airflow and maintaining stable conditions within the compressor. By controlling the bypass airflow, the valve ensures that the compressor operates within its designed parameters and reduces the risk of stalling.


The Variable Bypass Valve on the Boeing 737 is an essential component that contributes to the overall efficiency and performance of the aircraft’s engines. By regulating the bypass airflow, the VBV helps maintain stable combustion, optimize fuel consumption, and prevent compressor stalls. Understanding the functionality and importance of the Variable Bypass Valve allows us to appreciate the complexity and precision involved in the design and operation of modern aircraft engines.

For More: What is ELCCR* on Boeing 737? (Electrical Liaison Change Commitment Record)