What is ISLN on Boeing 737? (Isolation)

The Boeing 737 aircraft is a widely used narrow-body airliner that has revolutionized the aviation industry. With its efficient design and impressive range, the Boeing 737 has become a staple for airlines around the world. To ensure the safety and performance of this aircraft, various systems and components are implemented, including the Isolation System (ISLN). In this article, we will explore what isolation means in the context of the Boeing 737 and how it contributes to the overall functionality of the aircraft.

What is Isolation?

Isolation, in the context of the Boeing 737, refers to the separation of different electrical, hydraulic, and fuel systems to prevent any unwanted interactions that could compromise the operation of the aircraft. The purpose of isolation is to contain any failures or malfunctions within a specific system and prevent them from affecting other critical systems.

To achieve effective isolation, the Boeing 737 utilizes various components and techniques that are specifically designed for this purpose. These include:

  • Isolation Valves
  • Crossfeed Valves
  • Isolation Diodes

Let’s delve into each of these components and understand their role in maintaining isolation on the Boeing 737.

Isolation Valves

Isolation valves are critical components in the isolation system of the Boeing 737. These valves are responsible for separating various systems, such as hydraulic and fuel systems, to ensure that a failure in one system does not impact the operation of the others.

In the event of a failure or an abnormal condition in one hydraulic system, the isolation valve automatically closes to prevent the hydraulic fluid from flowing into the other hydraulic systems. This prevents contamination and ensures that the failed system can be isolated and addressed separately without affecting the overall performance of the aircraft.

Similarly, in the fuel system, isolation valves play a crucial role in preventing fuel leaks or malfunctions from spreading to other fuel tanks. These valves can be manually controlled or have automated features, allowing them to respond proactively in case of any detected abnormalities.

The isolation valves on the Boeing 737 are designed with robust features to ensure their reliability and effectiveness. They undergo rigorous testing and comply with stringent safety regulations to meet the high standards set by aviation authorities.

Isolation Diodes

Another crucial component in the isolation system of the Boeing 737 is the isolation diode. Isolation diodes are electronic devices that allow the flow of electrical current in only one direction, preventing backflow or unwanted interactions between electrical systems.

These diodes are strategically placed in the electrical system of the aircraft to ensure isolation between different circuits. They play a vital role in preventing electrical faults or failures from spreading to other systems, thus enhancing the overall safety and reliability of the Boeing 737.

Isolation diodes are engineered to withstand high voltages and have low resistance in the forward direction to allow efficient electrical flow. They are designed to handle the demands of the aircraft’s electrical system and provide reliable isolation under various operating conditions.

Crossfeed Valves

Crossfeed valves are essential components in the fuel system of the Boeing 737. These valves enable the transfer of fuel between different fuel tanks, allowing the aircraft to maintain proper weight distribution and fuel balance during flight.

While the primary function of crossfeed valves is not isolation, they indirectly contribute to the isolation system by providing the capability to isolate fuel tanks when necessary. By closing the crossfeed valves, potential fuel cross-contamination or leaks can be contained within a specific fuel tank, preventing the spread of any issues to other tanks.

The crossfeed valves on the Boeing 737 are designed to be reliable and responsive. They are operated by the flight crew and can be manually controlled in case of any required isolation procedures.


In summary, isolation plays a vital role in the overall functionality and safety of the Boeing 737 aircraft. Through the implementation of isolation valves, isolation diodes, and crossfeed valves, the Boeing 737 ensures that failures or malfunctions in one system do not adversely affect other critical systems.

By isolating different systems, the Boeing 737 can contain and address specific issues without compromising the operation of the entire aircraft. The rigorous testing and adherence to safety regulations ensure that the isolation system on the Boeing 737 is reliable and capable of handling various scenarios.

The implementation of isolation systems demonstrates Boeing’s commitment to safety and their continuous efforts to enhance the performance and reliability of their aircraft. The isolation system is just one of the many intricate components that contribute to the success of the Boeing 737 and establish it as a leading aircraft in the aviation industry.

For More: What is OPAS on Boeing 737? (Overhead Panel Arinc 629 System)