What is SL on Boeing 737? (Service Letter)

What is a Service Letter on Boeing 737?

A Service Letter (SL) on the Boeing 737 is a communication tool used by Boeing to provide information and guidance to airlines, maintenance organizations, and other stakeholders about specific issues, updates, or recommended actions related to the aircraft. SLs are not mandatory compliance documents like Airworthiness Directives (ADs) but are rather advisory in nature.

Service Letters can cover a wide range of topics, including system enhancements, modifications, or inspections. They are typically issued when Boeing identifies a need to address a specific concern or improve the performance of the aircraft. These concerns can arise based on feedback from operators, maintenance data analysis, regulatory requirements, or evolving industry practices.

Each Service Letter contains detailed instructions and recommendations on how to implement the changes or actions outlined in the document. It may include step-by-step procedures, part replacements, inspections, or other necessary tasks. The information provided in SLs is based on Boeing’s engineering analysis and expertise, ensuring that it is accurate and reliable.

It is important to note that Service Letters are not openly accessible to the public. They are exclusively distributed to individuals or organizations with a direct interest in the Boeing 737, such as airlines and maintenance providers. However, some summaries or excerpts of SLs may be available through various sources, including aviation publications and forums.

Importance of Service Letters

Service Letters play a critical role in maintaining the safety, reliability, and optimal performance of the Boeing 737 aircraft. By addressing specific concerns and providing guidance on necessary actions, SLs ensure that operators and maintenance crews stay up-to-date with the latest information and practices recommended by Boeing.

One of the key benefits of Service Letters is their proactive nature. Boeing continuously monitors the performance and maintenance data of its aircraft, allowing them to identify any potential issues or areas of improvement. By issuing SLs, Boeing can provide timely solutions and recommendations to operators, preventing larger problems from arising and minimizing any impact on operations.

SLs also facilitate communication and collaboration between Boeing and its customers. They serve as a channel for Boeing to share important information and updates with operators and maintenance organizations, ensuring that they are aware of any necessary actions or changes related to their Boeing 737 fleet. This collaboration enhances the overall safety and efficiency of the aircraft.

Furthermore, compliance with Service Letters can also have regulatory implications. Regulatory authorities, such as the Federal Aviation Administration (FAA), often rely on Boeing’s guidance and recommendations when evaluating the airworthiness and safety of the aircraft. Therefore, it is crucial for operators to comply with the instructions outlined in the SLs to meet regulatory requirements and maintain the necessary certifications.

How are Service Letters Distributed and Implemented?

Service Letters are typically distributed electronically by Boeing to its customers and other authorized parties. The recipients of SLs are responsible for disseminating the information to their respective maintenance and operational departments to ensure appropriate action is taken.

When a Service Letter is issued, Boeing provides clear instructions on the implementation process. This may include information on the applicable models and serial numbers of the affected aircraft, the timeline for compliance, and any necessary supporting documentation.

Depending on the nature of the SL, the required actions can vary. In some cases, it may involve a simple inspection or verification process, while in others, it may require component replacements or modifications. The complexity and duration of the required tasks depend on the specific issue being addressed.

Once the necessary actions are completed, the maintenance organizations or operators are required to document the compliance in their maintenance records. This documentation serves as proof that the required actions were taken and can be reviewed by regulatory authorities during inspections or audits.

It is crucial for operators and maintenance organizations to closely monitor the issuance of Service Letters by Boeing and ensure timely compliance with the instructions provided. Neglecting to act on SLs can have implications on the safety and airworthiness of the aircraft, as well as regulatory compliance.


Service Letters are an essential part of the Boeing 737 ecosystem, providing important information and guidance to operators, maintenance organizations, and other stakeholders. They ensure that the aircraft remains safe, reliable, and complies with regulatory requirements. By promptly addressing concerns and recommending necessary actions, Service Letters contribute to the continued success of the Boeing 737 aircraft.

For More: What is MOD on Boeing 737? (Modify)