How do a service letter and service bulletin differ.
In the United States and presumably in other countries, there is no Government oversight on how a manufacture communicates to aircraft owner/operators except that they must issue some form of mandatory compliance "Service Bulletin" for Part 135/121 commercial operators. There is no industry guideline for what a document is called, or the information it conveys.
Manufactures often use an assortment of documents and the type of information in each document is left up to the company. For example, Continental Motors uses almost every kind of document I have ever seen...
- "Service Bulletin" i.e. SB "used by most manufactures..."
- "Critical Service Bulletin" i.e CSB 01-1 "Fuel Pump Inspection..."
- "Service Bulletin Mandatory" i.e M64-18 "Field Conversion to Turbocharging..."
- "Service Information" i.e. SIL99-2A "Current Listing of Sealents..."
- "Service Information Directive" i.e. SID97-4C "Cylinder Bore And Piston Fit..."
- "Service Letter" i.e 556c "Shouldered magneto drive shaft bushing..."
There are others I don't have copies of such as information sent to marketing, promotions, or financial departments. Additionally, the FAA will require specific forms to be used as part of the approval for Part 135, 119, and 121 operating rules. For example some type of "trace-ability" paperwork is required to be attached to all materials and parts. A "trace-ability" document provides manufacture, model, mil-spec, fire resistance, etc.
It is interesting to note that despite the universal use of the Red, Yellow, and Green parts tagging system, their is no FAR regulation requiring it's use.
It's usually a matter of priority and importance. A Service Letter is normally used to pass information along to the industry from the manufacturer and are not mandatory. Service Bulletins are highlighting more important service difficulties found in the field. They can be labelled as Mandatory by the manufacturer, but still are not required to be performed on privately owned aircraft. Commercial operators are normally required to comply with mandatory SB's by the regulatory authority (FAA, TC, EASA, etc.). Mandatory SB's are often the basis of Airworthiness Directives published by the regulatory authority.
If you want to check them out for yourself to get an idea of how they differ, check out Piper's technical documentation repository here: