The short answer
A service bulletin is a notice from an aircraft, engine, or component manufacturer describing recommended inspections, modifications, or product improvements. For most privately operated aircraft, service bulletins are advisory unless an Airworthiness Directive or the aircraft's own maintenance program makes them mandatory.
Categories and weight
Manufacturers grade bulletins by urgency — mandatory, alert, or optional in their own terminology — but a manufacturer's "mandatory" label does not by itself create a legal obligation for Part 91 operators. The FAA makes a bulletin's actions compulsory by issuing an AD that references it.
Even when optional, service bulletins are valuable maintenance intelligence: they document known issues, improved parts, and inspection techniques for the fleet. Records of which bulletins have been complied with are a meaningful quality signal in an aircraft's history.
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.