An Airworthiness Directive (A.D.) is a directive issued when the FAA realizes that a perilous condition exists in a product (aircraft engine, airframe, appliance or propeller).  They notify aircraft operators and owners of potentially unsafe conditions requiring special inspections, alterations, or repairs.

A Service Bulletin (S.B.) is a notice to an aircraft operator from a manufacturer informing him/her of a product improvement. An alert service bulletin is issued when an unsafe condition shows up that the manufacturer believes to be safety-related as opposed to a mere product improvement.

Service bulletins often result in the issuance of Airworthiness Directives by the FAA. An airworthiness directive references the alert service bulletin to comply with the AD.

Having realized that there were distinct levels of seriousness to a service bulletin, manufacturers started categorizing them as optional, recommended, alert, mandatory, informational, etc. It was left to the manufacturers to classify a service bulletin as they considered best because there was no terminology standard.  Differentiation between non-mandatory service bulletins is done and decided only by the FAA.

Although a service bulletin may be categorized as mandatory by the manufacturer, it is crucial to know that compliance with service bulletins isn’t necessarily required under the FARs (Federal Aviation Regulations) unless the service bulletin includes or is accompanied by an airworthiness directive.

As opposed to service bulletins, airworthiness directives affect the safety conditions of a flight. It’s for this reason compliance becomes mandatory.

So, just because the FARs don’t necessarily demand an aircraft owner comply with service bulletins, does this imply an aircraft owner or operator can ignore service bulletins? Not necessarily. The inaction may come back to haunt the aircraft owner at some time in the future. Therefore, it is always important to keep in mind that manufacturers issue service bulletins because they believe compliance will make their products safer.

However, compliance with service bulletins translates into higher costs to the aircraft owner. Whether it requires the performance of a more detailed and elaborate inspection or replacement of a component, the service bulletin’s recommendation simply means that the aircraft owner will dig deeper into his/her pocket in paying for labor or parts. As a result, most aircraft owners reject or defer compliance with service bulletins to save money.

This is a usual scenario, especially where the aircraft owner feels that the aircraft is safer even without compliance. In fact, if a service bulletin doesn’t have an airworthiness directive, the FAA doesn’t deem its recommendations to be mandatory or necessary. So, why should the aircraft owner pay extra costs for maintenance or parts that may not make the aircraft safer?

If the safety of flights is not an alarming issue, an aircraft owner may decide to perform a cost-benefit analysis to compare the benefits of complying with a service bulletin and the cost of compliance. This analysis assists an aircraft owner in deciding whether to comply or not to comply with a specific service bulletin.

However, there is a way for the manufacturers to make a service bulletin regulatory for all the affected aircraft. Requirements referred to or called out in the TCDS (Type Certificate Data Sheet) or within the airworthiness limitations, part of the aircraft maintenance manual, are, unexceptionally, mandatory.

The bottom line? Compliance with an A.D. is exclusively mandatory; compliance with an S.B. is not mandatory unless the service bulletin includes or is accompanied by an airworthiness directive.