Service Bulletins and Airworthiness Directives

When an aircraft is down for regularly scheduled maintenance or other routine services, operators typically use that downtime to also make enhancements in the cabin and install updates in the cockpit. On some occasions that downtime is used to incorporate actions addressed in a Service Bulletin (SB) or an Airworthiness Directive (AD).

As a rule, SBs  and ADs  are safety-related changes that apply to airframes, propellers, engines, radios, or any of the other myriad parts of an aircraft. Both documents are the result of either discovering a safety-related anomaly during a part’s production, flaws uncovered during maintenance, or during an investigation of an incident or accident.

An SB originates with the original equipment manufacturer (OEM) and is distributed to all known owners and operators of the affected component. Although an SB may be labeled by the OEM as “urgent”  or  “mandatory,” aircraft operators don’t necessarily have a legal requirement to comply, unless the SB is part of the resolution to an AD or otherwise required by the operator’s maintenance program. This is typically the situation for business jets.

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The cost associated with compliance (acquiring the new part, its installation and possibly extended downtime) is one reason why an  owner/operator may decide not to comply, if that is an option. Non-compliance with an SB has been found to be a factor in an incident or accident. In some instances, the OEM bears the cost under warranty coverage or customer loyalty.

Notably, an SB does not always address an unsafe situation. It can also convey an OEM developed change that improves operational efficiency, and sometimes these are labeled as “service letters” or “service information.” Unlike SBs, Airworthiness Directives always address an alleged unsafe condition, there compliance is mandatory and they are issued by a regulator.

In the U.S,  Airworthiness Directives are published by the Federal Aviation Administration (FAA).  Because ADs are issued by reference to the affected part or operational parameter and not normally sent to known users, owners and operators must be vigilant to check for relevant directives online at dedicated government web sites. Non-compliance invites serious repercussions, such as fines and/or having an operating certificate or flying license suspended.

Some ADs are recurring and thus may require maintenance action in between regularly scheduled inspections. For maximum safety and regulatory compliance, keeping track of these recurring ADs is essential.

Most directives require initial compliance after a certain number of flight hours or on a future date. Others, particularly “Emergency ADs,” require compliance “before further flight.” In this case, regulators usually allow an owner/operator to get special approval for what’s called a “ferry” permit to return the aircraft to home base if it’s on a trip (but only with essential crew on board) or to fly from home base to an appropriate maintenance facility.

Airworthiness Directives are not always published as a final rule. It behooves U.S. owner/operators to keep tabs on pending ADs published online in the Federal Register as a Notice of Proposed Rulemaking (NPRM). These NPRMs give interested persons about 30 days to comment on specific aspects of the directive, such as intended compliance schedule or the accuracy of cost estimates. Comments rarely prevent a proposed AD from being issued, but they can modify the required changes.