Focusing on semantics over content misses the point and could lead to potential safety hazards.

The overhauled engine had been running fine, with a normal break-in pattern. Then, at about 170 hours, the owner received an email from his parts supplier alerting him that one of the through-bolts he purchased for the overhaul had been engineered incorrectly and that he “could possibly” have one of the suspected parts.

The manufacturer had issued a mandatory service bulletin requiring that the affected through-bolts be replaced, and the supplier assured the customer the new part would be on its way soon, at no charge.

This scenario sounds good—until you realize that although the replacement bolt may be free, the labor cost to install it is ­double or triple the cost of the part itself, and the manufacturer won’t be reimbursing you for the expense, even though it’s their fault you need the labor in the first place.

The Purpose of Service Bulletins

At this point, you might be wondering whether our bolt customer was required to change the suspected part based on a service bulletin mandate. If the aircraft is being used in accordance with 14 CFR Part 91, the answer is no, as long as it’s not part of an issued airworthi­ness directive (AD).

But is it a good idea to ignore a service bulletin? Before we answer that question, let’s explore why service bulletins are issued.

Manufacturers issue service bulletins to make owners aware of maintenance issues, manufacturing deficiencies, or product improvements that could affect safety. If the bulletin is highlighted with words such as “mandatory,” “emergency,” or “alert,” the manufacturer is telling you the information is of significant safety importance but hasn’t risen to the level of an airworthiness directive—at least not yet. The bulletin alone isn’t mandatory for Part 91 operators even though it uses powerful words implying that it is.

In the bolt customer’s case, the manufacturer couldn’t tell him whether the newly overhauled engine had a defective through-bolt; the company could only say the part was purchased during the time frame in which defective parts were manufactured. So, should the suspected through-bolt be replaced?

Doing so would require removing several other assemblies in the way, parts such as the oil cooler, governor, and engine baffling, thus creating more work.

Furthermore, the engine was running fine with no indication of an impending problem. We’re talking about hours of maintenance labor costs and aircraft downtime.

Expect the Unexpected

In the end, the bolt customer decided to have the maintenance performed in accordance with the mandatory service bulletin. He expected the identified bolt would be removed and found to be a good, unaffected part. After completion, the work could be signed off as performed and everyone would be happy.

The part on the right (above) was improperly engineered, with its center dowel section out of position. (HAI/Zac Noble photo)

But that’s not what happened. The through-bolt was removed, but even though the engine had been running well and hadn’t presented any signs of possible problems, the part was indeed inaccurately engineered, as seen in the photo at right. (The bolt pictured on the right is the defective one.)

The bolt’s center dowel section, which keeps the engine case from vibrating and chaffing during operation, was about half an inch out of position, potentially depriving one of the case halves of the dowel’s support, thereby allowing it to vibrate. This, in turn, could have led to aluminum fretting and allowed the crankshaft bearings to slip, resulting in oil-lubrication and cooling problems. The end result could have been a loss of engine power.

Whose Guidance Are You Following?

Back to our question of whether complying with a service bulletin is required.

I hope you answered with a hearty YES. If you didn’t, I hope you at least think it’s a good idea.

Allow me to ask another question: If you’re not following the manufacturer’s guidance, whose guidance are you following? Consider your answer carefully.

Bottom line: Follow the manufacturer’s recommendations for maintenance. They have the expertise to issue the correct guidance. If that guidance comes in the form of a service bulletin, make sure it doesn’t pertain to your aircraft or appliance before you disregard it.

Service bulletins are intended to inform aircraft owners of problems the manufacturer has identified with their product, or of enhanced ways to care for one’s machine. Ignore them at your peril.

Fugere tutum! 

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Zac Noble

Zac Noble

Zac, HAI’s director of maintenance and technology, joined the association as its deputy director of flight operations and technical services after 11 years of flying in the air medical sector. A US Army veteran, Zac’s aviation career spans more than 35 years. He is a dual-rated ATP, a dual-rated CFII, and an A&P with IA privileges.

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