Maintenance manuals tell the mechanic when to perform an inspection or service, and how to perform it. Many mechanics believe that the regulations require them to follow the book exactly. But in an excellent column on this murky subject, mechanic and aviation author Mike Busch sums up the regulatory requirements nicely:
The manufacturer's “how-to” instructions are compulsory, but the manufacturer’s “when-to” instructions are not.
Let's say, for example, that the manual requires the aircraft’s spark plugs to be removed and regapped every 100 hours. If a mechanic decides to service the aircraft’s spark plugs, he must do it exactly as instructed in the aircraft manual. The regulations, however, do not require the mechanic to follow the manufacturer's instructions at all concerning when or how often to service the plugs, regardless of how much time the engine has accumulated. As Busch explains:
No manufacturer can mandate any maintenance requirement on a part 91 aircraft owner; only the FAA can do so.
There is another part of the story, however, that Busch's column doesn't address. The FAA regulations are bare minimum requirements only. If an accident occurs because the mechanic failed to comply with the manufacturer’s recommendations, questions can arise as to whether the mechanic was negligent – that is, not reasonably careful -- and thus liable to those injured as a result. A jury may conclude that, though the regulations didn't require him to, a reasonably careful mechanic would have followed the manufacturer's recommendations anyway. After all, does a reasonably careful mechanic believe he knows better than the manufacturer?