PMA Parts

[PT20J]

There are three NTSB investigators in my Lycoming factory engine class. I learned something interesting about PMA parts.

Suppose a manufacturer reverse engineers an OEM part and gets a PMA to produce it.

Then suppose that the OEM part is found to have a design defect and is subject to an AD requiring replacement with an improved part.

Now, suppose you had installed the PMA copy of the original defective part.

The AD doesn’t apply to your part (different part, different manufacturer). The PMA manufacturer is not obligated to change its design unless an AD is issued against it. So, you might have a legal but defective part installed and there is no easy way to know that.

Apparently, this has actually happened where PMA parts have caused accidents. 

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[kortopates]

this is very common between Superior PMA parts for Lycoming and Continental parts. Remember the recent Lycoming rod bearing debacle AD, those with PMA Superior bearings where exempt from the AD. But in that case it was perfectly legit not to include Superiors because the issue was a manufacturing defect as opposed to a design defect.
Can’t actually think of a design defect AD along these lines but you raise a great point. Would have liked to think that the FAA is smart enough to include (Superior) PMA parts in their assessment when issuing AD’s on OEM parts.


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[Igor_U]

This is to be expected as PMA part has a different p/n and even looking the same, might not have the same issue due to different manufacturer or process. Also, it does not have to be exact copy of the part. Good examples are LASAR parts like Manual Gear lock is a their own design and is machined out of Al plate while original part is, I believe, casting.

 

 

[PT20J]

It is certainly true that PMA parts can be an improvement over the OEM equivalent. It likely comes down to how the part was designed. Did the design start from scratch analyzing all the loads and other parameters, or did it just assume that the OEM part was correct and make a carbon copy including any defects?

So, a PMA part could be better, worse, or the same as an OEM part. And there is really no way to know.

Personally, for critical components, I’m betting that the OEM has the most data and experience.  It seems that building aftermarket cylinders has proven to be difficult. 

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