Charging System Repair or How to Spend 12% as Much By Personal Involvement in Aircraft Maintenance

Simple is good

After reflection I realize that while avoiding waste and saving money is important to me, there’s another and perhaps more compelling reason I like to be directly involved in anything done to my plane: it’s safer to know exactly what was done and how it works, down to a detailed level.

I have a very simple 175K km motorcycle in Europe that prior to my buying it had a reputation for leaving the owner stranded. Since then I’ve found it to be reliable over 17 years of use, only once having a major issue. I think the reason is being aware of all its functions so that one can tell early when something is going out of spec. Obviously the simplicity of the machine in both cases makes that easier.

Here’s the technical conclusion, for those who might be interested. Pictured are the old (top) and new (bottom) brush mounts. You can see black rings around the rivets where the original connections got hot, melting the plastic surrounding the riveted connection. Eventually the extruding black plastic loosened and then broke the electrical connection to one brush lead, the excitation current went to zero and the alternator failed.

The only difference I could see between old and new ‘improved’ Hartzell unit is that the brush lead wires are now epoxied down to provide strain relief. I don’t think that would have any effect on the failure mode I saw here, but regardless my plane is now back together, charging the battery and airworthy. It might be worth me pulling the brushes every so often to assess their condition, it’s only a few minutes work if you don’t remove the alternator itself.

The appearance of electrical parts, windings etc on many machines does tend to reflect them being hand made, for the most part. I’d bet those rivets that came loose on the old part were set by hand by a $20/hr worker and it’s not a great design regardless.

If you wanted to clean this up the assembly could be done in one piece, versus two pieces connected with the riveted leads. But without changing the existing housing design a little bit it couldn’t be pushed into place that way.

Agreed – the ‘improvement’ is a negligible improvement, it’s likely just an artifact of a changed part number to reflect what is really only a ‘process improvement’ (adding a couple of dabs of glue). $170 for this part is absurd, but that’s par for the course in aviation and OTOH I’m happy it was such an easy fix for me.

It’s interesting to me that the lugs under the rivets are relatively large in diameter. I’d guess that this is for cooling of the connection. The riveted lead wire arrangement is not a fundamentally good design and I bet that as a result there’s been a series of tweaks to make it work better. It would have been better to redesign the whole thing especially since the rest of the alternator looks good – I love the accessibility of the brushes for service. It’s likely that redesign options were limited by the existing PMA approval and nobody wants to spend more money.

You can spend a lot of time looking at such a tiny little thing but that is actually what technology is made of, not hand waving

For those who don’t mind digging into an issue deeply…

The reason why the brush mount design in post #12 looks a little ad hoc is because it’s not part of the original Lycoming-supplied Prestolite alternator design. The replacement design was introduced in 2005 as an ‘upgrade’ to the alternator line being produced at that time by Kelly Aerospace, under FAA PMA. That’s why my failed brush holder produced much more recently than 2005 looks like the 2023 replacement except for the glue strain reliefs. The original pre-2005 design was one piece and as I suspected a one-piece design is difficult to install or remove. So Kelly came up with the two piece design with leads between the two halves, the riveted attachment of which caused my excitation circuit failure after about 200 hrs. I can’t see the way the original style brushes/leads are attached in the Kelly SB but I wouldn’t be surprised if the original one-piece design might have been more durable.

Kelly SB Explaining History Here
SIL_A_116_pdf

Nowadays the PMA and facility are owned by Hartzell, but the design has been unchanged since 2005. Given the premature failure I’ve seen with a two-piece brush mount of the same ‘improved’ type that I’ve just installed as a replacement, I find this tedious archeological research interesting Next time the cowling comes off, I’ll be pulling the brush block and looking for signs of the same failure mode repeating itself.

What I find more of an issue in owning a light aircraft is not the lack of mechanic skills availability to maintain them in the field, so much as the sometime lack of engineering skills availability to sustain the designs and produce parts of the same quality that were available decades ago. Getting back to the advantages of being involved in maintenance as an owner/pilot, looking new parts over carefully before installation is a valuable thing to do.