Help interpreting Sensenich prop model #?
I replaced my prop a few years ago because the one that was on the plane was deemed unserviceable.  I don't remember exactly why, but I think the gist was...
/community/styles/poa/apple-touch-icon-180x180.png
Pilots of America
I replaced my prop a few years ago because the one that was on the plane was deemed unserviceable. I don't remember exactly why, but I think the gist was that umpteen years of having nicks filed out at annual eventually caught up to it, or something along those lines.

The old prop model # was 76EM8S5-2-60.

The new prop model # is 76EM8S5-0-60.

The only difference is that -2- went to a -0-.

I remember confirming at the time that 76EM8S5-0-60 was the proper model for my application, but I didn't pay attention to the old prop model number. However, today I was looking through my logbooks and noticed the difference, so out of curiosity I went to the Sensenich site for the decoder ring, which is:

So...does that mean my old prop was "reduced 2 inches from basic diameter" of 76 inches...meaning it was 74" in diameter? (What's the diff between a 76xxxxx-2-xx and a 74xxxxx-0-xx propeller??)

If so, then does that mean I was illegally flying my aircraft for ~10 years, because the TCDS specifies a prop that is "not over or under 76 inches"?

?????

FWIW: I did not notice a dramatic change in engine RPM at full throttle prior to takeoff when the prop was switched. I *did* feel like I could tell a difference in acceleration on the runway...but of course I didn't do any objective before/after measurements. The rational side of me figured it was just my imagination dreaming it was pulling more strongly...you always want to think you made your plane ~10AMU better when you make that kind of investment, right?

A 74.....-0 and a 76.....-0 are the same by the numbers,

To be clear, you meant to say that a 76xxxxx-2-xx and a 74xxxxx-0-xx are the same diameter? (I.e., they're both 74"?)

A 2" diameter reduction should give you roughly 100-200 rpm increase.

I *should* have noticed an RPM drop like that with the new prop. I didn't, but that doesn't mean it didn't happen--might just mean I was oblivious, though a change that big should have driven me out of the correct static runup range and it didn't. I'll go back and look at the engine data from that timeframe and see if I can see anything there.

Thus, my surprise and confusion re: the previous prop.

There are a couple Sensenich docs on this. I found one memo here. As to your previous prop, it did not meet the TCDS requirements. If your aircraft allowed 74" props then it would have been okay. Never really ran across this issue but if you want more verification perhaps a call to Sensenich tech support?

A) Does such field work change the model number?

The Sensenich repair manual gives the requirements based on the repair. However, only a CRS can perform this type of work.

B) Is that routinely done by a reputable prop shop if it violates the TCDS for the application plane?

Most props have their own type certificate to include your 76E series. So all work performed is to meet that TCDS and not the aircraft TCDS so no violation. And that prop is still serviceable and can be installed on an aircraft that permits a 2" diameter reduction. There are a number of aircraft that allow this reduction and still meet the aircraft TCDS limits. The memo above gives one such example.

Most props have their own type certificate to include your 76E series. So all work performed is to meet that TCDS and not the aircraft TCDS so no violation. And that prop is still serviceable and can be installed on an aircraft that permits a 2" diameter reduction. There are a number of aircraft that allow this reduction and still meet the aircraft TCDS limits. The memo above gives one such example.

That makes sense, of course, but that's not really what I meant. What I meant was, if someone pulls the prop off their plane and sends it to a reputable/certified/whatever the right word is for "does the right thing" prop shop, wouldn't such a prop shop be reasonably expected to take pains to stay within the limitations of the application as well as the limitations of the prop itself? (I'm actually now wondering if maybe that's what happened when my prop got replaced--I know it was sent off to a prop shop, I think to have it repainted, and I wonder if the prop shop called my mechanic and said, "Um...you know this prop isn't legal for that aircraft, right?" Maybe that conversation got "softened" when the news was passed on to me. I wish I could remember that situation more clearly, but I don't.)

If I sent my prop off for refurb and the prop shop sent it back with a $2000 bill and said, "We shaved two inches off it," I'd be...disappointed. Surely the application limitations are routinely gathered when prop shop generates the work order or otherwise discussed with the customer before doing work that might violate them?

But...even if the prop shop did shave off two inches...presumably that would have at least been communicated back to the customer at the end of the job, and the A&P/IA should have identified the discrepancy prior to reinstalling it and signing off that the aircraft was airworthy?

I'm having a hard time figuring out what might have happened here, other than some sort of gross oversight or negligence.

(The installation of the 74" prop happened before I bought the plane, so is not something my current mechanic did.)

That makes sense, of course, but that's not really what I meant. What I meant was, if someone pulls the prop off their plane and sends it to a reputable/certified/whatever the right word is for "does the right thing" prop shop, wouldn't such a prop shop be reasonably expected to take pains to stay within the limitations of the application as well as the limitations of the prop itself? (I'm actually now wondering if maybe that's what happened when my prop got replaced--I know it was sent off to a prop shop, I think to have it repainted, and I wonder if the prop shop called my mechanic and said, "Um...you know this prop isn't legal for that aircraft, right?" Maybe that conversation got "softened" when the news was passed on to me. I wish I could remember that situation more clearly, but I don't.)

If I sent my prop off for refurb and the prop shop sent it back with a $2000 bill and said, "We shaved two inches off it," I'd be...disappointed. Surely the application limitations are routinely gathered when prop shop generates the work order or otherwise discussed with the customer before doing work that might violate them?

But...even if the prop shop did shave off two inches...presumably that would have at least been communicated back to the customer at the end of the job, and the A&P/IA should have identified the discrepancy prior to reinstalling it and signing off that the aircraft was airworthy?

I'm having a hard time figuring out what might have happened here, other than some sort of gross oversight or negligence.

(The installation of the 74" prop happened before I bought the plane, so is not something my current mechanic did.)

Some of your questions might be answered if you go back and look at why the prop on your airplane was changed and the 74” propeller ended up on there. My guess is that some event happened that required a prop replacement and someone found the one that you had and thought it was a suitable replacement.

All aircraft have stories, some of which are more colorful than others.

wouldn't such a prop shop be reasonably expected to take pains to stay within the limitations of the application as well as the limitations of the prop itself?

The actual repair would dictate what limitations needed to be adhered to... no pain required.

But...even if the prop shop did shave off two inches...presumably that would have at least been communicated back to the customer at the end of the job, and the A&P/IA should have identified the discrepancy prior to reinstalling it and signing off that the aircraft was airworthy?

I would think so. But I believe it was more an innocent oversight than anything intentional. Not all prop model numbers follow the same format and since all the numbers are the same except one I can see that happening. However, why this detail was missed during the subsequent annual inspections is another matter as technically conformity is a part of any required Part 43 inspection. But sheet happens and now its fixed.

76xxxxx-2-xx and a 74xxxxx-0-xx are the same diameter

But the effective pitch of the 76xxxxx-2-xx is higher than the -0- prop.

Both have 60 inches of pitch, measured at 75% of the full length. So at 28.5 inch radius versus 27.75 inch radius. And since the pitch reduces as you go out, the -0- prop has less pitch at 28.5 inches and further out.

The pitch does not reduce as you go out. The blade angle (measured in degrees) does, but the pitch (the theoretical distance the prop moves forward in one revolution, measured in inches) is constant.

Pitch = 2*π*r*tan(B) where B is the blade angle and r is the radius.

That said, some prop makers play with the pitch along the blade for different reasons, so measuring at 75% gives a more or less standard basis for comparison.

Some of your questions might be answered if you go back and look at why the prop on your airplane was changed and the 74” propeller ended up on there. My guess is that some event happened that required a prop replacement and someone found the one that you had and thought it was a suitable replacement.

All aircraft have stories, some of which are more colorful than others.

I did go digging through the logs, and my first pass through I missed where my previous s/n prop was first installed. Took me some hunting to find it buried in a longer entry. There's an entry for when the prop before *it* was removed for overhaul. The reinstall initially says the same prop was installed, but that's lined through and the s/n of the prop I just replaced is written in. The prop part number at that point has an -0-, not a -2-, so it's the correct prop model. I also found a yellow tag stapled at the back of that log book with the model and serial number of that then-new prop, also with the correct -0- model number.

The next log book has the prop model and serial # written in on the information page at the front of the book, along with the aircraft and engine model and serial numbers. On this page, the prop model number is written with a -2-.

So now I wonder if someone made an error when populating that information page in the new logbook, and any subsequent references to the -2- model number were just copied from there?

I did go digging through the logs, and my first pass through I missed where my previous s/n prop was first installed. Took me some hunting to find it buried in a longer entry. There's an entry for when the prop before *it* was removed for overhaul. The reinstall initially says the same prop was installed, but that's lined through and the s/n of the prop I just replaced is written in. The prop part number at that point has an -0-, not a -2-, so it's the correct prop model. I also found a yellow tag stapled at the back of that log book with the model and serial number of that then-new prop, also with the correct -0- model number.

The next log book has the prop model and serial # written in on the information page at the front of the book, along with the aircraft and engine model and serial numbers. On this page, the prop model number is written with a -2-.

So now I wonder if someone made an error when populating that information page in the new logbook, and any subsequent references to the -2- model number were just copied from there?

What is stamped on the prop itself (both P/N and S/N)? It wouldn’t be the first time I’ve heard of a logbook not accurately reflecting what is installed on the aircraft. It is also a good lesson on why mechanics should verify that the records match the configuration of the physical aircraft when performing an inspection.

Regardless, the error got caught and corrected. That’s the important part. My best continues to be that some event occurred that necessitated the prop removal (such as a prop strike) and a different one that was believed to be the same was installed. Or the prop shop did the best they could with what they had to work with on the instruction of the owner and it ended up being a 2 instead of a 0.