Question on Prop Re-pitching Legality

[farmrjohn]

If either a TCDS or STC states only a specific prop, i.e. DCM69/48 and specific static rpm, i.e. 2350-2450 are allowed, is it legal to re-pitch from the /48 to achieve the desired static RPM?

Does that change the designation of the prop, i.e. to a 69/50 if static was too high, or 69/46 if static was too low?

If the static remains OK, is it legal to increase the pitch to minimize exceeding redline RPM?

Separately, if the STC an engine change was based on (but did not apply directly to that aircraft) called for one specific prop, i.e. DCM69/48 and the installation description in the 337 had a different prop installed, i.e. DCM69/50, which would govern for a replacement prop?

And further, if that replacement prop was legally re-pitched 30 years ago to 69/51, could that be a justification to install a new 69/52 if the static RPMs were on the high side of the range and the throttle needs to be pulled back to avoid exceeding redline in cruise?

(edited format for clarity)

 

[Let'sgoflying!]

I'm seeing five separate questions.
Consider rolling these out in point form to help us address them?
Or, perhaps the most important question first?
Have you considered running this all past a prop shop, they will likely have good info on this, FJ.

 

[farmrjohn]

I'm seeing five separate questions.
Consider rolling these out in point form to help us address them?
Or, perhaps the most important question first?
Have you considered running this all past a prop shop, they will likely have good info on this, FJ.

Edited, thanks.

The prop shop wasn't that helpful with the questions basically deferring to what I want. My IA is good with replacing like for like, in this case a re-pitched 50 to 51. I'm wonder if that re-pitch 30 years ago was legal since the 337 says 69/50 and the STC it's based on specifically states only 69/48. There are also some background issues with the initial order back in May as per my "rant" post. I'm re-visiting the original order since time savings are apparently gone.

 

[Bell206]

From strictly a configuration point of view....

If either a TCDS or STC states only a specific prop, i.e. DCM69/48 and specific static rpm, i.e. 2350-2450 are allowed, is it legal to re-pitch from the /48 to achieve the desired static RPM?

No. It would require an approval to use a prop other than the 6948. And only a CRS shop can re-pitch as it is considered a major alteration.

Does that change the designation of the prop, i.e. to a 69/50 if static was too high, or 69/46 if static was too low.

Yes.

And, if the static remains OK, is it legal to increase the pitch to minimize exceeding redline RPM?

No.

Separately, if the STC an engine change was based on (but did not apply directly to that aircraft) called for one specific prop, i.e. DCM69/48 and the installation description in the 337 had a different prop installed, i.e. DCM69/50, which would govern for a replacement prop?

??? STCs are applicable to the type certificate being altered. Most engine change STCs are applicable to the aircraft and its that install data that should be reflected on the 337. Don't quite follow the question.

if that replacement prop was legally re-pitched 30 years ago to 69/51, could that be a justification to install a new 69/52 if the static RPMs were on the high side of the range and the throttle needs to be pulled back to avoid exceeding redline in cruise?

The only legal replacement prop you can use is what is listed in the STC from what I read unless I'm missing something?

 

[GeorgeC]

"I can't make the static rpm requirement with an approved prop, will my airplane be less broken if I switch to an unapproved prop?"

 

[Dan Thomas]

I wonder how accurate the tachometer in this airplane might be. If it's overreading but the error hasn't been caught, the OP might be making expensive mistakes.

And, as others have said, reducing pitch of an approved prop to achieve static is overlooking a sick engine.

 

[farmrjohn]

??? STCs are applicable to the type certificate being altered. Most engine change STCs are applicable to the aircraft and its that install data that should be reflected on the 337. Don't quite follow the question.

The only legal replacement prop you can use is what is listed in the STC from what I read unless I'm missing something?

The 337 field approval was based on the information from an STC but that STC did not apply to the airplane. While the STC does call for a 69/48 the 337 was approved by the FAA with a 69/50. This is the quote: 2. Installed TCM factory rebuilt, zero timed engine model O-200-A serial # 285295-R and new McCauley Prop model #1A101-DCM6950, serial # GN010 in accordance with instructions and procedures contained in STC # SA-2628WE with one deviation. 3. Deviation consists of removal of pull type starter part #1109659 with old engine.

I wonder how accurate the tachometer in this airplane might be. If it's overreading but the error hasn't been caught, the OP might be making expensive mistakes.

And, as others have said, reducing pitch of an approved prop to achieve static is overlooking a sick engine.

I'm assuming the tach information is correct. The original tach is still installed along with a CGR-30P (approved as primary) which is based on pulses in the P-lead vs. a mechanical cable. The CGR-30P reads higher than the original so I would not be reducing pitch to make static. Static is right at the top of the range on the CGR and middle of the range with the mechanical tach. The engine will exceed redline in level flight with full throttle so I would be looking for more pitch as the original owner did in 1994 when they had it re-pitched as a 69/51, which is what the log book says. (The engine was swapped out from a C-90 to O-200 thanks to Chevron and their Watsonville episode).

 

[Dan Thomas]

(The engine was swapped out from a C-90 to O-200 thanks to Chevron and their Watsonville episode).

And the C-90's prop was installed on the O-200? The reason for overspeed becomes a little more obvious. Was there an STC for the engine change? It should have prop change information as well.

Is the tach calibrated to the O-200's higher redline?



If the airplane's tach is still the C-90's, I can see where the overspeed comes from.

 

[farmrjohn]

And the C-90's prop was installed on the O-200? The reason for overspeed becomes a little more obvious. Was there an STC for the engine change? It should have prop change information as well.

Is the tach calibrated to the O-200's higher redline?

View attachment 132194

If the airplane's tach is still the C-90's, I can see where the overspeed comes from.

There was no STC for the engine change, the 337 entry states they used the instructions and procedures from the STC. They did include the operating instructions and limitations from the STC in the records. The C-90 prop was removed and a new DCM6950 installed. The tach is calibrated for the 2750 max rpm, which full throttle will exceed in level flight

 

[Bell206]

The 337 field approval was based on the information from an STC but that STC did not apply to the airplane. While the STC does call for a 69/48 the 337 was approved by the FAA with a 69/50.

This makes more sense now. So your only legal option now is the 6950 listed on the 337. Any deviance from that model will require a new 337 for the prop spec change. And to add, this is why you have an RPM issue as the prop was not matched to the engine as would have been required under the STC.

FYI: it was this use of STC data with a field approval that gave rise to the regulation requiring a permission letter from the STC holder to use their data.

I'm assuming the tach information is correct.

Don’t assume. However, keep in mind there are properly certified engine/prop combos out there that will give you more static RPM than permitted and it falls to the pilot to maintain those limits manually.