STC-PMA approved - available with or without the STC?

[Jim Logajan]

The 1956 C-172 I bought has an old Delco Remy pull starter for its Continental O-300. It sometimes acts up. A search for solutions uncovered a document authored by Ron Wanttaja that helps diagnose and possibly fix problems with such starters. However, I'm leasing the plane to a flying club so I think it best to install a modern pushbutton lightweight starter to make things easier for everyone flying the plane. I believe this starter will work:

BC320-1 Continental Starter (STC/PMA) - B&C Specialty Products

The B&C Starter for Continental engines (C-75, C-85, C-90, C-145-2, O-200, IO-240, O-300-A, -B, and -C, and GO-300-A, -B, and -C) solves all the problems that other starter designs have had: worn-out clutches, broken pinions, nagging oil leaks, and catastrophic failure modes. Featuring the...

bandc.com

But I don't quite understand this part:
"The B&C Starter for Continental engines is STC-PMA approved for use in certificated aircraft, and is available with or without the STC."
So my question is: Under what circumstances, if any, would I need STC paperwork for my aircraft if the part is also available under a PMA? Who would need to install under STC? (It looks like the STC was issued in 1994 and the PMA approved in 2005.)

 

[Richard_Allred]

How do you like the 1956 172's? How are they compared with like 1970's 172's?
Does it feel like an old classic airplane?

 

[Ron Wanttaja]

But I don't quite understand this part:
"The B&C Starter for Continental engines is STC-PMA approved for use in certificated aircraft, and is available with or without the STC."
So my question is: Under what circumstances, if any, would I need STC paperwork for my aircraft if the part is also available under a PMA? Who would need to install under STC? (It looks like the STC was issued in 1994 and the PMA approved in 2005.)

I wrote that 20 years ago, but I think my intent was to say that owners of Experimental aircraft could buy the starter without an STC, but owners of a certified aircraft aircraft would have to buy the STC version.

Ron Wanttaja

 

[Jim Logajan]

I wrote that 20 years ago, but I think my intent was to say that owners of Experimental aircraft could buy the starter without an STC, but owners of a certified aircraft aircraft would have to buy the STC version.

Ron Wanttaja

Sorry, I should have been clearer in my post - the italicized quote came from the B&C web page, not from your web page. I linked to your page because I considered it very useful if I chose to try and adjust the existing starter than replace it.

The B&C starter was first STCed and they later got it PMAed for the same engines (per the FAA the PMA appears to refer back to the STC.) I can find a lot of articles explaining STCs and PMAs but haven't found anything that explains the deal when a part has both - or why one would want STC paperwork if the part already has PMA approval.

 

[Jim Logajan]

How do you like the 1956 172's? How are they compared with like 1970's 172's?
Does it feel like an old classic airplane?

I did not notice much difference in flight characteristics, though the early 172s use a Johnson bar to actuate the flaps rather than an electrical switch. Also, access to the baggage area requires folding down the back seat, while later models have a side baggage door. No rear facing window is not noticed during flying. Other than the flap handle the thing I noticed is the forward visibility seemed much better - probably because of the panel curvature vs later squarish panels.

 

[rotorwrench]

So my question is: Under what circumstances, if any, would I need STC paperwork for my aircraft if the part is also available under a PMA? Who would need to install under STC?

I can find a lot of articles explaining STCs and PMAs but haven't found anything that explains the deal when a part has both - or why one would want STC paperwork if the part already has PMA approval.

In general, if a vendor develops a part (article) that could be used across a diverse set of aircraft type designs, the vender may elect to combine individual methods from the STC, PMA, and even a TSO approval process, to complete the overall approval for their specific article. For example, on Concorde batteries you will find an STC, PMA, and TSO approvals on the same model battery.

Now which approval to use is determined by the part installer which could be the owner/pilot, mechanic, or a mechanic with an IA if the install is considered a major alteration/repair. In the case of your starter, unless the installation is a “plug and play” type replacement, your only option would be to use the STC.

It’s a much deeper rabbit hole to go down on the “why” multiple approvals, but if you have more questions post them.

However, I'm leasing the plane to a flying club so I think it best to install a modern pushbutton lightweight starter to make things easier for everyone flying the plane.

FYI: while lightweight starters have their place, they are not very tolerant of repetitive start attempts in short time periods. You may want to re-look into installing one for a flying club where the potential to have multiple, different starting techniques exists that may shorten the starter’s lifespan if the starter limitations are not followed precisely.

 

[Jim Logajan]

It’s a much deeper rabbit hole to go down on the “why” multiple approvals, but if you have more questions post them.

No more questions - thanks for trying to explain. The starter issue may be something else:

I measured the airplane's battery voltage yesterday with a multimeter and it registered only 11.34V (Uavionix AV30 reported bus voltage of 10.4V.) It is a new Concorde RG-25 installed June 11, 2023 by the previous owner. Based on bus voltages shown in 4 photos (11.2, 10.8, 10.9, and 10.4V) of the Uavionix AV-30 by the previous owner and myself over about three months I suspect the battery may have a problem holding a charge. Or generator losing its charging capacity. I had intended to eventually buy a battery minder anyway so have ordered a BatteryMinder128CEC2-AA-S5. Will see if it recovers the battery. Otherwise battery may still be under partial warranty if it is at fault. I suppose I could insert an ammeter between the battery and ground to see if the plane has some tiny current draw from something. Seems unlikely.

 

[rotorwrench]

I suppose I could insert an ammeter between the battery and ground to see if the plane has some tiny current draw from something. Seems unlikely.

In my experience, parasitic drains are the #1 battery killer on aircraft. A 50 milliamp drain will drop your RG-25 to 0% capacity in less than a week. If you want try and save your battery, the minute your battery minder shows up, pull the battery and charge it per the manual in an ambient temp stable environment.

Once its fully charged, let it rest for at least 12 hours then check the open-circuit voltage. If less than 12.4-12.5 then I would see if you can get a warranty claim given its less than a year old. Usually after a full charge and rest if the voltage is less than 50% capacity it’s a sign of excessive sulfidation which I don’t think the Minder is capable of reversing.

Regardless, I would definitely use your VOM if it has an amp function to check for a parasitic drain at 1st chance and follow it up with a review and check of your complete electrical system if you don’t want on going issues.

 

[Jim Logajan]

Haven't had a chance to test for parasitic current drain yet, but while waiting for the charger to arrive I've been curious about the generator and regulator systems. I ran across this nicely done tutorial on Youtube on how vibrating point regulators work:

 

[Jim Logajan]

In my experience, parasitic drains are the #1 battery killer on aircraft. A 50 milliamp drain will drop your RG-25 to 0% capacity in less than a week. If you want try and save your battery, the minute your battery minder shows up, pull the battery and charge it per the manual in an ambient temp stable environment.

Once its fully charged, let it rest for at least 12 hours then check the open-circuit voltage. If less than 12.4-12.5 then I would see if you can get a warranty claim given its less than a year old. Usually after a full charge and rest if the voltage is less than 50% capacity it’s a sign of excessive sulfidation which I don’t think the Minder is capable of reversing.

Regardless, I would definitely use your VOM if it has an amp function to check for a parasitic drain at 1st chance and follow it up with a review and check of your complete electrical system if you don’t want on going issues.

Late follow-up:

No parasitic drain detected on the battery.
After the charger reported it was fully charged I disconnected it and a multimeter measured 13.23V across the battery terminals (not considered valid meaurement for various reasons, but decided to do it anyway.) Fifteen hours later it measured 12.92V. A day after that it was still at 12.92V.

So I turned on the master with (I believe) only the uAvionix AV-30 drawing power. Under its own load the AV-30 reported bus voltage of 11.7V while at the same time the multimeter measured 12.5V across the terminals. Tried the starter with the magnetos off and it turned over nice and brisk. After a bit of turning things on and off and checking current draw I shut everything off. Voltage across the battery terminals went up to 12.8V without any load.

Next day I did some runups and a single pattern circuit. Bus voltage remained near 11.6V and ammeter indicated the generator wasn't generating. I've never flown a plane with a bus voltage indicator so plead ignorance on not realizing it should probably read over 13V at high engine RPMs - taking into account the ~7% voltage divider drop I noted earlier. And if you look at photo of the panel earlier in the thread you'll see the ammeter hiding just under the pilot yoke control bar. Very difficult to see. Anyway, that's my lame excuse for why I didn't diagnose a generator problem much earlier.

Had a mechanic check the generator, voltage regulator, and wiring to diagnose the specific problem. He texted:
"Hi James, it's [...] I’m going to order a voltage regulator for your aircraft, it seems it is not sending the proper field voltage to the generator to produce proper output voltage. I did polarize the generator with no success, all fuses were good. I won’t be able to get the regulator here until Thursday. Thank you"
When I asked what regulator he said "It’s a newer version, Zeftronics G1200N"
I'd already run across positive recommendations for Zeftronics regulators and had in fact priced that specific model "just in case." Hoping it resolves the problem.

 

[rotorwrench]

Under its own load the AV-30 reported bus voltage of 11.7V while at the same time the multimeter measured 12.5V across the terminals.

This part caught my attention. There are only 2 reasons to have this voltage difference: either the AV-30 is inaccurate or you are physically losing 60+% of your battery power between the battery and the bus. You may also want to track this down first before replacing the regulator.
If your buss voltage is truly 11.7 with 12.5 at the battery, using the diagram below, check for voltage drops starting at the batt positive post across all connections to the buss. You should have the same voltage from the battery to the buss +/- .2-.3V.

 

[Jim Logajan]

This part caught my attention. There are only 2 reasons to have this voltage difference: either the AV-30 is inaccurate or you are physically losing 60+% of your battery power between the battery and the bus. You may also want to track this down first before replacing the regulator.
If your buss voltage is truly 11.7 with 12.5 at the battery, using the diagram below, check for voltage drops starting at the batt positive post across all connections to the buss. You should have the same voltage from the battery to the buss +/- .2-.3V.

View attachment 515

The regulator arrived this morning and was already installed and tested by the time I saw your post. The mechanic said the old regulator was only delivering 12V or so. I had already agreed with him on replacing with the solid state regulator rather than adjust or repair the mechanical one. The new one solved the charging problem: AV-30 reported bus voltage went to above 13.3V at cruise RPM while ammeter showed good charging current.

The manual for the AV-30 says the Bus Voltage accuracy is +/-1.0V
That seems awfully wide, but would put the values I measured within the stated accuracy.
Still, I will do some voltage drop measurements to cross-check the accuracy of the AV-30.

With regard to the starter problem in my original post: previously with the weak battery the engine would start or do nothing - not even any noise. (Corrected the latter by manually moving the clutch plunger with one hand while moving the prop so the clutch gears would smoothly mesh prior to starting from the cockpit.) With a properly charged battery the starter motor now either starts or the clutch slips or grinds. The mechanic ran into the same thing and says that should be fixable by adjusting the actuation arm, which he'll do tomorrow. (According to Wanttaja's web page any grinding noise might indicate damage. Hmmm.)

 

[Jim Logajan]

Belated report on the bus voltage reported by the uAvionix AV-30: it appears it is reporting about 0.5 to 0.6V low. My multimeter shows that much higher when measuring at various points behind the instrument panel and at the cigarette lighter.