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prop RPM cycling

jacksel

Well Known Member
I have an MT P-420-3 governor installed on my RV-6 with a Lycoming 0-360A1A engine. 11 years and 700 hours of reliable service. Recently I had 2 experiences where the propeller RPM began cycling. Pretty sure it started when I abruptly brought the power back to idle. It stopped cycling when I added a little power. Unfortunately I didn't have the presence of mind to note the specific tach readings. Has this ever been reported before? And should I be concerned?
 
I have an MT P-420-3 governor installed on my RV-6 with a Lycoming 0-360A1A engine. 11 years and 700 hours of reliable service. Recently I had 2 experiences where the propeller RPM began cycling. Pretty sure it started when I abruptly brought the power back to idle. It stopped cycling when I added a little power. Unfortunately I didn't have the presence of mind to note the specific tach readings. Has this ever been reported before? And should I be concerned?

Based on the part number you provided (MT P-420-3), I think you may have a Jihostroj prop governor that was labeled and sold as an "MT".

Look here at the bottom of the page:

http://www.propellergovernor.com/faq/

Here is what it says...

Is JIHOSTROJ governor MT governor?
No! And never had been!

Some time ago MT-Propeller distributed JIHOSTROJ governors of model lines P-ABC-XXX and P-ABC-X/Y. It was labeled by MT as MT product, but always this governor was designed and manufactured at JIHOSTROJ. At the present time MT is distributing governors of its own design and manufacture and JIHOSTROJ does not distribute governors via MT! See explanatory chart.


This Jihostroj link: http://www.propellergovernor.com/reference/
References an RV7 with LYC.IO-360 Engine, Hartzell Prop and P-420-003/A Jihostroj prop governor.

The Jihostroj operation and installation manual for constant speed pro governors P-AB0-X/A & P-AB0-X/B states the following:

The overhaul interval (TBO) 1) for the governors is 2000 hours or 7 years if not altered by governor, engine or aircraft manufacturer. The exact value is stated and confirmed in governor?s
Quality Certificate.
1) This period includes both storage period and operation period


The same manual states the following under trouble shooting:

7. TROUBLE SHOOTING

7.1 PROPELLER SURGING ? POSSIBLE CAUSES:

7.1.1 Excessive transfer bearing leakage

Engines with excessive transfer bearing leakage can experience propeller surging since the governor may not be able to get enough oil pressure to the propeller.
Solution : Perform a transfer bearing leakage test per engine manufacturer?s instructions. If test indicate a high rate of leakage (even through it may still be on the high side of ?acceptable? tolerance), this maybe the cause. Install the suspect governor on a known ?good? aircraft. If problem disappears, engine work may be indicated.

7.1.2 Malfunctioning magnetos

7.1.3 Dirty engine oil
Contaminants in dirty engine oil can cause blockage of close tolerance passages in governor, leading to erratic operation.
Solution : Flush the governor in order to cleaning out contaminated oil.

7.1.4 Excessive ?play? in aircraft propeller control linkage
Excessive ?play? in the linkage between the governor and the cockpit control often leads to erratic operation. Specifically, if the propeller RPM is suddenly changing and holding a new setting of its own, this could indicate loose linkage.

Solution : Trace linkage and locate unsecured sections and tighten-up as needed. Please note that although linkage may appear to allow full governor control while the engine is off, it may not in the air. Engine vibration and ?stretch? of the mount during operation can often aggravate the condition. Therefore, it is important the entire length of linkage be properly secured, even if the ends alone are tight.

7.1.5 Excessive propeller friction
(NOTE : This is rarely the cause of RPM malfunction.)
Propeller may be overly-resistant to pitch movement. This can be caused by either excessively shimming of the propeller blades, or internal corrosion or part failure, causing binding.

Solution : Check amount of propeller blade ?play? as defined in the propeller manufacturer?s operating manuals.

7.2 RPM DRIFT ? POSSIBLE CAUSES:

7.2.1 Internal oil leakage
Governor unable to reach sufficient control pressure.
Solution : Contact propeller governor service center for governor internal leak test. 7.2.2 Extremely high temperature Extremely high temperature changes oil viscosity. The governor may not reach sufficient control
pressure.
Solution : Check oil temperature. Correct cause of high temperature.
7.2.3 Governor wear Excessive wear of internal parts.
Solution : Contact governor service center for bench test.
 
Just had same thing

I was just about to
Post the same issue. Uncomanded rpm surge.

My oil is dirty, how do I flush the governor to rule that out as a cause.

Paul
 
I have an MT P-420-3 governor installed on my RV-6 with a Lycoming 0-360A1A engine. 11 years and 700 hours of reliable service. Recently I had 2 experiences where the propeller RPM began cycling. Pretty sure it started when I abruptly brought the power back to idle. It stopped cycling when I added a little power. Unfortunately I didn't have the presence of mind to note the specific tach readings. Has this ever been reported before? And should I be concerned?

First things first. Look for anything that would cause or allow the governor arm to move unintentionally. Check for any slack in the prop control cable and linkage. Check the mounting bracket on the governor for cracks and secure mounting. More info: Does it cycle only when you make a rapid power change or at other times? Are you suggesting 100 RPM fluctuations or 1000?

Don Broussard
RV-9 Rebuild in Progress
57 Pacer
 
Thank you for all the replies. This is what I received from MT Propeller USA:

Hi John, the P-420 series governor was manufactured by Jihostroj for MT-Propeller. MT now manufactures our own governors at Avia, which is owned by MT-Propeller. Jihostroj does not provide parts to us for these older governors anymore and as such we are unable to work on them. These Jigostroj governors are still produced and sold as PCU5000 governors today. You could contact Aircraft Propeller Service in Illinois who is the importer for these governors now and see if they are capable of working on them but I had been told by a customer recently that they did not service them. We offer exchange of the P-420 governors to our new model at a cost of $975. Normal price for the new governor is $1,800.

He also mentioned he suspected a sticking valve. I think I'm going to go for the trade-in.
 
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