Ask Paul: What’s causing loss of propeller control?

Question for Paul McBride, the General Aviation News engines expert: I have been maintaining a 1953 T-34 (Beech A45) with a Continental O-470-13 engine installed (original) for the last couple of years. The ownership group has been reporting a loss of propeller control after about 20 to 30 minutes into their flight, however it was happening intermittently and now it is happening almost every flight.

I installed a Dynon Skyview HDX system in the plane, so I am able to retrieve the flight data.

What we have been able to determine is that when the oil pressure drops below 35 psi (with a corresponding high oil temperature) the prop goes to an overspeed condition. What is interesting is that this problem occurs mostly during the summer months down here in South Texas. So, I have been trying to figure out what would cause this intermittently.

The history of the engine is this: Last overhaul was August 1995 with a top overhaul in 2003 with just over 532 hours since overhaul.

I have been using the maintenance manual troubleshooting guide to methodically troubleshoot this problem.

The work that I have done is as follows:

• Replaced leaking gaskets on the magnetos and magneto adapters
• Tested the oil transfer tube per Continental instructions and it passed
• The starter was leaking oil through its housing, so I had that overhauled
• Tested the vernatherm and it was functioning properly, nonetheless the oil cooler was overhauled, including the vernatherm.
• The cylinders, starter, and oil cooler work were done at the same time but wasn’t completed until March 2023 due to the cylinder overhaul taking so long.
• Switched from 50 weight oil to 60 weight oil and this seemed to work for several months in the fall. But last December it happened again and this time I had all cylinders removed and overhauled because during the last annual one cylinder’s compression was low but passed per Continental’s M-O Standard Practices manual (my thinking was that exhaust gas was heating up the oil and thinning it out and allowing the pressure to drop). Another cylinder was oil fouling the spark plug and the rest had some amount of oil leaking into the cylinder. You could see the pistons were black.

I think I covered everything on the outside of the engine, and I believe that the only thing left is in the engine case.

Is there anything that I might have missed or in the engine case that would cause an intermittent drop of oil pressure that would lead to a loss of propeller control?

Also, the prop governor had been overhauled and removed twice to be tested with no defects before I started working on the plane. The propeller was overhauled in 2014 and I had it resealed in 2022.

Lastly the plane has just over 60 hours of flight time since this all started.

Any insight that you might have is greatly appreciated.

Daren Urban

Paul’s Answer: Daren, thank you for your interesting question on your Continental O-470, and I’ll admit right now that my Continental engine experience is limited.

That being said, the problem you’re looking at actually sounds more generic to engine operation regardless of what engine model is involved.

After reviewing your troubleshooting approach and the corrective actions you’ve taken, I’d like to try something simple.

After I’ve rolled all of this over in my mind, I keep coming back to the question of the oil temperature and pressure. It’s not uncommon for engines to do strange things when the oil temperature gets hot, and the direct result is lower oil pressure. As you know, this can have a direct effect on the prop action. My suspicion is that the oil temperature is reaching a point that the engine oil is thinning out and causing the prop to react as you mentioned.

I’d suggest that you first begin by doing a close inspection of the complete engine installation, focusing on the location and condition of the engine baffles. The problem may be something simple like a misplaced or folded-over baffle, causing cooling air to be lost over the engine or oil cooler. I realize it’s difficult to know exactly how the baffles are located when the cowling is installed, so do your best to see that they are in their proper position.

Even though you’ve had the engine oil cooler overhauled and the vernatherm appeared to be functioning properly, I am still curious to know what the actual engine oil temperature is.

I’d suggest you use a calibrated oil temperature gauge in order to eliminate the aircraft system and confirm that the sending unit is also checked for accuracy.

If you can’t determine the actual oil temperature any other way, then you may have to resort to using a laboratory glass bulb thermometer, which I have mentioned in some of my past articles. This method works fine, but you must use extreme caution, or you’ll end up removing the oil sump to remove the mercury. That, I can promise, will increase your vocabulary and probably not in a nice way.

Daren, I think you will not be the only one who will experience unusual things regarding engine operation this summer with all of the extreme heat we’re seeing. I believe high oil temperature/lower oil pressure issues will move to the top of the squawk list as this weather continues.