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JohnB

Oil pressure decreasing with altitude in Bravo

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Huge favor I’ll ask from fellow Bravo owners. What are your oil pressure readings in Psi at altitudes above 13,000 feet? 18k feet? This is a question for Bravo owners.

in mine, I have noticed a steady decrease in oil pressure readings as I climb in altitudes just above 13k to around 53 psi with no other gauge changes. Haven’t gone higher recently since I’ve noticed this with more accurate gauges than my factory gauges were. I’ve had the oil pressure screw adjusted, spring replaced, gauge sensor  replaced with same so I’m wondering if this is just a lycoming issue in our bravos that doesn’t affect performance before throwing more repairs at it. What I would be hugely curious about if another bravo owner next time you’re flying above 12 k, if you could note what your oil pressure in PSI is at 12, 15, 18, and 21k if any of us are still flying that high that would be hugely helpful. 

Thanks in advance!

John

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John,

I believe there might be a similar discussion on this topic already... specific to Bravo owners too?

I’ll look to see if my search skills ar working good enough...

Best regards,

-a-

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Take a read on Dr. Dubin’s thread... Sounds similar to what you are describing...  :)

If it is the same issue... my memory is getting better!

Best regards,

-a-

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The pressure regulator balances engine oil pressure against the spring pressure and atmospheric pressure on the outside of the ball. The atmospheric pressure on the ball is about 15 PSIA at sea level and about 5 PSIA at 25000 ft. So there is about a 10 PSIA difference between sea level and 25000 ft. The bypass orifice has an area of about 1/2 sq in. So just because of atmospheric pressure changes you can expect about a 5 PSI difference in oil pressure. Oil temperature will also have negative affect on oil pressure. The oil cooler is less effective at higher altitudes. Have you tracked your oil temperature with altitude?

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I think this is a problem for people with digital oil pressure gauges. People with the old analog gauges sleep much better at night.

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3 hours ago, N201MKTurbo said:

The pressure regulator balances engine oil pressure against the spring pressure and atmospheric pressure on the outside of the ball. The atmospheric pressure on the ball is about 15 PSIA at sea level and about 5 PSIA at 25000 ft. So there is about a 10 PSIA difference between sea level and 25000 ft. The bypass orifice has an area of about 1/2 sq in. So just because of atmospheric pressure changes you can expect about a 5 PSI difference in oil pressure. Oil temperature will also have negative affect on oil pressure. The oil cooler is less effective at higher altitudes. Have you tracked your oil temperature with altitude?

Oil temperature stays rock soilid and doesn’t change with altitude. That is very interesting, and hugely helpful I like that explanation thank you!

3 hours ago, N201MKTurbo said:

I think this is a problem for people with digital oil pressure gauges. People with the old analog gauges sleep much better at night.

Ha! True, I never noticed this before I had digital gauges for oil pressure. I’m thinking the analog gauges don’t give you warnings like the digital ones do now which we don’t notice, at least that’s what I’m thinking could have occurred. But it should be reading the same with both digital and analog with your explaination above, except the analog doesn’t give you a caution warning when it dips down. 

 

Still hoping Someone still flies their bravo in the flight levels and can take a peek at their oil pressure gauge and share that number and their altitude. So at least I won’t be chasing a non existent problem. 

 

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5 hours ago, carusoam said:

 

Take a read on Dr. Dubin’s thread... Sound similar to what you are describing...  :)

If it is the same issue... my memory is getting better!

Best regards,

-a-

Yes read that post with interest. The author did the following after his overhaul (which sounds very expensive)

In my case, after flying the reman engine for 21 hrs, the engine was pulled and rebuilt w another new camshaft and new tappets, the original design type. After that, I can fly up to FL240 without any oil pressure drop. It certainly fixed the issue.
No sense having an engine and plane that can fly at FL240 but you need to fly with the oil pressure in the red or yellow....
 
Hoping to see if this can be fixed without having to rebuild an entire camshaft and tappets. Or if it even needs to be fixed? 
 
 

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Ok I’m raising the ante... I’ll buy beverages of choice whenever I get to meet you for the Bravo owner who can fill in this chart! PM ok if you don’t want to share! And if you could also let me know whether you have digital or analog gauges and if you’re on your original engine.

Altitude             Oil Pressure (psi)

12,000                 ____________

15,000                ___________

18,000                ____________

21,000                ___________

25,000                ___________

 

Thanks hugely!

 

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12 minutes ago, JohnB said:

Yes read that post with interest. The author did the following after his overhaul (which sounds very expensive)

In my case, after flying the reman engine for 21 hrs, the engine was pulled and rebuilt w another new camshaft and new tappets, the original design type. After that, I can fly up to FL240 without any oil pressure drop. It certainly fixed the issue.
No sense having an engine and plane that can fly at FL240 but you need to fly with the oil pressure in the red or yellow....
 
Hoping to see if this can be fixed without having to rebuild an entire camshaft and tappets. Or if it even needs to be fixed? 
 
 

I think @rocketman may have had help from lycoming on that... (this is an invite to see If Dr. D. recognizes your situation.  Maybe he has some input...?)

It seems Lycoming may have created a situation that only they can fix. (Possibly?)

 

There is a similar situation where a newly built IO360 generates too much oil pressure and causes the Mooney gauge to bust a redline... no technical issue other than the legal redline bust... it took a while, but I believe they came out with a solution here as well...

PP thoughts only, not intending to bash an engine builder...

Best regards,

-a-

 

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Got it! Looks like this is the same problem as the other thread.. moving to the other one.. thanks again all hugely helpful! 

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As always, thanks everyone for your input! Well it looks like this is not the same issue as @tls pilot had, as my tappets are the original straight ones, but in speaking with Lycoming, they are now putting parabolic tappets on all of their new FRM engines today as they feel they have fixed the parabolic tappet issue that was around engines in 2009. I think I have this finally figured out, but if someone more wise at engines than me has a better theory, I'm all ears! Here's what I've done to look into this issue:

  • I had my oil relief spring replaced with a new stiffer one PN LW18085.
  • Adjusted oil pressure screw upwards, this changes the startup pressure but does not change pressure at altitude
  • Checked my digital EIS with a known external pressure sensor and verified almost equivalence within specs..
  • Made sure all of my screens were clear of of obstructions, spring/ ball assembly clear..
  • Oil pressure still only goes down at increasing altitude above 10.k
  • Oil temp remains fine between 180-192. leaving cowl flaps open allows oil temps to stay lower and therefore oil pressure higher

After all of this replacing, I have come to the conclusion this is a manifestation of recognizing oil pressure on a digital readout that blinks at you compared to the old needles that don't blink at you. I found a picture from last year at 17k that shows my oil pressure needle at top of yellow, bottom of green or close, and I've taken my bird up to 25k with no problems whatsoever. AND Ive seen a few pictures of some other mooneys on social media who were taking pictures and posting the speed they're doing at altitude, and likely not noticing their oil pressure gauge,  and I note that their oil pressure above 10k is at the bottom of the green/ top of yellow, or yellow on most. It's  hard to discern the difference between a pressure of 56 or 53 on a analog needle, but easy with digital.

So I think I'm going to go with @N201MKTurbo's theory, that it is simply a manifestation of lower atmospheric pressure on the outside of the ball on the relief spring (or some other atmospheric pressure reduction explanation) compared to the atmospheric pressure at lower altitude, as this is the only variable that changes with my engine is altitude. Another possible theory my mechanic pointed out that on our oil pressure sensor on a Bravo is located on a Y port along with out wastegate controller, so his theory was that the wastegate at that location may be pulling more oil on closing at altitude therefore reducing local pressure which I think is logical as well.  Lycoming doesn't buy either of those two theories but no other suggestions. So "I'm going with the altitude theory, as I've heard from a few others that are brave enough to say that their oil pressure does tend to go down at higher altitudes. My fix? Leaving the cowl flaps open at higher altitudes if the blinking of the oil pressure light bothers me. :)

Thanks all!

John B

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I believe your spring is the correct later style PN. 

You could pull the oil pressure source from another oil galley. I once had an oil pressure gauge tapped near a governor line, it fluctuated a bit. We changed the pick up and it read normal. 

Maybe you could crank the oil P to 100 psi for T/O or whatever your high limit is.

-Matt

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