Oil pressure too high
I think most engineers work on the principle that things should get better over time, so may set an initial high oil pressure, that will become good at 200 hours.
The other way is to set the correct pressure, then as the engine runs-in, increase the now low oil pressure. However it is easier to remove a shim, than to find a shim in the parts bin, to add one shim. It seems as if that C150 now has tight running clearances, I wonder if the correct non-detergent oil was used during the run-in period...?
Those types of oil pumps are capable of very high pressures, and are used for hydraulic ram applications where the pressures are in the thousands of PSI. It is only the pressure relief valve that limits any destructive high pressures, which could result in a drive failure to the oil pump.
.
The other way is to set the correct pressure, then as the engine runs-in, increase the now low oil pressure. However it is easier to remove a shim, than to find a shim in the parts bin, to add one shim. It seems as if that C150 now has tight running clearances, I wonder if the correct non-detergent oil was used during the run-in period...?
Those types of oil pumps are capable of very high pressures, and are used for hydraulic ram applications where the pressures are in the thousands of PSI. It is only the pressure relief valve that limits any destructive high pressures, which could result in a drive failure to the oil pump.
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You then go on to describe the method by which, in my experience, such things are usually formally addressed - such as checking Tq loading on MRGB attachments etc, etc after x flight hours or doing other post-install verrification/adjustments against approriate usage parameters.
But that's not saying that what you suggest above doesn't happen..... and probably more often than we think!!!!!
Cheers, H 'n' H
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I think most engineers work on the principle that things should get better over time, so may set an initial high oil pressure, that will become good at 200 hours.
The other way is to set the correct pressure, then as the engine runs-in, increase the now low oil pressure. However it is easier to remove a shim, than to find a shim in the parts bin, to add one shim. It seems as if that C150 now has tight running clearances, I wonder if the correct non-detergent oil was used during the run-in period...?
.
The other way is to set the correct pressure, then as the engine runs-in, increase the now low oil pressure. However it is easier to remove a shim, than to find a shim in the parts bin, to add one shim. It seems as if that C150 now has tight running clearances, I wonder if the correct non-detergent oil was used during the run-in period...?
.
The first method seems like a match here as oil pressure is reportedly going down slowly over time. Taking apart piloting aspect of it prohibiting takeoffs at pressures outside of green arc, is that really healthy to run the new engine on a pressure that is 20 PSI lower than the end of the scale? How about risks to oil system components described by PDR?
Now, the periodicity between checks would be such that, under normal run-in conditions, with the intial point set at Release (of the aircraft from maintenance), say, near the top of the green arc that, by the time the check is called up, it would still be in the green arc, but now at the lower end. If, due to a problem, the rate of pressure decrease was such that it went below the lower limit before the check was called, it would be assumed the a/c would be snagged anyway. If that was the case, a new check at "y hours" would be set up after re-adjustment and, if the pressure had dropped rapidly in the first instance, "y hours" may well be a reduced periodicity so that it could be monitored more closely by the engineers. If it continues to drop rapidly (ie goes below the green arc in under "y hours"), it should be snagged again and now time to do some fault investigation. So, in theory, the oil should never be outside the green arc and, during run in, should drift down at a fairly predictable rate (stats have their uses for design engineers!) from the high end of the green arc to the low end. It should never be planned to go outside the green arc. If it does, snag it!
Turning to the practical realities of running at a consitently low oil pressure? Not good! Things like extra wear as where oil is being forced into bearing journals, piston rings, valve guides etc, etc at a low pressure, it may not penetrate fully or as efficiently thereby providing partial lubrication leading to greater wear, higher operating temps etc, etc. Would your scenario cause an instant "smoking hole" - maybe not. Depends on a lot of things. But you are certainly helping to bring a very expensive bit of kit to an early end of life which no owner would want. Just like your car really. But here speaks an AV. Maybe the grubbers can add to this discourse!
Oh, and my bit about the "set it high and let it drift down" (i.e. initially well above the green arc and then letting it drift down into the green arc) being an "approved method" in my last Post is tongue-in-cheek - just to ensure I'm not mis-quoted!
But, if someone has a formally approved schedule which clears "driftdown" from above the green arc as being OK I'd be happy to upgrade my knowledge!Cheers, H 'n' H
PS Sorry such a long Post!!!
Last edited by Hot 'n' High; 3rd Jul 2019 at 09:36. Reason: Apologies for length of Post!!!!!
Why would the oil pressure drift down over time and need "readjusting"?
The oil pressure is set by the action of a spring pushing a piston into a seat. The only thing that would cause the pressure to decrease would be the springt relaxing. The spring (if correctly designed) shouldn't be loaded anywhere near its elastic limit and so shouldn't "relax" unless it actually fails (in which case it would need replacement rather than readjustment).
As the engine wears the clearences will increase, and that will result in an increase in oil FLOW, but as long as the pressure release valve is still being pushed above its seat there should be no change to oil PRESSURE. That's why we have an oiul pressure relief valve in the first place.
PDR
The oil pressure is set by the action of a spring pushing a piston into a seat. The only thing that would cause the pressure to decrease would be the springt relaxing. The spring (if correctly designed) shouldn't be loaded anywhere near its elastic limit and so shouldn't "relax" unless it actually fails (in which case it would need replacement rather than readjustment).
As the engine wears the clearences will increase, and that will result in an increase in oil FLOW, but as long as the pressure release valve is still being pushed above its seat there should be no change to oil PRESSURE. That's why we have an oiul pressure relief valve in the first place.
PDR
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The Type Certificate Data Sheet says:
So any "drift down" would have to be starting at a high pressure of 60 PSI, and drifting below 30 PSI would not be airworthy. The oil pressure of an airworthy engine will not drift down. Should it begin to be lower, the engine is in need of maintenance for that. The most valuable use of the oil pressure indication to the pilot is not an "engine maintenance condition", but rather a loss of oil pressure, due either to inadequate oil quantity, or a blockage in an oil passage (both of which I have seen).
I have only ever seen high oil pressure with much too cold oil ('should not have even started the engine). I would be more suspicious of the oil pressure indicator than the oil pressure relief valve in the engine.
NOTE 3. Oil pressure limits: Normal operation 30 - 60 psi Idle 10 psi Maximum (cold oil) 100 psi
I have only ever seen high oil pressure with much too cold oil ('should not have even started the engine). I would be more suspicious of the oil pressure indicator than the oil pressure relief valve in the engine.
If the oil quantitry is "inadequate" the oil pressure gauge will give no warning until it gets sufficiently "inadequate" to cavitate the oil pump. At that point the oil system pumps air to the main and big-end bearings and the engine is (to use the technical term) "completely phucked". If there is a blocked oil gallery the oil pressure gauge won't show it because the back pressure will simply cause the relief valve toi open further and dump the oil into the scavange system. In these circumstances the engine will also be toast, but it may last a little longer than it would with a cavitating pump.
The main failure modes that the oil pressure gauge will indicate are an oil pump or pump drive failure and worn bearings. Oil pressure that starts in the normal range but drops to low levels when the oil is warmed (especially if iit's very low at low or idle revs) usually indicates imminant failure of big-end bearings or slightly less (but equally terminal) failure of the main bearings. Either arising is a "land in the nearest suitable field and be prepared for a sudden complete enegine failure" scenario.
PDR
The main failure modes that the oil pressure gauge will indicate are an oil pump or pump drive failure and worn bearings. Oil pressure that starts in the normal range but drops to low levels when the oil is warmed (especially if iit's very low at low or idle revs) usually indicates imminant failure of big-end bearings or slightly less (but equally terminal) failure of the main bearings. Either arising is a "land in the nearest suitable field and be prepared for a sudden complete enegine failure" scenario.
PDR
Guess what, it all depends and it seems everyone is a bit right on this - I guess it is down to specifics for aircraft and engines. From a bit of browsing - and no apologies for length, nor the formatting (which is doing my head in!) but I have highlighted a few bits to help(?).
1. From the Lycoming O-320 Operators Manual:-
“3. LUBRICATION SYSTEM.
I could not find anything on when adjustment would be needed, just a table of various Max, Min, Idle pressures.
2. These guys identify a general comment right at the start:-“Lycoming generally sets the operating pressures for cruise rpm on their factory-rebuilt engines to between 75 to 85 psi. Most new, rebuilt or overhauled engines require a slight adjustment of the oil pressure to finalize the setting once the engine break-in process is complete.”
They also discuss, at some length, the following under Oil Pressure Relief valves, including why adjustment may become necessary:-
“The older styles were adjusted by removing the housing and spring and adding or subtracting washers behind the spring to increase or decrease pressure. The oldest style housing was short and had an adjustment of zero to three washers maximum. The longer housing allowed up to nine washers maximum to increase spring tension. Each added washer increases oil pressure approximately 5 psi. On the externally adjustable models, one turn in (clockwise) increases oil pressure approximately 5 psi.
There are also springs of varying tensions and lengths which can be interchanged if the above adjustments do not yield the desired results. Some of the springs are color-coded to help differentiate them from one another. The most commonly used ones are the white LW-11713 springs (thick, heavy springs that are used to increase oil pressure at all settings), the 68668 (purple springs that are short and have much less tension than the others), and the 61084 non-color-coded spring that is standard equipment on most regulators.
One of the more common problems with the oil pressure regulators is with the seat that the steel ball contacts every time it closes. The seat is simply a machined aluminum section of the crankcase itself on most engine models, and over time it can become worn, especially if the ball is not contacting the seat dead in the center. If oil pressure varies excessively with engine rpm, especially at lower engine speeds, the regulator ball and seat may not be closing properly. Poor contact allows some of the oil to bypass back to the sump when it shouldn’t. If the cast aluminum seat has an irregular wear pattern in it, Lycoming recommends rigging up a makeshift tool out of an old ball welded to a steel rod that is thick enough to be struck with a hammer, then inserting the newly made tool squarely against the seat and giving it a couple of sharp hammer strikes to reform the seat, allowing a tighter fit between a new ball and the seat.
The field method of repairing a worn or non-concentric seat that most mechanics employ is to use the same tool mentioned above, but instead of striking it with a hammer, they use a tiny bit of valve grinding compound on the ball to re-lap the seat. Care must be taken to prevent the compound from getting into any of the oil passageways during the process, but overall this method tends to work well to reform the seat and regain a good seal between the ball and seat. Some of the earlier engines did have a replaceable seat insert that could be changed out and replaced if it was worn, but the most common seat is the cast aluminum type mentioned above.”
They also mention that the Pressure Transducer:-
“Some aircraft use an oil pressure transducer or sending unit that looks similar to the oil pressure switch used for Hobbs meter installations. It is a unit that has an oil pressure line piped into one side, and electrical wires connected to the other side. Pressure is converted to an electrical signal and wires are run to a gauge that displays the oil pressure reading. The oil pressure in most Lycoming engines is taken off the top rear accessory case. The oil pressure fitting has a reduced orifice in the outlet to the gauge. This helps prevent catastrophic oil loss if the oil pressure line or gauge begins to leak. Carbon or dirt can sometimes clog the orifice and cause an abnormally low oil pressure reading.”
They then go on:-“Troubleshooting oil pressure problems
Most oil pressure problems can be adjusted back to normal with the regulator or traced to a malfunctioning regulator or gauge. Sometimes, the trouble is a little more difficult to repair. The first step in correcting abnormally high or low oil pressure should be to double-check the pressure reading with a separate pressure gauge to confirm that the oil pressure really is too high or low. Check the oil temperature, too. Low oil pressures will produce increased oil temperatures, and vice versa; overly high temperatures thin the oil and can cause a lower-than-normal oil pressure reading.
Excessive internal engine clearances due to excessive wear or a bearing failure can become so great that the output of the pump is insufficient to fully pressurize the oil system. This is typically a worst-case scenario and lower oil pressure readings occur gradually over time. Excessive oil pump clearance between the impellers and the housing can also cause degraded oil pressure output. Oil viscosity plays a role in oil pressure as well. A slightly lower than normal oil pressure may be caused by using too thin an oil depending on where the plane is operated. A clogged suction screen or partially blocked passage between the screen and pump can also cause low oil pressure.
A higher-than-normal oil pressure reading, especially one that occurs suddenly, can be indicative of a blockage somewhere in the system, usually downstream of the pump. Oil pressure readings should be consistently monitored so that any deviation from normal operation can be detected and remedied quickly. Consistent, normal oil pressure from startup to shutdown helps assure that an engine will run reliably for a long time.” https://www.cessnaflyer.org/maintenance-tech/item/1149-hone-in-the-range-lycoming-oil-pressure.html
And there is a useful thread which includes:-
“I have a Continental (smaller O-300) engine, and I was incredibly surprised to learn just how sensitive the relief valve mechanism is. You "adjust" the oil pressure on the engine by adding or removing washers from under a spring, which keeps the piston/plunger (the sliding valve) pressed against a valve seat. My O-300 was running 10-15% above the minimum, so I thought I'd add a washer or two in there to bring it up just a little.
Two additional washers resulted in a startup pressure spike (80-100 PSI), so I immediately shut down, removed one washer, and tried again. This time it only went to 70-90 PSI, so I shut it down again and replaced the one extra washer with a thin washer. The thin washer still spiked the pressure, so I had to go back where I had started, with the same number of washers as it had, and the same 10-15% over minimum that I had.” https://www.flight-mechanic.com/reciprocating-engine-lubrication-systems-oil-pressure-regulating-valve/
So in summary:-
During the 50 hour run period, the oil pressure PRV may need to be adjusted.
That there are several different ways to adjust them, depending on make.
That the adjustment is usually quite sensitive.
That wear and tear of the PRV itself can necessitate some mid-life adjustment.
Any pressure transducers can get partially blocked so lead to low readings.
But the unanimous message is, ensure it is always in the correct operating range or snag it!
Finally, so far (a) it seems impossible to find any simple definitive statements and (b) seems everyone is right as even just the above supports many of the statements made throughout this thread. But - the one common theme everywhere, if it ain’t green, ground it!
I’m all Googled out here and my brain is now hurting so I will retire from this thread now! Finding a definitive answer is like finding the Holy Grail and I'm neither Indiana Jones nor a Monty Python! And more to the point, you'll be totally fed up with me .... if not from wayyyy before this post!
Oh, I have to say it - for "washers" you can read "shims" when adjusting things - well, generally!
Cheers, H 'n' H
1. From the Lycoming O-320 Operators Manual:-
“3. LUBRICATION SYSTEM.
- Non-Adjustable Oil Pressure Relief Valve – The function of the oil pressure relief valve is to maintain engine oil pressure within specified limits. The valve, although not adjustable, may be controlled by the addition of a maximum of nine STD-425 washers under the cap to increase pressure or the use of a spacer (Lycoming P/N 73629 or 73630) to decrease pressure. A modification on later models has eliminated the need for the spacers. Particles of metal or other foreign matter lodged between the ball and seat will result in faulty readings. It is advisable, therefore, to disassemble, inspect and clean the valve if excessive pressure fluctuations are noted.
- Oil Pressure Relief Valve (Adjustable) – The adjustable oil relief valve enables the operator to maintain engine oil pressure within the specified limits. If pressure under normal operating conditions should consistently exceed the maximum or minimum specified limits, adjust the valve as follows:
I could not find anything on when adjustment would be needed, just a table of various Max, Min, Idle pressures.
2. These guys identify a general comment right at the start:-“Lycoming generally sets the operating pressures for cruise rpm on their factory-rebuilt engines to between 75 to 85 psi. Most new, rebuilt or overhauled engines require a slight adjustment of the oil pressure to finalize the setting once the engine break-in process is complete.”
They also discuss, at some length, the following under Oil Pressure Relief valves, including why adjustment may become necessary:-
“The older styles were adjusted by removing the housing and spring and adding or subtracting washers behind the spring to increase or decrease pressure. The oldest style housing was short and had an adjustment of zero to three washers maximum. The longer housing allowed up to nine washers maximum to increase spring tension. Each added washer increases oil pressure approximately 5 psi. On the externally adjustable models, one turn in (clockwise) increases oil pressure approximately 5 psi.
There are also springs of varying tensions and lengths which can be interchanged if the above adjustments do not yield the desired results. Some of the springs are color-coded to help differentiate them from one another. The most commonly used ones are the white LW-11713 springs (thick, heavy springs that are used to increase oil pressure at all settings), the 68668 (purple springs that are short and have much less tension than the others), and the 61084 non-color-coded spring that is standard equipment on most regulators.
One of the more common problems with the oil pressure regulators is with the seat that the steel ball contacts every time it closes. The seat is simply a machined aluminum section of the crankcase itself on most engine models, and over time it can become worn, especially if the ball is not contacting the seat dead in the center. If oil pressure varies excessively with engine rpm, especially at lower engine speeds, the regulator ball and seat may not be closing properly. Poor contact allows some of the oil to bypass back to the sump when it shouldn’t. If the cast aluminum seat has an irregular wear pattern in it, Lycoming recommends rigging up a makeshift tool out of an old ball welded to a steel rod that is thick enough to be struck with a hammer, then inserting the newly made tool squarely against the seat and giving it a couple of sharp hammer strikes to reform the seat, allowing a tighter fit between a new ball and the seat.
The field method of repairing a worn or non-concentric seat that most mechanics employ is to use the same tool mentioned above, but instead of striking it with a hammer, they use a tiny bit of valve grinding compound on the ball to re-lap the seat. Care must be taken to prevent the compound from getting into any of the oil passageways during the process, but overall this method tends to work well to reform the seat and regain a good seal between the ball and seat. Some of the earlier engines did have a replaceable seat insert that could be changed out and replaced if it was worn, but the most common seat is the cast aluminum type mentioned above.”
They also mention that the Pressure Transducer:-
“Some aircraft use an oil pressure transducer or sending unit that looks similar to the oil pressure switch used for Hobbs meter installations. It is a unit that has an oil pressure line piped into one side, and electrical wires connected to the other side. Pressure is converted to an electrical signal and wires are run to a gauge that displays the oil pressure reading. The oil pressure in most Lycoming engines is taken off the top rear accessory case. The oil pressure fitting has a reduced orifice in the outlet to the gauge. This helps prevent catastrophic oil loss if the oil pressure line or gauge begins to leak. Carbon or dirt can sometimes clog the orifice and cause an abnormally low oil pressure reading.”
They then go on:-“Troubleshooting oil pressure problems
Most oil pressure problems can be adjusted back to normal with the regulator or traced to a malfunctioning regulator or gauge. Sometimes, the trouble is a little more difficult to repair. The first step in correcting abnormally high or low oil pressure should be to double-check the pressure reading with a separate pressure gauge to confirm that the oil pressure really is too high or low. Check the oil temperature, too. Low oil pressures will produce increased oil temperatures, and vice versa; overly high temperatures thin the oil and can cause a lower-than-normal oil pressure reading.
Excessive internal engine clearances due to excessive wear or a bearing failure can become so great that the output of the pump is insufficient to fully pressurize the oil system. This is typically a worst-case scenario and lower oil pressure readings occur gradually over time. Excessive oil pump clearance between the impellers and the housing can also cause degraded oil pressure output. Oil viscosity plays a role in oil pressure as well. A slightly lower than normal oil pressure may be caused by using too thin an oil depending on where the plane is operated. A clogged suction screen or partially blocked passage between the screen and pump can also cause low oil pressure.
A higher-than-normal oil pressure reading, especially one that occurs suddenly, can be indicative of a blockage somewhere in the system, usually downstream of the pump. Oil pressure readings should be consistently monitored so that any deviation from normal operation can be detected and remedied quickly. Consistent, normal oil pressure from startup to shutdown helps assure that an engine will run reliably for a long time.” https://www.cessnaflyer.org/maintenance-tech/item/1149-hone-in-the-range-lycoming-oil-pressure.html
And there is a useful thread which includes:-
“I have a Continental (smaller O-300) engine, and I was incredibly surprised to learn just how sensitive the relief valve mechanism is. You "adjust" the oil pressure on the engine by adding or removing washers from under a spring, which keeps the piston/plunger (the sliding valve) pressed against a valve seat. My O-300 was running 10-15% above the minimum, so I thought I'd add a washer or two in there to bring it up just a little.
Two additional washers resulted in a startup pressure spike (80-100 PSI), so I immediately shut down, removed one washer, and tried again. This time it only went to 70-90 PSI, so I shut it down again and replaced the one extra washer with a thin washer. The thin washer still spiked the pressure, so I had to go back where I had started, with the same number of washers as it had, and the same 10-15% over minimum that I had.” https://www.flight-mechanic.com/reciprocating-engine-lubrication-systems-oil-pressure-regulating-valve/
So in summary:-
During the 50 hour run period, the oil pressure PRV may need to be adjusted.
That there are several different ways to adjust them, depending on make.
That the adjustment is usually quite sensitive.
That wear and tear of the PRV itself can necessitate some mid-life adjustment.
Any pressure transducers can get partially blocked so lead to low readings.
But the unanimous message is, ensure it is always in the correct operating range or snag it!
Finally, so far (a) it seems impossible to find any simple definitive statements and (b) seems everyone is right as even just the above supports many of the statements made throughout this thread. But - the one common theme everywhere, if it ain’t green, ground it!
I’m all Googled out here and my brain is now hurting so I will retire from this thread now! Finding a definitive answer is like finding the Holy Grail and I'm neither Indiana Jones nor a Monty Python! And more to the point, you'll be totally fed up with me .... if not from wayyyy before this post!
Oh, I have to say it - for "washers" you can read "shims" when adjusting things - well, generally!
Cheers, H 'n' H
Moderator
If the oil quantitry is "inadequate" the oil pressure gauge will give no warning until it gets sufficiently "inadequate" to cavitate the oil pump.
If there is a blocked oil gallery the oil pressure gauge won't show it because the back pressure will simply cause the relief valve toi open further and dump the oil into the scavange system. In these circumstances the engine will also be toast, but it may last a little longer than it would with a cavitating pump.
If an oil pump has been damaged internally, this might show as reduced pressure, but for my experience of five years of tearing down Continentals, the oil pump has to be pretty bad before the oil pressure indication shows any difference, probably because the oil pressure relief valve was now closed all the time, and the pump could not even make the minimum pressure any more. My O-200 had always indicated on the low side of the green, though with suitable oil, never less than green. I had it overhauled, expecting to see a difference, it was the same. I had the accuracy of the oil pressure gauge checked, it was fine. It's run very well for more than 3000 hours I've flown it, so I'm not worried about low green oil pressure - it's in the green.
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Why would the oil pressure drift down over time and need "readjusting"?
The oil pressure is set by the action of a spring pushing a piston into a seat. The only thing that would cause the pressure to decrease would be the springt relaxing. The spring (if correctly designed) shouldn't be loaded anywhere near its elastic limit and so shouldn't "relax" unless it actually fails (in which case it would need replacement rather than readjustment).
As the engine wears the clearences will increase, and that will result in an increase in oil FLOW, but as long as the pressure release valve is still being pushed above its seat there should be no change to oil PRESSURE. That's why we have an oiul pressure relief valve in the first place.
PDR
The oil pressure is set by the action of a spring pushing a piston into a seat. The only thing that would cause the pressure to decrease would be the springt relaxing. The spring (if correctly designed) shouldn't be loaded anywhere near its elastic limit and so shouldn't "relax" unless it actually fails (in which case it would need replacement rather than readjustment).
As the engine wears the clearences will increase, and that will result in an increase in oil FLOW, but as long as the pressure release valve is still being pushed above its seat there should be no change to oil PRESSURE. That's why we have an oiul pressure relief valve in the first place.
PDR
And thanks again for lots of interesting and insightful info. It is true that many pilots including me do not shine on systems knowledge. For an average charterer like me it is not trivial to get insights, so knowledge remains at or below PPL/ATPL theory courses level as most of stuff gets forgotten over the years. Following this discussion I even wanted to buy IO-240 maintenance or troubleshooting manual for some extra education, but that seems not a trivial task to do. Without close contact with mechanics it's not straightforward to improve the knowledge either.
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There has been a lot of discussion about this but to me the only way to locate the problem quickly would be to rig a calibrated direct reading oil pressure gauge into the oil system.
Once this is fitted run the engine on the ground and observe the pressure readings.
Compare the direct reading gauge results with the aircraft oil pressure gauge and the certified requirement ( TDS) across the engine RPM range.
This simple process will locate the problem system be it the engine or the oil pressure indication system.
Once this is fitted run the engine on the ground and observe the pressure readings.
Compare the direct reading gauge results with the aircraft oil pressure gauge and the certified requirement ( TDS) across the engine RPM range.
This simple process will locate the problem system be it the engine or the oil pressure indication system.
One of the best bits of advice I got early in my career was to note the exact position of critical engine gauges especially the oil pressure. One day a student and I were climbing out for an aerobatics lesson in the club C150A. On this airplane the oil pressure stayed pretty constant at about 2/3 scale up the green arc, however i noticed it was in the lower half of the arc and then watching the needle it made a slight but noticeable drop. I told the student to immediately turn back to the airport. By the time we got to short final for a downwind landing the oil pressure was indicating zero so I shut down the engine and we made an uneventful power off landing.
It turned out that a tooth on the oil pump drive broke off and over the course of a very few minutes the skipping gear ground off the other teeth until the pump was no longer turning. If we had continued the climb to the aerobatic box we would have had a forced landing in relatively inhospitable terrain instead of an uneventful landing on the runway back home.
After that I would randomly cover the engine instruments with my hand and ask the student to tell where the needles were sitting. "in the green" was not a satisfactory response.
Engines almost always give you some warning of an impending mechanical failure and any sudden change in oil pressure or oil temperature almost always is the first indication your day is going to get interesting.....
It turned out that a tooth on the oil pump drive broke off and over the course of a very few minutes the skipping gear ground off the other teeth until the pump was no longer turning. If we had continued the climb to the aerobatic box we would have had a forced landing in relatively inhospitable terrain instead of an uneventful landing on the runway back home.
After that I would randomly cover the engine instruments with my hand and ask the student to tell where the needles were sitting. "in the green" was not a satisfactory response.
Engines almost always give you some warning of an impending mechanical failure and any sudden change in oil pressure or oil temperature almost always is the first indication your day is going to get interesting.....
Interesting. How stable would you expect the oil pressure to be? I recall pulling back the revs to around 1,500 as I started the descent and on checking I saw the oil pressure at the very bottom of the green; a needle's width inside. I pointed this out and the instructor said it was because the engine revs were low. Indeed, flying with this a/c since then I see that the oil pressure is closely linked to engine revs, always within the green, but only just at the low end.
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flying with this a/c since then I see that the oil pressure is closely linked to engine revs, always within the green, but only just at the low end.
So if the oil pressure indication is obviously changing in proportion to engine speed, though remaining in the green the whole time, you should understand that things are okay, though not ideal. For ideal, the oil pressure indication will remain constant, and ideally in the higher portion of the green at all engine speeds. An engine whose oil pressure varies with engine speed within the green is going to get you home, but I would not buy it! The one proviso to that is viscosity breakdown in the oil, which I described earlier. The engine was perfectly fine, the oil was no longer functioning.
FWIW, a retired Continental Engineer told me, in the context of my O-200, that as long as the full power oil pressure was at least 30 PSI, and the idling oil pressure was at least 10 PSI, the engine would run safely. This corresponds to what the type certificate for the engine, and the C 150 flight manual state. Obviously they allow for pressure variations with engine speed.
The engine relies partly on oil pressure, but more on oil flow. The oil flow provides a film of lubrication, and carries away excess heat, and the pressure provides some resistance to metal to metal contact against engine internal mechanical forces, and provides the required pressure for the hydraulic valve lifters. If there is a concern about low oil pressure (as opposed to no oil pressure), and the quantity is sufficient, the first, and lowest cost check is to change the oil, and run the engine again to see if the problem is improved.
One of the advantages of always flying the same airplane is that you should have a very good feel for where the needles sit and so if there is a noticeable change then it should be relatively obvious. If you are flying club airplanes then this obviously is harder to do as you will likely be flying a different air frame every flight. Nevertheless it is worth paying attention. I would not continue flying an airplane that had oil pressure that varied with engine RPM even if the pressure stayed in the green. As Pilot DAR pointed out the oil pump should have more than enough output to hold the oil pressure steady in the green at any normal operating RPM ( eg anything above 1000 RPM or so). There is something wrong that should be investigated.
One thing that I have noticed in light aircraft pilots is that there seems to be a poor practical understanding of the relationship between oil temperature and oil pressure. With cold oil I expect to see an oil pressure near the top of the green at initial start up with the oil pressure dropping to the normal steady state value for that aircraft as the oil warms up. If I have to do a prolonged climb I expect the oil temp to rise and see a slight drop in oil pressure with the hot oil losing some viscosity. I don't want to see oil temp close to the red line so I would take action to reduce the oil temp if I were to see this. Anything that did not conform to this, like for instance low oil pressure with cold oil would merit investigation
In a related topic oil consumption is an important indicator of engine health. A sudden rise in oil consumption is almost always the engine telling you something is very wrong. I once had an engine failure in a heavily loaded Piper Navajo. The choice was fly 100 nm over mountains to the nearest big airport or land at the small airport I just took off from. This involved an NDB approach to minimums with a circle to land, which was I elected to do. To say it was a "significant emotional event" in my flying career would be rather an understatement....
Anyway it turned out 2 other pilots had put a total of 4 litres of oil in that engine in the last 6 hours and did not think it worth informing maintenance
This was a totally avoidable failure
One thing that I have noticed in light aircraft pilots is that there seems to be a poor practical understanding of the relationship between oil temperature and oil pressure. With cold oil I expect to see an oil pressure near the top of the green at initial start up with the oil pressure dropping to the normal steady state value for that aircraft as the oil warms up. If I have to do a prolonged climb I expect the oil temp to rise and see a slight drop in oil pressure with the hot oil losing some viscosity. I don't want to see oil temp close to the red line so I would take action to reduce the oil temp if I were to see this. Anything that did not conform to this, like for instance low oil pressure with cold oil would merit investigation
In a related topic oil consumption is an important indicator of engine health. A sudden rise in oil consumption is almost always the engine telling you something is very wrong. I once had an engine failure in a heavily loaded Piper Navajo. The choice was fly 100 nm over mountains to the nearest big airport or land at the small airport I just took off from. This involved an NDB approach to minimums with a circle to land, which was I elected to do. To say it was a "significant emotional event" in my flying career would be rather an understatement....
Anyway it turned out 2 other pilots had put a total of 4 litres of oil in that engine in the last 6 hours and did not think it worth informing maintenance
This was a totally avoidable failure
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>Anyway it turned out 2 other pilots had put a total of 4 litres of oil in that engine in the last 6 hours and did not think it worth informing maintenance
not great. Far from sensible. But surprisingly close to allowable.
Assuming a 300hp IO540 the max permissible consumption is 0.97Qt / hr or 2 3/4 litres in 6 hrs.
Source : https://www.lycoming.com/sites/defau...onsumption.pdf
not great. Far from sensible. But surprisingly close to allowable.
Assuming a 300hp IO540 the max permissible consumption is 0.97Qt / hr or 2 3/4 litres in 6 hrs.
Source : https://www.lycoming.com/sites/defau...onsumption.pdf
Yesterday, Saturday afternoon, as I started to taxi, I noticed the O200 oil pressure was at the top of the scale. Turned and shut down. A wire had broken at the sender. As it was an LAA Permit aircraft, and an Inspector was at at the hangar, it was fixed within 40 minutes, and readings were normal in the subsequent flight.
