Oil pressure too high
 

I've been chartering a plane (engine Continental IO-240) and had to abort the flight because of oil pressure on run up being at 80 PSI (green arc is 30-60 PSI, upper limit is 100 PSI). The engine is relatively new, maybe between 100 and 200 hrs. I've been flying the same plane previously before engine was replaced and oil pressure never stayed so persistently outside of green arc. On a normal day, it used to stabilize within green arc immediately after startup. POH says oil pressure may go beyond green arc prior to reaching normal operating temperature, and also warns against applying full power at oil pressure above 70 PSI at low OAT (0 deg celsius or below), and that's about it.

Talking to another pilot who flew the same plane it appeared that the problem is known, oil pressure sits at 80 PSI and goes down in flight very slowly to slightly above 60 PSI. It appears that problem is not seen as no-go and mechanic has 'approved' the plane as good to go. I eventually suggested the owner to revert to maintenance shop to ensure that oil pressure is back where it should be. I am not a mechanic, but my common sense understanding - which is perhaps wrong - is that oil pressure above normal may indicate some latent issue in the oil system that may show up in the future in worse form. Also, if it is indication issue, then pilot has no reliable oil pressure indication which is no-go too.

It would be interesting to hear opinions on this situation. Assuming this is not an indication issue, how exactly high oil pressure has negative impact on engine operation? Also, I find POH warning on no full power above 70 PSI at 0 degree celsius or below quite interesting. Struggling to understand what would be the difference in applying full power at 0 degrees vs 20 degrees celcius when oil pressure is at 80 PSI?

High oil pressure means one of two things:

1. The oil is cold
2.The oil pressure relief vale is either sticking shut or has the wrong spring.

When the oil is cold it is too viscous to lubricate properly (it won't wet to the surfaces and form protective films). That's why they tell you to wait for the oil temp to reach the operating range before running at any significant power levels, and it's also why high pressure is acceptable until the oil warms up (but noting that you shouldn't be running at much more than fast tickover until it does). oil pressure/temperature errors in very cold OAT are a concern because they may indicate that the thermostat to the oil cooler is stuck open, so the oil is not warming up and may even get colder (more viscous) when you climb into colder air. That way lies a high risk of increased engine wear/damage, and a moderate risk of catastrophic engine failure.

If it's the oil pressure relief valve that's stuck shut (or has too strong a spring due to an manufacturing/overhaul/maintenance error) then you will subject the oil system to too higher a pressure, and risk damaging the oil filters and/or blasting out any core plugs in the oil galleries. In either case the risk varies from a persistent oil leak and lack of filtering to a sudden and catastrophic total loss of oil when a core plug fails and all the oil makes a break for freedom through the resulting hole.

If this is a new or newly overhauled engine which has been like this since installation I'd be taking it up as a warranty issue with whoever supplied/overhauled it, and I wouldn't be happy flying it. I'm actually surprised that the RTS has been signed with that kind of defect.

Even if it's just an instrumentation issue it's a no-go item IMHO because, as you say, this means that the pilot cannot rely on one of his/her primary engine health indicators.

£0.00007 supplied,

PDR

If the gauge is electrical, it might be a bad earth return connection, in some obscure place.
Or other electrical fault. A disconnected starter engaged light wire caused a drop in oil pressure when it shorted.

I would suggest that before the oil pressure valve is adjusted/ replaced that the oil press.gauge and transmitter are check calibrated,and the filter is checked/replaced as well...You may also want to check it is using the correct grade of oil...especially after overhaul....It is important that you do not use any significant power until the oil temp. is up to 40 deg C,no matter what the OAT is....Remember,the mechanic doesn`t usually fly.......

Correct grade of oil? I can't speak for the IO-240, but it's cousin the O-200 recommends 80 weight. If someone has poured in 100 weight, and it's cold high oil pressure is likely, at least until it warms up. Oil pressure spring is only an issue if the engine has been apart recently. Gauge error is certainly a possibility to investigate.

Had exactly this myself! And I was amazed/shocked by what it revealed. Same thing, came in to fly a plane and high oil pressure. Shut down and enquired. Usual blah of "been like this for ages", "maint have checked it so good to go", "what are you worried about?", "Been like that since the new engine went in so it's fine!" etc, etc.

Since I had the a/c booked - I just snagged it! Being an Aero Engineer the people I rented it from moaned but duly accepted it was my flying that I was giving up on so reluctantly allowed me to pop it over to maintenance. Lots of "I don't know why you did that!", "what's the problem?, .... i.e. trouble maker (a frequent charge leveled at H 'n' H :E). Within an hour it was back "S". When maintenance dropped it back I just asked the Engineer who'd taxied it back over (their Boss man this time) what the "snag" had been and all was revealed. It seems all new/recon engines have the oil pressure transmitter shimmed to the max and so over-read on release. Part of the fit/runup checks is to look at the oil pressure on a test gauge once running the aircraft before release and to then remove the required no of shims until a/c oil pressure shows the test gauge pressure. Disconnect test gauge and the jobs a good ‘un! Quite simple.

There were some very red faces tho as (a) the owners clearly had never gone back to ask about it/snagged it despite their claims that they’d checked and that the Engineers were happy, and, (b), it also seemed a certain pair of bods in the Maint facility had just had a “Chat, less tea, less bikkies” with their Boss for not doing the approved install process and then signing it off! He assured me the rest was OK! Good job as, again in this case, it had flown a good number of hours (100+) since the donk change! Bonus was, when I then did a short hop in it with the time I had left for my slot – it was called an “Air Test – have this one on us!” by the owners when I got back. ;)

Not sure about your case CFO, but I know where me money is with this one too! Cheers, H 'n' H

Good story, HnH, thanks for sharing! It only confirms my happiness to do maintenance myself - "on n'est mieux servi que par soi-même" as the Romans used to say - even if it confines me to a rather humble craft and corresponding humble flights.

No probs Olie! I was just sad that no-one had been "professional" enough to sort it on Day 1 and that I'd had a bit of grief off the owners for snagging it .... to start with!

And no such thing as "humble craft"! Anything that gets someone off the ground should be afforded the same respect! And can be even more fun!!!! :ok: H 'n' H

As far as I'm aware you can't "shim" the pressure transmitter (it's a hydraulic transducer that responds to pressure) - the thing you shim is the oil pressure release valve spring. Adding more shims puts more pressure on the release valve piston and thus increases the pressure at which it opens. I can see the logic of putting a lot of shims in for the first run to ensure a healthy oil pressure for those important first few minutes of running, but as it's the actual oil pressure which is excessive (not just the gauge reading) it should be reduced to the specified range as soon as practicable because running in that condition will have all the consequences previously described.

But I am confused - if this was a "new" engine then the oil pressure should have been set at the factory because engines have to be bench run in a dyno (actually several bench runs at a range of power settings and durations) before being released for sale. If it was an overhauled engine it should also have had a number of bench runs before release - these are certification requirements. If a new or overhauled engine was installed without a correctly set oil pressure relief valve then the company who made/overhauled it seem to have been playing fast and loose with release certificates...

PDR

Hi PDR, TBH, I'm not sure exactly which bit had the extra shims fitted (this was 20+ years ago now and I have difficulty remembering anything at all these days) so I'd not use my post ILO the Maintenance Manual. :p

Point was, the engine came with a full set of shims fitted (shims I do recall being mentioned) to the oil widger-thingy-ma-jig which were then removed once the donk was on the aircraft to "calibrate it" to the aircraft using a calibrated pressure gauge and that was all part of the install - hence why it was such a quick fix. But I guess my real point is, if it's wrong, don't take people's word for it being "OK" etc, etc, etc - just make a bit of a fuss and just get it sorted – or walk away! I've had a few cases where things have been plain wrong, often straight out of maint, but have taken very little time to sort. But I often have found pilots have accepted the issue without snagging them for a while until I ended up flying them!

Now, as to whether that was just the gauge system, or to take into account the effects of oil coolers etc in the system and to what the shims were fitted to etc, etc I know not – the transmitter is what my memory tells me I was told but I could be wrong. It’s the “If not right, get it fixed!” message which is important and I wanted to highlight. Sorry I can't add more to the Techie detail but, being of the LEC/AV "persuasion", I've always left the messy stuff to the Riggers/Sooties/Grubbers who appear to enjoy being up to their armpits in gunk! What’s the saying, “If there are dogs around, why bark yourself?”!!!!!

And on that last bomb-shell - hat, coat and exit sharp Left before the nasty old Riggers/Sooties/Grubbers catch up with me!!!!!! :E

Cheers, H 'n' H

...and *my* point is that none of that should have happened AT ALL. The oil pump and the adjustable oil pressure relief valve are part of the engine change unit, not the aeroplane. For a new/overhauled engine the oil pressure should be set up for the long before installation in an actual aeroplane, because nothing in the external system (even if the existing oil cooler and oil tank were retained rather than changed with the engine) should have any effect on the oil pressure relief setting. External oil plumbing can affect oil *flow*, but not oil *pressure*, hence this claimed need to calibrate it in the aeroplane against a calibrated pressure gauge, together with this bunkum about engines "being supplied with all the shims fitted" just doesn't stand scrutiny. For a new/overhauled engine there is simply no reason why the crew installing the engine in the aeroplane should ever need to mess about with the pressure relief valve settings. There are a few engine "repairs & rectifications" which might require recalibration of the pressure relief valve itself (not the gauge) to give the correct pressure (rather than the correct reading), but that wasn't how this incident was described.

I guess what I'm saying is that you were hoodwinked into accepting a smart line of B/S as an excuse for what was actually a serious maintenance error - one that could easily have led to an in-flight engine failure and a need for a non-practice forced landing.

PDR

I fitted a used, replacement engine to one of my cars. It ran fine for a couple of thousands of miles but one very cold morning immediately on starting, in a few seconds it dumped all of the contents of the sump on my driveway. After cleaning everything up (took most of the day, the inside of the engine bay was dripping with oil, let alone the mess on the driveway), I found that the oil filter canister was loose but otherwise seemingly OK. I couldn't understand why it had come loose because I work on all my own vehicles, I've been doing oil and filter changes for almost five decades and never suffered this problem before. However, seeing as the engine was almost due an oil change I simply carried out that procedure (hardly any oil left in the sump!) and fitted a new filter, this time making absolutely sure it was very secure. Two weeks later later the same thing happened again, a hundred and fifty miles from home. This time the oil filter canister base was bent, so that the "O" ring wouldn't go back in. The canister "wobbled" as I unscrewed it. The car had to be recovered on a trailer.

The only thing I could think of was that the pressure relief valve in the oil pump had stuck shut. I dismantled the lower front of the engine, took off the oil pump and sure enough, the pressure relief piston was seized solid in the closed position.

I'd hate that to happen in flight....

LOL! Let's not fall out over this PDR! Maybe, maybe not! He (the engineer) seemed genuine when I asked. But, yes, it could all have been all BS as you say. 20 years down the line, I can't be bothered to track him down and ask.

However it mattered not to me anyway as, after all (and this is what I'm saying), it was me who was the first person in 100+ hrs to put my foot down about flying it when 100+ hrs worth of others had flown it prior to me pitching up. I snagged it as soon as I saw it - copping some sig grief in the process from the owners - who then, it turned out, had lied to all those (and me) who had had the "its' been checked by maintenance and it's fine!" line trotted out to them! Even as an AV, I wasn't buying that old gag! They were very nice to me after that .... but I moved on anyway shortly after that!

The Engineer who "hoodwinked me"(? so what if he did on the details - I never flew it in that state so it would never have killed me - only, maybe, those who flew it with the high oil pressure indication - but why did they all take it when it was out of limits?) also apologised that it had been released in that condition so he held his hand up to errors on his side and he was most displeased it had ever left his organisation. He seemed genuine - and I took what he said at face value. He also could not believe that no-one else had reported it in over 100+ hrs given the fix (whatever it was ... and I'm going to avoid any mention of "shims" from now on!!!!!) was complete in less than 1 hour. That made him annoyed too! Aviation is, after all, looking out for ourselves and others so if it was really dangerous, many, many people had ignored the chance to get something possibly serious fixed!

I don't disagree that I may have been fed a line and that maybe it was more serious than I was led to believe - immaterial to me at the end of the day. As I say, I refused to fly it and only asked a simple question and took the answer at face value when it returned. That is what I repeated earlier. But the aircraft was now "S"! If it was the case that something "really unsafe" had happened, that should have been a frank 1-to-1 twixt owners and maintenance. The likelyhood of that happening? Mmmm! One hopes lesson were learned on both sides. Hence, back to poor old CFO's dilemma. My point is, "If it's wrong, don't fly it!".

Don't wish to fall out PDR as you seem someone of "sound reason" on here! I'll let others go into the "It may be this" or "It may be that"! Off to water the garden now the sun is going down as the Boss is away for a couple of nights and her "pride and joy" has been entrusted to my not-so-green fingers. Apparently, if any plants at all die in the next 48 hrs I will be held entirely responsible and shall die a slow lingering death! Mrs H 'n' H is not to be messed with when it comes to "her garden"! :(

Cheers, H 'n' H

Great discussion and info, thanks all! To be honest, the whole thing makes me sad when I think of it for a minute. Maintenance shop clears the plane that should have never been cleared. POH says 'oil pressure good' in before takeoff checklist, but it gets ignored. What else could possibly go wrong? Will keep an eye on the situation and will keep the thread updated for completeness.

Aw heck, me neither! Not looking to fall out or grandstand - just emphasising the point that if the PoH givens a maximum oil pressure (or anything else) then taking the mechanic's word that "it'll be all right mate, trust me" might not be an execise in good judgement!

PDR

Phew! Glad about that! :ok:

Exactly! In my case it seems that mechanics had never even been asked by the owners! All 'em lovely Green Arcs ain't there just to look pretty!

PS You'll be relieved to know the garden has survived the first 24 hours! :}

I am not the slightest bit surprised to hear the airplane in question was flown a 100 hours with the oil pressure reading too high. One of the things I find rather discouraging is that there seems to be a significant problem with the culture in flying schools and clubs. It is IMO the toxic combination of pilots not making the effort to learn their airplane systems combined with a willingness to accept broken airplanes without complaint. Sadly it starts with instructors not doing their job

2 examples

1) A while ago I was asked to do a couple of flight instructor ratings for a local school. On the first flight in the C 150 I let the student do the takeoff and gets us to the practice area so we could start with learning how to teach Attitudes and Movements. On the way out I thought the student was doing a very good job of staying coordinated. When I took over I found out part of the problem, as soon as I let go of the controls the airplane turned sideways. In 30 years I had never seen an airplane so badly out of rig. When we got back to the school I asked the engineer what was up and she said in the year she had been there no one had ever complained :ugh:. After she went through the airplane and adjusted all the control surfaces the airplane flew perfectly hands off :ok:

2) A different school and another C 150. I was asked to fill in for a sick instructor. We go out and the engine seems a bit rough on runup. The student apples full power and it is immediately obvious we are a good 100 RPM low on the required minimum static RPM so I take over reject the takeoff and taxi back. When I explain why we rejected he says "What's static RPM ? ":hmm: . I snag the airplane and t turns out one cylinder has an exhaust valve that was probably only a few hours from total failure. As I am leaving the dispatcher says "Oh the instructors have been complaining that that airplane has been really doggy for weeks ! " :rolleyes:

Bottom line learning your airplane systems is a choice.You can make the effort to really understands how things work so you know when things are not doing what they are supposed to do.....or not,and then make good decisions.

When I had a car dump all its oil the garage diagnosed the pressure relief valve problem immediately, the first (and only) time it happened.

#NotAllSchools

On the climb out I pointed out to the instructor that the oil pressure was rather higher than usual (outside the green but below the red).

He took control, flew a circuit, and we landed, lesson scrubbed.

Next time I flew that aircraft the needle was in the green.

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.
.

I assume that's an approved method in the respective install procedures and that a suitable placard is placed in the cockpit as part of that approved procedure for the duration that the readings are incorrect? ;)

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!!!!! :sad:

Cheers, H 'n' H

Not sure if I understand you here but there is a subtle difference between starting with the pressure above the green arc and drifting down into it (scifi) and what should happen which makes use of the drift down as well so I'll cover tht first. The pressure would initially be set to be at the high end of the green arc and, over time, it may/would drift down. The periodicity for the check could either be "after x hrs, carry out a recheck of oil pressure and reset to (say) middle of the green arc if necessary" or it may even be an "on condition" callup - usually the former as it forces a check as it has already been noted how many pilots are happy to fly aircraft "out of limits"! :cool:

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! :ok:

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!!! :p

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 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.

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

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.

1. 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.
2. 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:

With the engine warmed up and running at approximately 2000 RPM, observe the reading on the oil pressure gage. If the pressure is above maximum or below minimum specified limits, stop engine and screw the adjusting screw outward to decrease pressure or inward to increase pressure. Depending on installation, the adjusting screw may have only a screw driver slot and is turned with a screw driver; or may have the screw driver slot plus a pinned .375-24 castellated nut and may be turned with either a screw driver or a box wrench.”

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! :E

Cheers, H 'n' H

There are only two types of "adequate" for oil quantity, and the other is "inadequate", If the quantity is inadequate, the pump will not draw, and there will be no oil pressure, and continued operation will ruin the engine. If the oil quantity is adequate, the oil pump will create pressure and flow, and the indicator should indicate that. There's really not much in between these two conditions.

An O-200 I used to fly had had a starter clutch let go inside, and distribute pieces throughout the engine, including oil galleries. The engine was removed from service, and overhauled - I know, 'cause I test flew it upon reinstallation. I had oil quantity ('cause I checked) and oil pressure ('cause it indicated), but in flight, the oil pressure suddenly went to zero in downwind, so I returned immediately for an idle landing. After several ground runs where oil pressure was indicated, and a little head scratching, I flew it again, and it was fine. The following week, the owner returned it with intermittent zero oil pressure indication. The engine was taken apart again, and a piece of starter clutch was found to be lodged in an oil gallery, and blocking the pressure indication, the engine had been fine for oil pressure and supply, (other than a bit of metal being somewhere it should not have been!) The engine shop had to absorb the cost of a re-n-re, and second disassembly. After that it was fine.

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.

Since I've not flown that plane with high oil pressure, my information is not first hand and may be confused. My understanding is that pressure was not dropping through the single flight, but very gradually dropping over the hours, although I may get confuse that too. In any case, the owner is now convinced to ask maintenance shop to conduct oil system inspection and to get oil pressure back into green arc where it belongs to. Let's see how that goes. I am sure once it is back to normal everyone including me will keep a very close eye on oil pressure needle and not tolerating any instability.

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.

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.....

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.

The engine oil pump, is, of course, a hydraulic pump, whose output (flow and pressure) is in direct proportion to engine speed. The oil pressure relief valve is designed to assure the minimum required pressure is maintained, while excess oil pressure is dumped harmlessly back into the system at low pressure. Other factors affect the oil pressure, in particular oil viscosity vs temperature. It's hard to maintain high pressure with a very low viscosity oil in a [somewhat] leaky system.

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 :ugh::mad:

This was a totally avoidable failure:=

>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

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.