High Oil Pressure

[GroundBound]

Yesterday, flying the club Archer, I noticed the oil pressure was close to the red line MAXIMUM, although it remained just within the green arc. The oil temperature remained as normal - in the middle of the green.

This surprised me, as I have never had an indication of high oil pressure before, and most engine related problems seem to be related to low pressure.

I checked the oil on the pre-flight - it was OK (mid-way between min and max). Pressure was normal during engine run-up. Outside temperature was warm, 27 C - but I would not expect this to cause an increase in the oil pressure.

So what might cause a high oil pressure - to the point where I was becoming quite attentive to the gauge? Blocked filter or pipes is the only thing I can think of.

Any thoughts?

GB

[Superpilot]

You have an airplane approaching what 20 years old? Of course it's most likely to be a crap guage.

You know it's amazing, you can pick up a 15 year old 4 door family saloon these days for just under £1,000. Naturally you'd expect everything to work. Including the fuel guage, the oil and water coolant temp gauges and they are all extremely accurate.

Now swap the 15 year old car for a 15 year old plane. Suddenly ones expectations become very different. My group PA-28 (around 18 years old), has not a single guage that you could confidently say was reliable. Not one. Yet it passes it's annuals and so forth each time. One of the problems right now is an OP guage with a mind of it's own. Now tell me that's not a safety issue! :ugh:

[IO540]

A crap gauge is most likely, but it could be real and since this is a serious safety issue it must be investigated before further flight, IMHO.

I've had this too, a persistent problem in fact, and it is always the oil pressure transducer (sensor).

Since one obviously cannot depart with a suspect oil pressure situation, I was going to fit a backup gauge but under G-reg the paperwork alone (CAA Level 2 company had to do it) was about £2000. Under N-reg it's a bit easier; perhaps £500-£1000 for the paperwork.

[A and C]

Has the aircraft been into the sheds for an oil leak problem around the crankcase or cylinder base gaskets ?.

If this is the case then I would suspect that the crankcase has fretted and the engine is not far from a major failure.
I would not fly the aircraft untill I had looked at the engineering history, if there has been any work on the crankcase tie bolts the engine is likely to require overhaul.

[jetflite]

A pressure relief valve in the system should ensure that the oil does not reach an unaceptably high pressure (red range). A high oil pressure may cause parts of the system to fail, however it is not unusual to see the needle in the top half of the green range, It also varies from aircraft to aircraft and depends on the setting of the relief valve.

SO in my theory if your oil in the preflight check was in between the max and min, and your temp gauge showed a constant reading through out you may have to tell the club to check their pressure relief valve setting,

This can be dangerous if set to high not allowing the excess pressure to escape the operating limits will soon be exceeded, best bet is to write a note on the MR, this way other pilots wont be as nerved as you were seeing a high oil press. and to ALSO let the club know!!!!

Cheers,
JF:ok:

[A and C]

In theory you are correct, but without looking at the maintenance history you can't tell why the oil pressure is on the high side.

If the oil pressure has suddenly increased there is a reason for this and the reason must be known, with the OAT stated I would expect the oil PX to be on the low side (normal) when the engine is at idle (hot) but to rise to green band when in flight. If there is a presure rise then there is a reason for this and it is likely to indicate major mechanical distress.
This needs investigation unless you want to see the engine stop suddenly when at high power.

IO-540 is quick to critisise the indication system but I have found these remarkably accurate over the years and unlikely to change the PX indicated suddenly without good reason, they are not super accurate but after all they are only indicating the oil pressure on a former water pump engine !

[GroundBound]

A&C - I'm not an engineer, and the aircraft belongs to the club, so they deal with all the maintenance - there's nothing in the Trouble Report log, though.
I did leave a note in the flight log - and informed the club, just in case. However, when I told them the pressure was normal during the run-up and the temperature remained the same throughout I had the distinct impression they didn't believe me (body language!).
I fly the aircraft fairly regularly and I have never seen the oil pressure guage this high - almost touching the red marker (and allowing for some parallax) - it usually sits plumb in the middle of the green arc.
I'm due to take the aircraft on a long trip away to Germany this weekend, so I am still a bit concerned. :ooh: :ooh:
GB

[IO540]

I agree that the purely mechanical ones are very reliable. I would imagine the original poster had one of those, which is why I suggested taking the unusual indication seriously.

Then there are the relatively simple electrical ones with a resistive transducer; they are not so reliable.

Then there are the electronic ones (like mine) which have a resistor bridge transducer, feeding an amplifier comprising of 3 op-amps, etc, and they are less reliable still.

[Say again s l o w l y]

Whilst guages are often unreliable, you ignore them at your peril.

Have a chat to those who fly the a/c most often to see if they have noticed anything, but always remember that as PIC it is your right to ground a machine if you consider it unsafe. It may not make you popular, but any issue like this should be investigated. I don't like seeing any indications near red lines so if the guage is knackered, then this at least needs to be checked.

[Paris Dakar]

GroundBound,

I may have misinterpreted your post but are you saying that the needle sits in exactly the same place once the engine has started or does it start off normally and climb to the high pressure reading?

It could well be a problem with the pressure valve but I would get it checked out straight away. As for flying to another country in the aircraft - I'd be reluctant to even do a circuit in it until an engineer has resolved the problem :ooh:

[LowNSlow]

A & C I'm intrigued now. I am ignorant of the innards of a flat four engine, I'm more used to inline fours which don't have the split crankcase of the flatties. Why would work to repair an oil leak result in high oil pressure apart from the obvious one of somebody dropping a bit of the old gasket and / or sealant into an oilway?

Wouldn't a partially blocked oil filter be the more likely cause a high oil pressure reading or is the pressure sensor generally downstream of the oil filter thus causing a low pressure reading as the filter gradually chokes up?

[A and C]

The problem is that if the crankcase has fretted around the main bearings this has the effect of releaving some of the tension on the cylinder tie bolts, this in turn means that the cylinders leak at the base.

The crankcase can also leak at the joints as the fretting gets worse.

The big problem is that if the engineers dont pick up the fretting and just re-torque the cylinders this pulls the crankcase down tight and as it takes up the gap left by the fretting it pinches the main bearing shells reducing the clearance between the main bearing and the crankshaft, the detectable result of this is a rise in oil pressure due to the decreased bearing clearance.

If the bearing clearance is reduced there is a good chance that it will overheat.

There are two failure modes the first is that the main bearing seizes and the engine stops, usualy at low power.

The second failure mode is a bit more of a problem because this happens at high power, first the main bearing seizes onto the crankshaft but because of the high rate of crankshaft rotation it pulls the bearing pegs out of the crankcase and the bearing shell rotates in the case lubricated by the oil feed.
Unfortunatly the big ends are lubricated by oil that is fed via the main bearings and now have no oil feed, now in a very short order the big end bearing seizes solid and a con rod exits the crankcase at some speed !

I think that you can now see that what seemed like an indication problem may will be an indication of a much bigger problem and so if I was to fly in this aircraft I would want to know the maintenance history over the last 100 hours to assure myself that no work had been done to the tie bolts and cylinders, the chances are that it is an indication problem but I would be asking questions.

[IO540]

Isn't this engine type dependent??

On an IO-540, the oil pressure regulator valve (which incidentally is adjustable) is a spring loaded bypass valve, and it is this valve that regulates the oil pressure.

The bearing clearances do not affect the indicated oil pressure i.e. the oil pressure in the oil distribution galleries. Obviously if there was a large oil leak after the regulator valve (for example in the oil cooler; a prime candidate for a catastrophic leak if there ever was one) then the valve would be unable to maintain the oil pressure and the indicated pressure would drop.

Curiously if you get a major oil cooler leak, the oil pressure should drop but the oil temperature will not change - until the oil has been exhausted. This puts paid to yet another OWT: "if the oil temperature is OK, don't worry about oil pressure fluctuations".

That's my understanding.

[LowNSlow]

A&C thanks for the explanation, it makes a lot of sense.

IO540 if the bearing supply is downstream of the pressure relief valve then there is a potential for an initial high pressure caused by restrictions through the bearings. This will then be relieved by the valve doing it's job which then results in a reduced feed to the bearings resulting in the seizure descrived by A & C.

Fortunately i don't have an oil cooler on my Auster!

[A and C]

In answer to your question no!.

This is not type dependant but Lycomings seem to be particualy suseptable to crankcase fretting you have not grasped the nature of the problem.

The reduction of the bearing clearance increases the oil pressure due to the fact that the regulator valve can't regulate the pressure becaure it would have to "dump" more oil to the return side of the system than it was ever intended to.

So as you can see the bearing clearances do affect the oil pressure if they are outside the range that was intended by the engine manufacturer the oil PRV is in the system to "trim" the oil pressure to deal with the variations in pressure ( volume) due to the speed of the oil pump (engine RPM) and the viscosity (temp) of the oil.

If your Lycoming has had some cylinder work and exibits any of the symptoms that I have stated such as oil leaks from the crankcase jonts or cylinder base then you need to have the engine looked at, I have seen engines with sealant placed along the top joint of the crankcase to keep the oil in................................. the aircraft is usualy for sale !

The only answer to the fretting promlem is to have the crankcases repaired but it is important to catch the problem quickly as if the fretting is to deep it will write off the crankcase.
I have had to remove from service two engines in the last two years due to fretting one was at an early stage and the crankcase was repaired the other one's crankcases were B.E.R.

In the case of both engines full overhaul was the best economic course of action but it would have been better if one of the engines had been removed from service sooner because I might have saved the crankcase.

[B2N2]

Short answer is....don't play engineer.
The real advantage of always flying the same plane is that you get to now it.
What temp is normal under what conditions, what pressure is normal etc.
The changes you describe in a short period of time....NO GO.
Have it checked out by somebody who is good at what they are doing.
Engine failures are something that you train for not practice.

[IO540]

OK I can see that if the resistance (to oil flow) following the pressure regulator valve increased sufficiently so that it took the valve outside the range it can regulate, that would increase the indicated oil pressure.

However, has this been proven to be the mechanism for the high indication? Presumably, it could be proven by adjusting the valve, using its external adjustment. If adjusting it doesn't do anything then the hypothesis would be right.

I think there is a lot to be said for having a second oil pressure gauge. Most planes have a switch (driving a light) but that normally operates at a very low level and is useless until the engine is about to pack up.

[Gertrude the Wombat]

I once noticed the oil pressure just over the red line on climb-out.

I was flying with an instructor.

The instructor took over, flew a circuit, and we landed and swapped it for a different aircraft.

Yes, it turned out to be just a duff gauge or sensor or somthing, but you can't tell that in the air can you. I think I would have decided to land straight away if I'd been on my own, but I definitely would in future.

[GroundBound]

Hmmmm, seems everyone has the same reservations I had.
It doesn't look as though anything has been done (I said they didn't believe me), and the a/c has been flown by several people since then - apparently without any concern. :rolleyes:
So, at the weekend we took said a/c some several hundred miles into Germany and back, and the oil presure behaved itself - still a little high, but not close to the red line.
Seems now, within the club, I have a acquired an invisible sticker with "stupid pilot" on it - sigh!
Hopefully it is a suspect guage and the engine will operate OK, but it still gives me a slightly uneasy feeling.
GB

[IO540]

I don't think you are a stupid pilot at all.

The attitudes of so many people in GA are "if it starts and takes off, who gives a toss?". The practices in much of the flight training business don't exactly help, with some practices (especially on fuel management front) coming as close as they can to all four wheels falling off.

The only time this changes is when one buys one's own plane, which of course the vast majority never do.

[Paris Dakar]

GroundBound,

I concur with IO540.

I would/will take the same action as you have. Perhaps the pilots that flew the plane since, didn't even notice the gauge - it may have been a case of 'yep, that looks about ok, let's get on with flying'?