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Corrosion in low time and 'unused' engines


skydog
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I am considering an engine that was brand new 4 years ago was in a plane that was not flown for a year and then for the following year did only about 13 hours. The plane has since been flown albeit not a lot but regularly like one hour per month.

 

During the year the plane was not flown the owner ran the engine on the ground for about 20 minutes once a month.

 

I hear about corrosion that can occur in idle engines and reduced "life".Can anyone tell me, , about the probable condition of this engine and if the no fly period has caused damage to parts of or corrosion to the engine?

 

What is the real worth of running the engine on the ground, I thought they had to be flown at 170 degrees oil temperature to get acid and water out of the oil and engine.

 

Comments?

 

 

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Corrosion.

 

20 minutes running moght do more damage than not running it at all. The engine should reach about 85 degrees celcius,(or more) oil temp and this would need about an hour of flight time. The camshaft is high in the crankcase and gets moisture (condensation )effects. The engine should be inhibited, during long periods of inactivity. If the oil was freshly changed, that would help. The latest Shell oil has additives that help to limit Lycoming corrosion .Nev..

 

 

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Guest drizzt1978

I supose if you get it cheap enough....you could run it...do a compresion test...then have the oil sent to a lab...if you were happy with that.....do circuits for a few hours send oil to lab again...if its all looks ok im guessing it would be.??? What do you think?041_helmet.gif.78baac70954ea905d688a02676ee110c.gif

 

 

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All good advice, check compression leakage and do an oil analysis, also how does the engine look externally, any corrosion here? If the price is right, I would get a lame to top it and check crankcase out while cylinders are off. This exercise will cost a few grand if nothing is found and a TAD more otherwise.. but you will have piece of mind.

 

 

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Inspection.

 

You can inspect the cylinders thru the plug holes. A compression check would do no harm. The real concern is the camshaft lobes. If they are rusted they can scuff the followers/lobes. and you can contaminate the motor with metal.

 

The latest IO 233 has roller followers, (just mentioning this)

 

. An engine LAME could check this for you. The engine is probably a good thing. Would be a shame to spoil it. Apart from the cost of any repairs that might result from the damage, you don't take chances with aero engines. Good luck. Nev..

 

 

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Thanks guys. I guess the key is getting it cheaply enough. So a check thru the plugholes is a borescope I take it.

 

Spose the top has to come off to see the lobes though.

 

The engine looks AOK externally. Oil analysis checks would be essential drizz yes would do that for sure, thanks.What do others think about that idea after a few hours flying check again or would you put new oil in and check that?

 

 

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Where abouts was the engine stored... Ie, near the coast, or more inland?, is it humid or dry?.... reason i ask is because if it's near the coast(a lot more humid), you've got more moisture in the air, therefor making it more prone to corrosion... thats what I've found with vehicle engines anyway.

 

Cheers,

 

 

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Moisture in Lube oil.

 

Moisture accumulates in lube oil when the oil temps are too low to "boil it off". This moisture comes from the process of burning the fuel. (hydrocarbon). The carbon burns to CO2 and the hydrogen combines with oxygen to form water H2O. (Pretty basic stuff). Blow-by past the pistons allows the oil to pick it up. For every litre of paraffinic hydrocarbon fuel burned there is more than one litre of water formed. If you want to get the precise amount get the basic chemical equation and apply the atomic weights to the components and do the calculation. (or take my word for it).

 

Due to impurities in the fuel (Sulphur,etc. ) and the CO2 ,acids are formed in this moisture, which attack the surfaces of engine internals. Cloudiness in the oil is a sign of moisture retention. Not all of the oil in an engine has to reach the boiling point of water but (depending on the particular design), a temp of say 85 to 90c, should be indicated and other parts of the engine would then achieve temps above this (locally) and get rid of the water, through the engine breather. Nev.

 

 

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Yes I know what you mean....I just asked because an engine on the coast is... how will I put it... has more corrosion, than an engine in the desert...for example...

 

 

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Environment.

 

Look I don't disagree with that .The salt and humidity do a lot of damage and a non-saline desert environment is a great preserver of old aeroplanes. I've seen rain get into stored motors /gearboxes and they ruin easily. Rotecradialengines have a reasonable bit of information on storing engines on their website. The worst thing you can do to a Jabiru engine is to leave it out in front of the clubhouse , then start it up and taxi it to the hangar, and leave it fot a month or so. Guaranteeed to rust the cylinders. Nev..

 

 

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Skydog. To properly see the cam lobes will require splitting the case as the camshaft is parallel to the crank in the centre of the case. Not a cheap job, and by the time oil analysis reveals iron in the oil from a camshaft, the damage has probably been done. It may just be possible to see parts of the cam with a cylinder removed, but I wouldn't like to rely on it giving the all clear, it may just show that damage has been done.

 

 

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Shutting down the engine correctly has a big part in stopping corrosion in the top of the engine. from memory Lyc recommends that you idle the engine at 1200rpm to settle the temps for 3 minutes and then run it up to 1600 for one minute to bring the oil pressure up and fill the galleries and flood the cam area and then straight to idle cutoff without reducing the rpm. might be 1800rpm but you get the point?

 

you know you have problems when you cut open the filter and can read part numbers on the metal bits.

 

Ozzie

 

 

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I knew it was around those figures been a few years since i worked on them. but i was always amazed at how many pilots never knew this or if they did never followed that procedure.

 

ozzie

 

 

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From memory this procedure came in with demise of 87 oct Avgas. The engines didn't take kindly to 100 LL. and this helped with lead plug fouling.(it actually has quite a lot of lead)

 

Seems that it was the start of lots of new problems for the engine; (some may have been design features remember the 0-320 HAD!); fouled plugs, sticky valves etc., at least they fixed the the front crankshaft seals; what a mess that used to make.

 

 

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I remember reading something about the 'aromatics' used in 100ll caused a lot of problems with high acidic buildup in the oil. which jogs a figure of at least 15 minutes of operating the engine after full working temps obtained to evaporate moisture and minimise the buildup of sulpher and acids from the fuel. 50hr oil change too i think

 

when did they change to 100LL? i can't remember.

 

Ozzie

 

 

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Ozzie,

 

Your comment below stumps me, it does sound rather counter intuitive (but maybe so). I had a quick re-read of the Lyc. Operational notes but could not find reference to what you are saying. I have never come across this concept nor have I ever heard others mention it. I am not at this stage discrediting what you are saying but it does sound very strange indeed and I would like very much to hear the background science to explain it.

 

Shutting down the engine correctly has a big part in stopping corrosion in the top of the engine. from memory Lyc recommends that you idle the engine at 1200rpm to settle the temps for 3 minutes and then run it up to 1600 for one minute to bring the oil pressure up and fill the galleries and flood the cam area and then straight to idle cutoff without reducing the rpm.

Could you please advise where I might find the reference.

 

djpacro,

 

This reference;

 

Page 66 of http://www.lycoming.textron.com/supp...Operations.pdf (3.5 MB) refers to what ozzie said with one difference - take it back to 1000-1200 rpm for shutdown.

Is as far as I can tell, has nothing to do with oil preservation of the inards of an engine and simply a tried and tested method for aviod plug fouling.

 

Am I missing something?

 

 

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No problem

 

this procedure was shown to me by an old LAME/FAA Repairman and repeated by other lames were i used to work some time ago.

 

It was never mentioned that it was a procedure to keep the plugs clean but rather for 1. And 2 was a benificial by product of one.

 

1 it allowed the temps to stabilize and settle down and was better for the engine rather than just landing and switching off the engine. If the engine is still 'hot' and it is just switched off the different materials cooled at different rates and things like the guides could nip up on the valve stems and leads to cracks in the head ect.

 

i saw this happen on a 3.3 jab engine that was fitted to a storch that had a poor cooling not to mention the aircrafts speed in cruise and climb that is about 40kts slower compared to other aircraft this engine is fitted to. He stopped to refuel tried to restart and the engine was locked up in the rear cyls guides. I have read this in other various engine ops manuals including high performance car engines. ie turbo charges.

 

Turbines are also subject to a 3 minute cool down at ground idle.

 

2. oil pressure at idle is lower than at 1600rpm so it goes to say that there is less oil flow at idle than at 1600rpm. so running at the higher rpm will supply more oil into the galleries and into the cam deck area as well as assisting in cooling this would give the cam and followers a good coating of oil including the case top.. true it will mostly all drain down but it will have to help if everything is lubed right up rather than if it had lower oil flow and just shut down.

 

My experience with Lyc engines has been limited to the 250 and 300hp models in Islanders. (jumpships). as these had no cowl flaps they really needed to be 'driven down' to stop cool shock which cracked heads, the MP was reduced 1" per 1000ft when you decended under 5000ft. Then the 3 minute high idle for shutdown. This was the owners procedure and when they were flown like this they never popped a cork. The others were schofields fleet of 'weekend warriors'. when i reread the lyc ops manual pfd last night it seemed a lot bigger than the last time i read it. i never recalled the lead bit which is why i asked as to when 100LL became the norm, some operators would lean a bit more to burn it off.

 

One other thing that i was taught was warm up from cold. Some one mentioned idle @ 1200rpm.

 

Q. what is your oil pressure showing at this rpm on a cold start?

 

A. bettcha it's way up in the red.

 

pretty hard to pump that cold molases so the pressure goes way up and the by pass valve will open and very little oil will find it's way to were it is needed.

 

so pretty important to 032_juggle.gif.8567b0317161503e804f8a74227fc1dc.gif rpm and oil pressure until you can get the pressure to drop a bit to get the rpm.

 

Time changes methods used in practise due to improvements in design and manufacture not to mention the accumilated knowledge gained over time so i'm always open to updating, so back to you

 

ozzie

 

 

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G'day Ozzie,

 

Thanks for that detailed response.

 

The purpose of my query is to establish how a correct v incorrect shut down technique makes a difference to internal engine corrosion. And what exactly is a correct or incorrect shut down? And why?

 

I guess what I was looking for as a matter of first order was verifible hard data, ie. that from test cells or specific flight testing. If you are able to give me a direction regarding this I would be most appreciative.

 

Whilst I understand and appreciate the merits of temperature stabilisation, I do not see how this has any influence on internal corrosion over the following weeks/months of inactivity.

 

As a side note and with regard to the other areas of operation you have detailed in your response I would like to add comment if I may.

 

My comments can be verified by most common engine installations and very basic standard instrumentation.

 

With reference to naturally aspirated engines, time spent at above a typical idle rpm will serve no purpose other than to drive the engine temperatures back up again after landing and exasperate the purpose of the exerise.

 

Following a standard circuit and final approach the engine will never be cooler than at the intial touchdown. Although a quick tight circuit may see further cooling after touchdown. This is intuitively obvious as the pressure deck differencial is still relatively high (compared to ground ops) and the horsepower output (heat potential) very low. Once on the ground poor deck pressure diferential is aparent and thus poor cooling. In a normal operating environment the reduced power during the 3 odd minutes of the approach followed by the roll through to the apron is sufficient to deal with temperature stabilisation. Turbo charged engines are the same however well meaning soles have created a thriving adherence to an OWT (old wives tale) that states that in order to cool your oil and avoid coking one should run their engine on the ground for a prolonged period to heat the engine back UP from the temperature achieved at touchdown. Having said that there is still importance in ensuring that turbine/compressor temperature diferential is minimised, and this may require some thought through the descent circuit/long final and roll through.

 

Whilst on OWT it is a myth that lead can be burnt from the plugs, it can't, however carbon can be. The fellows you refered to as weekend warriors were either dealing with carboned up plugs or misinformed.

 

A simple exercise to verify the above comments on temperature is to run your engine (into wind) at idle for as long as it takes to achieve an absolute max oil and CHT value. Then shut down and then when back at ambient do the same but at say 1600 RPM and see which produces the highest values. I don't think many will need to do this to know which one will be the higher.

 

As for the 3 minutes cool down for turbines, this is more to do with the coking of the fuel nozzles by the residue of Jet A on the nozzle tips inside the baking temperatures of the combuster after shut down. This coking inturn causes poor atomisation resulting in poor combustion and in the worst case torching of the stators/turbine.

 

Whilst your comment that 1600 RPM produces greater oil flow is true it is not correct to think that the difference in flow is as simple as thinking 1600 RPM is 1.6 times greater than 1000 RPM, it is more likely 1.25 times greater. This hardly constitutes a "flooding" oil flow.

 

I find it difficult to accept that after flight and within the few minutes to the apron that the internals of the engine have shed significant quantities of oil as to leave them extrordinarely exposed.

 

Originaly you stated that running the RPM to 1600 RPM cleared the oil galleries of air. I have never heard or read of this, and I am suprised to hear that once pressure has stabilised after start up that there could be significant air in the system when back at idle, particularly after flying. Surely if this were the case foaming would be an issue as would flickering oil pressure at the gauge.

 

One other thing that i was taught was warm up from cold. Some one mentioned idle @ 1200rpm.Q. what is your oil pressure showing at this rpm on a cold start?

A. bettcha it's way up in the red.

It shouldn't be if the engineers have shimmed the vernatherm correctly and the pilot has adhered to the POH regarding pre-heating the engine/oil in very cold temperatures. There is also written engineering comments that too low an idle speed >1000 RPM results in poor internal splash efficiency with thick oil. However I imagine this would be quite subjective depending on internal design.

 

Ozzie,

 

Whilst it appears I am discrediting the theory you have espoused, I am simply following my nose on learnt knowledge. If you have references to suport your statements I would be more than prepared to be enlightened. Verbal comments from LAME's don't cut it, I have heard some of the biggest OWT propergated from some LAME's.

 

Best Regards

 

Mick

 

 

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wow thats hard on the head at 7am.

 

there is reference to old wives tales somewere . it concerned engine operations such as this and how the procedures started,

 

it stemmed mainly from older operators who carried over procedures from the very early days when design and materials as well as oil were not as good as they are today.

 

I learnt from older lames that who originally trained in the UK US as well as here. They regulary conflicted each other in areas..Things do change and have rapidly done so since the internet has come along . But i do belive them as they were the experienced ones. I am not a chemist or metalugist so the deep down reasons are past my knowledge and really not interested in it other than in the direct line of work. ie service and fix.

 

air in the oil galleries hope not. oil pressure in the red is in the red avoid it. who preheats an engine here in Oz on a winters morning when it is sitting in a paddock.?

 

when you taxi in the gauges indicate the cht cyl head and oil temps to be dropping but the actual core temp is still high and bringing the temps back up then shutting down will give a more even cooling down. what do continental say about this?

 

time for brekky

 

Ozzie

 

 

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Couple of comments.

 

Some turbines, where high thrust settings have been used on approach are required to be run at lower thrust for a period after landing prior to shut-down, to stop the turbine blades increasing the tip clearance in the case. Repeated in flight shutdowns from high power settings,(Take-off power) will deteriorate engine efficiency to a noticeable extent. Same reason.

 

Radial engines that I have been involved with were shut down on the mixture control from a higher than minimum idle speed, and thereby ingested clean unburned air to scavenge the combustion by-products from the cylinders. I believe the DC-3's (that I did not operate, so I have no personal experience) were recommended to advance the throttles during the run-down to aid this process. This only effects the "top end" of the engine (combustion area).

 

Something is very wrong if air is ever in the oil galleries, (except at start-up). This is why you should start up with a small amount of throttle only , then advance it to the recommended warm up RPM.

 

We don't have the situation with our aircraft that exists with some types where with full flap out and gear extended, the engines on approach are often at fairly high power settings, and the lowest speed that they fly . Cowl gills are set at trail, for best drag/cooling, on approach. They are set to "open" after landing, where the extra drag doesn't matter. Turbochargers glow bright red and exhaust valves and ports get very hot. It is better to allow these temps to reduce slowly with low power settings, (A lengthy taxi might do this) than to cut the engine and cause a certain amount of shock cooling, and migration of some of the heat to the "core " of the engine, where seal life might be affected, particularly with "close cowled' engines Nev..

 

 

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Blade scrap as it is known for turbines. core temps!!

 

did i say air in the oil gallaries? i was referring to flow in the galleries and into the cam deck area.

 

facthunter has summed up the cool down much more clearly than i.

 

look in thru the gills of a chieftain cowl on takeoff at night you can see the turbine wheel thru the cheery red housing. amazing

 

Ozzie

 

 

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Ozzie,

 

My bad, when you stated "fill the oil galleries" I incorrectly assumed you meant that there was a lack of oil in the galleries, sorry.

 

Lycoming recommend a short period at 800 - 1200 RPM after touchdown for the purpose of stabilisation, generally the taxi time will suffice. Running it back up to 1600+RPM is counter productive for stabilisation. However as facthunter has pointed out perhaps a 5 second burst as idle cut off is selected may have merit regarding clearing corrosive by products. When back at my computer I will try and attach some material on this.

 

Facthunter,

 

I agree on the blade creep issue, had forgotten about that, generally not an issue with the turbines I fly.

 

Your comment regarding scavenging clean air through the cylinder sounds logical and may well point to some good old data that might be around on this. I sometimes used to this very same thing with the likes of the PW985/1340 but this was more of a case where idle cut off was less than perfect. I had not heard it mentioned (or have forgotten) on removing by products, but does have a logical ring to it.

 

Regarding high power approaches, it should be easy enough to plan most approaches such that a large power setting is not required, occasionally it might happen but not in a normal planned approach. I have flown some pretty draggy types and aiming for a 3 degree profile only requires <40% pwr. The cool down rate from TOPD in a turbocharged machine (or any for that matter) should resemble a somewhat parabolic curve, slow to start increasing toward the the touch down. This way as the yeild strength of the alloys increases with reducing temperatures so does the cool rate. The target result is to arrive at the parking bay with everything at a minimum, not always possible, but mostly. I might add at this point I do not fly turbocharged aircraft, turbines and supercharged yes, but no turbo's, however the principals are well written about.

 

For an excellent education on turbochargers I can recommend John Deakins 5 part series on the subject, this can be found (I hope still) on the Avweb site, Pelicans Perch, archives.

 

Regards

 

Mick

 

 

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