Coking in Arriels.
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Coking in Arriels.
Is there any documented evidence that Prist reduces coking in Arriel engines?Apart from the anti-freeze capabilities it is supposed to prevent coking and act as a bacteriacide agent. All this , along with the 30 second cool down also achieving the same, is all hearsay. It would be nice to get some written proof.
My DO does not believe that Prist is required in helicopter fuels(go figure--but the man is a Fixed Winger -can't blame him for his misspent youth) and is planning on doing away with Prist in bulk fuel purchased by the company. Apart from the fact that the RFM for the Astar calls for mandatory Prist when temps. drop below 0 degree C. a reinforcement argument, at least for Astars, would be nice.Anyone?
Alt3.
My DO does not believe that Prist is required in helicopter fuels(go figure--but the man is a Fixed Winger -can't blame him for his misspent youth) and is planning on doing away with Prist in bulk fuel purchased by the company. Apart from the fact that the RFM for the Astar calls for mandatory Prist when temps. drop below 0 degree C. a reinforcement argument, at least for Astars, would be nice.Anyone?
Alt3.
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I can't give a direct answer to the coking question however I previously worked for a company providing helicopter support to the UK police using AS355N and used AS355F1/F2 as back-up aircraft. About nine years ago we were directed by the company not to use any remaining aerosol canned stocks of PRIST ourselves at point of refuel, because it was too toxic. If my memory is correct the N didn't need PRIST but the other models did. It wasn't a problem to us because the supplied bulk fuel at base had FSII aditive but I don't know how other units dealt with it.
The two types I've flown since don't need it as they have fuel warming in their systems; the RFM being the documentary authority I refer to.
It's the fuel temperature that counts; will your Astar sit out in freezing temperatures, getting cold soaked? If so, surely the additive is needed. If the RFM calls for it, so would the insurance company. When making aviation decisions, I always ask myself "what would the board of inquiry say?"
The two types I've flown since don't need it as they have fuel warming in their systems; the RFM being the documentary authority I refer to.
It's the fuel temperature that counts; will your Astar sit out in freezing temperatures, getting cold soaked? If so, surely the additive is needed. If the RFM calls for it, so would the insurance company. When making aviation decisions, I always ask myself "what would the board of inquiry say?"
Depends on your engine - Arriel 1C needs Prist but if later models with an oil/fuel heat exchanger you don't. Anyway what part of "the RFM states" does your DO not understand?
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Ok, stupid question/s time............ 
Are we suggesting that the PRIST, that is used as an anti-freeze for the fuel will possibly, as a secondary effect, assist the engine oil's ability to withstand coking?
Coking being a term used for the oil not being able to transfer heat away sufficiently to a point that it turns to carbon? I am confused (easily done mind you).
My experience is that use of a 3rd Generation HTS turbine oil (Mobil 254 or equivalent) would be the primary anti-coking preventative measure.
Are we suggesting that the PRIST, that is used as an anti-freeze for the fuel will possibly, as a secondary effect, assist the engine oil's ability to withstand coking?
Coking being a term used for the oil not being able to transfer heat away sufficiently to a point that it turns to carbon? I am confused (easily done mind you).
My experience is that use of a 3rd Generation HTS turbine oil (Mobil 254 or equivalent) would be the primary anti-coking preventative measure.
Its a long time since I did the Arriel course but I do recall the following:
The 30 - 45 second rundown is an issue. Any LONGER than that at idle is a problem. Longer is NOT better. In this cooldown period the rear bearing temperature will reduce but after that period it is anyones guess. There is a graph around somewhere in the training notes that shows what happens, the temperature invariably goes back up.
Either prior to shutdown or running at idle on the ground, the problem is poor scavenge of oil from the rear bearing area and also oil that is prone to carbon formation. Mobil Jet 2 is not a good oil in this respect in this engine. Change to a better oil. 254 appears to work OK. Exxon (now BP) 2380 always seemed to be a good oil in this engine. Avoid "ground idle" operation. If you have been idling, run up to flight again before shutting down or even shutdown from flight so long as the aircraft has been in flat pitch for the prescribed period. This will help scavenge the rear bearing area. In some installations there is no "ground idle" for this engine.
Another problem is pilots transitioning to the Arriel from Allisons. The procedure is nearly the complete opposite and it is not unusual to find people doing a 2 minute rundown!
As to whether FSII has any benefits regarding the oil system coking, put me in the skeptics line.
The 30 - 45 second rundown is an issue. Any LONGER than that at idle is a problem. Longer is NOT better. In this cooldown period the rear bearing temperature will reduce but after that period it is anyones guess. There is a graph around somewhere in the training notes that shows what happens, the temperature invariably goes back up.
Either prior to shutdown or running at idle on the ground, the problem is poor scavenge of oil from the rear bearing area and also oil that is prone to carbon formation. Mobil Jet 2 is not a good oil in this respect in this engine. Change to a better oil. 254 appears to work OK. Exxon (now BP) 2380 always seemed to be a good oil in this engine. Avoid "ground idle" operation. If you have been idling, run up to flight again before shutting down or even shutdown from flight so long as the aircraft has been in flat pitch for the prescribed period. This will help scavenge the rear bearing area. In some installations there is no "ground idle" for this engine.
Another problem is pilots transitioning to the Arriel from Allisons. The procedure is nearly the complete opposite and it is not unusual to find people doing a 2 minute rundown!
As to whether FSII has any benefits regarding the oil system coking, put me in the skeptics line.
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Sorry for the confusion about coking. Entirely my fault. I meant fuel coking at the injection wheel. I believe there is a service letter out there somewhere that addresses this issue.
Thanks for all the answers so far.
Alt3
Thanks for all the answers so far.
Alt3
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clarification
Alouette3,
Maybe I can help clarify a few things to help you out. I work with the Arriel engines everyday and I am very familiar with the coking issues. First of all as you stated there are two types of coking, oil and fuel. It seems you are concerned with the fuel coking so we will address that one first.
Since the Arriel has a centrifugal injection type fuel system we have a injection wheel that rotates and sprays fuel into the combustion chamber. In order to get the fuel into this rotating wheel we have a stationary manifold with labrynth seals that mates up to it and supplys the fuel. In between the lips of the labrynth seals is where fuel can coke up and cause a drag on the engine. Typically this doesn't cause any significant issues in flight but it can lock up the engine after shut down making a restart difficult if not impossible. So far this phenomenon is very isolated to a few areas of the world. Out of thousands of Arriels operating it seems only ones in certain areas experience it.
There was a time early on that it was suspected that prist may help the situation however I was never able to get any definite clarification from the operators who tried it as to if it worked or not. We also fond that it tends to happen more during the winter season (if memory serves me correctly) and less in the summer. This may have been were the prist solution came from. If they started using it in spring and the occurances slowed or stopped that could make one think they have cured the problem only to find it comes back when the temps change. I would suggest at this point that prist is used for it's intended purpose only and not for fuel manifold coking prevention.
There was also another additive that was tested and approved specifically for coking prevention and that is Turboline plus 100. It did seem to have an effect at one time but I don't know if it ever actually cured the issue. There were events of coking while using the additive but I don't think it was confirmed that the operator used it 100% of the time.
There was another bout of coking in Hawaii a year or so ago and it seemed to be a fuel quality issue of some sort. Only the operators on one island seemed to have it and there was some issues with the fuel delivery system on that particular air field. I won't comment any more on this as I was not part of that investigation first hand (the others I was).
Now just to clarify the oil coking issue while we are here. The Arriel 1 engines have had issues in the past with gas generator rear bearing coking hence the requirement to clean the rear bearing periodically. There were a few modifications that seemed to all but do away with this issue in conjunction with the use of an HTS gen III oil such as Mobil 254. This phenomenon stems not quite from the reduction of scavenge oil flow at low speeds but the lack of cooling air around the bearing area. At ground idle the temps tend to decrease for the first 30 seconds or so but afterwards they can creep back up. If the bearing temps are high and the engine is shut down you can see whare this would creat more propensity for coking in that area. The modifications I mentioned earlier (TU274,281,283,and 284) help increase the volume of cooling air in the rear bearing area among other things, which significantly helps. Nowadays if you have these mods you don't have to clean the rear bearing unless there is an issue and only the oil tubes leading to the bearing area have to be checked for coking periodically.
To summarize:
Fuel coking
Forget about prist as it is not proven for this purpose. If you are having fuel coking issues contact TM to find out the latest fix. They may recommend Turboline plus 100 additive and will most likely want to test your fuel supply.
Oil coking
Stick to 30 second cool down period. If you need to ground idle for longer periods go back to flight idle and stabilize before performing a normal shut down sequence. Use and HTS oil if possible (although with the new mods I have a few operators that are having good luck with older generation oil, even aeroshell 560!).
If you have any further questions feel free to send me a PM and I can help you out as best I can.
Max
Maybe I can help clarify a few things to help you out. I work with the Arriel engines everyday and I am very familiar with the coking issues. First of all as you stated there are two types of coking, oil and fuel. It seems you are concerned with the fuel coking so we will address that one first.
Since the Arriel has a centrifugal injection type fuel system we have a injection wheel that rotates and sprays fuel into the combustion chamber. In order to get the fuel into this rotating wheel we have a stationary manifold with labrynth seals that mates up to it and supplys the fuel. In between the lips of the labrynth seals is where fuel can coke up and cause a drag on the engine. Typically this doesn't cause any significant issues in flight but it can lock up the engine after shut down making a restart difficult if not impossible. So far this phenomenon is very isolated to a few areas of the world. Out of thousands of Arriels operating it seems only ones in certain areas experience it.
There was a time early on that it was suspected that prist may help the situation however I was never able to get any definite clarification from the operators who tried it as to if it worked or not. We also fond that it tends to happen more during the winter season (if memory serves me correctly) and less in the summer. This may have been were the prist solution came from. If they started using it in spring and the occurances slowed or stopped that could make one think they have cured the problem only to find it comes back when the temps change. I would suggest at this point that prist is used for it's intended purpose only and not for fuel manifold coking prevention.
There was also another additive that was tested and approved specifically for coking prevention and that is Turboline plus 100. It did seem to have an effect at one time but I don't know if it ever actually cured the issue. There were events of coking while using the additive but I don't think it was confirmed that the operator used it 100% of the time.
There was another bout of coking in Hawaii a year or so ago and it seemed to be a fuel quality issue of some sort. Only the operators on one island seemed to have it and there was some issues with the fuel delivery system on that particular air field. I won't comment any more on this as I was not part of that investigation first hand (the others I was).
Now just to clarify the oil coking issue while we are here. The Arriel 1 engines have had issues in the past with gas generator rear bearing coking hence the requirement to clean the rear bearing periodically. There were a few modifications that seemed to all but do away with this issue in conjunction with the use of an HTS gen III oil such as Mobil 254. This phenomenon stems not quite from the reduction of scavenge oil flow at low speeds but the lack of cooling air around the bearing area. At ground idle the temps tend to decrease for the first 30 seconds or so but afterwards they can creep back up. If the bearing temps are high and the engine is shut down you can see whare this would creat more propensity for coking in that area. The modifications I mentioned earlier (TU274,281,283,and 284) help increase the volume of cooling air in the rear bearing area among other things, which significantly helps. Nowadays if you have these mods you don't have to clean the rear bearing unless there is an issue and only the oil tubes leading to the bearing area have to be checked for coking periodically.
To summarize:
Fuel coking
Forget about prist as it is not proven for this purpose. If you are having fuel coking issues contact TM to find out the latest fix. They may recommend Turboline plus 100 additive and will most likely want to test your fuel supply.
Oil coking
Stick to 30 second cool down period. If you need to ground idle for longer periods go back to flight idle and stabilize before performing a normal shut down sequence. Use and HTS oil if possible (although with the new mods I have a few operators that are having good luck with older generation oil, even aeroshell 560!).
If you have any further questions feel free to send me a PM and I can help you out as best I can.
Max
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Max:
Thank you very much for your very detailed and extremely helpful response.I will now be able to respond to some of the myths that are floating around my base.I certainly appreciate the technical input,which was very easy to understand.It certainly adds to the need-to-know portion of my mental library.
Best wishes,
Alt3
Thank you very much for your very detailed and extremely helpful response.I will now be able to respond to some of the myths that are floating around my base.I certainly appreciate the technical input,which was very easy to understand.It certainly adds to the need-to-know portion of my mental library.
Best wishes,
Alt3
Maxtork,
Excellent info which raises another question in my mind, hopefully one you can help with. If I understand correctly, shutting the engine down while the rear bearing temperature is increasing can cause the oil which is in contact with the bearing to "carbonise" when the oil flow across the bearing slows & stops. Let me set the scene for my question - I am conducting a vertical descent into a confined area & the engine oil temperature increases above 80 deg causing the oil cooler fan to come on. I settle the helicopter on the ground, reduce the engine RPM to 70% Ng & start my 30 sec count. If after 30 secs the cooler fan is still running & the engine oil temp is say between 70 & 80 deg, should I shut the engine off or leave it running to supply oil flow to the rear bearing until the temp reduces to the point where the fan switches off? I stand by in anticipation of your wisdom.
P.S. - We run Mobil 254 oil.
Excellent info which raises another question in my mind, hopefully one you can help with. If I understand correctly, shutting the engine down while the rear bearing temperature is increasing can cause the oil which is in contact with the bearing to "carbonise" when the oil flow across the bearing slows & stops. Let me set the scene for my question - I am conducting a vertical descent into a confined area & the engine oil temperature increases above 80 deg causing the oil cooler fan to come on. I settle the helicopter on the ground, reduce the engine RPM to 70% Ng & start my 30 sec count. If after 30 secs the cooler fan is still running & the engine oil temp is say between 70 & 80 deg, should I shut the engine off or leave it running to supply oil flow to the rear bearing until the temp reduces to the point where the fan switches off? I stand by in anticipation of your wisdom.
P.S. - We run Mobil 254 oil.
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Thanks MT, great response. I have not seen coking issues for ages and now I know why!
I was scratching my head until Alouette cleared up his terminology.
Good question Heliduck, some leave the battery on, and hence cooling fan, even after shutting engine down until temp drops and fan cuts out, can't see any reason for it myself but...........?
I was scratching my head until Alouette cleared up his terminology.
Good question Heliduck, some leave the battery on, and hence cooling fan, even after shutting engine down until temp drops and fan cuts out, can't see any reason for it myself but...........?
"some leave the battery on, and hence cooling fan, even after shutting engine down until temp drops and fan cuts out, can't see any reason for it myself but...........?"
Yes, another conundrum! I would think that leaving the cooling fan going after shutting the engine down will only cool the oil in the cooler, & subsequently the reduced temperature oil from the oil cooler drains back into the resovoir, reducing the temp at the switch & shutting the fan off. I don't think this will help the rear bearing at all, but if you have a good battery I can't see that it would hurt anything either.
Yes, another conundrum! I would think that leaving the cooling fan going after shutting the engine down will only cool the oil in the cooler, & subsequently the reduced temperature oil from the oil cooler drains back into the resovoir, reducing the temp at the switch & shutting the fan off. I don't think this will help the rear bearing at all, but if you have a good battery I can't see that it would hurt anything either.
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Heliduck, TCF
I dont know how much wisdom I can provide but I'll give you my best effort. As for the situation you described I would not be overly concerned with extending the ground idle time due to a slightly higher oil temp. The reason I say this is, we are avoiding the long ground run times not for oil temps but bearing temps. Oil temp is measured, on the inlet to the engine so if we squirt a hot bearing with oil that is a few degrees warmer to begin with the cooling effect will surey be less but I don't know if it would be enough to make a significant difference.
The bearing cavity is cooled through oil as well as through an airflow that comes from outside the engine just behind the module 3 near the T4 probes and is drawn through by venturi effect. When the engine is at slow speeds like at ground idle that venturi effect is reduced and cooling effect is also which can lead to higher bearing temps. Once the engine is shut down oil flow stops and the residual oil is left to cook away on the bearing. Even if the oil temp that is going into the engine is nice and cool.
It would be interesting to take a look at the rear bearing in your engine after a period of time to see if it has any coking accumulation if this is a flight profile that you use routinely throughout the life of the engine. My personal opinion would be that unless you are getting carbon build up in the filter, start failing oil flow checks, or find significant carbon build up in the rear bearing oil tubes I wouldn't be too excited. Nothing is for certain as there are lots of variables but thats my initial take on it from what you explained.
As for letting the fan run after shut down you are correct that it would only be cooling the oil in the cooler which, since oil flow through the engine is stopped, will never reach that hot bearing.
Wisdom or BS...you get to decide
Max
I dont know how much wisdom I can provide but I'll give you my best effort. As for the situation you described I would not be overly concerned with extending the ground idle time due to a slightly higher oil temp. The reason I say this is, we are avoiding the long ground run times not for oil temps but bearing temps. Oil temp is measured, on the inlet to the engine so if we squirt a hot bearing with oil that is a few degrees warmer to begin with the cooling effect will surey be less but I don't know if it would be enough to make a significant difference.
The bearing cavity is cooled through oil as well as through an airflow that comes from outside the engine just behind the module 3 near the T4 probes and is drawn through by venturi effect. When the engine is at slow speeds like at ground idle that venturi effect is reduced and cooling effect is also which can lead to higher bearing temps. Once the engine is shut down oil flow stops and the residual oil is left to cook away on the bearing. Even if the oil temp that is going into the engine is nice and cool.
It would be interesting to take a look at the rear bearing in your engine after a period of time to see if it has any coking accumulation if this is a flight profile that you use routinely throughout the life of the engine. My personal opinion would be that unless you are getting carbon build up in the filter, start failing oil flow checks, or find significant carbon build up in the rear bearing oil tubes I wouldn't be too excited. Nothing is for certain as there are lots of variables but thats my initial take on it from what you explained.
As for letting the fan run after shut down you are correct that it would only be cooling the oil in the cooler which, since oil flow through the engine is stopped, will never reach that hot bearing.
Wisdom or BS...you get to decide
Max
More to that story.................
In some operations where the ambient temps are low and the aircraft is mainly flown from A to B and very little in the hover the fan probably never runs.
Depends on the aircraft model as well but on a 1B engine this is pretty normal.
Downside is you may not realise this until you need it! Always used to short the temp switch on a preflight to be sure. Thats the one just behind the oil tank.
In some operations where the ambient temps are low and the aircraft is mainly flown from A to B and very little in the hover the fan probably never runs.
Depends on the aircraft model as well but on a 1B engine this is pretty normal.
Downside is you may not realise this until you need it! Always used to short the temp switch on a preflight to be sure. Thats the one just behind the oil tank.
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Ok, my turn to clarify a post....
My query on oil cooler fan was not meant in any way as an aid to reduce #8 brg coking, just a bit of thread drift....
Me personally, I switch the battery off once the blades have stopped, regardless of fan operation.
MT - Those temp's/times and a chart you mention are interesting. I have seen a chart on the RR250 series where the core wheel temps have stabilised and plateau after 1 min 45 secs but BHT round it up to 2 min 'cool down', but do not recall any raising of core temps by going past that.
My query on oil cooler fan was not meant in any way as an aid to reduce #8 brg coking, just a bit of thread drift....
Me personally, I switch the battery off once the blades have stopped, regardless of fan operation.
MT - Those temp's/times and a chart you mention are interesting. I have seen a chart on the RR250 series where the core wheel temps have stabilised and plateau after 1 min 45 secs but BHT round it up to 2 min 'cool down', but do not recall any raising of core temps by going past that.
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TCF,
I remember seeing that chart for the RR250 years ago as well. I thought I remember someone telling me that they at one point asked for a 3 minute cool down just so they would get an honest 2 minutes. After some research they decided that 3 minutes actually was better! That could be just my crappy memory though.
My info on the Arriel came from one of the engineers years ago who is no longer with us. It would be nice to have that type of chart to reference now using the newest modifications. Thats always been one of TM's marketing points over the RR250 was that it was a direct flow engine as opposed to a reverse flow and that was supposed to make it run cooler.
Max
I remember seeing that chart for the RR250 years ago as well. I thought I remember someone telling me that they at one point asked for a 3 minute cool down just so they would get an honest 2 minutes. After some research they decided that 3 minutes actually was better! That could be just my crappy memory though.
My info on the Arriel came from one of the engineers years ago who is no longer with us. It would be nice to have that type of chart to reference now using the newest modifications. Thats always been one of TM's marketing points over the RR250 was that it was a direct flow engine as opposed to a reverse flow and that was supposed to make it run cooler.
Max
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Thats always been one of TM's marketing points over the RR250 was that it was a direct flow engine as opposed to a reverse flow and that was supposed to make it run cooler
