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Allison/RR 250-C20 Engine Coking

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Allison/RR 250-C20 Engine Coking

Old 4th Aug 2019, 22:40
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Allison/RR 250-C20 Engine Coking

I have 3 OH58A+ helicopters with the C20 in them. Have had them different amounts of time. Our oldest ship we've had for about 15 years. Our newest ship we've had about 4 or 5 years. Starting to have engine coking problems. Already had the engine out on one to have the bearing replaced. Got so bad we couldn't spin the rotorblades by hand at all. Our newest ship is starting to stick lightly after being flown and then cooling down, and our oldest ship is doing the same thing.

We follow the 2 minute cool down religiously. Running mobil jet 2 engine oil.

Thoughts/recommendations?
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Old 4th Aug 2019, 23:10
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After shutdown, all is stopped, pull the ignition, ensure fuel is off, and motor it for 20 secs - helps cool it off faster, limits the amount of residual oils that turn to coke.

Walk the blades backwards when tying down and before startup, though it sounds like you are doing that.
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Old 5th Aug 2019, 02:46
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Thanks for the tip! Will give it a try.

And yes, we spin blades backwards religiously also. Once right after blades stop turning, and then once 5-10 minutes later after clean up of the aircraft before we push into hangar. Each time 4-5 full rotations.
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Old 5th Aug 2019, 04:00
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Please excuse a question from a fixed-wing pilot. Why do you spin the blades backwards?
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Old 5th Aug 2019, 04:55
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Could there be any changes to the fuel that your supplier hasn’t told you about? I’m thinking if it was the oil, there would be reports from all over the world about it.
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Old 5th Aug 2019, 06:24
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I was told by a Rolls Royce AMC to never ever cool the engine by pressing the starter as it will damage the wheels ! Turn the blades backwards. Never had a problem in 25 years with 10 different engines , We use mobil 254 !
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Old 5th Aug 2019, 06:50
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Originally Posted by India Four Two View Post
Please excuse a question from a fixed-wing pilot. Why do you spin the blades backwards?
This will turn the power turbine and break away any carbon build up in the labrynth seal. If the blades are rotated in normal direction the freewheel unit wont let the turbine rotate.
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Old 5th Aug 2019, 06:58
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Thanks Falcon Al.
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Old 5th Aug 2019, 07:33
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Mr Hughes, how can turning the engine over damage the wheels?
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Old 5th Aug 2019, 08:12
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Originally Posted by Falcon Al View Post
This will turn the power turbine and break away any carbon build up in the labrynth seal. If the blades are rotated in normal direction the freewheel unit wont let the turbine rotate.
Is this only for that type of aircraft or does it apply generally to all types?
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Old 5th Aug 2019, 08:25
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"Mr Hughes, how can turning the engine over damage the wheels?"

Cooling too quickly. The purpose of the 2 minute rundown is to stabilise the temperature, not reduce it.
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Old 5th Aug 2019, 08:40
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From Rolls Royce C20B Customer Service Letter CSL 1178, which was issued back in 1993. "It has come to Rolls-Royce’s recent attention that a number of Model 250 operators have adopted another engine manufacturer’s cool down procedure intended to reduce coke/carbon formation in the turbine area.

We understand that this procedure calls for the hot engine to be motored with the starter immediately following shut down to provide additional oil flow for cooling the turbine bearing/shafting area.

Operators must understand that this procedure is not recommended by Rolls-Royce and can accelerate rim cracks of the first and second stage turbine wheels due to the thermal shock of introducing a flow of cool air over the turbine wheels.

Operators wishing to reduce coke/carbon formation in the engine should follow Rolls-Royce’s currently published recommendations:


Use MIL-PRF-23699 Rev. G or subsequent oils.

Cool engine for two minutes at a stabilized idle prior to shut down.

install STC approved scavenge filter.

Change oil at recommended intervals.

NOTE: This CSL is not intended to change the currently published procedure for restart shortly after shutdown by cooling a warm engine below 150°C prior to light off through motoring.

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Old 5th Aug 2019, 08:41
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Originally Posted by HeliboyDreamer View Post
Is this only for that type of aircraft or does it apply generally to all types?
It was more of an issue with earlier model Allison C20s. Includes B206s not sure if the early H500Cs were affected but it is the same engine.

... and there have been ongoing issues with later versions of the engine. I remember Dick Smith queried this way back when he owned a 206.
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Old 5th Aug 2019, 12:11
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First post here guys. Just wanted to add a little first hand experience if it may be helpful. My company operates a fleet of around 10 OH-58s / 206s. We do adhere religiously to the 2 minute cool downs, but do not spin the blades backwards after shutdown. (Other than perhaps half a revolution just to tie it down) We have been using Mobil Jet 254 for the last 12+ years and haven’t had any coking issues.
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Old 5th Aug 2019, 14:49
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I had similar issues with a 500C many years ago. Aircraft was fitted with a C-18.
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Old 5th Aug 2019, 15:33
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Does the engine smoke on shutdown?
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Old 5th Aug 2019, 16:39
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Motoring the engine to cool it will do SFA wrt to coking and only promotes rim cracks as per the quoted CSL.

Change to a better oil - Mobil Jet II is marginal at best - works in some engines but not all - try 254.

Is your scavenge strut clean? Have a really good look - it is not round in shape but has 2 flat sides and 2 radius sides.

Low scavenge flow does not help. How many hours has your gearbox and pumps done? There is a procedure to test the scavenge performance from memory.

Flooding of the space will cause build up on the lab seals. I think there is a check valve in the pressure side that works to shut off the oil as the pressure drops below a certain value on shutdown. This check valve can leak. May be wrong as it is about 30 years since I worked on a 250 but hey................

Scav filter will clean the lumps out and is worth every cent.

It's a classic old engine - pay attention to the details though and they work fine.

Last edited by RVDT; 5th Aug 2019 at 16:53.
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Old 6th Aug 2019, 01:23
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Fuel data

When we had coking issues, we switched to a different fuel vendor. The ‘Gum’ value on the fuel data sheet (name?) was too high and was believed to be the source of the coking.

On a different note, my C20J smokes on shutdown every time.....specifically out of the left exhaust stack. Any cause for concern?
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Old 6th Aug 2019, 04:21
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Originally Posted by HeliboyDreamer View Post
Is this only for that type of aircraft or does it apply generally to all types?
Heliboy, this procedure applies to pretty much all single-engine helicopters with so-called "free turbine" engines - in other words, helicopters that don't have a clutch between the engine and transmission.

When the engine turns, it drives the transmission through a freewheeling unit. Like the hub on the back wheel of a bicycle, this device "decouples" the engine (pedals) and allows the transmission (back wheel) to keep turning if the engine stops. So on the ground when the engine is off, you can turn the blades in their normal direction and nothing happens. But if you turn the blades "backward," then the sprag clutch in the freewheeling unit engages and turns the engine backward too. IF you feel resistance when turning the engine, then it's "Houston, we have a problem."

It works this way in the Bell 206, 407, MD/Hughes 500's, and the French Astars/EC120/130, etc. Sometimes, if carbon has formed in the bearings or seals within the engine, it will resist turning - backwards *or* forwards. There is usually a checklist item that specifies that the rotor must be turning by such-and-such engine RPM. If it doesn't, it could mean that the interior bits of the engine are suffering from this "coking" problem we've been discussing.

It's all basic stuff to those of us who've been in the business for a while, but I'm sure it confounds newbies as it did me when I was first learning about these crazy machines, which was back when helicopters were still powered by steam engines - which, by the way is where we get the expression "steam gauges" from to describe mechanical, analog gauges. But you didn't ask about that.
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Old 6th Aug 2019, 06:02
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Yes, infernal combustion engines...
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