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Allison/RR 250-C20 Engine Coking

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Allison/RR 250-C20 Engine Coking

Old 6th Aug 2019, 08:17
  #21 (permalink)  
Join Date: Feb 2008
Location: Good Question
Posts: 70
Originally Posted by RVDT View Post
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.
We swapped a fleet of at least 20 C20B from Jet II oil to Jet 254 in the eighties, the trouble we had from carbon coming loose and blocking the torquemeter orifice in the gearbox, it was unacceptable, we were pulling engines out all the time to open the gearboxes and clear the orifice.

BUT, we never had carbon problems after that.

In hindsight our company sShould have fitted a Facet scavenge filter kit to all aircraft before changing oils, but hindsight................ and costs.
PEASACAKE is offline  
Old 6th Aug 2019, 08:32
  #22 (permalink)  
Join Date: May 2007
Location: Merriott, Somerset, UK
Age: 74
Posts: 223
I operated C18's...........50 years ago, and we had coking problems.
I find it amazing that the problem still exists.
Tony Mabelis is offline  
Old 7th Aug 2019, 18:13
  #23 (permalink)  
Join Date: Oct 2006
Location: USA
Posts: 331
Originally Posted by Rotorhead84 View Post
Starting to have engine coking problems. Thoughts/recommendations?
Has there been a change to your scheduled maintence plan or procedures like oil changes or cleaning of the 6/7 bearing strut?

Normally coking doesn't just "appear" across the fleet unless something has changed.

As an FYI: generation III oils are better and an external filter highly recommended which can also allow you to extend your oil changes if you so wish.
wrench1 is offline  
Old 7th Aug 2019, 18:21
  #24 (permalink)  
Join Date: Mar 2004
Location: hayling island
Posts: 249
Oil is cheap change it regularly and can't understand why it is coking as you say.
No problems with our C20B's or C20R's can't understand it are you burning JETA1?
timprice is offline  
Old 12th Aug 2019, 18:45
  #25 (permalink)  
Thread Starter
Join Date: Jan 2011
Location: USA
Age: 35
Posts: 27
Originally Posted by Salusa View Post
Does the engine smoke on shutdown?
None of them do, no.

Originally Posted by DOASO View Post
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?
How did you go about figuring this out? Our fuel is delivered by the same company as always, but could have switched where they got it from.

Originally Posted by RVDT View Post
Scav filter will clean the lumps out and is worth every cent.
I believe the helicopter that we ended up changing the bearing on had this filter. The oil filter that rides behind the rubber engine oil tank in the aft cowling behind the hampster wheel, correct? Our other two ships do not have this filter.
Rotorhead84 is offline  
Old 19th Aug 2019, 08:18
  #26 (permalink)  
Join Date: Oct 2018
Location: Cracow
Posts: 14
Hi, maybe this article will be interesting for you aerossurance.com/helicopters/coking-causes-power-loss/

Safety recommendation to reduce probability of occurrence is periodic cleaning of PT front bearing aasembly oil jet and oil supply pipe. But I am not sure if there is an access on your engine for similar maintence. Of course the most important is engine thermal stabilization at shutdown.
tom_pilot is offline  
Old 2nd Sep 2019, 19:52
  #27 (permalink)  
Thread Starter
Join Date: Jan 2011
Location: USA
Age: 35
Posts: 27
Is there a specific written procedure from the manufacturer about switching oils?
Rotorhead84 is offline  
Old 3rd Sep 2019, 07:57
  #28 (permalink)  
Join Date: Aug 2007
Location: The South Coast
Posts: 98
The Rolls Royce Customer Service Letter (CSL 1208) recommends the following -

In order to minimize the shedding of previously accumulated carbon/coke build--up, it is recommended that the switch to an HTS oil be accomplished only on new engines or engines that are newly overhauled or repaired where the hot oil wetted surfaces have been cleaned. In service engines should be changed over gradually during operation by the ”top--off” method as described below.
If a switch to HTS oil is desired, and the engine is not new or freshly overhauled, it is recommended this change be accomplished gradually using the ”top--off” method. The ”top--off” method decreases the likelihood of shedding of previous carbon/coke build--up. In the ”top--off” method, the HTS oil is added as oil is consumed by the engine or a quart of oil is removed at 10 hour intervals and replaced with the new HTS oil. At the next scheduled oil change, normal oil change can be accomplished utilizing HTS oil. When the engine is returned to service, watch for impending bypass indications, and check the scavenge oil filter and engine oil filter on a regular basis for carbon/coke deposits. These indications could be a sign that carbon/coke, which was formed during previous operation, is being dislodged and circulated through the system. The accumulation of dislodged carbon should diminish quickly. Maintain filters as required.
The use of an HTS oil does not eliminate the requirement to run the engine at idle for two minutes in order to reduce oil wetted turbine skin temperatures and stabilize turbine wheel rim temperatures.

Chris P Bacon is offline  

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