bad carb heat design

[johnr]

Hi all CTsw owners,  I am attaching a copy of an email I sent to Flight Design over 1 year ago. I never got a reply.

I had an oil sample checked by a specialist laboratory & found that the oil was contaminated with sand. Has anyone else

had oil tested & discovered that the oil was contaminated?

 

 

sand in oil.rtf

[Ed Cesnalis]

| I apply carb heat down wind on my pre landing, any dust in the system will be ingested.

 

You have a Rotax 912 and fly where there is a lot of sand.  I question if there is any need for carb heat  on normal landings from downwind.

 

How many CT pilots routinely use carb heat?

[Roger Lee]

Unfortunately many carb heat designs bypass the air filter in many planes. Personally I don't use carb heat in my dry climate and I know you live in a dry climate and I don't think it's needed. I don't think I know of anyone out here that uses carb heat. I think it should be used on a case by case basis and not every flight just because someone said so. I would only use carb heat if I suspected carb ice and not in a routine use. That would limit your exposure.

If you are talking about silicon then you will find it in all samples. It will never be zero. It can come from several sources including dusty sand. Filters can only pull out so much depending on the size of the particles. Nothing is 100%.

 

Read this:

 

http://www.machinery...icon-oil-source

 

http://www.blackston...con-bugaboo.php

 

So long as it is lower than what the analysis calls for your good to go.

[Tom Baker]

I don't know of any carb heat system that uses filtered air when carb heat is applied, and that is by design. There are some that do have a coarse screened intake.  Typically carb heat is also an alternate air source if the filter becomes blocked for any reason. If there was a secondary filter it could be block from the same thing that is blocking the primary filter.

 

Personally I think I would be looking for a different source of the sand rather than being ingested through the intake. This is based on the tight tolerances between the piston and cylinder wall on the 912.

[IrishAl]

I don't know of any carb heat system that uses filtered air when carb heat is applied, 

 

What about electric or warm water carb heat systems?

 

Johnr - maybe your best route to a solution might be to seek permission to install one of these systems; then you could use filtered air all the time.

[Roger Lee]

The hot water one if you use a carb heat is the best pick. The hot water one draws on none of the planes resources, it's always on and it doesn't effect the air/fuel mixture. The electric draws too much juice.

[FastEddieB]

For whatever its worth, I've never used the carb heat in my Sky Arrow during routine operations, and have never had any indications of carb ice.

 

At least in my installation, does not seem overly prone to carb ice - YMMV.

 

That said, I think the thread title is misleading. Virtually all carb heat and alt air systems in small planes use unfiltered air by design, at least in my experience. When you absolutely, positively want to be sure air can get to the engine, you really don't want anything in the way of that.

[CT4ME]

I'm not surprised that you didn't get a response.  Your Judge, Jury & Executioner tone, with the threat to go to the equivalent of the FAA and get the fleet grounded wasn't a good way to explore the issue.

 

Many CTsw owners fly around Arizona, with its share of dust/sand.  I've not heard of any sand contamination in the oil.  But like many have said, we don't use carb heat.

[Tom Baker]

What about electric or warm water carb heat systems?

 

Johnr - maybe your best route to a solution might be to seek permission to install one of these systems; then you could use filtered air all the time.

 

I have never seen or heard of electric carb heat, and I don't think the hot water system can be applied in flight?

[IrishAl]

The hot water system uses the engine coolant to keep the the physical carburettor warm, which prevents ice build up, so it's permanently on and doesn't compromise performance because it doesn't heat up the air.  

 

The electric systems perform the same function, but draw about 3A if I recall rightly.  

[IrishAl]

Eddie - I'm surprised you don't use carb heat in your wet climate.  There must be a significant number of days each year where you encounter a small difference between air temp and dew point - ?

[FlyingMonkey]

I'm in Georgia (USA, not Eastern Europe!) -- High humidity environment, but the temperature usually comes up with the humidity.  If the humidity is high, the temp is usually 80°F or more, and not really conducive to icing.  I have never had any indication of carb ice on my CTSW.  I have put the carb heat on a few times as a precaution when the air temp was around 60°F and humidity relatively high with small temp/dewpoint spreads.  My understanding is those are the conditions where the Rotax 912 is most prone to carb ice.

 

Most of the time my carb heat knob collects dust.

[Ed Cesnalis]

I have not yet used carb heat.  That is because I have not yet encountered small dew point spreads or rough running that could indicate carb ice.

 

With a Lcycoming I pulled carb heat each time I did a big throttle reduction.  With the Rotax I will only use if needed or when landing with a small dew point spread.

[Craig]

I suspect the Rotax having dual carbs may contribute to its lesser, though not impossible likelihood of carb ice (half the volume of air per carb vs a single carb setup)?  I remember seeing carb ice for on automotive carbs w/ a common intake so all the air volume was through a single carb, but never on a motorcycle where there was 1 carb/cyl.  Just a theory... and yes, I know the CTLSi is fuel injected and doesn't have this problem, and a SR22 cannot be landed with the throttle set at idle.

[IrishAl]

That a good point Craig.

 

However, despite that, there's still a surprising number of random 912 engine shut-downs that end up being attributed to icing.

 

I think of the guy in a CTLS who throttled back to start a descent (in Illinois IIRC) after a long cross country and his engine quit without any warning as soon as he pulled the throttle.  He landed in tall corn with his elderly wife and both were unhurt.   On inspection, the engine started right up, so it was put down to icing.  He said he never had icing before in many hours in that aircraft.

 

I think of the Tecnam Sierra used for training.  One day, it just quit on short final and crashed in a playpark.  Again, it started right up upon inspection and again it was attributed to icing, yet it had done many hundreds of approaches without a single icing incident.  

 

Yes, Rotax icing events are rare but when conditions are just right, they do occur, and they can be really dangerous.  

 

In my view the evidence does seem to suggest that not applying heat every time you pull the throttle is playing a lottery,  even though it's one with long odds.  

 

For this reason, I'd advocate making it a habit  - something you do every time, because sooner or later......it might just happen to you, especially if you don't live in a dry state like Arizona.

[ct9000]

I have had carb ice quite a few times and it comes on quickly . The air temp can be higher than you think and it still can happen if the humidity is high enough. As far as unfiltered air goes , I have never seen filtered air for carb heat . I have seen water heat used but it rarely works , you need the blast of hot air to clear to clear the ice quickly . I understand that it is a legal design requirement for certified aircraft to have a temperature rise in intake air of about 50 deg, c.

If you fly through mist then smoke eg after a forest fire the smoke particles will block your filter real quick then tour only get out jail card is carb heat - alternate air.

[FastEddieB]

The air temp can be higher than you think and it still can happen if the humidity is high enough.

 

 

I believe the maximum temperature drop in the carb throat is in the vicinity of 60ºF. Doing the math, it can be 90º outside and you can still get carb ice. 

 

Only had it once for sure in a Cub out of Homestead FL. Major insidious power loss - at first I thought it was a creeping throttle. Full carb heat momentarily killed the engine. On landing, there was frost all over the carb, which was ice cold on a moderately warm day.

[Ed Cesnalis]

I am re-thinking my pattern and think I will add carb heat to all of my landings. 

 

1 reason is to protect on those landings at the coast where dew points can be high.

 

2nd reason is just to bring carb heat back into common usage so it is more likely to occur to me if I encounter icing in cruise.

[FlyingMonkey]

In the CT there is no reason not to use carb heat; I have noticed zero rpm or power drop on or off (and yes, it's working).  I'd run it all the time except then I'd have the unfiltered air issue.  I just hit it when conditions seem ripe for it.

[Tom Baker]

In the CT there is no reason not to use carb heat; I have noticed zero rpm or power drop on or off (and yes, it's working).  I'd run it all the time except then I'd have the unfiltered air issue.  I just hit it when conditions seem ripe for it.

Andy, you are making a statement based on your airplane. The CTLS has a different carb heat system compared to the CTSW. It does provide enough heat to produce a RPM drop.

[johnr]

Hi . I have read the posts , for those interested here are more details.

 

The intake for carb heat is in a vulnerable position, low down on the front of the cowl. I believe it collects dust like a vacumn cleaner even with carb heat off.

It would make little difference when carb was applied, the dust in the inlet tract will be ingested. Dust/sand would be much reduced if the carb heat was

drawn from under the cowl or from the cockpit.

I only became aware of this when I had the oil tested.How many owners have had their oil tested in an approved test facility.

 

This is the warning that was given after the oil test.

DIAGNOSIS
 Dirt level (alumina + silica) high. Please check recorded
viscosity against requirements.
Action: Check all dirt access points (breathers/seals).
Recommend checking if system oil pressures are
within normal parameters. Check for excessive engine
knock at stall speed. Recommend checking oil
filter/screens for abnormal wear particles. Change oil
only if found necessary to flush contamination from
compartment. Resample at a reduced service interval
to monitor and establish wear trend. Note: Lead can be
added to some oil to improve performance. Note 2: Oil
type stated as, Synthetic V Twin. end of report.

 

The air filter is correctly installed, it is the reuseable type this is cleaned & reoiled with filter oil annually

which is about every 75 hrs. All the hoses were checked & found to be correctly fitted with no damage.

 

I was taught to always check carb heat as part of the preflight & to apply it down wind. I do not have the ability to estimate the changes

in  temperature & humidity during flight  that can occur very quickly near the coast, so I must agree with my instructor.

 

I imported my CTsw from the US. When I registered it in Australia I contacted Flight Design to inform them that I was the new owner

& could they send me any SBs . This is a requirement by the Australian Aviation Authorities. I never received a reply  even after

repeated requests. I have tried the "please sir can you" approach so I felt justified to try a hammer!. It made no difference I was

still ignored.

The CT was once an exceptional aircraft when compared with the others available, this is now changing as other manufacturers lift their

game. I was contemplating upgrading the CTsw to the Ls. That is very unlikely now. I have seen the Syncro & will get to fly one in the near

future.  <http://www.flysynthesis.com/dealers/lista/index.html>

johnr

[Tom Baker]

Unless your CTSW has been modified your description of the carb heat system is inaccurate, because the heated air is taken from inside the cowling.  All of the CTSW's I have seen take their filtered air from the top center of the cowling. The filter sits in a fiberglass air box. The carb heat/alternate air is taken from the engine compartment through a door on the bottom of the air box near the firewall. It is not connected to the lower opening on the front of the cowling.

 

The CTLS on the other hand uses a standard Rotax air box, that does take heat directly from around the muffler. The inlet for its air is the small opening on the lower cowling.

[Top Cat]

I don't have a carb heat control in my RV-12 despite it having the same 912 ULS engine that others are flying in different designs.

 

I'm curious as to why that is? Is it because of the Vans design or because the Rotax is not usually subject to carb ice?

[Tom Baker]

I don't have a carb heat control in my RV-12 despite it having the same 912 ULS engine that others are flying in different designs.

 

I'm curious as to why that is? Is it because of the Vans design or because the Rotax is not usually subject to carb ice?

 

There are others around that are the same way, Tecnam comes to mind, It is just the way they chose to do things.

[IrishAl]

Lots of other manufacturers use the same setup as on the RV12.   They omit the airbox (which draws outside air directly into the engine) and simply attach an air filter onto each carb, enabling the engine to draw in air from inside the engine cowl.  This air has already been warmed by the radiators, engine and exhausts, so it's considered to be safe, hence, no carb heat control.

 

However, this setup does cost you hp, maybe 3 or 4% I believe.

 

I suppose the reasons for adopting this setup could be space limitations, cost, weight, simplicity, etc.

 

I've often wondered why more manufacturers don't adopt a ram air system, which should substantially improve power output over the rated amount, and this gain would be practically free.    Anyone know why this is not more commonly adopted?