Carb Ice and 912 Rotax's

[redozbris]

Hello All!

 

Just a quick question for those in the know, I am curious about 912's and Carb Icing, the Tecnam that I fly, doesnt have Carb Heat, the carburettor's are mounted above the engine, near the firewall, I am guessing that the ambient heat will keep this problem at bay, but am always interested in hearing any thoughts..

 

I am also curious if anyone has encounted any carb icing 'adventures', and if they would like to share them...

 

ReD

 

 

[eastmeg2]

Hi Red,

 

I think you will find that most 912's have an anti-icing collar on the engine side of each carb', which coolant passes through to warm it.

 

I've flown in a lot of drizzle for long distances and have had the occasional engine cough in those conditions as a bit of ice breaks off and goes through. Just remember to give the throttle a bit of a blip every couple of minutes to help break any ice away.

 

 

[redozbris]

I havent noticed any collars other than the flexible joints (from the air filter etc), but there may be something under the bowl... will have to have a closer inspection :)

 

 

[Wayne T Mathews]

Good morning Red,

 

Most conventional aircraft engines use "RAM Air" direct from some sort of NACA scoop to the carby. The air is cold, therefore more dense, which allows us to get the maximum power out of our engine that we can. However, when there is moisture content in the air, and the temp of the air passing through the carby venturi drops to zero, ice forms in and around (usually downstream of) the venturi, making it hard for the engine to "breath". We counteract this in most aircraft engines by fitting/having "CARBY HEAT" or "ENG ANTI ICE". The carby heat usually takes air from around the warm muffler and directs it to the carby inlet, therefore raising the air temp through the venturi to above the freezing level and preventing ice formation. However, the warm air is less dense, therefore the engine does not supply maximum power with carby heat on.

 

Rotax 912s don't use RAM air, they use air that has already been slightly warmed by passing over and around the engine. Mr. Rotax has worked out that with the air intakes at the back of the engine, getting carby ice is extremely rare. So he normally doesn't fit carby heat to his engines.

 

Having said that, it is possible during a power off descent to cool the engine sufficentlly to have air going through the carby at a low enough temp to induce carby ice. A practical remedy/protection for this event, is to smoothly increase, then decrease, the power for a bit every thousand feet during the descent, which normally keeps the temps out of the critical range. I'm not a fan of "blipping" the engine, certainly not every couple of minutes during the cruise. It is a statistical fact that most engine failures occure during or immediately after a power change. And blipping the engine during cruise makes about as much sense to me, as changing down a gear for a bit while driving, so as to give top gear a rest. (believe it or not, I know a bloke who used to do that)

 

Keep the sunny side up,

 

Wayne.

 

 

[Guest davidh10]

As per Eastmeg's post. The 912 on Airborne trikes has a heating collar on the carby. It does not heat the air, but heats the throat and spindle to prevent freezing. Thus it does not cause power loss as with the conventional aircraft "ram air" method. You cannot turn it on or off. It is on, permanently. If you cannot see it, just follow the water pipes, they attach at the top, just on the engine side of the carby.

 

Different to a front facing engine position, on the trike, the air filters, and thus intakes are ahead of the engine, and thus do not get any pre-heated air. Advice given to me was to keep engine revs at or above 3,000 rpm if the outside air temperature is 13 degrees or below, however that is general advice and you also should look at your icing probability chart (see VFRG), using wet and dry bulb temperatures or dry bulb and humidity readings.

 

I've not experienced icing, and my engine runs a bit on the cool side of normal, but the description of the cough as the engine swallows a bit of ice is a typical symptom. Having said that I fly in all conditions and temperatures. It isn't an issue if you are above the freezing level.

 

 

[Wayne T Mathews]

As per Eastmeg's post. The 912 on Airborne trikes has a heating collar on the carby. It does not heat the air, but heats the throat and spindle to prevent freezing. Thus it does not cause power loss as with the conventional aircraft "ram air" method. You cannot turn it on or off. It is on, permanently. If you cannot see it, just follow the water pipes, they attach at the top, just on the engine side of the carby..

Thank you for that David,

 

I'm not familiar with trikes, and wasn't thinking of pusher configurations when I wrote my previous post. Putting a heated collar on such engines makes perfect sense.

 

In early August, our Tecnam Seirra had an incident with a new pilot flying. Coming in to join the circuit, the engine coughed and burped when power was applied to level off. The pilot elected to not continue the circuit, and landed immediately on the vacant runway in front of him. Not a bad decision given the circumstances. On the ground, the engine ran normally and everything was fine. No fault was found with the engine during subsequent ground runs and test flight. A rare case of carby ice was diagnosed.

 

However, in mid September, I went and got the aircraft and ferried it back to Gunnedah for it's 100 hourly. During the after take-off checks, when I turned the boost pump off, the fuel pressure dropped towards zero. I turned the boost pump back on. The fuel pressure recovered, and I climbed to altitude before investigating the problem. Turns out we had an intermittent engine driven fuel pump (EDP). We changed the EDP and haven't had a problem since. Intermittent faults are rare, and a pain in the butt to diagnose, but they do happen. And carby ice ain't the only thing that'll cause an engine to cough and burp.

 

Keep the sunny side up,

 

Wayne.

 

 

[facthunter]

The failed fuel pump is a good illustration of "when you move something and something happens just after, maybe it's got something with what you juat did". If you swap a fuel tank, don't assume it is ok till you have "proved" it. Nev

 

 

[eastmeg2]

In response to Wayne's post #4, I use the word "blip" very loosely. I go from cruise power to full power and back to cruise over about 2 to 3 seconds. The idea is that the movement of the butterfly valve will help break away ice inside the carburettor and to do this without unduly stressing the engine, gearbox or prop.

 

 

[Wayne T Mathews]

In response to Wayne's post #4, I use the word "blip" very loosely. I go from cruise power to full power and back to cruise over about 2 to 3 seconds. The idea is that the movement of the butterfly valve will help break away ice inside the carburettor and to do this without unduly stressing the engine, gearbox or prop.

I'm sorry Eastmeg2, I do not like that idea. I agree there is a slim chance it may dislodge some small piece of ice. But with a heated collar around the carby outlet, it's a mighty slim chance.

 

What it will do though, is waste fuel. It will reduce your engine's life by virtue of the expansion and contraction stresses you inflict every couple of minutes. And it will tire you more than needed because you're putting in unnecessary engine, and therefore flight control, corrections every couple of minutes.

 

What does your aircraft's operating/flight manual say about setting and maintaining cruise power? Does it say to increase and then decrease power over about 2 to 3 seconds every couple of minutes? If it says to do that, then fair enough. But I'll bet a dollar, sight unseen, that your aircraft's operating/flight manual doesn't say that.

 

Keep the sunny side up,

 

Wayne.

 

 

[Yenn]

With Cessna's I was of the opinion that you needed to advance the throttle from idle in a descent every few minutes, just to check that carby ice hadn't occured. I did have to abort a take off due to carby ice in a C150, ran OK until I got to just about lift off speed, then lass of power. The same day a twin had the same trouble, but he ended up through the fence. I've had carby ice in a Rotax 503 a couple of times, put it down at Eagle Farm the first time. Not the real Eagle Farm which is Brisbane International now, but my neighbours place.

 

 

[Tomo]

Got carby ice in a Rotax 912 while taxing once, nearly stopped on me! (it was a cold morning!!) But this one has carby heat so that fixed the problem after a minute or so.

 

 

[eastmeg2]

Hi Wayne,

 

The idea originated from at least one trike CFI. I don't recall which as I've known a few. More likely something would be in a Rotax engine manual, but not that I've come across . . .

 

Although the trike does have a hand throttle that can be set I tend to keep using the foot throttle even when flying x-country, so the additional workload isn't that great. I'm not entirely sure I understand what you mean by the extra expansion/contraction forces induced by a couple of seconds of higher power setting, not due to engine temps anyway.

 

 

[Guest davidh10]

Hi Wayne,The idea originated from at least one trike CFI. I don't recall which as I've known a few. More likely something would be in a Rotax engine manual, but not that I've come across . . .

It may or may not relate, but I've had similar advice in only one specific situation, and that was performing a simulated engine out (ie. engine at idle) in atmospheric conditions which were high probability for carby icing (dry bulb > 0 and <=13 degrees + very high humidity). In that case, it was to every now and then just bring the engine up to 3,000rpm for a few seconds. Thus assists to reduce engine cooling, plus exercises the butterfly. This situation was however not normal flying and thus not something that will be repeated often.

 

 

[facthunter]

I've given this matter a bit of thought. There was one engine that ran quite rich on low throtttle settings ( A 10 Mk 2 gipsy major in a Chipmunk). It was always recommended to give them a burst every 1500' on an idle descent to prevent PLUG FOULING. which that engine was good at.

 

I reckon the practice might hark back to that. Instructors can be a funny lot, doing things because that is the way it was done way back, without quite knowing why, sometimes.

 

An idling engine will cool off, but there is little fuel being atomised and although the manifold vaccuum is quite high, not a lot of air is expanded and cooled adiabatically. Mostly the heat is drawn from a shroud on the outer skin of the muffler to heat the carburetter. The muffler would cool quite quickly as it is only steel sheet and quite thin. A high relative humidity provides the moisture and you can get ice developing at over 25 degrees celcius. The carburetter is a good freezer with the atomisation of fuel and the expansion of gas to cool it. Nev

 

 

[Wayne T Mathews]

Thank you Eastmeg2 and Davidh10 for your input in this discussion. David, what you have said is, in my opinion, correct. All reciprecating engines that I'm aware of, will benefit from the technique you have described when descending from altitude with the throttle closed (for whatever reason and/or weather conditions) because it helps to control/reduce the engine's rate of cooling. And then there's the added benefit that if the engine does "quit" during the descent, you'll know about it earlier than you would have, had you not done this gentle "test".

 

The idea though, of increasing to full power and then back to cruise power every 2 - 3 minutes during cruise is not something I agree with. Please bear in mind that if your flight manual says to do it, then so be it... Do it. Beat your engine up... But if your flight manual doesn't say to do it... Don't.

 

At the risk of having the aeronautical design engineers amongst us cut me a new ring gear, let me try to explain in a way normal people can understand, why we shouldn't "pump" the throttle during cruise. Every time we go up to full power and back to cruise, we're whipping the engine. The internal temps increase and then decrease, even though we don't see it on the guage. There's lots of different/dissimilar types of metal in our engine. They expand and contract at different rates, and they remember, because all metal has a memory.

 

Keep the sunny side up,

 

Wayne

 

 

[jwn57030]

I believe I experience carb ice with the rotax 912. The weather that day was around 40F and 40-50% humidity. Before I took off I had gotten the engine to the min temperature of 120F. After that I still had to keep the oil heat on for most of the flight. I was doing a short cross country to an airport about 40 minutes away. About half the way there I noticed the oil temperture had gotten to 200F so I decided to turn off the oil heat. The engine seemed to be running fine up to that point. At the point I started decending towards the airport I noticed the oil temperture had dropped all the way to 150F. At that point I turned on the carb heat and oil heat. When I was on final I got a little hiccup from the engine. After I landed I taxied and parked for about 15 minutes. Unfortunately its all the time I had before I had to get back. Durring that time I felt the carburetor and it was ice cold. Before I took off again I restarted the again. It was running a little rough, so I let it idle with the carb heat on. As I did this it seemed to have some more hiccups. Interestingly the fuel pressure also jumped up while this was happening. Still in the normal range but much higher than I had ever seen it. After a few minutes of idiling the hiccups seemed to go away and the engine began running normally and the fuel pressure went back to normal. I did a run up and the engine seemed fine so I flew back home with no further incident. I did keep the oil heat on the entire time to make sure I kept the engine temperture within normal operating range.

 

 

[cscotthendry]

I experienced what I believe was carby icing in my Airborne trike on a couple of occasions. It showed up as slightly rough running and the engine refusing to go above 5,000 RPM. The guy who bought the trike from me experienced it worse than I did (he lives in melb and I'm in bris) and went through the engine like a dose of salts and discovered that Airborne hadn't purged the air out of the heating collars when they built the trike. Once he purged the lines he had no more troubles.

 

I would agree with the idea that pusher installations would be more susceptible to icing than tractor mounted engines.

 

Wayne: With regard to the intermittent fuel pump. I hear you mate. I had an old FX Holden ute once that had a pinhole in the fuel pump diaphram (or so I was told by a mechanic friend some time after I got rid of the beast) and the engine would intermittently and unpredictably stop. I can't remember all the things I tried to fix the problem, but plugs, leads, points, cap, rotor, condensor, new carby etc all come to mind. I think I swapped out everythine EXCEPT the bloody fuel pump!

 

 

[Ultralights]

i have had carby ice cause an issue with my 912, long descent from 8500 ft to wollongong, near idle power, OAT about 20 deg, and light whispy clouds/mist at certain levels.

 

ran rough for a quite a while, i have never had carby ice with a 912 in nearly 1000hrs, so didnt think of carby ice, landed, removed the cowls looking for a loose plug lead or fouled plug, and quite quickly noticed a nice ring of condensation ice formed on the outside of both carbys.

 

on the shock/ descent cooling stuff, i believe it is a bit of a myth, i will not conduct an engine run until oil temp is at a minimum of 50deg C. engine runs complete, line up for take off, oil temp tends to settle around 60 to 70 degC, full power, take off, power back a little to climb power, temp will then climb to 90 degC or 110C if a long climb past 3500ft. cruise power even after 5 hrs, temps still around 90degC, long descent from 8500ft to 1500ft, idle power, oil temp drops to about 70degC or maybe 65 if OAT is between 0 and about 10. so even after the longest descent at idle power, the engine is still warmer than it is when asked to work it hardest on take off.

 

someone else i know has a RV10 has shown similar results with data logging that shock cooling just isnt happening with his IO540, and my 912ULS in the Savannah. the descent recorded in the RV was from flight levels to circuit height of 1500ft, after ATC forgot to give descent clearance requiring a far steeper than usual cruise descent.

 

 

[Sapphire]

Reading the posts, the carby head method is described as "working most of the time" and if certain engine control procedures are carried out. The same applies to most Rotax 447 and 503 without any carby heat and the carby is in the full airstream. They do come down due to icing.

 

 

[bilby54]

Hi Red,

 

Any engine that is fitted with carburettors is susceptible to icing whether it is a Lycoming or a Rotax.

 

The Rotax design with the intake at the rear of the engine helps considerably to reduce the amount of icing but does not eliminate it.

 

The idea of moving the throttle was an old remedy when icing was suspected and it was moved rapidly to try and dislodge any ice - and if the pilot was in a bit of a panic then that was easy to do!! Carby icing can be hard to detect in the cruise and is usually found by having to increase the throttle slightly to maintain power until you run out of throttle.

 

Carby icing happens more often in the descent as the throttle is closed causing a larger pressure drop across the butterfly with a resultant drop in temperature. Nothing quite like opening the throttle on short final to find nothing happens!

 

 

[Sapphire]

Reading the posts, the carby heat method is described as "working most of the time" and if certain engine control procedures are carried out. The same applies to most Rotax 447 and 503 without any carby heat and the carby is in the full airstream. They do come down due to icing.

[alf jessup]

Never had any carb icing situations with my 912 Trike with the heated rings in over 500 hrs, had a couple on my 2 stroke over 200 hrs.

 

I never do idle decents in anycase unless of course high on final, usually plan my decents from cruising height over distance at a slightly lower rpm than normal cruise.

 

Tecnam has manual carby heat which I usually pull on just prior to turning base on downwind while running at higher rpm then pushing off halfway down final, on long descents under 3500 rpm I use carby heat haven't noticed any carby ice with it so far.

 

Alf

 

 

[danny_galaga]

Sorry for the Necro. Has anything changed with the 912 since the last post here? My kit (tractor) comes with carb heaters, but if it really doesn't need it then id rather reduce the complexity a bit, and the weight, and gain an  xtra horse or two

Edited December 17, 2022 by danny_galaga

[RFguy]

You wont gain horsepower if the carb heat is not in circuit when you dont need it.
And using warm cowl air when not required  is leaving horsepower (air density)  on the table. So I would suggest just leave the carb heat there in case one day you need it.

 

Edited December 17, 2022 by RFguy

[IBob]

What RFguy said.
If you have the option, there is no way I would leave it out, Danny.
I've had icing cruising just below low cloud level in mild weather (figures!) and I know of another member here had icing on finals, which is exactly when you don't want it.
FWIW the Sav has an airbox temp sensor and if in doubt, I now follow the advice of the second example above, by keeping the airbox temp not less than 20'C.