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carb ice revisited

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Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat Aug 16, 2008 8:51 am Post subject: carb ice revisited

Hi Guys, 



FYI 



I know I had a hard time convincing some that the Rotax 912's with the altitude compensating variable-venturi carbs are not susceptible to carb ice. 



Here is a quote from "Aircraft Powerplants" by Bent/McKinley a recognized text approved by the FAA. (Light reading when I have nothing better to do.  ) 



Quote: 

" The variable-venturi and pressure injection carburetors are relatively free from carburetor carb icing troubles." 



This is the big difference that pilots have to come to understand when they come from GA aviation that have a standard carb verses a variable-venturi carb. A few authors I have read recently would like to hear from anyone that has had a documented carb icing with these carbs. Fixed venturi carbs are far more susceptible to icing. 

That said a doctor teaching me a class once said, "Never say Never". 

So, if you want to spend the money for a fix for the the improbable carb ice issue you can buy one of two types. These are heated rings that go between the carb and the intake mount. One is an electrically heated ring. The other is hooked into your hot water/coolant and circulates through the ring. The water one is better as it does not put a hit on your electrical system.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

imap8ntr(at)cox.net
Guest

Posted: Sat Aug 16, 2008 9:30 am Post subject: carb ice revisited

Roger

One day when I did my run up on the ground,  I got a progressive drop of 

over 150 in RPM at idle.  I repeated this twice.  I figured that after a 

long taxi at low throttle set me up for carb ice.  I turned on the carb heat 

and the rpm soon went back to normal.  Waited about 10 seconds or so and 

turned off the carb heat again and again the idle dropped.  I turned on the 

carb heat again for about 20 sec or so and then turned off the heat.  The 

idle then remained steady at the proper idle rpm.  Had no further 

occurrances that day.



I also had the above scenario occur one other day and the heat worked like a 

charm.



Ivan

Phoenix,  AZ



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JimT

Joined: 27 Feb 2006
Posts: 22
Location: Auburn, ME

Posted: Sat Aug 16, 2008 5:23 pm Post subject: carb ice revisited

OKay , a very timely post.  Is there any data that the electric ones work.  One only draw 1.5 amps per carb.

 

 

 http://www.ulparts.com/SearchResult.aspx?CategoryID=94

 

 

 Another is listed from ireland,     http://www.xairireland.com/access-h.htm#carbheat

 

  I am ready to buy some kind of carb heat for my searey project with a 912s, but would love a little guidance.

 

 Jim Timoney

   

 

 --

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat Aug 16, 2008 6:52 pm Post subject: Re: carb ice revisited

Hi Tim,



Good question. The electric ones do work, but you still have the amp draw and managing the wires and making sure it is always working. The coolant hot water are better and more consistent and easier to manage over time. You can buy them through CPS.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Sun Aug 17, 2008 3:32 am Post subject: Re: carb ice revisited

Roger,

In the nearly three years we've operated our Allegro 2000 w/ 912UL, I've had carb ice develop twice in flight. Both times it was at normal cruise power around 5,000 rpm on warm humid days. Noticing it quickly, it is easy enough to clear by cycling the throttle open/close a couple of times. If you don't notice it quickly and don't clear it out, it can build up pretty fast and lead to serious loss of power. 



It is not a bad idea to install carb heat but we chose not to do so... yet.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

peteohms

Joined: 31 Oct 2007
Posts: 224
Location: Leander, TX

Posted: Mon Aug 18, 2008 9:12 am Post subject: carb ice revisited

Roger,



My 912 does the same thing every time I do my run-up on the first flight of 

the day. I really don't think it is ice.  I think because the carb heat 

bypasses the air filter that the carbs are getting more air.  If I run the 

engine at 3500 rpm for a half minute the problem clears up.



Pete

Hell Paso, TX



---

_________________
Pete
Leander, TX
Kitfox III, 912ul, Grove

imap8ntr(at)cox.net
Guest

Posted: Mon Aug 18, 2008 12:51 pm Post subject: carb ice revisited

Roger, Pete

Also to consider is the fact that my carb is an altitude compensating carb. 

Because of that there is normally no drop in RPM when turning on carb heat 

in the absence of ice. The two times presumable ice occurred was on a medium 

temp day,  humid,  at low throttle setting.  If that is not a set up for 

carb icing I dont know what is.



Ivan

---

peteohms

Joined: 31 Oct 2007
Posts: 224
Location: Leander, TX

Posted: Mon Aug 18, 2008 2:11 pm Post subject: carb ice revisited

Ivan ,



Well, every day is humid here lately.  Guess you could be right.  It never 

happens after the engine gets good and warm.



Pete

Hell Paso, TX

Kitfox III, sn 1000, 912, Grove



---

_________________
Pete
Leander, TX
Kitfox III, 912ul, Grove

imap8ntr(at)cox.net
Guest

Posted: Mon Aug 18, 2008 4:03 pm Post subject: carb ice revisited

That venturi in the carb can get pretty cool with evaporation of the fuel 

inlet and it would not take that much to close off the opening.  I would not 

think that Rotax would have carb heat if it was not necessary or useful.  In 

my flying if I suffer a power loss I would definitely try carb heat.  I 

never had the problem during the warm weather in AZ.  I can remember that 

the day it happened it had been foggy, 100% humidity and expected the carb 

ice.  When I did my run up and the RPM dropped for the first time ever,  I 

just said OH there it is.  I did recheck doing the run up twice to make sure 

it did not ice up again and all was ok.  I was especially vigillant on my 

final.



Just to make sure I have the Bing Altitude compensating carburetor.

Ivan

---

avid2008

Joined: 16 Sep 2008
Posts: 2

Posted: Fri Sep 19, 2008 12:09 pm Post subject: Re: carb ice revisited

Roger, I have always been told that R912 is pretty much ice-proof. Last winter I discovered the contrary although it must be said in all fairness that conditions were prone to icing.



I purchaised  a Rotax 912 powered Avid Flyer and was ferrying it  accross Europe in the company of the previous owner. It was around 0°C and very humid; the guy was running it at 4500rpm. After about 20min of flight I noticed that revs were around 4200 and quite uneven; I pushed in some more throttle with no effect, then after some 30-60secs suddenly revs came up to 5000. We kept it at 4800-5000 for the rest of the day( 6hrs/2 landings) without any further problem.

The next day the valley where we stopped for the night was filled with stratus and occasionally there were some drops of rain. Hardly a VFR type of weather but we decided to try to take off.  While warming up the engine I noticed that it was running VERY rough - I couldn't check above 3500rpm as the brakes were not strong enough to hold the plane, but anything below 3500rpm was very uneven. Not being certain if there is everything OK with the engine and having marginal weather we decided to postpone the takeoff.

Next day I called a local FBO mechanic (I had to go back home by train) and asked him to check the engine. The weather was fine and he didn't have any problem.

I feel very strongly we must have experienced carb icing.



I did 70hrs with this plane since that day and I must say that on a couple of occasions I had a suspicion of icing (not as apparent as described above, but a slight irregularity below 4000rpm) in humid conditions. Adding power has always improved the situation.

I must say that the Avid Flyer has huge air intakes on both sides of the prop, so presumably there is no wam air stagnating under the cowl (as it is frequently the case of modern planes with sleek cowlings). Warmer air in the carb area can probably help, although it is not a 100% solution (FAA wants to see, if memory serves me well, at least 70F air temp increase to be considered as an efficient carb heat).

Martin

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Fri Sep 19, 2008 4:47 pm Post subject: Re: carb ice revisited

Hi Guys,



Please don't take my next comment as being a smart ass, but how do you guys know if it was really icing without any confirmation and have you ruled out all other possibilities? Without confirmation it still is speculation. These are not standard carbs like the ones we were raised with during our life, but quite different. These are variable venturi carbs. That in itself makes them less prone to standard icing effects.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Sat Sep 20, 2008 3:21 am Post subject: Re: carb ice revisited

Roger,

You are not a smart ass. You just live in a place where 50% humidity seems like a steam bath. Back east, where we have enough rain to allow trees to grow, 50% humidity seems down right arid.



I've been flying since 1966 and I know the symptoms of carb ice. That is how I know I've had it. It is true that the Bing carbs are LESS LIKELY to ice up but they can and do under high humidity conditions. I've had it happen to me on two separate occasions, both of which were in conditions highly conducive to carb ice. Although it is rare enough that I don't bother with a carb heat system, it does happen here in the land of "normal" humidity .

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Sat Sep 20, 2008 6:39 am Post subject: Re: carb ice revisited

Hi Thom,



If you ever make it down to the Tucson, Az. area give me a call. I'll be happy to take you and whom ever up in my Flight Design CT for a flight around the area in some of that dry air.     



I have a friend that lives in New York that I'm sure he would be happy to take you for a cruise in his CT.



Invites good any time.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Sat Sep 20, 2008 12:17 pm Post subject: Re: carb ice revisited

Thanks for the offer, Roger. If I get down your way again, I'll look you up.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Sat Sep 20, 2008 1:28 pm Post subject: carb ice revisited

So true. But sho0rt of landing immediately and pulling off the air screens

there is no other way to verify ice in the carb other than through symptoms.



Noel 



--

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

avid2008

Joined: 16 Sep 2008
Posts: 2

Posted: Sat Sep 20, 2008 11:45 pm Post subject: Re: carb ice revisited

, but how do you guys know if it was really icing without any confirmation and have you ruled out all other possibilities? Without confirmation it still is speculation. 



Roger, the reason I related the circumstances of my trouble(s) was to let you guys estimate if it was or if it wasn't carb icing. I don't see what else it could have been.

your statement that Bing carb is ice-proof is also a speculation without confirmation, it works both ways.

Martin

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