Carb icing

[kodoz]

I know it's a subtle difference, but what reason did you learn for not using carb heat below -10C (or, -4C in the RHC POH)?

[Wally]

Choices 1, 2 and 4 are illogical. Number 5 is practical but doesn't address the issue, so number 3 is 'more correct'. At some point, the amount of water available to create an ice problem will be insufficient, if the books says -10C is that point, that's it.

[ironranger]

I think number 2 warrants some consideration since there can be frozen moisture in the air in the form of ice crystals and snow which could melt and freeze in the carb. I was flying today with lots of ice crystals in that air and left the carb heat latched shut. it was -13C.

 

IR

[Goldy]

it was -13C.

IR

 

Even though I voted #3, it is true that existing ice could melt..that being said, I beleive there should also be an answer # 6...

 

#6- You fly in So. Cal so you really don't get to worry much about it being -10C !!!! What the hell is -13C anyway?

 

Once a year, it gets really cold in Los Angeles. Sometimes it even drops below 50F !

Edited December 20, 2009 by Goldy

[lelebebbel]

I asked the same question to the Robinson Customer Service last year, here's their reply:

Hello xxxx,

 

Your readings seem to be normal. As stated in the R22 POH carb heat should be applied between 27C and -4C. Colder than -4C carb heat not required.

Your CHT and OAT with a cold engine should be approx. the same.

Please refer to the R22 POH.

 

Best Reagrds,

Chris Sennett

RHC Tech Support

 

 

----- Original Message ----- From: me

To: <customerservice@robinsonheli.com>

Sent: Monday, December 15, 2008 9:39 PM

Subject: R22 - Carburetor heat use in sub freezing temperatures

 

 

> Hello,

> I have heard and read a lot of conflicting information about the use of carburetor heat in the R22 during cold weather, specifically temperatures between about -3C and -10C deg.

>

> The scenario I had on a flight today was: temperature -6C, dew point -13C without visible moisture.

> CAT was about -10C to -12C at 104% RPM with no carb heat applied (sitting on the ground), definitely within the yellow arc.

> With full carb heat applied, we barely got about +4 to +5C CAT throughout the flight (getting colder at higher altitudes), still within the yellow arc at times.

> We left the carb heat full on throughout the flight. This was a R22 Beta.

>

> My questions:

>

> 1) Did the carb heat perform as expected, or should I have been able to get a higher CAT with full carb heat applied? (i.e. possible leak in the CAT airduct?)

>

> 2) should I have left the carb heat off, and would the CAT have dropped below the yellow arc once airborne?

>

> 2b) if yes, does it make a difference if the resulting CAT is below the dew point (like today, with a DP of -13C)

>

> 3) with both OAT and CAT well below freezing (but not below the yellow arc on the CAT gage), does adding carb heat increase the risk of carb ice formation due to melting of possible ice crystals in the air?

>

> 4) when climbing into higher altitudes and colder temperatures, is there a certain temperature at which the carb heat should be turned off, and is it safe to do so in flight, letting the CAT decrease from "above yellow" to "below yellow"?

>

> My students and me would really appreciate it if you could provide some clarification.

> Thank you!

>

Edited December 20, 2009 by lelebebbel

[Hovergirl]

I agree that when it's cold enough outside there will generally be very little moisture in the air so much less chance of icing. My general rule is to keep the needle out of the yellow if possible -- if it's below the yellow keep the CHT off. I just estimate whether it's possible to get it out of the yellow with full carb heat, and if not, or if it's really close to the bottom of the yellow, I just keep the carb heat off.

[kodoz]

Choices 1 and 2 are explanations that I've gotten in the past from CFIs, and I agree, they don't make sense. The first choice ignores the relationship between temperature and dew point. The second is incorrect because humidity exists as water vapor (molecules that aren't "frozen" or "liquid").

 

Choices 3 and 4 are explanations I've seen in print, and I think both are correct. At cold temperatures, air holds less moisture, and at very low temperatures, the air can't hold enough moisture to deposit much ice in the carb (this is the FAA and AOPA explanation). The "not sticking" explanation is given in Principles of Helicopter Flight and Weather Flying. The way I understand it is that, at very low temperatures, whatever ice is deposited in the carburetor is completely frozen and doesn't adhere well. Warm up the air some, and you get some condensation and some ice deposition--the condensation helps the frozen stuff adhere, and you have a potential problem. An analogy would be trying to make a snowball with dry Utah powder versus moist Cascade cement.

 

Either way, Robinson says that below -4C (Robinson POH), no carb heat.

 

Answer 2 is not completely without merit for visible moisture. Back to Weather Flying--if you're flying in dry snow or cirrus clouds, you're dealing with ice crystals that won't stick well (although I'd still defer to whatever the carb temp is reading, since how much heat you add also factors into the equation). In this case, if the carb temp is cool enough, the frozen moisture passes through the carb without a problem.

[amphibpilot]

<snip>

 

Either way, Robinson says that below -4C (Robinson POH), no carb heat.

 

<snip>

 

Where, exactly, does the POH say this? My current POH does not make such a statement. And I have definitely needed to use carb heat below -4C OAT.

Edited December 22, 2009 by amphibpilot

[kodoz]

Where, exactly, does the POH say this? My current POH does not make such a statement. And I have definitely needed to use carb heat below -4C OAT.

 

Normal Procedures, p 4-11, use of carb heat assist. Please follow up and let us know if this has changed, because this is also what they were teaching at the safety course as early as February.

 

While you're at it, what's your opinion on why carb heat is required below -4C? Did you need carb heat at that temp because you were experiencing signs of carb icing?

[lelebebbel]

Pat Cox of Robinson just sent me a clarification email re. the email posted above:

Season's greetings Dirk,

 

I believe I just read your posting on the Vertical Reference forum at:

http://helicopterforum.verticalreference.c...amp;#entry89297 and dated Dec 19 2009 at 21:38.

 

The emphasized "Colder than -4C carb heatnot required" in the forum is, by itself, misleading. Chris' 16 Dec 2008 10:25 AM email was in reply to your specific scenario of :

-6C OAT,

-13Cdewpoint,

and no visible moisture.

 

The R22 POH's FAA-approved pages do NOT list specific OAT or dew pointranges for carburetor icing; instead, the POH says "When conditions conducive to carburetor ice are known or suspected to exist, such as for,rain, high humidity, or when operating near water, use carburetor heat asfollows: ....

 

"The POH does recommend "...the [carb heat assist] control knob be unlatched (to activate carb heat assist) whenever OAT is between 80°F (27°C) and 25°F(-4°C) and the difference between dew point and OAT is less than 20°F (11°C). Readjust carburetor heat as necessary following any change inpower."

 

Also, the Beta II's carb heat assist does not relieve the pilot of responsibility for monitoring the CAT gage; the carb heat assist just reduces the frequency ofmaking large carb heat adjustments.

Please also refer to R22 POH page 5-1 "Note" concerning hover performance with carburetor heat, and Safety Notice SN-25.

 

Here are some carburetor icing links you may find useful: http://www.aopa.org/asf/epilot_acc/nyc02fa025_1.html http://www.caa.co.uk/docs/33/ga_srg_09webS.../carb_icing.pdf

 

Happy holidays andFly safely, Patrick Cox

RHC Tech Support

[ironranger]

Based on Robinson's recommendation of:

 

"the [carb heat assist] control knob be unlatched (to activate carb heat assist) whenever OAT is between 80°F (27°C) and 25°F(-4°C) and the difference between dew point and OAT is less than 20°F (11°C)."

 

should carb heat be used based on the following metar?

 

KHIB 231353Z AUTO 10007KT 10SM OVC070 M08/M11 A3034 RMK AO2 SLP305 T10781106

 

The temp is below -4C but the spread is only 3degrees C. Does it make a difference if there are ice crystals or light snow in the air.

 

IR

[amphibpilot]

R22 POH page 4-11 dated 13 OCT 2000 is current, and it contains no statement concerning use of carburetor heat versus OAT. Note the emboldened emphasis in the following quote:

Normal Procedures, p 4-11, use of carb heat assist. Please follow up and let us know if this has changed, because this is also what they were teaching at the safety course as early as February.

 

While you're at it, what's your opinion on why carb heat is required below -4C? Did you need carb heat at that temp because you were experiencing signs of carb icing?

This is not just a matter of semantics, as the POH subsections are for two different items:

"Use of Carburetor Heat"

and

"Use Of Carburetor Heat Assist"

 

Altho both subsections are on page 4-11, "Use Of Carburetor Heat" contains No mention of OAT; OAT is only mentioned in the following "Use Of Carburetor Heat Assist" subsection.

 

The last sentence in the "Use Of Carburetor Heat Assist" subsection is notable: "Readjust carburetor heat as necessary following any change in power."

 

And yes, I have required carb heat below -4C due to signs of carb icing in an R22 (the conditions did not catch me unaware - I wanted to see how resistant the Lycoming's oil-warmed induction system was to carb icing compared to a Continental engine I often fly with).

Edited December 23, 2009 by amphibpilot

[amphibpilot]

RHC's references to websites in lelebebbel's 23 Dec post were truncated. I checked with RHC and they should be:

 

http://www.aopa.org/asf/epilot_acc/nyc02fa025_1.html

 

http://www.caa.co.uk/docs/33/ga_srg_09webSSL14October.pdf

 

http://www.atsb.gov.au/aviation/editorials...ttor-icing.aspx

 

http://www.atsb.gov.au/media/56519/carb_icing.pdf

 

 

Happy Christmas to all!

[crashed_05]

I'm a brand new helicopter pilot and I've always wondered what I should do if the temp was right at -4C..carb heat on or off (R22)? Well when I took my checkride a few weeks ago I had that senario. I was going through the start-up checklist and when I came to the carb heat check I hesitated for a moment, then told the examiner what the POH says and told him I'd leave it off. He asked me if I could see any visible moisture in the air and I said no. He then stated that I don't need the carb heat then and that there are more mishaps that occur from using carb heat than there are from not using carb heat (being that you have less pwr available with carb heat). Throughout the flight, the OAT fluctuated between -3C and -5C and I left the carb heat down and locked the whole time.

 

What are y'alls thoughts on that?

[Hovergirl]

He then stated that I don't need the carb heat then and that there are more mishaps that occur from using carb heat than there are from not using carb heat (being that you have less pwr available with carb heat). Throughout the flight, the OAT fluctuated between -3C and -5C and I left the carb heat down and locked the whole time.

 

What are y'alls thoughts on that?

Seriously? Accidents due to carb heat usage? I would think you'd have to be operating pretty close to the limit if that 1" of power lacking was enough to cause an accident. Is this a problem for people in piston helicopters and I just haven't heard about it?

[apiaguy]

It is an interesting question... (carb heat usage causing accidents vs not using and icing up)

 

When you have a full rich mixture and low power setting and then add full carb heat you can load up the engine and cause it to die... usually after loading up for a length of time and then trying to give it more power (think power recovery autorotation)

 

How often does an engine see carb ice? Alot depends on the engine... some aviation engines are known to be prone to carb ice for whatever reason more than others. I could see the R22 being an ice builder based on it being derated and uses less than wide open throttle settings regularly.

On the other hand, helicopters use alot of power (compared to fixed wing cruise) and that power creates heat and can help reduce carb icing incidents.

 

I hate carb heat and why robinson doesn't make the R22 fuel injected is beyond me. I almost never use carb heat in airplanes unless 1)I am in a very long decent at less than cruise power (say 5000 foot decent) 2) there is visible moisture, be it fog, rain, mist, dew point spread is close etc..

 

I don't have any answers... just an interesting debatable point

 

I understand what the examiner was saying... like on a clear blue day in the desert... would I use carb heat.. absolutely not.

[avbug]

Carburetor ice is perhaps one of the single most misunderstood topics in aviation, where piston engines are concerned. I've watched people make forced landings before, following an encounter with carburetor ice, who were completely baffled as to the cause of their power loss, who had no idea they could get carb ice at 90 deg F, or who thought carb ice couldn't form at low temperatures.

 

Carburetor heat is there to be used as needed, when needed. Do NOT box yourself into a corner by thinking it only applies at certain temperatures. The only consideration with respect to carburetor ice is carburetor air temperature. Way back when, the use of carburetor air temperature gauges was standard. In fact, I spent a lot of time adjusting carb heat in big radial engines, to maintain a carb air temperature within the "green" range, in flight and on the ground, in the summer and in the winter.

 

You don't have that option in most light airplanes and helicopters. You have general guidelines, and common sense. In order to make the most of either one, you need to understand what's going on that carburetor. I didn't answer the question in the poll, because none of the answers given are congruent with the question.

 

Can precipitation, moisture, ice, etc, be heated by carb air heat, only to refreeze in the carburetor again? Yes.

 

Can the ambient temperature be cold enough to preclude carburetor ice? Yes.

 

Can the air be dry enough to preclude carburetor ice? Yes?

 

Can carburetor icing cause a power loss, rapidly, on a clear day? Yes.

 

Can leaving carburetor heat on cause significant performance loss, enough to cause an aircraft mishap? Yes.

 

Can failure to use carb heat cause power loss, leading to an aircraft mishap? Yes.

 

Is there more than one type of carburetor icing? Yes.

 

Is there more than one way to use carburetor heat? Yes.

 

Properly used, carburetor heat employs an air temperature gauge and is applied in increments to adjust the temperature of the airflow through the carburetor venturi to an acceptable range. Again, in light aircraft, we don't have that option, so it's either full open, or full closed for carb heat. The most basic rule of carburetor heat is that if you think you need it, you should apply it fully.

 

If you wait to apply carburetor heat, you may wait too long.

 

If you apply carburetor heat and you do have icing, expect your engine operation to become rough and a power loss to occur, until the carb icing has been cleared.

 

If you apply carb heat and nothing happens, then you probably didn't have carb icing, but you may be acting to prevent icing from forming.

 

As you know, inside your carburetor, a pressure drop occurs. When the pressure drop occurs, a temperature drop also occurs. As the pressure drop occurs at the carburetor throat or venturi, this lower pressure is used to draw fuel through a "jet" or nozzle opening, which then mixes with the airflow and vaporizes. This action also causes a temperature drop, and introduces additional moisture into the airflow. Open or partial throttle icing occurs at this time.

 

When at idle, a different fuel source is used; the idle jet...completely different than the way fuel is introduced when the throttle is open. In this case, the throttle plate in the carburetor is closed, and a very small amount of air is allowed to leak past it. This small leakage area acts as a venturi of sorts, and a fuel jet is located just past this point...this is the idle jet. Carb icing which takes place during idle operations or low power operations is not the same as icing that takes place with the throttle open, and icing that occurs may take place under different conditions.

 

Icing which occurs at idle doesn't take a lot of ice to cause the engine to die; opening the throttle means that a different fuel jet is used and different airflow occurs, and the problem may be alleviated with only a stumbling of the engine as ice flows through from the idle jet. Conversely, a power loss at idle might not be recognized right away, and may mean that the engine dies completely when the throttle is opened as there may be no fuel during the transition from idle jet to throttle-open operation. One may see a stumble, a cough, and then no power at all, requiring a restart.

 

Leaving carburetor heat on as power is increased may prevent or remove ice, but it also robs power. Someone asked if the loss of 1" of manifold pressure is significant enough to cause a mishap. The answer is a resounding "yes," because leaving carburetor heat on is the same to the engine as experiencing a much higher density altitude. It also causes a much richer mixture. Hot engine induction air (carburetor heat) means thinner induction air, which means a richer mixture. It means less power, and it's the same as introducing higher density-altitude air, as well. It's not just a 1" manifold pressure loss, necessarily. You're looking at a combined power loss from the change in mixture and the increased air temperature...and it's the potential torque loss that's really of interest.

 

While one might not normally expect carburetor ice at freezing temperatures, don't bank on it. Carb heat is there to be used; don't simply assume that because it's cold outside, you won't get carburetor ice. Some of the worst carb icing I ever had was in an airplane, but occurred during freezing weather. It involved carb ice which formed very rapidly and resulted in a total power loss. I applied carburetor heat, but the cable broke, and I ended up making a descent out of a cloud and a forced landing on a mountain gravel strip, as a result. Never make assumptions that because temperature is below XXX value, it's too cold or the air is too dry for carburetor ice. If you suspect it given engine roughness or a power loss, be prepared to use carb heat and see. Making assumptions that you can't get carburetor ice is an act which narrows your response options, and closes your mind to the possibilities around you. You may fly for some time and never see carb ice, or you may be operating in conditions in which you see it regularly.

 

These conditions may range from warm summer days in Florida (great place to get carb ice, incidentally), to cold winter evenings in the rocky mountains (also a great place to get carb ice). You may operate every day with no indication of ice, then go for a morning flight after a light rain shower, and find that the increased humidity is giving you icing fits. Be prepared for carb ice, and don't get locked into thinking "in the box" only. The icing might not be there, but it might just be found "outside the box" when you least expect it. The secret, then, is to expect it anyway, and plan accordingly.

[67november]

Excellent post Avbug, very informative.

 

thx

[RockyMountainPilot]

Avbug,

 

You don't seem too familiar with the R22 and since that is the aircraft we have been discussing, I will correct a couple of the things you mentioned.

 

Properly used, carburetor heat employs an air temperature gauge and is applied in increments to adjust the temperature of the airflow through the carburetor venturi to an acceptable range. Again, in light aircraft, we don't have that option, so it's either full open, or full closed for carb heat.

 

The R22 does have a CAT gauge and the carb heat can be applied incrementally to keep temps out of the yellow arc.

 

Someone asked if the loss of 1" of manifold pressure is significant enough to cause a mishap.

 

There is no loss of manifold pressure when carb heat is applied. Manifold pressure remains the same because the pressure going to the engine is unchanged. This is true for all carburetor equipped aircraft that I know of, and in fact, the manifold pressure might increase in some aircraft because you bypass the air filter with carb heat. Not so in the R22 because the carb heat still goes through the filter. However, as you mentioned, the heated air is less dense and more rich which will reduce power. But the R22 is derated by manifold pressure, so increasing the manifold pressure by 1 inch, (.9 in the Beta II's I believe) will return the power to around the same level as before the carb heat was applied. So Hovergirl is absolutely correct. Unless you don't have an extra inch to spare because of high density altitude operations, you won't see any loss of available power in an R22, R44 and other manifold pressure derated helicopters such as the 300C.

[crashed_05]

I understand what the examiner was saying... like on a clear blue day in the desert... would I use carb heat.. absolutely not.

 

I think that is the point my examiner was trying to get across...don't rely solely on what the temp is, but also observe other conditions such as temp/dp, visible moisture...

[kodoz]

I think that is the point my examiner was trying to get across...don't rely solely on what the temp is, but also observe other conditions such as temp/dp, visible moisture...

 

 

Right. Same thing I learned. I'd include in that the carb icing risk charts provided by the FAA and AOPA when estimating your risk.

 

Avbug, thanks for your thoughtful post. Not all of it is relevant to the R22, but still a lot of good info.

[crashed_05]

Seriously? Accidents due to carb heat usage? I would think you'd have to be operating pretty close to the limit if that 1" of power lacking was enough to cause an accident. Is this a problem for people in piston helicopters and I just haven't heard about it?

 

Its a problem in piston helicopters especially when operating at high DA. According to the R22 POH, use of carb heat reduces HOGI/HIGE ceilings by 2000ft. If the performance charts show that you have a HIGE ceiling of 8000ft and you figure you have plenty of pwr for a landing in a confined area at 6500ft, you may be in for a surprise if you neglect to turn off carb heat on final.

[RockyMountainPilot]

Its a problem in piston helicopters especially when operating at high DA. According to the R22 POH, use of carb heat reduces HOGI/HIGE ceilings by 2000ft. If the performance charts show that you have a HIGE ceiling of 8000ft and you figure you have plenty of pwr for a landing in a confined area at 6500ft, you may be in for a surprise if you neglect to turn off carb heat on final.

 

Most pilots wouldn't crash. I have run into that scenario before and you simply pull in max power and if you are still descending, you pull a little extra collective to cushion the landing. Even properly trained students will do this automatically.

 

Accidents due to carb ice are more likely than to carb heat being left on.

Edited December 31, 2009 by RockyMountainPilot

[lelebebbel]

most pilots wouldn't crash

You think so?

Hundreds of helicopters have crashed on landing because of insufficient power available.

 

If it is marginal and you pull the revs off at 10ft agl you might be fine - if you react right and you can stop the yaw.

But if it already starts to droop at 20 or 30ft AGL, you are in for an interesting ride. And if you were on the way into a confined area in a 22, that will be a ride into the ground.

 

Aways know at what MAP you will be at full throttle when operating at altitude. Keep an eye on that gauge.

And if things are so marginal that carb heat use will make a landing impossible, maybe find somewhere else to land?

Edited December 31, 2009 by lelebebbel

[RockyMountainPilot]

A pilot who lets the RPM droop is more likely to crash because they have poor decision making abilities and/or poor skill. That means they are more likely to crash for any reason.

 

If a helicopter has insufficient power within its normal operation and it crashes. It is the pilot who crashed it, not the insufficient power. You always have the option of going around.