Carb Icing

[Guest prop]

I have a question regarding Carb Icing and Carb Heat. I have heard that in some situations of temp and humidity, it is non advisable to pull carb heat. That being when there is a danger of already frozen particles, being melted by carb heat and then freezing again when in the Venturi.If this is the case, then in a older Cessna 172, which suggests carb heat when the throttle is pulled, ie. on approach, would it be prudent to leave it off then and what would be the consequence of just leaving it off. Or is there one?Further, since anything below -5 or so would already be frozen particles anyway, there would be no danger of carb ice, so why would you be pulling carb heat anyway? Unless you are setting up for approach which then, is in relation to my previous question.Darryl

[michal 44]

Follow aircraft manufacturer's recommendations when to apply carburator heat. If they want you to pull it when flying with lower RPM (like on approach) - do it. They know what the engine/carburator needs, do not invent your own rules, it can cost you dearly, specially when not far from the ground.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[Guest prop]

I agree... but explain to me then, if it was a situation of cold weather and pulling carb heat would warm the ice crystals and then have them freeze again as they passed into the cold venturi... what then?? Why would you pull carb heat?Darryl

[michal 44]

>I agree... but explain to me then, if it was a situation of>cold weather and pulling carb heat would warm the ice crystals>and then have them freeze again as they passed into the cold>venturi... what then?? Why would you pull carb heat?>You have to read more on the subject. You are making assumptions that simply are not true (about those ice crystals). The tiny ice crystals in the air is not the danger you (or your engine) is facing.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

[Guest Adverse Yawn]

Negative,First some facts:* Firstly the temp in the carb venturi falls due to the lower pressure and the vapourisation of fuel. This causes approximately a 25-35degC temp drop in the carb.* Full carb heat warms the air by about 150degC* Water vapour in cold air does is not ice (regardless of the temprature), the droplets that form the vapour in sub zero OAT are called Super Cooled Water Droplets or SCWD. SCWDs are very unstable and will turn to ice as soon as anything touches them, such as impacting the throat or butterfly valve of the carb or hitting wings leading edges, etc.* Clear ice (the dangerous stuff) forms most readily between 0 and -10 degC. It can also down to -15degC* Pure rime ice (non lethal stuff) forms from about -10degC to -25degC. Rime ice is weak and fluffy and will not stick in the carb.* Below -25degC icing is very unlikely.From this it is possible to see that carb icing is most problematic when the Outside Air Temp (OAT) is about +25 to +35 degC as that puts the carb throat temp at between 0 and -10 degC. You add 150degC in the form of full carb heat there is no way that ice will form.However, if you apply full carb heat momentarily, all you will do is move the ice. You must apply carb heat on for a good while, if there is cie present the engine may sound very rough momentarily as it ingests the ice, then the revs may rise and then rise again when you turn off carb heat.Applying partial carb heat should only be done when a carb temprature guage is available as partial heat can increase a carb throat of say -35degC (no icing) to -5degC (heavy icing!!) rather than taking it above 0degC.Carb icing is most likely when the engine is at a low power setting, such as on approach. It is very important that you apply full carb heat in such a situation. By the time you discover you have no power it will be too late to do anything about it because the heat-exchanger/exhaust manifold could have now cooled too much and applying carb could not only simply be ineffective but also act like partial heat and add ice!!!Hope that helps.

[LAdamson 240]

And to add,The carb. is bolted to the warm oil sump on Lycomings that you'll often see on Pipers, which is a reason you won't see carb heart in the landing checklist, like you might see on the Cessna checklist with a Continental engine. This keeps the carb. a bit warmer, but technically looses a bit of power due to the warmer air induction than the Continental.And then you'll see arguments of using carb heat regardless, but I fall into the "camp" of not using it, unless recommended, because carb heat "robs" power in a go-around if you forget to remove it. Where I fly, is high altitude mountainous area, where all available power is needed in a go-around.L.Adamson

[Guest DreamFleet]

Yup, agreed, Larry. However, since Electronics International had a great promotion going on last month, where I can get a free fuel flow transducer and an extra probe of my choice, I chose the carb temp probe to add to the UBG-16 engine analyzer I got for my Dakota. Always wanted to see the temp in the carb, and now I can! ;-)http://www.buy-ei.com/UBG-16_Bar_Graph_Engine_Analzyer.htmRegards,http://www.dreamfleet2000.com/gfx/images/F...R_FORUM_LOU.jpg

[Guest Adverse Yawn]

Agree with Larry. Always refer to the POH. It is theorectially possible to cause detonation in the engine if high power settings and carb heat are used.

[AlaskanFlyboy 7]

Our Piper Warrior IIIs call for carb heat on approach if temperatures are below 10

[Guest Adverse Yawn]

Makes sense, the temp drop in the carb could be less at lower power settings. My post is a generalisation intended to dispel the idea that using carb heat properly causes ice. Always operation in accordance with the POH for your aircraft.

[Guest SPOFF]

>I have a question regarding Carb Icing and Carb Heat. >I have heard that in some situations of temp and humidity, it>is non advisable to pull carb heat. That being when there is>a danger of already frozen particles, being melted by carb>heat and then freezing again when in the Venturi.>>If this is the case, then in a older Cessna 172, which>suggests carb heat when the throttle is pulled, ie. on>approach, would it be prudent to leave it off then and what>would be the consequence of just leaving it off. Or is there>one?>>Further, since anything below -5 or so would already be frozen>particles anyway, there would be no danger of carb ice, so why>would you be pulling carb heat anyway? Unless you are setting>up for approach which then, is in relation to my previous>question.>>DarrylCarb ice actually forms on the INSIDE walls of the carb venturi. This occurs when NO heat is being supplied to the mouth of the carb, under certain conditions of humidity, temperature, and velocity of the air entering the carb. In airplanes, and automobiles, carb heat is usually supplied from ductwork surrounding the exaust manifold..which adds warmed air to the airflow being sucked into the carb. After a period of time, without heat, ..and usually at reduced air inlet velocity, the ice forming on the inside walls of the venturi will "build up" to the point where not enough air is getting to the engine to support the required combustion, because of the restriction caused by the ice. NOT Good. The outside air temperature does not have to be near or below freezing, but visable water vapor..such as clouds or fog suggests that carb iceing is possable.Internal combustion engines can actually tolerate water entering the engine through the carb if it's not excessive. Some older high performance aircraft engines actually had "Water Injection" as a method to control "Detonation"..which is another topic. As mentioned above, ICE in the carb reduces air flow which the engine won't tolerate. Reduced throttle settings are when carb heat is usually used..as when setting up for landing. Hope this helps...Maybe a little wordy. SPOFF