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Folks with digital carb temp - how do you use it?


DXB

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Given my carb'd O-360, I've read all I can about carb ice, and now I have a carb temp number available to me on my recent JPI install, but I'm unsure how to use the gauge. Prior to having the  gauge, I left carb heat off in climb and cruise (vfr), turned it full on when reducing power for descent, and then turned it off on short final in preparation for go-around.   I've read carb ice is a non-issue at carb temps outside the 22-40F range, but my gauge typically reads in the 20s-30s F in cruise.  So I've been turning carb heat partially on in cruise to raise temps into the low 40s- this is a simpler approach than assessing ice risk in each case based on OAT and humidity, and it seems to come with no performance cost. Mike Busch even says a smidge of carb heat can improve mixture distribution and thus makes it easier to lean aggressively in cruise.  I still turn it full on for descent, when I have more to focus on than dithering with partial carb heat.  No instrument rating yet, but I will also need to know how to use it in IMC eventually.    

I'm finding very little written on how to actually use carb temp gauges, so my question to the carb'd Mooney community is: how do you use your digital carb temp gauge, and why?   

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Important things to know...

1) water always freezes at 32°F...

2) the temperature ranges usually given are for OATs for people with unknown carb temps.

3) the procedure to always use full carb heat is also for not having Carb Temp info.

4) the reason for the large range to begin with is the adiabatic cooling that occurs with the drop in pressure air is subjected to while passing past the carb's butterfly valve when it is partially closed.  Low MP generates a lot of extra adiabatic cooling

5) warm moist air entering the carb can turn to ice.  With a carb temp gauge you should be able to know if the carb air is above freezing or not.  If it is close add some heat to maintain a warmer temp.

6) in the winter there is so little moisture in the air.  Adiabatic cooling won't cause so little moisture to cause a challenge when it gets cooled more adiabaticly.  This moisture goes through the engine and doesn't stop...

7) consider comparing the range of temps where ice occurs on the wing.  Similar low pressure adiabatic cooling occurs there as well.

See if Hank has more to add.  He has a C with a Carb Temp indicator.

Best regards,

-a-

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I don't have much personal experience using a carb temp gauge, but there is some info here:. https://www.faasafety.gov/gslac/alc/libview_chapter.aspx?id=10520&chapter=En+Route.  It will depend on humidity and visible moisture.  Looks like you want to keep the carb temp above 32 if in visible moisture or above 50% humidity, unless the carb temp is below 15.  I would tack on a little margin for error, typically 2 degrees.  Obviously the guys using a temp gauge can chime in for better info.  I do have some experience using an ice detector (Ice Man as I recall) on a 182 we installed it in.  It had an optical sensor in the Venturi.  There was a knob that you fine tuned.  Basically, turn the knob until the ice light turned on, then back it off until the light just turned off.  Then, if the light came on, it meant the optical detector was getting frosty...add heat.  Seemed to work well when new, I don't know about the long term reliability.  It was a more direct indicator of carb ice.

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My Carb Temp Gage isn't digital, and it has an orange stripe to indicate the possible icing zone. The cooling comes as air moves through the venturi; as it expands, velocity and temperature both decrease. If it's moist out, you can have ice. Our Mooneys, though, have the carb right beside the oil sump, greatly reducing the risk if icing. Talk to someone who flies a 150/152 with a Continental engine, they are ice machines!

I have experimented using partial carb heat in cruise, trying to reach LOP. It never worked for me. Then last year, we did additional doghouse repairs and rebuilt the carb heat box (which reached the point that it stayed wide open, killing power and climb!) and now I'm not only 10-15 mph faster but I can go almost 25°LOP without carb heat or roughness, just the typical (significant) loss of speed. No fuel flow, though, so I'm unsure of the benefits.

I always hit Pitot Heat before entering clouds, and if the Carb Heat is in the orange zone, I'll pull enough to move it out. Took my IR ride in April '10, and may have done this three (3) times. Because I descend power on, I don't use carb heat then, I just push the yoke for 500 fpm and let the speed build to make up for the slow climb; it generally stabilizes around 170 mph. As I come down, I periodically back the throttle off and enrichen the mixture to maintain whatever my cruise setting were for MP and EGT. 

Perhaps I'm lazy, or was transitioned poorly, but I also almost never use carb heat in the pattern, either. I do like to be down to at least flap speed before entering downwind, and prefer to fly at 90 mph and roll wings level on final at 85 mph. Gear goes down abeam the point of intended landing. Then I slow to 75 - 3 mph per 300 lbs below gross at that point and set her down. I do religiously check carb heat operation at engine start, though.

This may not help you with your digital Carb Heat gage, though. It's just another benefit of analog . . . See it just left of the yoke? It's the only thing I have not in English units. My OAT reads in °F, this crazy thing is in °C. I'm not running carb heat in this photo, but if I was in a cloud, especially near freezing, I'd pull enough to get the temps up to about here (once that I recall, over GSP at Thanksgiving, OAT had fallen from 52° to 32° in about fifteen minutes, and I was grazing the bottom of the overcast at night, headed north into the NC mountains).

You'll figure it out over time.

image.jpg

Edited by Hank
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Just now, BDPetersen said:

The drill in the DC-3, Wright engines, was to set 40C when using carb heat. I know that sounds like a different world, but I find a lot of the temps on the round engines correlate to the stuff we fly now. Get a good temp margin above freezing.

Do you mean 40F?  Full on carb heat doesn't achieve 40C for me.

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If we convert Hank's anaolgue instrument it seems that the yellow is from 14F to 50F.  The high side seems conservative and seems to be more appropriate to OAT vs carb temp.  BD must have been talking F since 40F seems to be a safe temp.  It seems that a safe zone might be 35 to 40 F when relative humidity is above 50%.  It is interesting that your guage reads 20s to 30s in cruise, but I think you have to consider humidity.  So, it would seem like 20s to 30s is ok if relative humidity is less than 50%. Add heat when in clouds or humidity above 50%. As you are probably aware, a key indicator is loss of power, so an occasional glance at MAP is a primary indicator of ice.  If MAP dropped and the throttle hasn't moved, consider ice a possibility.  

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Dude, my Carb Temp gage maxes out at 50°C. There's a huge amount of energy required to reach 40°C = 104°F. Besides, in the DC-3, was the carb attached to the oil sump? As long as I don't get any ice, I'm happy, and I'm pretty sure at 5-10°C, I won't. As far as I can tell, I've never had any carb ice--no loss of power, no loss of MP.

i don't even reach 40°C running WOT for takeoff on sunny summer days.

Edited by Hank
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32 minutes ago, takair said:

If we convert Hank's anaolgue instrument it seems that the yellow is from 14F to 50F.  The high side seems conservative and seems to be more appropriate to OAT vs carb temp.  BD must have been talking F since 40F seems to be a safe temp.  It seems that a safe zone might be 35 to 40 F when relative humidity is above 50%.  It is interesting that your guage reads 20s to 30s in cruise, but I think you have to consider humidity.  So, it would seem like 20s to 30s is ok if relative humidity is less than 50%. Add heat when in clouds or humidity above 50%. As you are probably aware, a key indicator is loss of power, so an occasional glance at MAP is a primary indicator of ice.  If MAP dropped and the throttle hasn't moved, consider ice a possibility.  

Yeah I wondered about the range on Hank's gauge too.   50F seems like a very wide margin.   Here's another PMA'd analog gauge where the yellow arc runs from 5-41F. http://www.aircraftspruce.com/catalog/inpages/carbtempkit.php  I don't understand how the numbers get picked, and without a clear temp guideline, it's hard to derive benefit from having the gauge.      

 I do understand why the carb temp may need a margin above freezing to preclude carb ice- the air at the position of the temp probe may be warmer than the temp on the metal of the venturi nozzle, where  freezing is the biggest problem.   But there seems little agreement on what a safe temp is.  And although our engine setup with the carb under the oil sump supposedly limits such risk, the actual temp readings in my carb with the heat off in cruise seem to suggest otherwise.   It would be nice to have a defined carb temp that you could use to simply  "set and forget" partial carb heat in cruise, irrespective of OAT and humidity.   A fixed target temp might also provide predictable fuel vaporization and thus add speed and consistency to leaning procedure.   

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I think that is why the carb ice detectors are nice.  Ice in cruise is rare, but that is also why it takes folks by surprise.  With an ice detector, it becomes a simple decision to add heat.  Red light on, add heat.  I don't think they are very expensive, but proper install is critical.  With carb temp, you are still ahead of the majority of carb aircraft operators.  Some aircraft are more prone to carb ice than others.  I have not heard of many Mooney incidents related to carb ice, which may indicate it is more tolerant than others.  You might do a accident search to see if this is valid.

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23 minutes ago, DXB said:

Yeah I wondered about the range on Hank's gauge too.   50F seems like a very wide margin.   Here's another PMA'd analog gauge where the yellow arc runs from 5-41F. http://www.aircraftspruce.com/catalog/inpages/carbtempkit.php  I don't understand how the numbers get picked, and without a clear temp guideline, it's hard to derive benefit from having the gauge.      

 I do understand why the carb temp may need a margin above freezing to preclude carb ice- the air at the position of the temp probe may be warmer than the temp on the metal of the venturi nozzle, where  freezing is the biggest problem.   But there seems little agreement on what a safe temp is.  And although our engine setup with the carb under the oil sump supposedly limits such risk, the actual temp readings in my carb with the heat off in cruise seem to suggest otherwise.   It would be nice to have a defined carb temp that you could use to simply  "set and forget" partial carb heat in cruise, irrespective of OAT and humidity.   A fixed target temp might also provide predictable fuel vaporization and thus add speed and consistency to leaning procedure.   

That would be a tricky bit of programming. As you fly along on an XC, weather changes, temps go up,and down, humidity changes especially flying to the coast. When you set partial carb heat, it's just so many amps through the circuit, you're not setting a temperature to hold. If you pull carb heat to 10°C once, and leave it there, who's to say that two hours later it may not be 25°C if flying to Florida, or 0°C if flying to Maine? I've seen OAT change more than that in fifteen minutes, and pulled some carb heat because of it.

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The intent was to offer one technique used to manage carb temp in icing conditions. It seems it may not be that applicable to Mooney/Lycoming world. If we are confident we are not threatened by ice, then a temp gauge is a waste of time. If, on the other hand, ice is a concern and merits a carb temp indicator, then go for the heat and a temp that will provide a margin. In the -3 we could set 40C.

I agree that ice is rare in a Lycoming.

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14 hours ago, Hank said:

That would be a tricky bit of programming. As you fly along on an XC, weather changes, temps go up,and down, humidity changes especially flying to the coast. When you set partial carb heat, it's just so many amps through the circuit, you're not setting a temperature to hold. If you pull carb heat to 10°C once, and leave it there, who's to say that two hours later it may not be 25°C if flying to Florida, or 0°C if flying to Maine? I've seen OAT change more than that in fifteen minutes, and pulled some carb heat because of it.

I agree completely Hank- one would have to periodically adjust the carb heat plunger  to maintain a given carb temp, even in cruise flight at stable power settings.  I guess what I'm after is an informed basis for a temp to maintain that eliminates the need for subtle considerations regarding OAT / humidity, providing an adequate margin under all conditions under cruise power, without making it run ridiculously rich.  Doing so might make leaning more predictable as a side benefit.   

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I pulled my old AOPA Air Safety Foundation Safety Review and saw only one mention of carb ice related accident.  Unfortunately my copy is almost 20 years old, but that was still 30 years of data.  As a side note, this was a great resource when I first bought the plane.  It helped me learn from the mistakes of others....I still do....and I still make my own mistakes.  Don't know if AOPA publishes that anymore.  An updated version would be nice.

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I've only been flying since '07, but the Skyhawk I got my license in was destroyed a few years later, just a couple hundred hours after OH and new interior, due to the licensed pilot's improper response to carb ice. He pulled throttle and tried to make a downwind landing just a couple of minutes after takeoff, was too fast for the 3000' field, and tried a late go around. Stalled and went down in the trees. He and his wife in the right seat survived with only minor injuries.

If you do get carb ice (unexplained loss of power or MP), pull Carb Heat all the way out and leave it there. If ice is melting, it will get rougher before it gets better. Any time you add (or remove) carb heat, you will need to relean the engine as the addition of warm air will make it richer (or lowering the air temperature going into the carb will lean it out). Once it clears up, reduce heat using the Carb Temp gage to a value you're comfortable with (higher than before if already using it). Me, I'll keep it above the orange stripe, I think 10-15°C should be plenty. But then again, I've never had carb ice. No Carb Temp gage? I dunno, ask a CFI, I bought my Mooney five weeks after my PPL checkride, I don't remember what to do without one.

Correction to a previous post:  don't know what's in the DC-3, but our carb heat just sucks in warm air from inside the cowl instead of outside air. There is no "heater" per se like is in the pitot. 

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1 hour ago, Hank said:

Rob, could you find that in the Nall Report? It comes out every year, should be on AOPAs site somewhere, maybe the ASI now run by fellow Mooney owner George Perry.

Do you mean the accident reports?  Probably.  What was great about the Safety Reveiw was the analysis and commentary.  It was very helpful when buying the plane and especially at breaking down some of the potential gotchas.

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I posted this on POA, and got a response already. An authoritative one, no less!

*************************

Re: Accident Data
Quote:
Originally Posted by Hank S  
An ongoing discussion about carb heat and proper usage brought up a question. One poster mentioned a 20-year-old copy of an AOPA Air Safety Foundation Safety Report that showed but one accident attributed to carb ice in the thirty year period reviewed.

Is more recent data available, without having to dig through the Nall Report looking for it? Any analysis available on carb ice accidents? I know of one, the plane I trained in about three years after I finished up. Wondered how many others there are, frequency, severity, etc.
I don't have any overall GA numbers, but I have analyzed some aircraft types for accident causes.

From January 1998 through December 2010, I show about 1600 Cessna 172 accidents. Twenty-four were either directly attributed to carb ice, or occurred in conditions conducive to icing with no other engine failure cause found.

From January 2000 to December 2010, there were about 600 accidents involving the PA-28-140, -161-, -180, and -181 models. Twelve involved carburetor icing.

From January 1998 to December 2014, there were about 3500 accidents involving Experimental Amateur-Built aircraft. 68 involved carburetor icing.

You can download the complete NTSB aircraft accident database at:

http://app.ntsb.gov/avdata/Access/avall.zip

Be advised it's about 250 MB.

Ron Wanttaja

*******************************

So for brand C and P, it appears to be 1-1/2 to 2% of accidents are carb-ice-related. For us, that would be limited to A, B, C, D and G models, maybe 4000 in the fleet? Don't know the overall accident rate, but 1-2% of the total accidents would be pretty small number for us.

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In 10 years of M20C flying I experienced carb ice once.  In the spring with light rain with clouds 500' overhead. RH was 100%.

MP dropping out slowly over time was pretty noticeable.  Power was restored by adding full carb heat.  Over time the MP would drop out again.

As a Mooney pilot, I would prefer to not use full carb heat for only one reason.  The intake is small and MP is less.  With the carb temp gauge it is much less of a guess what is going on and more of a read the gauge deal. Use of Partial carb heat without a gauge is dangerous because the resulting icing can overcome the carb with no way to Resolve the blockage.

'carb heat' is a bit of a mis-nomer. Warmed Alternate air source is more in line with what it does.  As Hank points out the air source is warmed by the cylinder cooling fins near (400°F) (fins not air temp). Not like heat coming from the heater muff wrapped around the 1000°F exhaust pipe.

Hope that helps shed some light,

-a-

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Here's the link to Don's site:  http://www.donmaxwell.com/publications/MAPA_TEXT/Carb%20Heat%20Maintenance/CARB_HEAT_CARE.HTM. Make sure yours is in good shape; much of mine was rebuilt last year. 

It's easy to test. Sometime in cruise, look at the Carb Temp gage and pull some carb heat. Watch the temp rise. Pull some more, the temp should rise more. Pull it all the way out carb temp should rise more. Push it back in and carb temp should fall back to what it was before. Note, though, that as you add carb heat, the mixture will get richer, and as you push it back in the mixture will get leaner, simply due to the temperature of the air going into the carb.

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8 hours ago, carusoam said:

As a Mooney pilot, I would prefer to not use full carb heat for only one reason.  The intake is small and MP is less.  

This is a great point that I hadn't considered until now.  When pulling on full carb heat in cruise , I'd assumed that the power drop was just from an overly rich mixture.  But that little intake at the back top right of the doghouse is a fraction of the size of the regular intake, which gets plugged by the flapper valve with full on carb heat. And it reaches the carb through a circuitous scat hose, adding further resistance.  I wonder regarding the power setting at which this flow restriction first comes into play- e.g. does it contribute to the rpm drop during runup?  

Regardless, I notice little if any power drop when cracking the carb heat open enough to raise carb temp from say 25F to 41F- so I assume flow restriction is not a big factor there- any partial obstruction of the regular intake may be offset by the added flow through the carb heat intake.  EGT drops a tad, and I lean a bit more to compensate, and then it seems to run identically.  

I wouldn't have the guts to crack open the carb heat slightly without the gauge for the reasons you cite - so the gauge does seem like a great benefit.  It's interesting that I read Mike Busch recommending doing this to help mixture distribution without mention of this concern of actually increasing carb ice.

Anyway, I love my gauge more after this discussion, and I think I'm going to use low 40s F as my minimum desired carb temp in cruise unless anyone sees a reason to do otherwise.

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18 hours ago, carusoam said:

Important things to know...

1) water always freezes at 32°F...

Water doesn't always freeze at 32F, it is relative to pressure as well.  This is one of the reasons why pressure carb's are fairly resistant to carb icing.

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2 hours ago, BDPetersen said:

I suggest referring to Don Maxwell's website for a discussion of the carb heat system on the B, C, & G as it applies to the source of the heat.

I wonder if it changed for later models.  1967 and before definitely had heat from the muffler shroud ducted to the carburetor air box.

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