Carb Icing PA28

[moonburn]

I would welcome any comments / info on the following instructions ,

" On the PA28 carb heat should be checked on the downwind and then selected off for the approach as selection to hot for the approach can lead to icing on this aircraft"
I cannot identify the source but believe this instruction to be based on a misunderstanding, however if anyone knows different please enlighten me as to the source of this information, I am not inviting complex theoretical discussion, simply trying to establish if there exists an official document with the above advice contained therein.
And thanks, yes am awaiting reply from Piper.

[Capt Pit Bull]

Have a search through AICs and GASIL (do they still have those? I've been out of the GA scene a while now). There is some good material on carb icing in general.

Depending on where you are in humidity/temperature/throttle setting carb heat can make carb icing more likely rather than less so.

Your quote doesn't sound very comprehensive though, in its rationale to fly an approach with it OFF, there's at least one concrete reason (the need for immediate full power for a go around) rather than one somewhat situational reason.

Caveat: All the above should be viewed with great suspicion as I haven't flown a piston for over 10 years (and I was crap at it then).

pb

[Genghis the Engineer]

It is, I think, a misinterpretation of the official Piper POH.

I don't have a PA28 POH to hand, but am certain that it says something along the lines of "you don't normally need it on approach and finals, so don't use it unless carb ice is clearly occurring".

Hopefully somebody about here has a POH and can post the exact words?

I certainly don't recall ever seeing any official words suggesting that use of carb heat can cause icing. I suppose, theoretically, an aircraft could have a finite supply of heat to give a blast of carb-heat, and so if you use it too much, you never have enough reserve to make any difference. But, I'm not sure if any of my own experiences would bear that out.

G

[Jimmy Do Little]

I've been out of the GA world for some time, but will offer these thoughts.

At low power settings, the accumulation of ice in the intake (carburator) could go un-noticed until the application of full power, where that ice would rear its ugly head. Therefore, the use of carb heat could prevent such an occurance.

However, in certain atmospheric conditions (cold and moist air) the application of carb heat could LEAD to the formation of ice (increase air temp to cause otherwise frozen particals to melt and accumulate).

In my current world, we generally use "Carb Heat" (Engine Anti-Ice) when the TAT is 10 degree or less. In a GA type aircraft, your TAT is generally the same as your SAT, so that rule could prove somewhat valuable.



TAT = the tempurature of the air after consideration of aeardynamic heating.

SAT = the tempurature of the air when sitting still.

Finally, allow me to plagiarize our learned colleague

All the above should be viewed with great suspicion as I haven't flown a piston for over 10 years (and I was crap at it then)

.

[Mark1234]

I can't be bothered to look it up (sorry), but I recall reading words somewhere to the effect that if the OAT is very cold, there may be ice crystals in the air that will pass harmlessly through the engine (presumably melting in the process), UNLESS you should happen to use enough carb heat to warm the air into the icing range. Probably somewhat unusual though.

Round my parts we check downwind / base and return to cold for finals, purely 'cos under normal circumstance you get more oomph on a go-around with it cold.. UNLESS there's reason to think we are getting icing, in which case it stays hot. Carb icing can occur in relatively hot (+20C) temperatures given the right humidity etc.

[Jimmy Do Little]

I can't be bothered to look it up (sorry), but I recall reading words somewhere to the effect that if the OAT is very cold, there may be ice crystals in the air that will pass harmlessly through the engine (presumably melting in the process), UNLESS you should happen to use enough carb heat to warm the air into the icing range. Probably somewhat unusual though.

Absolutely correct. You can reference the US FAA website ( Tips On Winter Flying - En Route) for clarification, but in a nut-shell... (Remember, the yanks use degrees F)

Three categories of carburetor ice are:

• Impact ice formed by impact of moist air at temperatures between 15-32°F on airscoops, throttle plates, heat valves, etc. Usually forms when visible moisture such as rain, snow, sleet, or clouds are present. Most rapid accumulation can be anticipated at 25°F.
• Fuel ice forms at and downstream from the point that fuel is introduced when the moisture content of the air freezes as a result of the cooling caused by vaporization. It generally occurs between 40-80°F, but may occur at even higher temperatures. It can occur whenever the relative humidity is more than 50%.
• Throttle ice is formed at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the throttle valve. Since the temperature drop is usually around 5°F, the best temperatures for forming throttle ice would be 32-37°F although a combination of fuel and throttle ice could occur at higher ambient temperatures.

Cheers....

[bookworm]

but I recall reading words somewhere to the effect that if the OAT is very cold, there may be ice crystals in the air that will pass harmlessly through the engine (presumably melting in the process), UNLESS you should happen to use enough carb heat to warm the air into the icing range. Probably somewhat unusual though

Ice crystals in the air would be visible as cirrus cloud. That would indeed be "somewhat unusual" on the approach!

[Sciolistes]

On the PA28 carb heat should be checked on the downwind and then selected off for the approach as selection to hot for the approach can lead to icing on this aircraft

I believe the logic is that as the heat is supplied by the exhaust manifold, an idle engine would not only produce much less heat, but as stated above actually take the air temp into the icing zone. But I would have thought that one must have visible moisture as warmed air will be less humid than cold, regardless.

I certainly don't recall ever seeing any official words suggesting that use of carb heat can cause icing.

As above and to follow on from above, is that properly functioning Carb Heat will heat the air to circa 150 deg C, which should zap any ice. However, the warning was to not use partial carb heat for that very reason - unless you have a carb thermometer (I believe piston helos do). It is also the reason why a less than optimal carb heat system (often a corroded box) is a no go.

Ice crystals in the air would be visible as cirrus cloud. That would indeed be "somewhat unusual" on the approach!

Good point. I used to think the same thing, but I recently received instruction on the use of WX radar to the effect that they are less effective when aimed at altitude due to the CBs at that level being predominantly ice. What happened to supercooled water droplets?

[bookworm]

I used to think the same thing

Meaning that you now think that ice crystals are invisible?!

[Sciolistes]

Moonburn,

" On the PA28 carb heat should be checked on the downwind and then selected off for the approach as selection to hot for the approach can lead to icing on this aircraft" I cannot identify the source

I got curious and just checked, the source is the PA28 Pilots Operating Handbook. The guidance is followed by an explanation that the approach should be flown with it off in case of a go-around and engine detonation, not in case of increased risk of icing as previously postulated.

Can I add this http://www.caa.co.uk/docs/33/ga_srg_07webSSL14.pdf as another more authorative description of carb icing in general.

[camel toe]

"Ice crystals in the air would be visible as cirrus cloud. That would indeed be "somewhat unusual" on the approach!"

Not always, I have just returned from some winter flying in Canada and Ice Crystals were around right down to the deck.

Camel Toe

"Over Macho Grande?"

[belowradar]

I do have a POH for a PA28 and if you look at approach and landing you checks you fly the approach with carb heat cold i.e. not selected.

The carburretor is located in a hot area of the engine and not susceptible to icing

That's the aircraft manufacturers recommendations and CAA advise that only aircraft manufacturers checklists should be used.

[honda cbx]

Carb heat!!

I was taught to apply carb heat when doing downwind checks, then on finals, remove carb heat at approx 300ft in case of a go-around in which case full power would be required. Carb ice can form up to an outside temp of 80 degrees farenheit

[boofhead]

PA28 airplanes have the intake in a heated part of the cowl that precludes icing. Carb heat will not normally be required as a preventive measure in this type of airplane, and it will reduce power, or even, under cold conditions, increase the carb temp into a region where icing is more likely.
Cessna airplanes, using the same engine, have their air intakes more directly situated so that they get colder air and therefore need the precaution of heat when in possible carb icing conditions. If you have a carb temp gauge, use the heat to maintain the temp in the green.
Cessna recommends the use of carb heat right to the ground, not just on approach. In really cold conditions (it is minus 25 outside where I am right now) we use (Cessna) carb heat all the time on the ground, except for the start up, runup and shut down, and only push it to cold after setting full power on takeoff (follow the checklist exactly). The idea is to get a good blast of hot air into the engine, not just for carb icing but also to help the gas to vaporise. An overshoot will be started with hot air, for the same reason. In some airplanes (carburetted) not using heat will cause the engine to stop since the cold conditions make it impossible for the carburetor to hold the correct mixture. Without heat it would be impossibly rich and the engine will stall on application of power. I have had them stop running in cold conditions when power is reduced to idle on approach, even with full heat, so I always hold a little power, and that way I know the engine is still running.
Too much heat can be bad too. A DC3 crashed in Canberra many years ago because the crew put the heat controls in full hot and both engines detonated themselves to death. Followed by the airplane and crew. Operating the carb heat might need a corresponding adjustment of the mixture. The Beaver will die on approach if the mixture is not adjusted with application of heat, especially in really cold conditions.
There is always more to learn, no matter how simple the matter appears at first glance. For example, how do you know you have carb ice? Or that you don't?

[honda cbx]

boofhead

Hi mate, when you ask is it carb iceing or not ? In a pa28, first you should be aware of the weather conditions your flying in. Humid conditions are perfect for carb iceing! ie, plenty of moisture in the air. Next there would be a slight drop in revs accompanied by a rough running engine, now i hear you say, could be a lot of things! but as carb iceing is one of the most common reasons for that type of problem, application of carb heat would be my first reaction.And to leave it on for a good few minutes to dislodge any ice that may have accumulated. I would then move onto the next checks given time etc.

[j_davey]

I flew the pa28 in Florida for 5 weeks and I only ever used carb heat fit prolonged descents like simulated engine failures.... Never had an icing issue and my instructor told me it was not neccesary in that climate. This was April/may so temp was in the 80's ....

John.

[moonburn]

Sciolistes.

Thanks, I agree, I think that what we have here is a misunderstanding of the advice to avoid attempting a go-around with heat selected and I suspect that the POH checklist says select cold on approach following the same ( probably legally generated) reasoning.
The location of the intake is largely academic since the aim of every good induction system is to deliver large quantities of cool ( hence dense ) air to the engine and as evidenced by Piper's decision to fit a hot air selection on the aircraft in the first place. In fact one could argue that the location of the intake, quote " in a warmer part of the cowling" would constantly expose one to the danger of heating air into the icing range and could lead to detonation in very hot conditions. Perhaps Piper should have fitted a carb cooling lever ?
Most of the well meant advice recieved on this subject would appear to be apocryphal and in the absence of any official / documented evidence to the contrary I think I shall continue to be a tad suspicious of this item in the checklist , Never assume, Check ! it's kept me alive for a long time so far.

Pause for thought: Would those who follow the approach checklist to the letter advise their students not to select carb heat when closing the throttle
at 2000 - 3000 ft for a PFL ?

[Genghis the Engineer]

Absence of any official data?


Isn't the manufacturer's POH, duly authorised by FAA and CAA at least a teensie bit official?

G

[SNS3Guppy]

In my current world, we generally use "Carb Heat" (Engine Anti-Ice) when the TAT is 10 degree or less. In a GA type aircraft, your TAT is generally the same as your SAT, so that rule could prove somewhat valuable.

Whereas carburetor ice is a function of the temperature drop through the venturi in the carburetor, the use of ambient temp or even TAT is largely irrelevant. Carburetor ice forms well above 10 deg C TAT or SAT (or OAT, per most light airplanes). As high as 90-100 deg F, or about 40 deg C.

Carb heat use is probably one of the single most misunderstood concepts in piston aviation.

The guidelines offered by the manufacturer for the use of carb heat are just that...guidelines. One should always bear in mind when reading the airframe manufacturers information regarding powerplant operation that one isn't reading the powerplant manufacturers recommendations. Those can often be found elsewhere. This is particularly the case with respect to light aircraft and horizontally opposed piston powerplants.

Carburetor heat is there to be used when it's required, period. There's no one size fits all approach to the use of carb heat. Most light airplanes have no carb air temperature gauge, which is the only temp indication that is worthy of consideration when applying carburetor heat. Carb heat use, then, in a light airplane becomes more a matter of guesswork than anything. It's also either full on or full off.

Yes, carburetor heat can cause ice to form. Yes, carburetor heat can lead to detonation in certain powerplants under certain conditions. The power setting and mixture setting determines how carb heat plays into the equation; operations at high power settings close to peak mixture are candidate conditions for detonation. One should realize that carb heat also enrichens the mixture and provides a detonation margin under most conditions, despite an increase in induction air temperature doing the opposite. In a normally aspirated powerplant at 75-80% power settings...anything much above a few thousand feet of altitude...causing detonation is difficult if not impossible regardless of mixture position or carb heat use.

Should carb heat always be on or off for the approach, for a descent, or for power off operations? That really depends on the conditions which exist at the time.

[Centaurus]

The guidance is followed by an explanation that the approach should be flown with it off in case of a go-around and engine detonation, not in case of increased risk of icing as previously postulated.

Just be careful that the reverse policy applies if landing a Cessna single (150 and 172). The POH requires full carb heat on until after landing when it is selected in to Cold before taxiing. Same with go-around procedure in that it includes first full throttle and after that carb heat in to cold. The short time the throttle is full open with carb heat hot will presumably not cause detonation but will clear any ice that may have gradually formed on final.