Carb icing

[VictorGolf]

Chugging along in my aircraft this morning I noticed a slight roughness in the donk. As Stansted ATIS were giving a temp of 11 deg and a dew point of 9 deg a burst of hot air seemed to be called for and it did the trick. I then gave it a go for about 20 seconds every 15 minutes or so and the problem didn't re-occur. However the point of this post is to enquire if there are known engine/airframe combinations that are more prone to carb ice than others? I believe the small Continentals can be a handful , I fly behind a Lycoming 0-235-C2A but as the only other application is in a Brazilian crop-sprayer I don't suppose there is too much knowledge in the UK on the type.

[Crash one]

I have heard that the small Continentals are regarded as ice making machines, I'm a bit obsessive about it so I don't give my C90 much of a chance. Taxi on wet grass carb hot, climb out throttle back heat on for 2+min, every 10min after that.

[SkyHawk-N]

It's all due to the position of the carburetor and how much heat they absorb from the engine. On Lycomings carburetors are mounted on the oil pan and as a result run warmer and do not suffer too badly from carb icing. On the other hand Continental mounted their carburetors away from engine heat and suffer more from icing.

[12Watt Tim]

A classic case is the Cherokee series. The original 140s are susceptible, but the Warriors (160 hp) and Archers (180 hp) don't seem to get it airborne (anything not injected will on wet ground soon after start) at all. Don't know about any other carbed PA-28 engines.

[Big Pistons Forever]

The propensity for carb ice is determined by the installation on that particular airframe. The idea that Continentals are more prone to carb ice than Lycomings is an urban myth IMO. My experience in flying over 40 types of light carburated aircraft, is the two worst for carb ice were the BN2 Islander and the PA 23 Apache, both with Lycoming engines.

[Pilot DAR]

I do not support the belief that small Continentals are any more suceptable to carb ice than any other engine type. I can say that the O-200 installation in the Cessna 150 is a poor example of an effective carb heat system. Perhaps a less than ideal de icing system of the airframe design causes blame to the engine itself in that airframe.

My personal technique (borne of 2500 hours in my C 150 over 23 years) is to use the carb air temperature indicator (a vital safety instrument, in my opinion) to indicate how much carb heat to use to keep the carb air temperature out of the danger range. Other than that use, I do not touch the carb heat control at all during flight.

I do occasionally use the carb heat while taxiing, if conditions seem to promote carb ice, and otherwise only select it to check it's operation, pre flight.

I once had an engine failure upon selecting carb heat in flight. Nothing to do with ice. The change in ducted airflow allowed a mouse nest in an induction hose to be sucked right into the carb throat, and choked it completely.

Two additonal thoughts on carb heat use:

If you have chosen to use carb heat, consider doing the following right away: Reduce the throttle setting somewhat, to reduce the total mass airflow into the engine. This reduces the amount of heat which will be needed to raise the temperature of the air in the carb, and the metal carb parts themselves. It also reduces the total amount of moisture being sucked in. (this is a good reason for having an idea of the minimum power setting your aircraft needs to maintain altitude). Once set at this power setting, lean the engine as much as practical. Bear in mind, that for most engines, the power setting will now be in the range where leaning to peak is acceptable for continued operation. The hotter the exhaust, the hotter the carb heat air. The less volume of intake air, the hotter a given amount of carb heat will make that intake air.

If you're using carb heat, turn off cabin heat. Other than for aircraft with combustion heaters, it all comes form the same place, you may as well get as much as you can where you need it most...

Pilot DAR

[SNS3Guppy]

I fly behind a Lycoming 0-235-C2A but as the only other application is in a Brazilian crop-sprayer I don't suppose there is too much knowledge in the UK on the type.

What ag aircraft uses an 0-235 for a powerplant?

[VictorGolf]

Hi Guppy. I don't have my books to hand but I think it was an Aero Boreo(?), a sort of Super Cub lookalike. My thanks to the other posters for their useful comments on carb ice.

[robin]

Two additonal thoughts on carb heat use:

If you have chosen to use carb heat, consider doing the following right away: Reduce the throttle setting somewhat, to reduce the total mass airflow into the engine. This reduces the amount of heat which will be needed to raise the temperature of the air in the carb, and the metal carb parts themselves. It also reduces the total amount of moisture being sucked in. (this is a good reason for having an idea of the minimum power setting your aircraft needs to maintain altitude). Once set at this power setting, lean the engine as much as practical. Bear in mind, that for most engines, the power setting will now be in the range where leaning to peak is acceptable for continued operation. The hotter the exhaust, the hotter the carb heat air. The less volume of intake air, the hotter a given amount of carb heat will make that intake air.

If you're using carb heat, turn off cabin heat. Other than for aircraft with combustion heaters, it all comes form the same place, you may as well get as much as you can where you need it most...



Having flown behind VW and Continentals I am manic about testing for carb ice, having the donkey stop or perform badly a number of times through icing
I take a very simple view of prevention.

I've seen the ice form on the carb housing and supports on a hot sunny day!

I overuse the carb heat control if anything and leave out it for longer. It astonishes me to see other pilots use it only on take-off and landing and then only for a max of about 5 seconds.

I am on the alert for signs of icing and will taxi on hot, only returning to cold when lining up. In the air I will add power before using the control to warm the engine before hand and I'll usually fly at a higher power setting than normal so as not to risk icing at the butterfly valve.

The real problem comes in the descent - in icing conditions you never just chop the throttle but keep power on for as long as possible - in the 3 cases where I sat looking at a plank of wood whilst still in the air, all three were at short finals.

Its also worth noting that because of those experiences, most of my approaches are higher and closer in than other pilots so I have a fighting chance of gliding in.

It horrifies me to see the low dragged approaches some pilots use..

In short though, everyone should get to fly an 'ice-maker'. It will change your views about carb icing :ok:

[BackPacker]

will taxi on hot,

Although I agree with the rest of your post, this seems to be an exceedingly bad idea. In all of the aircraft I have flown, setting carb heat to hot also bypasses the air filter. This means that debris thrown up by the propellor, such as sand and gravel, can enter the engine without being filtered out. This can lead to all sorts of damage to valves, pistons, cylinder walls and so forth. Not good.

I realize that there are situations where taxiing without carb heat will lead to almost instant carb ice, and in that case you have no choice but to apply carb heat, or abandon the flight altogether. But to taxi with carb heat on by default, even when not required, sounds like an invitation to very costly damage.

[chevvron]

A few years back, I departed Glasgow Airport for the journey south over Beattock with the cloud just above the summit. OAT on departure was +4 deg.
As I climbed, I realised the engine wasn't revving as high as it should, and thought I could detect, or possibly imagine, a slight misfire.
Fortunately I was driving my car, so I was able to pull into the first layby I came to, and as I pulled up, there was definite black smoke from the exhaust accompanied by a misfire. Classic carb icing symptoms of course; I was using a lot of throttle on the climb and was about 1000ft amsl, so the inlet throat pressure would have been very low causing any moisture to condense and freeze.

[worrab]

Though carb icing is worse at higher temperatures and lower powers...


http://www.rnk.co.uk/himan/images/HiResOverheads/carb%20Icing.gif

[gasax]

My own experience of carb icing when taxiing always occured on grass and so issues about debirs ingestion are not really important. Damp grass in the morning would stop my C90 in less than 3 minutes.... Without carb heat the Gipsy often would not run at all under similar conditions

Apart from tropical conditions I cann't think of many places (short of a dry beach) where dust or grit would threaten an engine that much.

However Gipsy Majors are recommended to use hot air in dusty conditions (the air routing tends to throw dust out of the airstream into the carb) - but they are a bit of a rarity as GA engines go these days!

One of my friends has a proper carb ice detection (its electrical but visually detects the build-up of ice in the carb). The engine is an O-320 and it rarely causes concern from icing but the gadget does give a much better early warning than anything else I've seen. Its called a LAMAR Ice-MAn - I think!

[robin]

Quote:
will taxi on hot,

Although I agree with the rest of your post, this seems to be an exceedingly bad idea. In all of the aircraft I have flown, setting carb heat to hot also bypasses the air filter. This means that debris thrown up by the propellor, such as sand and gravel, can enter the engine without being filtered out. This can lead to all sorts of damage to valves, pistons, cylinder walls and so forth. Not good.

I realize that there are situations where taxiing without carb heat will lead to almost instant carb ice, and in that case you have no choice but to apply carb heat, or abandon the flight altogether. But to taxi with carb heat on by default, even when not required, sounds like an invitation to very costly damage.

Perhaps I didn't make it clear - I do that whenever I feel that conditions warrant it. On ordinary days when the risk is less then I don't.

One incident happened on a summer's day at lunchtime when the group temperature was in the high 70s. After taxying to the hold, I was kept waiting for over 5 minutes using carb heat for a lot of the time and increasing power to keep the engine warm. Even then, as I started the take-off, I lost power in the initial climb so put her back on the ground in short order.

[vee-tail-1]

The Limbach engine is an ice maker. There are almost no weather conditions in UK when it doesn't ice up. Taxy can be on cold but extensive running at 2000 rpm is needed during pre take-off checks to clear the ice. During cruise hot air is needed every 10 mins, and it is routine to pull the hot air control every 5 mins and watch the rpm. No drop or a rise in rpm is a sure indication of icing. I once assumed that cold air was OK with an OAT of minus 1 and flt level of 7,500 ft over the sea between Cornwall and Wales. The sudden onset of coughing and banging plus the possibility of a ditching sure changed my mind. :eek:

[Maoraigh1]

I frequently have carb icing on an O200 in a Jodel. Usually I need carb heat immediately after startup. It's worse in the summer, in north Scotland, as predicted by the carb ice diagram. I use mixture lean along with carb heat in the air. It can sometimes be difficult to decide if roughness is due to rich mixture or carb ice. I don't see how cabin heat, off the right exhaust, can affect carb heat off the left exhaust. I was taught carb heat every 10 minutes. I've seldom had carb icing with the C152 or Pa28 161.
PS. DAR should have done the carb mice check before take off - most pilots think this is for carb ice only, but if carb miceing is present, the engine will stop on the ground.

[Piper.Classique]

if carb miceing is present, the engine will stop on the ground.
Would a CAT gauge help here?

[Pilot DAR]

PS. DAR should have done the carb mice check before take off - most pilots think this is for carb ice only, but if carb miceing is present, the engine will stop on the ground.

Excellent!

My two cats have been firmly instructed that they are to earn their keep at the hangar, as well as the house!

Interestingly, I had checked carb heat operation prior to takeoff, as I always do, and did not detect any abnormallity on the ground. I would agree that a full power runup carb heat check probably would have dislodged the mouse nest at a much more opportune time. I'm always more pleased when my forced approaches are practiced, not actual!

The induction hose for the carb airbox inlet now has a criss cross lockwire "screen" to prevent a reoccurance of this startling situation.

Pilot DAR

[A and C]

I have to go with Big pistons on this, and think that the rate of ice build up has more to do with the air inlet fitted rather than the engine, so don't assume that the Lycoming is less likely to pick up ice than any other engine just because the carb is bolted to the sump.

The Lycoming O-360 fitted to the PA28 is much LESS likely to pick up ice than that when the same engine is fitted to the Robin DR400 ( I am talking engine / carb fit). The only thing that is not the same is the intake ducting so this must have an influence on the icing rate.

[Captain Stable]

One thing that I have not seen (somebody may have said it, if so I apologise) is that, in the air, select carb heat ON before reducing power - a good 10-15 seconds before doing so. Failure to do so (if you have ice forming in the carb and don't yet know it) will result in the ice jamming your throttle butterfly valve closed and, when the donk dies, you will have no means whatsoever of resurrecting it as the exhaust manifold will also have cooled by then.

I concur with those who say don't use carb heat while taxying. Use it on the ground while stationary by all means.

Check the graph in the books of temperature drop in the carb and the range of OAT's that will therefore most likely result in carb icing outside the green arc.

As for the rest of it, don't follow what Bob in the cafe told you he does. His aircraft is probably different from yours. Follow what your instructor told you and (hopefully) the POH also says.

[robin]

I concur with those who say don't use carb heat while taxying. Use it on the ground while stationary by all means.

Try that with an 'ice-maker' and you might see what we're getting at. Mine has cut-out taxying with reasonable power on damp mornings and evenings.

As for the rest of it, don't follow what Bob in the cafe told you he does. His aircraft is probably different from yours. Follow what your instructor told you and (hopefully) the POH also says.

Quite right. But I'd certainly listen to anyone who knew what they were talking about. Unfortunately instructors don't necessarily know the quirks of non-club aircraft.

I've flown instructors in mine just to demonstrate things they may not have come across in PA28s or C172s. Even the handbook may not address some of these areas adequately.

Other owners of your type might be a better bet.

[Captain Stable]

One other thing.

If you apply carb heat and get all the symptoms of ice melting, such as an initial drop in RPM and then a gradual increase, leave the carb heat on, for at least 30 seconds, and possibly much more - a couple of minutes or so.

You've just melted a load of ice in your carb and turned it to water. You don't want it freezing again.

Mention has been made of the temperature range.

Adiabatic cooling of the air in the carb (the cooling effect of coming through the venturi) is generally about 2-3 °C., but can be up to 5 °C. Most of the cooling effect is from the latent heat of evaporation of the fuel, and this is about 15 °C. Therefore, the total temperature drop in the carb will be about 20 °C.

At (for the UK) medium-to-high OATs such as 20 °C, anything above low humidity will reach saturation at 0 °C. Air with a relative humidity of 25% at 20 °C, or 50% at 10 °C, will reach saturation at 0 °C.

Check the graph worrab posted on page 1 of this thread.

At this time of year, carb icing is much less likely to be encountered. However, it is a good habit to behave as if it is, and always check for carb icing in FREDA checks, always check the carb heat in your power checks and downwind/prelanding checks, and always apply carb heat on final approach, cancelling as you cross the fence committing to land.

[Them thar hills]

Re the taxiing in hot air argument.
Having regularly flown numerous VW powered aircraft I can't think of one which had an air filter on the cold air supply. Most took the cold air from inside the cowlings, so it wasn't true cold air in every case. The hot air comes from a muff on the exhaust(s) which are generally outside the cowlings. So taxiing in hot air means you may pick up slightly more debris than by using cold air ! Applying hot air well before closing the throttle is good advice. In humid conditions it's a good idea to use slightly higher power settings to generate more heat in the exhaust system, also, icing is more likely to occur at small throttle openings.
I've never had a VW carb-ice problem, but other bits of "excitement", yes !