Commonsense attitude to Carburettor Icing
 

Extract from BASI Journal No.7 Autumn 1990.
Carburettor Icing:

Look again at the first little fact: `Ice is COLD and needs HEAT to melt it`.
So why one day that is conducive to icing, do we reduce power then pull carb heat on? Stupid really - we pull the power which reduces the heat available! The FAA in an Advisory Circular said: `Heat should be applied for a short time to warm the induction system BEFORE commencing descent.` Ever thought of putting the carb heat on as part of the base checks? Then you will be applying heat BEFORE power reduction.

And our second little fact: `somewhere you have a CLUE to tell you ice will form or has formed`.
.Did you get a definite RPM drop on the engine run?
.Did you check the carburettor icing probability chart to see if today is a good day for carb ice?
.Did you monitor your RPM, manifold and temperature gauges and determine what the indications could mean?
.Did you clear the carb of ice BEFORE you reduced power?
.Do you apply power FIRST in a go-around - then put carb heat to OFF?
............................................................ ......................................

The above recommendations were published in 1990. 27 years on they are still current.:ok:

And who would've thought that in the high tech world of aviation we'd still be fiddling with carburettor heat deep into the 21st century.

The last time I remember having to apply carby heat in a motor car was in the grand fathers '51 split windowed VW beetle. Or perhaps it was the great grandfather's Modet T!

That Journal article was really active-voicey and used words like "stupid", so I was too confronted and offended to work out what it meant.

(I still carry that carbie icing probability chart, even though I usually fly injected engines.)

Do you really think it makes that much difference? Carby heat then reduce power or reduce power then carby heat? All of 5 seconds difference.

My O360 has a carby temperature gague and when I reduce power in the circuit, the carby temperature increases without carby heat, usually to well within the green arc. Ice in cruise is not uncommon, as noted by a drop in MP. Ice on approach, never seen it in the last 8 years of ownership.

Early on whilst learning to fly there were two separate accidents near my home airfield which were both due to the engine stopping and insufficient height to make it to the strip.

Both pilots elected to put the aircraft down in paddocks. In both instances the aircraft were destroyed but crew and pax walked away pretty much uninjured - very lucky!

The engines in both aircraft were tested after the accidents and worked fine. Seeing these happen reinforced to me the importance of using carb heat. We were always taught (Cessna aircraft) left to right application, ie. carb heat on when turning base, before throttle pulled back. Or carb heat on when TOD, before pulling throttle back. :ok:

AAhhh, the use of 'that' carby heat knob....

Many moons ago in the NW of WA, a fairly new CPL fella was doing some mustering in a C-150. He had been taught to pull the carby heat knob whenever operating at low level.

So he did this for the week or two of the exercise, and later when the 100hourly was carried out, it was found that the 'new' engine had to be scrapped, due dust abrasion / scouring in all 4 cylinders.

It had never been pointed out to him, that the use of the carby heat, delivers 'hot unfiltered' air, directly into the carby....complete with all present dust particles....and these dust / dirt particles at his 'low level' had 'done the deed' to the donk.

Just sayin' is all.....

Cheers:ok:

The C65 in the Luscombe was particularly prone to ice and the whole circuit often had to be flown with carby heat on.

Kaz

That's because the problem was been engineered-out by having bimetal-strip-actuated selectors in the intake trunking that select warm air from above the exhaust manifold when the carb air temp drops too far, plus water-heated manifolds. Most aero engines are air-coooled these days, so the water-heated manifold isn't an option. Automated hot-air selection would become a safety-critical item needing intesive effort to design, certify and maintain - a manual "carb heat" selector is probably a far more cost-effective option.

25 years ago I built a Westfield 7SE kit car with a modified Ford Crossflow engine (roughly 160bhp, breathing through a pair of Webber 40DCOEs). To fit this lump in the car the carb intakes and air filters were largely outside the bonnet. When I first took this car onto a motorway I found that it started misfiring with reduced power after 5 miles or so of "cruise" running at 85-90mph^h^h^h^h^h^h^h^h 70mph ("cruise" power for a performance car would be less than 20%, so the throttles would be nearly shut), and if I continued the car would come to a standstill on the hard shoulder about 3 miles later. Looking under he bonnet I could never find anything wrong, and after 2-3 minutes the car would restart and drive normally.

This continued for several months and it had me metaphorically tearing my hair out until one day when it stopped for some reason I popped the cover off the front air filter to see if it was somehow clogged. Looking down the choke of no1 cylinder I saw a huge ice accretion almost completely blocking the choke. There was a similar one in the no.2 choke, and so I popped the other filter cover off and sure enough there were smaller ice blocks there as well. They all just melted away as I watched.

The webber manifold had no provision for heating, and the carbs were thermally isolated from the manifold anyway, so I ended up making a beaten-aluminium cover which extended the bonnet over the carbs and forced them to breath warm air from behind the radiator. After that the problem never recurred.

PDR

A value for money instructor there!

Why would they have scrapped the engine when a set of replacement barrels, pistons and rings would be far less expensive than a whole new engine?

Just sayin'...

PDR

Since when did the Carb Heat Air bypass the carby?

Just askin'

And just sayin

A exchange engine during a mustering season may be the fast option required.

Might have missed something -but I can't see where anyone Has said that! Selecting Carb Air HOT does bypass any normal air-induction filtration, usually taking hot air from adjacent to the exhaust manifold, to the carb. Quite counter-productive to have the hot air "bypass the carby" -at several levels, I would think.

So you don't think the dust and debris would get all through the engine in a hundred or so hours? Top end only, eh?
Maybe so, but I will bet against you on that one.
Twenty hours running is around 5,110,560 litres of dusty air for a 200 cubic inch, 4 cycle, engine at 2500 rpm. That's grade A, tasty, red dust.

I think he's suggesting that the dusty air would have somehow damaged the carb.

PDR

Well that's the only part of the engine that is normally fed by filtered air, so yes. Unless you're suggesting that dust particles large enough to not be immediately scrubbed into the oil filter can somehow get past the rings

PDR

Very few aircraft provide filtered air to the hot air intake. I had one once, a 1963 172 that used a second Bracket air filter for the hot air. It got surprisingly dirty.

Change of emphasis, perhaps, but I saw a 150 go into Moreby harbour probable cause carby ice on a hot png day. Humid tho. He was beating up boats along the reef on reduced power to get good looks at crews, when he opened the throttle the response was noted by absence. Trying to climb was the last mistake, he wallowed up to about 200', stall, spin, splash. Of course you can't get ice at 90+, can you?

Any that think dusty air wont effect a caby, its surfaces (and areas), bearings, bushes and orifices - will just do a Top.

Sure, dust will accrete, but it won't erode anything much. The only moving part that's exposed to this unfiltered airflow is the throttle butterfly, and I struggle to see how dust in a fast-moving airstream is going to find its way into the butterfly bushes. All the other moving parts are exposed to the dust to the same extent whether the carb heat is selected or not, so they are irrelevant.

You might get some blocked jets (although the air pressure is, by definition, pushing against the incoming dust rather than sucking it into the jets), but even if you did it would only need an overhaul (strip & clean) and wouyld not justify scrapping the carb.

So I still don't see it.

PDR

Feel free PDR1, to feed dust into any engine you own. I fail to see how this dust in the carby ****e can be extracted from Griffo's post, which is about dust damaging an engine.

Hot unfiltered air into the carby ..... then on into the engine. Not in my donk, even if it doesn't hurt the carby!!!!!! How else is the dust going to enter the engine if it doesn't go through the carby? I call bollocks to what you are saying anyway, dust will damage a carburetor eventually.