Carb ice checks - How often?

[bcfc]

Having flown on Saturday in lovely but serious icing conditions (as defined by safety sense leaflet 14A) I experienced mild carb ice in the cruise. I was checking every 5 minutes or so and leaving it hot for a good 15 seconds.

This got me thinking whether every 5 mins is enough. I tend to be a bit paranoid about carb ice and though every 5 mins overkill but saturday taught me otherwise.

What's your carb ice routine?

[Aussie Andy]

Hi there,

I was flying on Saturday and with all the fog about was certainly very conscious of the risk...

I just used my usual routine of every 15 minutes-ish during the FREDA checks. Didn't get any unusual rough running, so left it at that. Felt safe to me anyway!?!

Andy

[Whirlybird]

There's no harm in using carb heat more often, and I've been told if you think you have icing, you need at least 30 seconds each time. But in such conditions, every five minutes sounds like a good ide.? Why not? You don't need the extra power, unless you're about to do a go-around or similar. All it does is burn a very little extra fuel, and it may save your life.

Or is there something here I don't know about?

[2Donkeys]

There are no rules for the frequency at which you apply carb heat. There are simply conditions under which you are more and less likely to suffer. If the OAT is -40, you can probably safely assume that your Carb will not ice. If relative humidity is 5%, you can probably make the same assumption.

You should develop an understanding of the risks associated with the conditions in which you are flying, and apply carb heat as required, whether that is every 5 minutes or even more frequently.

When you do apply carb heat, as posted above, 15 seconds is probably too short a period of use. The risk is that you start to melt the ice sufficiently for the water to run back out of the way of the direct heat flow, where it re-freezes. Continued use of carb heat in that manner runs the risk of causing ice blockages that will not shift at all.

Some engine and aircraft combinations are more prone to icing than others, and are therefore more touchy if Carb heat is not used appropriatly.

Short answer... there is no single answer to the question. You need to know your aircraft and the conditions in which you are flying.

[I have control]

I agree with the comments that say maybe 15 seconds isn't long enough. I always been advised 30 seconds, and make sure to count slowly!

[adriannorris]

Some engine and aircraft combinations are more prone to icing than others

What makes some combinations more / less prone? If an engine installation can be made less prone to icing by careful airframe design, I would imagine that the US product liability lawyers would have made a big issue (and a lot of money) about it....?

If the solution is to put the air intake in a warm spot under the cowling, surely this would mean that there is a power reduction similar to using carb heat?

[Final 3 Greens]

bcfc

You have had some good advice from the forum.

I would just reinforce the view that the risk of carb ice is quite type/engine specific, for example a C150 with a RR/Continental 0-200 would be rated as being prone to carb icing, whereas a Pup with the same engine would likely be said to be less so.

The best course of action is to talk through your concerns with an instructor who has spent many hours in the combination that you are flying.

As to your actions, as they say prevention is much better than the cure.

I think that you have passed your PPL recently and are learning about flying - your cautious approach is entirely sensible and appropriate.

[A and C]

Under some conditions you may need the carb heat ON all the time to prevent the ice forming ,the important thing is that even if you have half a suspicion that the RPM has dropped then use the carb heat and check the RPM after you go back to cold air if it is higher then you have had ice and more frequent checks are the order of the day.

30 sec is the very least i would use , a post above has pointed out the dangers of short appication of carb heat.

[Code Blue]

I would echo A&C: If the risk of carb ice is high then leave it on all the time. You will need to lean a little more, but other than that it's one less thing to worry about.

If using intermittently, I would recommend at least 60 secs if not longer (The type I fly with carb ice risk is mainly Cessnas). If the engine roughens when carb heat is applied then you already have carb icing - see paragraph #1 :eek:

Rgds

CB

[bcfc]

F3G, yup I'm a low hour PPL and I'm taking a naturally more cautious approach as a result. Carb ice is one of the few elements of flying that I worry about. Its insidious but can be prevented, so its always nagging at the back of my mind. Bird strikes, complete engine failure, the wings falling off...well not much I can do to avoid these and I'll just make sure I'm prepared if any of this happens (except the wings bit :eek: )

Reading Flyer mag this month, the article of deisel engine a/c looked interesting. They use JetA at 30p/lt and don't need carb heat. So why aren't there more around?

Also, if anyone could tell me why a normally aspirated car engine doesn't suffer carb ice I'd be eternaly grateful.

Cheers...and stay warm!

[ 08 November 2001: Message edited by: bcfc ]

[Lowtimer]

BCFC -

Car engines certainly _can_ get carb icing but it's rare. Almost every petrol car since the widespread adoption of catalytic converters in about 1989 is fuel injected and uses a water-heated inlet manifold, which effectively eliminates the problem. Back in the '70s and earlier, in the days of the rear-drive Escort and Chevette, you often had a dual position intake nossle leading to the air cleaner, which breathed either cold ram air or (by manual adjustment) from a warm position behind the radiator.
I used to get carb icing years ago on my 1983 1300 cc carb-fed Fiat Uno. At that time there was a thermostatic flap operated by a bi-metallic strip which was supposed to blend in air between the hot and cold sources. The strip packed up and the carb would ice up a treat - classic aviation symptoms. I had to tell the Fiat dealer what to fix (it was under warranty) 'cos they didn't have a clue what carb icing was.
Good thread - by habit I'm a five-minute man myself, but with my skills test coming up soon I shall now be applying carb heat for longer periods (30 sec rather than 15 sec) during those checks. And I'd love to fly behind a nice modern turbo diesel running on jet fuel.

[Shaggy Sheep Driver]

When my wife was my girlfriend (still is, I suppose - I mean before we were married) many moons ago she had a Mini Clubman. It was an awful car in many respects (remember BL in the 70s?) and that suffered carb ice regularly.

The worst aeroplane for carb ice I've flown is the L4 Cub with a 65 horse Continental.

The Chipmunk has 'warm air' rather than 'hot', and I always put it into 'warm' as soon as I've taken off and leave it there. RAF Chippys were permanently wired to 'Warm' - but they had nice long runways! (actually the power loss isn't that great).

The Yak has a 'single point fuel injector and auto mixture device' which puts the mixture into the supercharger chamber, from where it is distributed to the 9 cylinders of the radial engine by inlet pipes. That can suffer from carb ice, particularly at low power settings. I've even had the engine cut on a damp morning due carb ice while doing the pre-flight checks.

You can tell when the air's moist if you are driving a Yak - you get vapour trails off the prop tips ;~)

SSD

[ 08 November 2001: Message edited by: Shaggy Sheep Driver ]

[Southern Cross]

SSD - re prop vapour trails: particularly prop hanging at the top of a stall turn! :D

The only time I have suffered icing issues in a Yak have been when flying glide approaches, when icing can occur to a limited extent affecting the ability to open the throttle.

[Shaggy Sheep Driver]

SC - yep, very pronounced. It's more visible from outside, though. I was watching a Yak waiting to taxy the other day and even at tickover there were little 'feathers' off each prop tip. Sometimes take off can be quite spectaculor, with a 'cylinder' of vapour off the loaded prop.

Had the carb ice can't move the throttle thing as well. Only on the ground, luckily !

SSD

[Who has control?]

Lowtimer is correct in saying that cars with cats need Fuel-injection, although not all have water heated manifolds. Cats are very temperature sensitive, if they are too cold, they don't work. Too hot - they melt. If the mixture is too lean, they melt and if its too rich - they melt. Carbs just aren't accurate enough.

Inelegant aero - cars can suffer from 'hot air uptake' where the air charging system draws in hot air from the engine compartment and there is a power loss. Cold air must then be ducted towards the air intake spout.

Engines thrive on cold, damp air. The colder it is, the denser it is so a greater fual/air charge is drawn into the cylinders. The dampness in the air is converted to steam and expands violently when the charge burns. You may have noticed that your car performs better under these conditions.

Changing the subject a bit - there are commerically available Carb-Ice Detectors that will light a bulb in the cockpit when ice is detected. Why? Why not use the current that lights the bulb to close a relay that switches a motor that operates a flap that ducts hot air to the carb? Any ideas?
:confused:

[poetpilot]

Carb ice in cars - yes, used to get it on an appropriately named Triumph Spitfire in the 70s.

And on A/c engines - have experienced it on an A65 in a Jodel a number of times, but not as yet on the A65 Luton Minor.

VW engines (on a/c) are also supposed to be prone, but I dont think I ever got it on my VP1 - however, the local Tipsy Nipper I believe is quite susceptible, and I've seen it come in a few times with the prop stopped.

RPM reduction on a VW is quite significant with carb heat applied, and the engine used to sound pretty rough when I applied it on the VP1.

[FlyingForFun]

Agree regarding carb icing affecting different types differently. Until recently, the only type I'd flown was Piper Warriors - never had any problem with carb ice except occassionaly just after starting up on a cold morning. Of course I did regular FREDA checks, and applied carb heat each time, but I became a bit complacent.

Just transitioned to Super Cubs. My instructor warned me they were more prone to carb ice than Warriors, but I didn't pay much attention - at least not until earlier this week. Instructor sent me to do some solo circuits - my last flight before being signed off. On one circuit, I was distracted on downwind by another aircraft, and didn't complete the BUMFICHH checks properly. Left the carb heat cold, and didn't notice until turning finals. Applied carb heat, and the RPMs immediately rose by about 200rpm - thas was simply from flying the base leg at around 1600rpm with no carb heat (in classic carb-ice conditions).

Take care out there!

FFF
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