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Flying above freezing level in clear conditions


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This is something I either never learnt, or forgot, from my RAAus cross country.. so seeking your collective advice.

 

I am contemplating a flight soon at 7500.  The GAF has conditions per the below image.

 

Aside from using the cabin heater, and not flying near any cloud, using carby heat on descent, is there anything else I should watchout for generally (eg icing on surfaces).  The engine is a 912.

 

image.png

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I must say I do find the thread drift on some of these topics confusing. We start out in a discussion about flying above freezing level in clear conditions, segway to rain being cloud or not and then

Thanks everyone.. flew back home over Mt Buller yesterday with no sign of icing. IMG_0746.MOV

OAT - not sure to be honest, probably 0-ish.  There is a cabin heater in the aircraft... but I encouraged my wife to wear warm socks.  She also brought a big loose "throw around", which frankly I foun

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You are very unlikely to experience airframe icing when in the clear (of cloud).

 

Don't know your aircraft, however many Rotax 912 engine inhalations have no external air delivery to the carburettors ie they get their air (warm) from within the cowling. In my aircrafts this usually gives a + 10 degree above ambient, making icing unlikely (but not impossible). My aircraft is not fitted with carb heat and to the best of my knowledge has never experienced it.

 

Altitude has, in itself, little to do with  carb icing  - check out the predictive icing chart.

 

The only time I experienced carb ice, was at Condobolin while taxying in a C172 

 

If you have carb heat - always apply befor any power reduction. Always carry some power on decent, to prevent rapid cooling of engine components.

 

Why have you set 7500as your  cruise altitude? Other than for known terrain/tiger country clearance, my cruise altitude is determined on the day, modified as needed during flight, according to the predicted/existing weather condition.

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Treating carb heat separately, the cloud base is where the dew point is the same as the ambient. Descending  the adiabatic lapse rate is going to make it warmer at approx 2 degrees C / 1,000 feet.. Descending is the way to avoid and remove airframe icing IF you have enough spare altitude. Getting into clear air usually stops more forming and sometimes will ablate the ice off but it can be slow and uneven. Small amounts of ice on wings can cause a large lift loss and raised stall speeds and a lot of vibration when on prop blades making it hard to read instruments. Stay away from this stuff.  Nev 

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Posted (edited)

I think the OM saw what I saw yesterday, something that I have not seen before in the GAF :

 

 * posting of a FZLYR  underneath a FZLVL * 

 

Is this  a cool pool of air / layer of air below the freezing level ?

IE from ground, for example... 18degC, 10,5,0,-1,2,3,4,0,-5,-10
IE two thermopauses where the temperature crosses zero ?

Edited by RFguy
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I think what is being asked here is;

If flying at 7500, with a FZLVL of 4500 and SCT AC/AS 7000 ABV 10000, and you remain clear of the cloud.

ie. you are above the freezing level and amongst moist clouds, but not actually in the cloud;

Are you going to get airframe icing?

My understanding is no, as you  are clear of the moisture (clouds), but you are awfully close to it.

 

And you are right, this isn't covered in the RAA syllabus as being VFR only we have to remain clear of cloud so the FZLVL isn't discussed.

When I've spoken to experienced GA, IFR rated pilots, asking the same question, the answer is typically;

"theoretically you won't get airframe icing if you stay clear, but it's remotely possible, and do you want to take that risk".

No one is willing to give a definative answer, probably for good reason.

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Ice does weird things even to cars aerodynamics, we had a minus 4 frost on Monday, my roof bars on triton only make a small amount of cabin noise at around 100km normally, they were solid with ice lumps on them, and the howl was deafening in the cabin all the way to work. They are shaped like an airfoil, but the upset was noticeable. If your wings/struts had that happen I don’t think it would be real great for airflow

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You can get supercooled water drops that need condensation nuclei to form. Also if you are flying in clear air at below zero and descend into cloud or drizzle you will accumulate ice even at oat's above freezing due to the plane being cold  soaked. Nev

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IF you do pick up some ice  coming down keep your speed well up as your stall speed is something different to what you had  before. Also don't fly without removing any ice on the plane if it's frosted in the morning..Nev. 

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And Also, because the area above your wing is a low pressure area, and the freezing point of water rises

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and clear of cloud of course applies only 1000 AGL / 3000 ASL, otherwise its  1000' vertical  (1500 m H) . unless special VFR, but since RA dont normally  enter CTA, that might not be relevant. well you might get permission to traverse CTA

https://vfrg.casa.gov.au/operations/general-information/visual-meteorological-conditions/

 



 

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2 hours ago, facthunter said:

…if you are flying in clear air at below zero and descend into cloud or drizzle you will accumulate ice even at oat's above freezing due to the plane being cold  soaked. Nev

That can happen on the ground. Coming home from the Alpine Rally thru late night fog, the fairing on my Ducati and my helmet quickly built up a thick coating of very rough ice.

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carby icing will be widely and wildy variable depending on configuration.....

 

We're discussing airframe icing here I beleive. 

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The answer is simple, keep out of cloud and precipitation where the OAT is +2 or less. When I lived in Kent in 1973 fog was ever present on freezing Winter mornings & it just froze on the car windscreen & my old 15 quid banger didn't even have a heater. They used to sell de-icing spray at service stations so I always kept a can in the glove box.

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When you drop the pressure on the top of the wing the air temp drops. If you are near the dew point there's a cloud forms there on big stuff.

 P1V1/T1 equals K.   Nev

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Posted (edited)

AOPA USA Article on icing

 

You’re unlikely to suffer from carb ice when operating in air temperatures below freezing. Cold air won’t hold much moisture, you’re at a much greater risk with high humidity around 15-20 degrees C. 

Edited by Roundsounds
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Posted (edited)
4 hours ago, RossK said:

The article is about Airframe Icing, not Carburettor Icing.

 

 

The original Q was about icing conditions (both airframe and carb were mentioned)

 

 I stated early on in this conversation - Airframe icing should be almost a non event/starter for RAA/VFR pilots. Not really worth discussing, Carburettor Icing on the other hand is a real danger to Australian pilots.

 

You would  have to go looking for airframe ice, in all the wrong placed for an Australian RAA/VFR  pilot to be, to find it.

 

 

Edited by skippydiesel
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2 hours ago, Roundsounds said:

AOPA USA Article on icing

 

You’re unlikely to suffer from carb ice when operating in air temperatures below freezing. Cold air won’t hold much moisture, you’re at a much greater risk with high humidity around 15-20 degrees C. 

A more Australian targeted advice would be the probability chart put out by CASA 

 

See https://www.casa.gov.au/files/carburettor-icing-probability-chart

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4 hours ago, skippydiesel said:

A more Australian targeted advice would be the probability chart put out by CASA 

 

See https://www.casa.gov.au/files/carburettor-icing-probability-chart

I think you’ll find the post was more related to airframe ice, which the AOPA USA article addresses aside from the Australian specific climate issues. 

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Looking at Light aircraft incidents/accidents carb ice still catches people out perhaps because there is no precipitation about. It's almost absent too with injection motors. Warmer air has more capacity to carry moisture. The earth's polar atmosphere's are more dry than many deserts air is..

  The rapid evaporation of fuel and the venturi action in a carburetter can cause icing at 24 degree's C. .and doesn't affect anything but the engine. Airframe icing will be rarer  but still occasionally causes angst even for high hours pilots who aren't flying low in the tropics all the time. Hail is another from of ice that will really  ruin your day. Nev.

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According to the CASA probability chart carburettor icing can occur right up to 38 degrees though I imagine that this would have to be under very high humidity conditions with partial power due to the cooling effect of a partly closed throttle butterfly.

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I would suggest the evaporation of the fuel is a bigger. factor With alcohol motors it's more noticeable again but weirdly, spraying alcohol into intakes is used to de ice them  as it lowers the freezing point of water.  Nev. 

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