How do I determine icing conditions

[PPL-Pilot]

This may sound like a silly question, and it probably is, but how do I determine icing conditions?

Taking today as an example, according to the Metar data for East Midlands it is +5C with a due point of +4C.

I am therefore assuming that at 500ft, the relative humidity will be 100% and above this altitude there is sufficient moisture to cause icing should the temperature drop below zero. On this same basis, icing conditions will be present above 2,500ft assuming ISA conditions with a temperature drop of 2C per 1000ft.

Is that correct or do other circumstances need to be taken into account such as presence of cloud etc?
Can icing conditions occur below 100% relative humidity?
Do icing conditions always occur when temperature is below zero?

Thanks
Jon

[PDR1]

A handy guide (especially page 3)

PDR

[scifi]

Doesn't the OP mean 'Airframe Icing'...?
.

[PDR1]

Oh - sorry. In that case here's another handy guide.

PDR

[PPL-Pilot]

Thanks, PDR, for the helpful links.

If I am understanding correctly, airframe icing can't occur in clear air as water vapour needs to be supercooled - that is, in liquid form below zero degrees celsius. Therefore, for the sake of general aviation, and when considering flight under IFR (with an IR(R) or similar rating), icing could be considered as flight in cloud (or precipitation) below zero.

Is that a fair (and safe) summary?

[Jonzarno]

Originally Posted by PPL-Pilot

Thanks, PDR, for the helpful links.

If I am understanding correctly, airframe icing can't occur in clear air as water vapour needs to be supercooled - that is, in liquid form below zero degrees celsius. Therefore, for the sake of general aviation, and when considering flight under IFR (with an IR(R) or similar rating), icing could be considered as flight in cloud (or precipitation) below zero.

Is that a fair (and safe) summary?

Basically, yes. If the temperature drops below about -20C, you’re also unlikely to get icing.

Also, especially in faster aircraft, you can get a small temperature increase from the friction of the airframe going through the air.

If using an OAT sensor in flight, you need to bear in mind the position of the sensor. There are two on my aircraft and they often read more than 5 C different from each other. The higher reading one is on the side of the cowling next to the engine and above the exhausts. Quite why they put it there, or why there are two different ones is a mystery...... On my aircraft, they display in different parts of the avionics and it’s important to know which one to ignore!

[NorthSouth]

Originally Posted by PDR1

Oh - sorry. In that case here's another handy guide.

"This article...does not consider the formation of frost which is created by sublimation, the process by which water vapour freezes directly onto sub zero surfaces."
A significant omission. Any aircraft left uncovered outside, or pulled out of a hangar into freezing conditions, will accumulate potentially dangerous amounts of ice by sublimation.
Sublimation works the other way too, with ice "burning" off as a result of airspeed. But not something to be relied upon.
NS

[scifi]

Hi NS, just to be pedantic, 'Sublimation' only occurs when going from Solid to Gasseous (to a higher energy state.). It is called 'Deposition' when going from Gas to Solid.
https://en.wikipedia.org/wiki/Deposi...ase_transition)


Our plane suffered from airframe icing when in its hangar; Water condensed on a roof girder, and dropped onto the wings, where it froze..!
.

[Gertrude the Wombat]

Originally Posted by NorthSouth

A significant omission. Any aircraft left uncovered outside, or pulled out of a hangar into freezing conditions, will accumulate potentially dangerous amounts of ice by sublimation.

Yeah, but you can see that and choose whether to de-ice or not go flying.

Once Upon A Time, when the instructor came out to the aircraft I said "there's frost on the tops of the wings". "So there is", he said, "now let's go flying."

DO NOT TRY THIS AT HOME, children - this guy was a test pilot, but you may not be.

[+TSRA]

Is that correct or do other circumstances need to be taken into account such as presence of cloud etc?
Can icing conditions occur below 100% relative humidity?
Do icing conditions always occur when temperature is below zero?

In-cloud icing is the most common location for icing conditions, save for the situation NorthSouth described. That can be solved by "cold soaking" the airplane - opening the hangar door and letting it "soak" in the colder air before pulling it out into any precipitation. This may still result in ice, but it is likely to be rime or mixed icing, or frost which is easy to get rid of. By not cold soaking the airframe, you're likely to get clear ice which takes a lot more glycol to get rid of. This all assumes active precipitation.

The other common area for in-flight icing that many pilots often forget is when flying in the clear, cold air below a front or TROWL. In this case the warm(er) precipitation cools as it falls, and can form ice when it hits the airframe. This can be aggravated if you're flying "the wrong way" and you're approaching the front from the warm side. This presents an interesting situation where the pilot thinks they are simply flying through the rain with a cloud base possibly thousands of feet higher, yet they're accumulating a serious amount of ice (think Freezing Rain, Freezing Drizzle, snow, hail, ice pellets, snow pellets, etc.).

Although this can happen behind a cold front, a warm front warrants a little more respect (in this specific, narrow regard) due to the slope of the front and the larger geographical area affected ahead of the front. Cold fronts do tend to have heavier precipitation, but it tends to be isolated to a few km. A warm front, on the other hand, can affect an area of 600 km or greater.

So.

1) Do other circumstances need to be considered? Yes. More than can be covered in a post. I would suggest a book titled "The Air Command Weather Manual," a review of Transport Canada's "When In Doubt" (When In Doubt), or a viewing of any (or all) of these videos and products: NASA Icing

NASA Icing for General Aviation Pilots

NASA Icing for Regional and Corporate Pilots

NASA Tailplane Icing Video - Glenn Research Center

When in Doubt - Wing Icing Contamination - Large Aircraft Program

2) Can icing conditions occur in less than 100% humidity? Yes.

3) Do icing conditions always occur when temperatures are below 0C? Yes and No. It is unlikely for airframe icing to occur above 0C outside of freezing precipitation. However, one must be vigilant in temperatures below 10C as local areas around the airframe and in the engine may cool to permit icing (think, carburettor icing).

[Chris Martyr]

Originally Posted by PPL-Pilot

This may sound like a silly question, and it probably is, but how do I determine icing conditions?Jon

Hey Jon , don't go looking too deeply into this , at least , not if you are a recreational PPL flyer.


Start when you get into your car . Do you need to scrape the windows ? It has nothing to do with how cold it is , just how much moisture is in the air.


What type of a/c are you flying ? It'll probably only have pitot heat.


Carburettor icing can happen at any time of year and is no big deal . Airframe icing is another matter and that can kill you.
Good old CAA Safety Sense Leaflets can be your greatest friend in dispelling any shall I/ shan't I doubts; Leaflet No.3 in particular .


Always remember . You're better off down here wishing you were up there than vice-versa..

[FZRA]

Originally Posted by Chris Martyr

Always remember . You're better off down here wishing you were up there than vice-versa..

Totally agree. However, I do think that a lot of pilots are put off flying in the Winter months (on what might be a gin-clear day) because they don't understand what "icing conditions" entails. I was always surprised at the number of aircraft parked up at our flying school on the most amazing of Winter days (including weekends) when the roads were icy and cars frosted over. Yet people would push their luck on a hazy "fish bowl" Saturday afternoon in the Summer when everyone else is up!

@Jon - If the aircraft is cold soaked outside overnight, and you've thoroughly de-iced all of the critical surfaces, and there is no visible moisture (i.e. cloud or fog) then you should be in for some cracking flying weather with great performance to boot (if the engines starts!).

[PPL-Pilot]

Thanks for all the responses which are really helpful. I also appreciate the latter responses, taking a pragmatic approach to the situation but for clarity, the reason I am asking the question is that I am starting out on the IR(R).

Whilst I have been able to make a reasonable assessment for VFR flying in winter previously, the added potential of flying into/through cloud under IFR conditions has made me consider more closely the potential for icing (especially given I am flying C152/PA28 aircraft which are not equipped for such conditions).

I am now much better informed than I was and appreciate that this is a potentially more complex area than I need to understand as a PPL - thanks for the responses.

Jon

[B2N2]

Just to keep things simple.
For ice you need two things: water(vapor) and temperatures below freezing ( air or airframe).

Dry ( clean ) air is clear.
Humid or moist air is hazy.
Increased haziness means increased humidity. Fog, mist and cloud are basically all the same, visible moisture.

Any time you’re flying in visible moisture with a measured temperature of 5 C or less you’re in icing conditions. Why +5? Because of a slight temperature rise caused by friction even at slow SE speeds and measuring and position errors of the probe.
An IR rating doesn’t mean you must fly in cloud, you may.
You may under conditions for which the aircraft is certified.
Most light single engine aircraft are really only suited to fly in ‘soft’ IFR or ‘gentlemen’s’ IFR.
Marginal VFR or simply just to climb through an overcast and cruise in clear air.
They lack the equipment to fly ‘hard’ IFR in snow and ice with weather down to the ground.
Avoid instructors that feel they need to prove something.

[Chris Martyr]

Originally Posted by B2N2

Avoid instructors that feel they need to prove something.

Boy , is that one ever true...


Only problem is , when one is in the earlier stages of ones flying career , it is not easy to differentiate which are which.


My advice to OP : Jon , concentrate principally on being a safe and competent instrument pilot first and foremost .
Only then can you start to focus on the types of aircraft which are approved to handle these types of conditions . Once you are satisfied that your level of competence and the aeroplanes level of equipment are in alignment , then you're making proper progress.


If you subsequently go on to be a career jet-jock , then it'll probably seem a lot more clear cut. Just make sure your nacelle a/ice and wing a/ice switches are in On/Auto .

[Sam Rutherford]

My experience is that it's very rare to collect ice above even just +1C, but something else to remember is that losing it can take a sustained period at even +5C or more.

As a general idea, when crossing the Atlantic, either summer or winter are okay (too warm or too cold for icing), the difficult periods are autumn and spring!

[rarelyathome]

Originally Posted by Chris Martyr

Carburettor icing can happen at any time of year and is no big deal:

Think that and you are an accident waiting to happen - the no big deal bit.

[+TSRA]

Originally Posted by Chris Martyr

Carburettor icing can happen at any time of year and is no big deal . Airframe icing is another matter and that can kill you.

I can only agree with your second sentence here Chris.

While you are correct that carburetor icing can happen at any time of the year, please don't suggest to anyone, especially younger (and often impressionable) pilots, that any situation that deprives an aircraft or its engine of performance is "no big deal."

I almost gave up on flying because of a bad carburetor icing encounter, and the ink on my PPL was not even dry yet. The engine stopped and I was gliding for two thousand feet before the carburetor heater started working and brought the engine back. I was not flying over the most inhospitable terrain in the world, but my options were limited to ditching in a lake or picking hilly fields just long enough to avoid breaking all my bones.

The problem? I had listened to a new instructor at the club who was teaching his students that, in certain conditions (such as I had on this particular day), the carb heat should be pulled half-way out to increase the air temperature and prevent carb ice. I was young and impressionable and listened to him. Looking back, it was the most inane suggestion I've heard from an instructor.

[Chris Martyr]

Sorry chaps , was I sounding a bit cavalier there ? that was most certainly not the intention [cue red-face]
I will come clean here , as I am the builder/pilot of a VW powered homebuilt and believe me , I am well versed in carb. icing situations . It's probably a similar situation with chaps who fly behind smaller Continental engines .
Luckily , my self fabricated carb. heat box is incredibly efficient [ask me how I know that] Carburettor icing scares the cr@p out of me , which is why I pull on my carb. heat at very regular intervals as a matter of course.
I apologise that I made light of that fact . It is a case of familiarity ,,,but certainly not contempt !


On a lighter note , I was being checked out on a C172 in Florida recently and on application of carb. heat , the Instructor remarked "Hey , y'don't need to do it that often".....................[Yeah,,OK mate ]
Maybe I should re-word as ; Carb. Ice,,potentially dangerous . But if one disciplines oneself in dealing with it , then with a serviceable aircraft , it's no big deal.

[Crash one]

I can understand where Chris is coming from.
I fly behind a Continental C90 (apparently an ice making machine).
So being well aware of that fact, flying recently towards low cloud, rain, foggy bits etc. I had the carb heat on, well in advance, rain streaming up the screen, down to 500ft, steering by the GPS, definitely "get home itis" situation.
The engine didn't falter. "No big deal", was not how I felt at the time, although that's how it turned out!
Carb ice can, with preparation, action, a working carb heat system etc, be resolved.
Airframe ice can make the thing unflyable long enough to kill you very quickly.