Define 'Known icing conditions'

[Nibbler]

I've not come across this before - can anyone clearly define what flying into known icing conditions means. For example if the air temp is -x at 1000ft but there seems to be no cloud about would this qualify within the definition?

[DB6]

No. Generally temp below OC and visible moisture (rain, cloud), although it can vary e.g. on the Saab 340 there is a venturi - cooling - effect in the engine intake so known icing is defined as below +5 with visible moisture. Also, terminology is often 'known or forecast' as it's difficult to confirm if cloud/rain is actually present outside the local area unless it's reported by another aircraft - which won't normally happen.

[SNS3Guppy]

A cloud isn't necessary for the formation of ice, or icing conditions. Flight below a shelf, or in clear air in freezing rain, for example, can lead to rapid ice buildup.

The definitions of what constitute known ice vary according to the legal jurisdiction providing the definition. By and large, you have known ice if conditions exist which are conducive to the formation of ice, or if ice is forecast, or if ice is reported.

We have very specific conditions spelled out for our own operations which provide for a visibility of a mile with temperatures less than 8 degrees C on the ground, and a total air temp in flight of less than eight...visible moisture, freezing rain, fog less than a mile, etc. However, we're legal to operate in known ice, and frequently do...our icing guidelines determine when we turn on our anti-ice and de-ice equipment.

In a light airplane, there's really no good way to carry ice. Some systems work better than others. TKS, for example, works well up to a point, and works while fluid remains and runback doesn't form a problem...but it has it's limits. Boots have limits. Blankets, hot plates, alcohol de-ice, and other systems all have their limitations.

In a light airplane, if you are in conditions which could cause ice or are conducive to ice, you're in known icing conditions...because such conditions are known to produce ice. If ice is forecast, then it's known ice...even if others are providing pilot reports of no ice. If you're in conditions in which others are reporting ice, but you're not...you're still in known ice.

You will find that the legal definitions will vary somewhat, but practically speaking, your chief concern is the ice itself. Ice can form suddenly and unexpectedly, and can build more rapidly than the airplane can handle...the definition of severe ice. Ice is nothing to fool with...even if it's thought to be trace or light icing, because that can change.

I've seen icing conditions go from trace to severe in a very short time period, with ice buildup increasing to one to three inches in less than a minute. I've also seen no ice turn into a real problem after passing through a seemingly benign little cloud. In fact, I was involved with a research project seeking out icing in flight, and can tell you that you might be very surprised how fast, and where, ice can form and build.

If you're in any conditions that have ice, might become ice, are reported to do so, or forecast to do so...take it seriously enough to know you're in known ice.

As for being in the clear...yes, you can get ice in the clear. Simply because you're not in a cloud doesn't mean you can't get ice.

[Pace]

Guppy

An aircraft approved into light icing really holds no ice (water) :) as other than reported ice and estimated levels there is no determining factor other than cloud and the moisture density within those clouds, temp and potential.

Ie potential can only be a forecast of a possibility and known ice can only be an airbourne report of known icing conditions again known can only mean that a level of icing accumulation is known to exist rather than has the potential to exist.

Hope you get my gist

Pace

[THE ORACLE]

The term 'Known Icing' refers to Ice certification as it relates to FAR/JAR 25, Appendic C. To be allowed to operate an aircraft into known icing conditions the manufacturer must certify the aircraft in accordance with this criteria.

In short, this appendix specified a certain liquid water content (LWC), cloud form (cumuliform/stratiform), and a mean droplet size diameter. It assumes two different conditions - one where you have intermittent icing conditions and one where the icing conditions are continuous.

Intermittent Icing Conditions - LWC 1.1 - 2.9g/m3; Cumuliform cloud (4,000 - 22,000ft/2.6NM). Mean droplet diameter 15-50 microns.

Continuous Icing Conditions - LWC 0.2 - 0.8 g/m3; Stratiform cloud (6,000 - 22,000ft/17.4NM). Mean droplet diameter 15-40 microns.

Freezing rain and freezing drizzle are two types of precipitation which fall outside the certification requirements.

Freezing Drizzle - LWC 0.6g/m3; Supercooled large droplets (SLD) with a mean droplet diameter of 50-500 microns.

Freezing Rain - LWC 0.3g/m3; Supercooled large droplets (SLD) with a mean diameter of 500-1200 microns.

FAR/JAR Appendix C was meant to cover 99.9 percent of possible icing conditions. Moderate icing conditions equal intermittent and/or continuous icing conditions.

There is no aircraft certified for operation in severe icing conditions.

[ExSp33db1rd]

....... Mean droplet diameter 15-50 microns........


Wot you're supposed to do ? Get out and measure them ?

[THE ORACLE]

No. Clearly, these are technical definitions.

From a pilot's perspective the aircraft's performance (sustainable rates of climb and sustainable indicated cruise speeds) with all required ice protections systems operating, together with assessment of observable/acceptable ice on the aircraft becomes the assessment criteria.

Turboprop transport aircraft often have a 'witness mark' on the propeller spinners. If ice is observed building up behind the witness mark and performance has/is degrading then the aircraft may be operating is a region of icing cloud which is beyond the 'known icing' criteria and the crew needs to commence a descent into warmer (i.e. safer) atmospheric conditions to remove the possiblity of an icing related in-flight upset.

QED!!

[bookworm]

Known Icing Conditions
Atmospheric conditions in which the formation of ice is observed or detected in flight.

AIM 7-1-22 (http://www.faa.gov/airports_airtraffic/air_traffic/publications/atpubs/aim/Chap7/aim0701.html).

[scooter boy]

Thankyou Bookworm!

This is why PIREPs are such a great thing to have available. It's always nice to know what others have recently experienced.
PIREPs are very commonly made in the USA but only rarely have I heard one made over here in Europe.

SB

[IO540]

There are two aspects to "known icing".

In the USA you can get busted for departing into KI (although cases of where this happened without an actual icing incident are AFAIK unknown). The regs on this have varied over time, anyway. I think bookworm has posted the latest version.

In Europe there is no known enforcement of such departures, nor any law that I am aware of, but there are categories of icing conditions for which an aircraft may be approved to fly in and where you are presumably supposed to exit such conditions should you encounter them while airborne.

One cannot map the FAA KI regs onto flight in Europe (even in an N-reg) because Europe has relatively crap weather services which in any case map poorly onto the U.S. ones on which U.S. legislation is based, has no PIREPs, and the UK has a real gem, courtesy of the supercomputer-equipped UK Met Office, in the form of a continuous forecast of icing in any cloud (regardless of temperature :ugh: ) on things like Form 215; the latter, if it appeared in a U.S. forecast, might at one time in the past have made all flight in IMC illegal except in planes which are certified for flight in icing ;)

The practical ways of dealing with icing conditions, flight planning and airborne strategies, is an altogether different subject from the legality of it. What do you fly, and what sort of profiles?

[bookworm]

In the USA you can get busted for departing into KI (although cases of where this happened without an actual icing incident are AFAIK unknown). The regs on this have varied over time, anyway.

There's certainly some history here. The definition of 'known icing conditions' appeared in the AIM relatively recently (3 or 4 years ago). I believe the FAA had always intended this definition, and had never intended it to refer to forecast or expected icing. An aircraft that is certified for flight in known icing conditions has been proven (subject to the usual caveats) to be able to continue flight in such conditions. An aircraft operating under Part 91 that is not certified for flight in known icing conditions cannot rely on that capability, and must exit such conditions immediately. Where forecast icing is relevant, the FARs are explicit e.g. 135.227c/d/e.

However, defences have been used, mostly by pilots operating under Part 91 who have behaved quite irresponsibly, suggesting that if the pilot could not be 100% sure that icing would occur in the conditions in question, they did not constitute 'known icing conditions' for the purposes of flight manual limitation, despite the fact that the aircraft was rapidly becoming an ice-block and the pilot had no contingency plan. Thus in supporting common sense in rejecting the appeals, the NTSB ALJs reached unhelpful opinions that clouded the difference between known and forecast icing, and those opinions became precedent. The FAA therefore sought to clarify the position with the AIM definition. Though in principle an AFM limitation is breached when an aircraft not certified for flight in known icing conditions starts to collect any ice whatsoever, I'm not aware of action taken against pilots who acted promptly to exit the hazardous conditions.

[PAPI-74]

Vis below 1 NM (approx 1 x runway lenght)
or
Visable mosture (rain/cloud/fog/mist/snow etc..)
Temp < 5 deg c

Simple as that.....

mist = 1000m - 5000m vis RH 95% or more
fog = zero - <1000m RH 100%

[IO540]

Vis below 1 NM (approx 1 x runway lenght)
or
Visable mosture (rain/cloud/fog/mist/snow etc..)
Temp < 5 deg c

Simple as that.....

mist = 1000m - 5000m vis RH 95% or more
fog = zero - <1000m RH 100%

Can you supply a reference for the above, especially one for the physics which covers the temperature range 0 to +5C?

[Fright Level]

At work icing conditions are defined as:

Icing Conditions exist when the OAT on the ground or TAT in flight is 10°C (50°F) or below and visible moisture is present (such as clouds, fog with a visibility of 1500m (1sm.) or less, rain, snow, sleet or ice crystals); or standing water, slush, ice or snow is present on the ramps, taxiways or runways.

The reason for using any temp at or below 10C is to allow for cooling effect in low pressure areas of the engine intakes. Even a fuel injected piston engine can suffer from inlet icing in the right (wrong?) conditions above 0C.

[IO540]

I think Fright Level you are talking about big jets.

Maybe above 0C but it would be ever so marginally above. A quick calculation (with the little Jepp circular TAS / mach heating calculator shows that at say 150kt the heating is only 1 or 2 degC and I would therefore expect cooling due to a pressure drop to be of the same order, or less.

Having said that I have often observed ice (on wing leading edges etc) disappear more or less instantly the indicated OAT moved above 0C. The probe actually indicates TAT rather than SAT because it is subject to similar aerodynamic heating as the wing.

So I think that at GA speeds, say below 170kt TAS, any cooling will be minimal and expecting ice below +10C is unreasonable. I have never seen any ice anywhere (I cannot see the fuel servo air intake; I accept that, but would certainly notice a loss of power because the fuel flow is displayed to 3 significant figures) at indicated (OAT probe) temperatures above 0C.

The pitot tube is a different proposition and I have seen some effects south of about +3C so I put the pitot heat on below +10C anyway. Maybe a very small blockage can form in there.

[Fright Level]

Yes, I was thinking of places where more substantial low pressures can occur (perhaps only the inlet manifold on an SEP).

[bookworm]

In terms of a wing, the difference between the lowest temp on the wing surface and the indicated temp is essentially proportional to wing loading. I ran the numbers once and found that while 1 degC is about all you'd expect for a light aircraft, a transport jet might see 10 degC. I don't know enough about the aerodynamics of engine intakes to know the magnitude of effects you'd expect there.

[FlyingOfficerKite]

The definition of 'known icing' or 'icing conditions' for a jet maybe +10ºC or below and visible moisture (which for a Boeing, in simple terms, it is).

Known icing for a SEP is more 'forecast icing' conditions coupled with use of the OAT temperature gauge - and of course ice on the airframe.

These are the criteria that I have always been trained to observe.

To fly above the forecast freezing level in an SEP in cloud would be a no-no, as would flight in cloud with an OAT gauge showing a temperature less than 0ºC.

More common sense than science in this case.

FOK :)

[IO540]

To fly above the forecast freezing level in an SEP in cloud would be a no-no, as would flight in cloud with an OAT gauge showing a temperature less than 0ºC.

It cannot be that simple, because such rules would terminate all non-deiced IFR flight for the entire year (not just winter) in Europe - because the airway MEAs in Europe, never mind levels at which you can get a Eurocontrol route into the system, tend to place you into potential icing conditions much of the time.

So, the sensible approach is to have an understnading of how it works, and a strategy to minimise the risk, and adequate escape routes if a problem is encountered.

For example, one sensible approach is to not go unless the 0C level is substantially above the MSA - unless VMC is assured. That gives you the obvious escape route: descend into warm air.

In normal IFR (airway) flight, one climbs as fast as possible to reach VMC on top, cruises in the sunshine enroute, and does a more or less continuous descent at the other end.

There is no denying that winters make it harder to do IFR trips (largely because one loses the "descend to warm air" escape route) but the potential icing conditions exist enroute just the same on the hottest summers in N Europe so one has to be running icing strategies all the time anyway. Personally I do not fly in IMC enroute for any significant time, always climbing up to maintain VMC "due weather", and would turn back if unable to climb high enough.

Obviously this requires an IR; IMCR holders are stuffed by being limited to be below the base of Class A and probably below the base of any other CAS, so could be picking up ice just by pottering about at 2400ft if the air is below 0C.

The other day I cancelled a short hop to Belgium. Not because the flight would have been a problem - the clouds were probably less than 2000ft thick at both ends - but because the forecast ground temp at the far end was -3C all day on the day of return and the plane was likely to be covered in thick clear ice (as it was a few days before) and I don't have one of those pump-up weedkiller bottle thingies for spraying deice fluid.

[Droopystop]

The aircraft I fly is fitted with icing detectors. From experience ice can be experienced at temperatures below about +2 deg C and can vary from trace to heavy very quickly even in relatively shallow clouds at GA altitudes.

To answer your "for example" CAVOK at -2 deg would not be icing conditions.

[FlyingOfficerKite]

In normal IFR (airway) flight, one climbs as fast as possible to reach VMC on top, cruises in the sunshine enroute, and does a more or less continuous descent at the other end.


We are talking SEP here aren't we?

What does one do if one meets icing conditions on the more or less continuous descent at the other end? (In the non-deiced SEP with, I suppose, the IR rated numpty in command).

Have you thought of any ideas for an epitaph?

Maybe one would be 'there are old pilots and bold pilots but no old bold pilots'

FOK :)

[IO540]

What does one do if one meets icing conditions on the more or less continuous descent at the other end? (In the non-deiced SEP with, I suppose, the IR rated numpty in command).

The ice comes off once you descend through the 0C level, which will be above the MSA because that is how you planned the flight :ok:

[FlyingOfficerKite]

and the weather conditions are always as planned.

Which is why there are no weather related incidents/accidents. NOT.

Anyway less of this 'Cloud Rain And Precipitation' and back to professional chat.

Cheers!

FOK :}

[Dr Eckener]

Known icing = conditions where you expect ice to form on the airframe. If the aircraft is not certified to be there then it does not matter whether it is light, medium or heavy. And microns? Whatever!

Keep it simple people.

[Pace]

The ice comes off once you descend through the 0C level, which will be above the MSA because that is how you planned the flight


That is a tricky one as usually you can work out the ground temperature and work out roughly where you expect the 0 deg temp to be.

Ie over fairly flat land be very cautious of taking off with temps at plus five or below. There again I have on numerous occasions taken off with plus 2 deg only to find at 5000 feet the temp is plus 4 ie where you get away from the frozen ground. the air above is warmer.

Then there are situations with true inversions were you can climb and have the temperatures well into the minus only to find they change well into the plus.

Knowing you can descend into warm air with an aircraft which is not anti/deice capable not only limits you when you can fly ie mostly summertime, but also brings in problems of either not being allowed to descend or not able to descend for terrain reasons.

I can remember flying into Edinburugh and put into a hold in icing conditions with other aircraft below waiting their turn on the ILS. Even in an aircraft with anti/deice ability the build up was so quick I explained the situation and was vectored out of the hold onto the ILS. On that occasion holding the ice till passing 1500 feet on the ILS.

Beware flying a non deiced/anti iced aircraft unless you are sure you have large margins in the temperature spread as at best you risk being very unpopular with other aircraft.

Pace

[adverse-bump]

The CAA consider it to be 10*C in visable moisture for TO or 5*C in Visable moisture at anyother stage of the flight.

[Pace]

The CAA consider it to be 10*C in visable moisture for TO or 5*C in Visable moisture at anyother stage of the flight.

Not sure what you mean by that? as if you are cruising above 12000 foot mountains 10 deg C on the ground is too cold to takeoff in and taking off in visible moisture means in cloud of varying density.

Pace

[IO540]

10*C in visable moisture

Interesting physics, very interesting physics... I await the Nobel prize for getting water to freeze at +10C.

Pace - you are right of course, but one can plan for where the 0C layer is likely to be, especially given a correctly executed IFR flight is 99% (or more) in VMC, above the clouds.

[Nibbler]

The flight in question was;

SEP, VFR, 370 miles EGHH to EGPG via CPT, WCO, Durham, Newcastle, SAB

0deg at 1000ft

Cloud front expected Hull / Humberside 12:00z moving South, possibly South West. Other areas expected clear.

- my concern was the whole flight would be in the minus at any level and therefore any moisture in the air anywhere on route and we'd have been in serious difficulties with no option to descend to warmer air.

Needless to say I did not want to fly this one but the other pilot kept harping on about - "known icing conditions would have to include cloud" and "this flight was good to go". In my opinion this was a very risky flight into known icing conditions given the weather factors and I refused to fly.

I am sure I made the right decision but I respect what everyone here thinks.

[IO540]

0deg at 1000ft

That one would be tricky; a no-go for me unless the clouds were obviously pretty thin and I was going somewhere nice (like Greece).

Never cut off the last escape route.

[Pace]

No go for me too in an aircraft with no deice anti ice capability. I am not a helicopter pilot but presume you guys always have the option to land in a field if the flight is not working out as you want which must be a reassuring option?

Ok a fixed wing has that option but with much higher risks and repercussions to a vertical landing on a chosen spot.

Maybe with that option in mind if I was a helicopter pilot i would attempt the trip. But in response to nibbler in a fixed wing with no deice/anti ice probably not with that temperature and weather spread

Pace

[IO540]

I tend to do much fewer long trips in the winter, not so much due to icing risks but due to the fact that there is nowhere easily reachable in a light plane (say 1000nm radius) where the weather is nice :)

I wouldn't like to do e.g. Trondheim right now, but to be honest even at the best of times that place is usually covered under so many fronts it is hard to even see Norway on the MSLP chart because there is so much stuff hiding it :)

[Dr Eckener]

adverse-bump. You confuse engine ice on jet engines (10C) with something else (5C?), neither of which are related to the question.

[bookworm]

Pace and IO-540

Are you taking account of the fact that Nibbler said "VFR"? The only significant threat from icing is freezing rain, and that depends on the nature of the front. It sounds like Nibbler would have to have penetrated the front VFR, in which case it doesn't sound particularly active.

[Pace]

adverse-bump. You confuse engine ice on jet engines (10C) with something else (5C?), neither of which are related to the question.

I think that is what he is referring to as with jets their climb rate is so fast that the temperature drops quickly. Also on some jets the engine anti ice takes some time to work from being switched on.

Pace

[Pace]

Pace and IO-540

Are you taking account of the fact that Nibbler said "VFR"? The only significant threat from icing is freezing rain, and that depends on the nature of the front. It sounds like Nibbler would have to have penetrated the front VFR, in which case it doesn't sound particularly active.

It was the frontal zone which caught my attention not the desire to fly the route VFR. 370 nm is a long way to go VFR and expect to remain VFR especially with frontal weather forecast enroute.
With a 0 deg at 1000 feet you might as well read that as to the ground.
So any icing encountered will remain.

Not a trip or situation to have flown in a non deiced/anti iced aircraft banking on staying VFR and out of cloud or visible moisture for the whole route.

Pace

[bookworm]

Which day was it Nibbler? I'll pull out the 215 and we can all take a look for ourselves.

[scooter boy]

"Not a trip or situation to have flown in a non deiced/anti-iced aircraft banking on staying VFR and out of cloud or visible moisture for the whole route."

Absolutely - even with full FIKI approved deicing the freezing rain associated with crossing cold fronts can be dodgy. We are talking frozed windscreen, 2cm of ice on the leading edges and icy chunks being thrown off the prop within seconds of entering the freezing precip. Best to go over the top of them when you can and only blunder through the guts of one amid the turbulent icy nastiness if absolutely necessary. Scud running underneath has its own inherent risks. I've flown across active fronts quite a few times but have never enjoyed it - over or around them is much more pleasant.

SB

[Nibbler]

It was Wednesday 10th December

[Vee1Kut]

Known Icing: Forcast and Pilot Reports of Icing.