Severe icing question

[General_Kirby]

Heard severe icing broadcast on LondonATIS today. How bad is severe? What would lead a flightcrew to report severe? And could icing actually bring down a modern jet if procedures werent followed etc?

[BOAC]

Very subjective, of course, but things like rain ice can exceed the performance of aircraft and rapidly obscure vision. It is probably defined somewhere obscure as a rate of accretion but not all of us can get out and measure that.

'Severe' normally signifies that it exceeds the anti/de-ice capabiliites of the aircraft and the normal action would be to vacate the area vertically or horizontally as fast as possible.

[safetypee]

There are at least two definitions of severe icing in aviation.
First that used primarily in met forecasting; in the UK this is defined in AIP Gen 3.5. (www.ead.eurocontrol.int/pamslight/pdf/4e415453/EG/C/EN/AIP/GEN/EG_GEN_3_5_en)

Second that which describes conditions encountered during flight and observed / reported by crew. This is defined in table 3.5.6.2 in the AIP, but note that there may be some differences between these reporting definitions and aircraft certification terminology (CS 25 / FAR 25 Appendix C), i.e. an aircraft may have an icing certification, but this does not imply that it can fly in all conditions, particularly not in severe icing.

Essentially severe icing relates to conditions which can overwhelm an aircraft’s anti / deicing system and thus the conditions must be exited immediately (diverts from the planned route). Thus this definition depends on the aircraft type and not so much on the type of icing or met forecast. Therefore in extremis, icing could bring an aircraft down, but it’s is more likely that individual systems would be rendered ineffective; e.g. engine, controls, instruments

BOAC, not so much a judgement, but a comparison; the rate of ice accretion is faster than the rate of removal by the aircraft system – airframe, engine, windshield, probes, etc; possibly as simple as ice remaining on a protected part of the airframe.

An ATIS broadcast can be of either form; the more likely is a warning issued by met, the alternative of a report from a crew (PIREP) would ideally specify the type of aircraft encountering the condition.

[General_Kirby]

Thanks for the info, I know aircraft were holding in the stacks a lot higher than usual, so to avoid the areas. I just wondered how the crew actually know they are in an area of (severe) icing, they can't see the leading edges etc so are they just going on ice build up on cockpit windows?

[Agaricus bisporus]

Judged by ice accretion on windscreen, window frames, wipers or other visible structure.
And usually, imho, substantially exaggerated just like turbulence reports. (ie "Light to Moderate" = light chop in reality)

[debiassi]

And usually, imho, substantially exaggerated just like turbulence reports.Ah I wish I had known that earlier. I have always thought to stay well away from supecooled precipitation. If I had known it was substantially exagerated I would have flown many more hours and saved on additional overnight stops???

Sarcasm apart, I would strongly recommend doing a little more research before making statements like that. Icing is probably the single highest factor to bring down aircraft. There isnt an aircraft in existence that is cleared to fly in severe icing!!!
If you see fzra fzfg fzdz whether light or not, stay well clear. An aircraft in supercooled liquid can lose its aerodynamic properties in minutes.
I wonder how much the car in this picture weighs??

Maybe in areas with this kind of freezing rain, they should ammend the forecast and state, +FZRA (Oh and were not kidding this time!!!)

http://i462.photobucket.com/albums/qq347/debiassi/SevereIcing.jpg

[Piltdown Man]

I agree icing is major threat to aviation safety but like turbulence, windshear and all other hazards reported in aviation, hopelessly overstated. A bit like fog warning signs on British motorways. The over-stating of hazard brings the reporting system into disrepute and possible complacency in dealing with the hazards these warnings are meant to be protecting us from. I think the phrase "Crying wolf" is appropriate.

[safetypee]

Ab, PM, rather than being overstated perhaps the icing hazard and others quoted are under-reported or poorly understood, particularly on a worldwide basis.
I recall that risk involves both frequency of occurrence and severity of outcome. Thus very few crews may have encountered severe icing – a rare event, but when encountered the effects can be major and develop rapidly.
Crews may not understand the relationship/disparity between the various definitions and thus reporting is inaccurate; also there have been reports of a widespread belief that all aircraft can cope with (any) icing which of course if far from fact.
If ice is forming on a heated windscreen, even at the edges, who knows how quickly this might spread all over the screen. Compare this with a car windscreen when entering a patch of freezing fog.

[Piltdown Man]

...involves both frequency of occurrence and severity of outcome.

I totally agree that that risk involves both frequency of occurrence and severity of outcome. Which is why the frequency of 'severe' is over reported. I'll happily accept that moderate icing, minor windshear (+/- 5-10 kts), and moderate turbulence are occasionally encountered. What I'll not accept is "Previous aircraft reported severe turbulence/icing/windshear" and you follow two minutes later and find nothing of the sort. We also have to accept that different aircraft deal with these phenomena in different ways. I've descended in a turboprop at max. continuous power due to icing (I'd call that severe) yet jets in the same area powered up straight through the same conditions. I've landed on "icy" runways only to be asked by ATC about the braking action. They are are disappointed when you reply "Sorry, didn't use the brakes". So in my mind, when I hear the word "severe" is has the same value as the words "horrendous" or "crisis" - it is just a superlative because the reporter rarely understands its meaning. Too often the word "severe" can probably be substituted with "some". It is only when these reports have context will they have some real value.

[low n' slow]

@ Pilotdown man: ...which is why you preferrably report type of A/C with that turbulence/ice report and why ATC are supposed to add that to their information to following A/C.

Also, in my experience, icing conditions can change very quickly and tend to be very local vertically.

/LnS

[Agaricus bisporus]

Dear me! How extraordinarily easy plain language seems to get misunderstood/misread on the internet.

What I meant - and what I thought I'd made clear - ie what was written - was that imho reports of "severe" icing or turbulence are usually exaggerations. I didn't use the word "overstated" with or without the word "hazard", and clearly my post made no implication whatsoever that I thought the effects of severe icing are overstated or exaggerated, as I has used the example in conjunction with the word "reports".

Clearer now?

[nnc0]

FWIW
Airbus uses the expression 'IF SEVERE ICE ACCRETION' in a number of ECAM procedures for conditional statement actions that require an approach speed additive for ECAM procedures that affect the wing anti-ice system.

ELEC DC ESS BUS SHED


IF SEVERE ICE ACCRETION
MIN SPD......................................................... ......VLS + 10 / G DOT

For purposes of AIRBUS ECAM application only, SEVERE ICE ACCRETION is defined as having approximately 5 mm of ice accretion on the airframe.