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Light twins and icing conditions

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Light twins and icing conditions

Old 2nd Nov 2008, 19:13
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Surely the categorisation of icing severity HAS to be based on the rate of accumulation and not the ability of a specific aircraft to deal with ice accumulations?
Should be? Yes. Is? No. Not yet at least. The link I gave above makes that very point.
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Old 2nd Nov 2008, 23:07
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Surely the categorisation of icing severity HAS to be based on the rate of accumulation and not the ability of a specific aircraft to deal with ice accumulations? I appreciate a pilot report on icing is subjective as in turbulence reports.
Absolutely not.

Icing severity is very much aircraft dependent. Icing severity in flight is a Pilot Report (PiRep) function, and is always subjective.

The amount of ice which accumulates depends on the aircraft structure, type of protection, etc. One airplane may be accumulating very little ice, while an airplane flying along side it may be experiencing severe icing.

Severe icing is that icing which is beyond the capability of the airplane to handle the conditions or the ice. Trace ice to one airplane may be severe to another.

I've operated during weather research flights in conditions which produced 3 inches of ice buildup. If you experienced this in many aircraft you would be unable to remove it, you would experience significant weight and aerodynamic penalties, and might very well consider it severe icing. So long as we could operate in those conditions and our ice protection kept our critical surfaces clean, we kept flying, and in fact sought out the maximum icing rate we could find.

By most accounts, that level of icing would be severe. For us, it was not.

Icing is always subjective. Furthermore, a change in the capability of the airplane may change a moderate icing condition to a severe one with no change in the amount of ice or the rate of accumulation.

I experienced a very significant buildup of ice in a large four engine radial powered airplane some years ago, with large horns or walls of ice extending off the top and bottom of the leading edge, into the slip stream, some eight inches or more. Strange looking twisted tentacles built forward into the slipstream off the propeller domes. The aircraft was controllable, but we evaluated our options. We were in the process of requesting a change and making that change when I began to experience aileron snatch with the controls attempting to jerk to the right. We descended only about 3,000' before much of the ice shed from the wings and the control issue immediately ended.

One could easily say that such a level of ice constitutes a "severe" buildup. It was definitely severe icing, and rapidly exceeded our ability to remove it, and then immediately thereafter, to control the airplane. This took place in a very short period of time. However, it's noteworthy that up until the aileron snatch was experienced, no adverse control effects were observed...quite likely this would have been severe ice much earlier for some airplanes, but this airplane remained controllable much farther into the icing process. We had no intention of seeing how far it would go; the ice buildup occured while we were maneuvering to get clear of the icing conditions. Ice, the rate of buildup, how it builds up, the type of icing, etc, affects different airplanes in different ways.

Ice that is melted off the leading edge of a wing but runs back and refreezes may represent a severe icing condition to that airplane, but not to other airplanes in the same area, experiencing the same conditions.

Icing is very much relative. One cannot make an assumption that any report of icing as trace, light, moderate, etc, represents the true icing conditions that one can expect to experience. Simply because that's what the other guy got...should never be taken on assumption to indicate what you might experience. One man's ceiling is another man's floor, and the same applies to aircraft. What's light to one can easily be severe to another.
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Old 4th Nov 2008, 22:42
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I remember an instructional video that said if you encounter ice buildup which is too much for comfort you can escape it by changing your altitude 3000 feet up or down.

Anyone else familiar with this theory?
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Old 5th Nov 2008, 08:15
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This is sometimes the case, sometimes not.

If you're close to MEA over mountainous terrain, quite possibly not.

The two greatest buildup conditions are either freezing rain with supercooled water droplets, or large supercooled water droplets near the -10 deg C range. A descent of 3,000' in either case may not be possible, and may not get you out of the ice; a climb likewise may do little to protect you.

The nature of the conditions determine your options. If you're in a layer and icing up, you may well simply be able to climb or descend out of the layer. Icing is commonly found greatest in relatively narrow bands...but one should never count on that. Further, even if one stops the ice buildup, one may be stuck with what's on the airframe and be unable to shed it.

Know in advance where your best prosepcts lie for escaping icing conditions. Know where the freezing level is, the extent of visible moisture or precipitation, and look over lifted indexes, pilot reports of ice, etc. This should always be a primary consideration, especially in a light airplane, remembering that your ice protection isn't there to allow you to stay in the ice, but to get you out of the ice. Plan accordingly.
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Old 5th Nov 2008, 09:24
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SNS3Guppy, sound advice .

I fully agree that the deicing is a get-out-of-jail-free-card.

Crossing fronts in IMC is where I have seen really rapid acculumations (centimetres in seconds) which I suppose has been freezing rain (looks like sleet from inside the cockpit until you lose forward vision). This is almost always in or around moderate turbulence and I am careful to not allow the aircraft to accelerate while descending to get rid of the ice but equally try to keep the speed from dropping too low in order to avoid tail stall. I think these are about the most challenging flight conditions that I have encountered.

Ice scares me and I do my best to avoid it, no deicing system can handle bad icing.

SB
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Old 5th Nov 2008, 09:36
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I would add to the excellent information from Guppy to also be wary of flying in conditions where you know you will hold ice to the ground or the MSA.

I can remember a trip in a light twin where I had encountered a lot of ice airways. I left the weather but still had a long way to go.

The speed had decayed so much that I asked the controllers whether I could descend below the airway for Ten minutes so that I could melt it all off the airframe.

I then climbed back up and setup a normal cruise.

Some ice will vapourise off once clear of ice and cruising but you can carry a substantial amount of airframe ice.

On another trip into Scotland I could hear large jets reporting ice in the holds and on approach. I was then given a descent to take up a hold. I delayed that descent for as long as possible to keep in clear air on top. Descending into the clouds into the hold it quickly became apparent that unlike the larger aircraft I could not hold for very long. Do make it clear to controllers if they ask you to do something which will cause problems.

Ice can be scary because it is inconsistant, can take you by surprise and can build very quickly. In larger aircraft which climb at 3000 fpm you can quickly climb through those ice layers. In light singles or twins you hang around in there with aircraft which are least equipt to deal with it.

Pace
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Old 8th Nov 2008, 11:21
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Icing....

Something tells me the instructor wasn't worried about the icing....
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Old 10th Nov 2008, 15:56
  #28 (permalink)  
Gizajob
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Dear All

Many thanks again for your valuable input - my friend has read this thread and learnt from it (as have I).
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