SLF Question: Icing

[FlyingConsultant]

[Hope this is the right forum]

Lots of conversation about icing re: the Buffalo crash. The specifics there will come out soon, so let's not talk about that here. I am just generally curious about icing. Could somebody explain a few things to me? I honestly thought that modern planes have de-icing system on the leading edge of the wing (if not somewhere else) that take care of this...

1. How as a pilot do you figure out that there is icing on the wings without looking? Obviously it's too late when you discover the lift is lost...

2. Clearly, there is a combination of humidity and temperature that creates icing. Is that the same for all airplanes regardless of wing design? Does it depend on speed?

3. Does ATC warn you about this, does the weather report gives the exact details far enough in advance, or is there often an "oh s&*t" moment where you discover the icing conditions on the way up or down?

4. My understanding is that you can "climb through" icing conditions (or go low where it is warmer). Is the ice just falling off up there where it is cold but dry?

5. What does the de-icing actually do? I though it was bleeding air from the engines to nozzles on the wing tip but some reading later, it seems that that's not true?

Just curious, thanks

[FlightTester]

I'll try to answer to the best of my ability:

1. How as a pilot do you figure out that there is icing on the wings without looking? Obviously it's too late when you discover the lift is lost... Usually on Part 25 aircraft there is an ice detect system - it generally puts on a warning indicator to alert the pilot to the presence of ice, the pilot then puts on the anti-ice or de-ice systems.

2. Clearly, there is a combination of humidity and temperature that creates icing. Is that the same for all airplanes regardless of wing design? Does it depend on speed?
Yes, and yes to some extent - as a rule of thumb higher speed helps to shed ice lower speeds help ice to accumulate

3. Does ATC warn you about this, does the weather report gives the exact details far enough in advance, or is there often an "oh s&*t" moment where you discover the icing conditions on the way up or down?
Yes - ATC warns operators of icing conditions, there are also PIREPS - pilot reports that pilots file when they encounter ice. There is seldom an "oh sh&t"

4. My understanding is that you can "climb through" icing conditions (or go low where it is warmer). Is the ice just falling off up there where it is cold but dry?

You can climb through icing provided that you aren't so iced up that climbing becomes impossible! Going low is also an option but not always a good one - sometimes the icing conditions exist all the way down to the ground

5. What does the de-icing actually do? I though it was bleeding air from the engines to nozzles on the wing tip but some reading later, it seems that that's not true?

Usually two forms of ice control - anti-icing and de-icing. Anti ice prevents the ice from ever forming - used in critical areas where having large chunks of ice break off would be damaging - think engine intakes. De-icing removes ice after it's formed. This is generally achieved through either heating of some sort (bleed air is favorite) through ducts or by inflatable boots. As the industry moves towards more electric aircraft there is a corresponding move towards electricly heated surfaces.

Hope that helps.:ok:

[411A]

FlightTester has provided a very nice explanation.
I would only add that on some types (and the L1011 is one of them) no anti-icing is provided to the tail surfaces, as flight test has proven ice does not form there to any great extent....strange as it may seem.

[asher88]

No tail anti-ice on the B747-200 either...

[fireflybob]

Perhaps we should mention that it's mainly the breakdown in the aerodynamic properties of the wing (and tailplane) by ice (thereby decreasing lift and increasing drag) which creates potential control problems rather than the weight of ice.

As has been previously stated some a/c do not need to have de-icing systems fitted to the tailplane.

There have been previous cases in aviation history where control of the a/c has been lost when landing flap was selected with tailplane icing. Essentially the tailplane forces balance the a/c in the pitching plane. When flaps are extended this affects the trim which in normal conditions is not a problem but significant ice on the tailplane coupled with the trim change can lead to loss of control.

[FlyingConsultant]

Thanks for the answers! Great crowd.

20 years ago I started out as engineer, so I have to ask...

Usually on Part 25 aircraft there is an ice detect system - it generally puts on a warning indicator to alert the pilot to the presence of ice

How does that work?

[deltahotel]

1. This is airframe icing we're talking about, which is caused by the impact of the ac with supercooled water droplets. The best way to detect is to look. straight wings - eg C130 you can see the wings from the cockpit. Swept wings you can't, hence the ice observation lights and a look through a pax window. On a 757 the windscreen wipers ice up at about the same time as the wings!!

2. Humidity and temp are linked. The worst clouds are convective (cumuliform), layer (eg stratocumulus) and orographic (forced to rise by terrain) as they hold plenty of water. Below about -10 the concentration of supercooled water droplets falls off. +5 to -10 is a bad temp range! The faster you go, the more droplets and more icing; however a swept wing is thinner so accretes less than a straight wing.

3. ATC may warn you based on pilot reports, or the forecast may give a sigmet warning.

4. You may be able to climb through an icing layer - best then to clear the ice off the wings. descending to warmer air may help.

5. Most a/c it's de ice, not anti ice, so you have to let it build up then clear it. Bigger types based on warming the leading edge with bleed air. Smaller types may have an inflatable 'boot' on the leading edge which is inflated/deflated in a cycle. Ground treatment (chemical) both clears existing contamination (de ice) and protects against for a period of time (anti ice).

Bottom line. Airframe icing is a bugger. Turboprops tend to get it worse due shape of the wing and the fact that they cruise around at altitudes where the big wet warm clouds live. Swept wing jets have a less affected wing and cruise a lot higher. Spent loadsa time keeping C130 and Jetstream wings clean, used it twice in 8 years on a 757.

Hope this helps

DH

[BelArgUSA]

As far as jets are concerned, you have to think either anti-icing or de-icing.
I flew airplanes that had to be "anti-iced", and some could be "de-iced".
The main difference, is the engines location.
xxx
In the 727, we had engines in the tail.
If some "ice cube" on the wing, if you activated "wing anti-ice", guess where it goes.
In the 727, we activated all anti-ice systems prior entering icing conditions.
Was so critical, even VHF dorsal (fuselage) antenna heat was ON at all times.
xxx
For the 707-747-DC8, we anti-iced engine nacelles and engine inlets/fans.
Again, was prior to entering icing conditions.
As far as "de-icing" the wings, was rarely used, we could start using later, as needed.
I remember using wing heat very few times. Maybe "holding" in icing conditions.
xxx
So, critical consideration is "where are the engines".
If DC9, MD-80, CRJ - engines in tail = operate wing/engine heat prior to icing conditions.
:8
Happy contrails

[Clandestino]

Ice detection system is usually a probe with heater and optical detection system on it - something like photo-cell. When it starts picking up the ice, alert is generated and heater turn on to discard the ice. It's then turned off and if the ice starts building up again alert is maintained and cycle starts anew.

ATRs use longitudinally oscillating rod protruding into slipstream. When ice builds up, the oscillation characteristics are changed and alert is generated.

1. A320s have icing probe between the windscreens, ATRs have ice evidence probe just outside the capt's window and Q400s have spikes on the windshield wipers. They are designed to act as "ice magnets". When they start picking up the ice, it's time to turn on the de-ice. Also there are electronic ice detector on Qs and ATRs

2. By definition it's less then 10°c with visible moisture. Actual amount of ice picked up by the aeroplane is type and speed dependent.

3. We get an idea about potential risk zones from weather forecasts and occasionally we are warned by ATC or preceding aeroplane.

4. if you get into clear cold air with t-prop, ice is very slowly melting and evaporating and most of the times small amount of ice picked up on the way up stays with you until you're back again in warmer air during descent. Inflatable boots just can't get rid of every single piece of ice but what's left after de-icing doesn't hurt performance much. Jets generally don't pick up much ice and have more efficient de-icing systems.

5. Jets use bleed air to heat the leading edges (outboard wing leading edges on A320 only). Turboprops (and some smaller jets) don't produce enough heated air for thermal de-icing, but rather use inflatable boots to mechanically shed the ice. And then there are exotics like: exhaust gases de-icing, wings weeping de-icing fluid, electromechanical de-icers, etc.

[FlyingConsultant]

Thanks all, I learned a lot.

Ice detection system is usually a probe with heater and optical detection system on it - something like photo-cell. When it starts picking up the ice, alert is generated and heater turn on to discard the ice.

ATRs use longitudinally oscillating rod protruding into slipstream. When ice builds up, the oscillation characteristics are changed and alert is generated.

Cool designs....

[Mungo Man]

On the Embraer 145 we have an anti-ice system. It uses an ice detector on each side of the fuselage below the flight deck windows. The detector has a probe that oscillates at a certain frequency. When the probe picks up more than 0.5mm of ice the frequency changes and it turns on the anti-ice system automatically and send hot air into the wing leading edges, the tailplane leading edge and the engine air inlets. Once on it stays on for a minimum of 60 seconds. The system is normally left in Auto, but with the caveat that the flight crew remain responsible for monitoring the accumulation of ice and the manual operation of the system.

I've heard a few people say they hardly ever use anti ice systems in 737s and 757s but in the Embraer it is very common for the system to activate. It can be a nuisacne because in the climb you lose some thrust, and it a decent it increases the flight idle hugely which can make slowing down in a decent difficult.