Few hangar flying topics evoke as much debate as the subject of airframe icing.  The shear volume of information and mis-information on this topic during these bravo-sierra sessions can be astounding!

I am particularly enthralled listening to flight instructors debate this issue.  One group group of CFIs insist that icing in the forecast is an automatic "no-go" decision.  They claim FAR 91.527 which prohibits flight into known ice conditions unless certain anti-icing equipment is on the airplane.

The other group is a bit more enlightened.  They recognize that ice will likely be in the forecast in the northern latitudes from November through March.  These folks don't lock their airplanes in the hangar during these winter months. 

So which group is the most prudent?

The best way to answer this question is to examine what meteorologists actually know about icing.  They know that icing, for the most part, occurs ONLY in the clouds.  They also know that not all clouds contain ice, even when their temperatures are below freezing.  In other words, there could be icing in one cloud and not in another.  There could also be icing in one part of the same cloud and not in another part of the same cloud.  Lastly, there could icing in one cloud at a given time and no icing in the same cloud six minutes later.

Clear as mud, right?

Here is a little more science about icing.  Clouds are nothing more than water droplets ranging in size from 10 micrometers to 450 micrometers.  To put this in perspective, a visible water droplet like a typical raindrop is about 400 micrometers in size.

The  tiniest water droplets, between 10 and 29 micrometers, are too small to pose any icing risk to airplanes.  Even if cooled below 32 deg F., they flow over, under, and around the airframe thereby causing no icing problems.

Similarly, the largest water droplets, say between 300 and 450 micrometers, even when cooled below freezing, form ice pellets and bounce off of the airframe.

The magic water droplet size for aviators is 30 to 300 micrometers.  These mid-range size water droplets have the potential of remaining in liquid form even though their temperatures are below freezing.  They are called "super-cooled water droplets."  They form solid ice when they impact the aircraft.

A couple of complicating factors . . .

Water droplets act differently depending upon the severity of freezing temperatures.  They can exist in super-cooled liquid form so long as the temperature doesn't dip below -20C.  Once the temperature gets that cold, they freeze as ice crystals and bounce off of the aircraft rather than sticking to it.

Another complicating factor is humidity.  Cold, relatively dry clouds contain super-cooled water droplets, but there may be too few of them to pose a threat to aircraft.  High altitude freezing cirrus clouds are examples of this condition.

The last complicating factor is stability of the air.  Airframe icing occurs primarily in unstable air.  Winter stratus clouds are generally quite stable.  Air within them rises at about one centimeter per second.  The chances of picking up ice in these kinds of clouds are very low.

[Photo left: Note ice build-up on the leading edge of the wing!]

Unstable clouds where air molecules inside them may be rising at, say, one meter per second can be a very high risk as far as airframe icing is concerned.  Here, tiny, non-risk water droplets of less than 30 micrometers in size are lifted inside the cloud and grow in size.  They reach their largest and most risky size of 200 to 300 micrometers right at the top of the clouds.

The "no go" group says, "Yup . . . there's that ice warning forecast.  Flight training canceled today!"

Hold it a minute.  Does the briefers comment constitute "known icing?"  Not necessarily.  That's why his cautionary comment is worded just the way it is.  He is saying that "at times" you may find icing in the clouds.  Conversely, it says that "at times" you may not find icing in the clouds. 

So what is a girl supposed to do?

Like all good girls, the proficient pilot must think!  She applies what she knows about the conditions in which airframe icing occurs.  She assesses the likelihood that she'll encounter icing during her planned flight.  Here are some of the factors she considers.  I call this "back door" analysis. 

You can see why icing is such a controversial topic among aviators.  The variables and options are numerous and there is no "one rule fits all."  Clearly, leaving the airplane in the hangar from November to April is not a solution.  Nor is blindly flying into freezing clouds.

This all comes under the heading of aeronautical decision making and risk management assessment - a topic that is just beginning to be talked about in flight training circles.

As for me . . .

Yes, I regularly fly in freezing clouds both in my known-ice certified T-210 and in training aircraft.  Have I encountered unexpected icing conditions in both kinds of airplanes?   Again, yes!

Have I ever had an icing emergency?  No . . . but on several occasions I did have to make a hasty retreat to my pre-planned back door!   The keys here are two-fold:  (1) take immediate action, and (2) always have a golden "back door."

A word about Icing PIREPs

One of the more serious "old wives' tales" put forth by, well . . . ignorant pilots is that "you will incriminate yourself if you file an icing PIREP from a non-known ice airplane." 

This tragic piece of mis-information probably leads to more icing encounters than anything else!  Your failure to call out icing leaves others behind you unaware of what lies ahead!

Nowhere has anybody been able to find a documented case where an air traffic controller initiated punitive action against a pilot for filing an icing PIREP.  In fact, most controllers have no idea if your airplane is certified for known ice or not!  In short, whenever you encounter even trace ice, advise ATC immediately!

Flight Instructors . . . avoid telling your students to remain on the ground anytime they hear the word "icing" in a weather brief.  Instead, teach them how to make responsible risk management decisions and how to engage in "back door" analysis.  Otherwise you leave them dangerously unprepared should they encounter unforecast icing on a long wintertime cross-country flight.

In summary, icing conditions are where you find them.  The important thing is to follow FAR 91.103 and secure ALL available information about the weather before your launch including recent PIREPS.  If you do not have any backdoors, don't launch!

For a more indepth discussion on icing, click HERE.