Topic

Icing

Topic Progress:

Brrr…..its cold!

airplane covered in ice

It’s safe to say this airplane isn’t going flying anytime soon.  In this TOPIC we are going to talk about the factors that make ice form on airplanes as well as the different types of icing, what to do if you suspect icing, and how to get rid of it.

First off, why is it so bad?  It seems obvious that ice and airplanes do not mix well, but there is a common misconception that the accumulation of heavy ice is simply too much weight for the aircraft to carry and thus it stalls.  In fact, the weight of the ice, while not helpful, is really not the main issue.  The bigger problem is that any sort of ice or frost on the lifting surfaces (note: I said lifting surfaces, not just wings) of an airplane will disrupt the smooth flow of air over the lifting surfaces, greatly increasing the speed the airplane must fly at to stay flying.

Ice may accumulate so much in fact (actually just 1/4″ may be enough) to disrupt the airflow so much that the aircraft must fly at a higher angle of attack, increasing drag, and with that increase in drag, the aircraft may not even have enough power to be able to stay aloft.  The aircraft may not have enough power to keep it at a high enough speed to fly and still maintain altitude, the bottom line is it is coming down (now it’s just a question of where it is going to meet the ground and in how many pieces).  What I want to communicate to you here is this: ice on airplanes changes the aerodynamic shape of the lifting surfaces, and not for the better.  The airplane with ice will now stall at a much higher speed, behave in strange ways, and stall at a different angle of attack than was originally designed.  Congratulations, if your dream was to be a test pilot for NASA, once you have ice on your airplane, you are one.

Types of Icing

For our purposes there are really only three types of icing that occurs on airplanes:

  • Clear Ice (think heavy solid ice like you get from the freezer for your drinks)
  • Rime Ice (think snow cone, its a little more airy and less dense than clear ice)
  • Mixed (it’s exactly what it sounds like, and shocker, it’s probably the most common type)

Broken down even further, you’re going to get this icing in one of two places:

  • Structural Icing (anything that sticks to the wings, airframe, basically any outside part of the airplane)
  • Induction system icing (ice that forms and blocks your air filter, or forms in the carburetor and must be melted with Carb Heat before it accumulates to such a degree the engine can no longer produce enough heat to melt it).  Since we’ve already talked about using Carb Heat in another LESSON, we are going to stick to structural icing for now.

Notice how no matter where the icing is accumulating, it, unfortunately, is not in your vodka glass.

Clear Ice

Clear ice forms when a liquid water droplet hits the airplane and only part of it freezes right away, then the rest streams back slightly in the wind and freezes.  The slightly slower process of freezing means air bubbles can escape the water droplet and the ice that forms is clear and very dense.  It is shiny and looks pretty, but aside from the weight of the ice on the airplane (the weight isn’t the big problem), the clear ice is still going to negatively affect the smooth airflow over the wing surface.  The biggest downside to clear ice is that it can be difficult to get rid of due to how dense it is.

Rime Ice

Rime ice forms in cooler temperatures when water droplets strike the airplane and rapidly freeze, trapping air bubbles in the mix.  This makes for a milky / opaque, brittle, thicker ice growth that can grow forward from the leading edges of the wings out into the air stream and seriously disrupt the airflow over the wing.

Mixed Ice

You guessed it!  This is a mix of rime and clear ice.  Again, as with all icing, this is really nasty stuff.  It is common with mixed icing to have very strange shapes, such as horns or other protrusions from the leading edge of the wing, that will substantially disrupt airflow over the wing.

Factors that affect icing

Things that affect what kind of ice and how much, if any, sticks to your airplane are:

  • Temperature
  • The moisture content of the air (or cloud)
  • Droplet size
  • SLWC (supercooled liquid water content)
  • Speed

When you are most likely to get it

There’s a number of ways to get icing on an airplane, but the most surefire way is to fly into visible moisture when the temperature is near, at, or below freezing (some will say “oh it’s too cold to get ice today”; however, I can tell you from personal experience, I’ve picked up icing with an OAT of -20 degrees C, yes the colder it is the likelihood of the moisture in the air sticking to the airplane does decrease, but be very wary of counting on it).  Visible moisture could be defined as: rain, clouds, or fog with visibility less than 1sm.

Just flying in clear air though does not guarantee you will not pick up any ice.  While it is incredibly rare to get ice when no visible moisture is around, we are all familiar with frost that forms on our cars overnight even when it is otherwise clear outside.  The same is possible for your airplane.  If you are flying up high in very cold air, your airframe and more importantly your fuel in your wings gets very, very cold.  Flying then with very cold fuel into moist air, whether the temperature is above freezing or not, is a good recipe for picking up some ice.  Be cautious when flying in high humidity after flying up high in very cold temperatures and “cold soaking” your airplane and its fuel.

Frost

As far as ice appearing out of thin air like frost does, let’s look at the technical definition of what we need for frost to form.  For frost to form the DEWPOINT must be below freezing. The Dewpoint is the temperature at which the air becomes saturated and moisture becomes visible.  If you have a surface like a wing sitting out on the ramp, it is very possible for it to radiate off heat at night and for it to drop in temperature below freezing, even when the air temperature may be above freezing.  Say the metal of the wing dips down to 29 degrees F and the OAT is 35 degrees F and dewpoint is 30 degrees F.  You can expect some frost to form for sure on that wing. Remember: Frost is not rain or visible moisture freezing to a surface, it is water vapor in the air going directly from a gas to a solid, skipping the liquid stage.  That is what differentiates it from just say freezing mist or rain.

frost on airplane wing

Freezing Rain

A sure sign that freezing rain exists is ice pellets falling at the surface.  Freezing rain is, you guessed it, rain that is liquid up high, then freezes before hitting the surface (or when it hits the surface).

Rain, of course, starts out as ice crystals and then falls into warmer air that melts it back into water.  This is fine when you’re flying and it’s 55 degrees F outside (no chance of icing).  However, when you have a temperature inversion (perhaps a warm front or just a warm layer of air aloft) and the temperature at the surface is at or below freezing, this poses quite the problem.  You could easily have liquid rain falling through the warm air, only to hit your airplane that is flying below in the colder air (perhaps 32F or 30F degree air) and then re-freeze when it hits the airframe.  Typically this will lead to clear ice forming very quickly on your airplane.  Moral of the story, if they forecast anything like this to be possible, or you see ice pellets falling at the surface, pour yourself a glass of hot apple cider with a shot of whiskey and turn on the flight simulator, you’ll only be pretend flying today.

temperature inversion ice pellets warm front

Supercooled Water Droplets

Supercooled Water Droplets are the things pilot’s nightmares are made of.

We all know water freezes at 0C or 32F, however in reality that is not totally true.  Very pure H2O (water) can, in fact, exist in a liquid form below its normal freezing point of 32F.  We call these droplets of water “supercooled”.  All it takes for these droplets of water to change state to solid (freeze) is either adding impurities, or agitation.  Check out the video below to see perfectly clear distilled liquid drinking water that is around 28 degrees F in a bottle.  Hitting the bottle (your wing striking the water droplets) serves as the catalyst to get the water to change state and freeze to a solid.  Even pouring the water out you can see how it rapidly builds up ice (imagine the doorstep of the truck is your wing and you are flying through rain that is actually supercooled water droplets) that could easily stick to your airframe and quickly turn your day very bad.

Mountain Icing

Moist air can easily be pushed up the side of a mountain and climb well above its peak.  When the temperatures are right, these rising currents of air can support large quantities of moisture and supercooled water droplets.

The most severe icing occurs above the crests and on the ridges’ windward side. This zone usually extends to about 5,000 feet above the mountaintops, but can extend much higher if cumuliform clouds develop. Icing with mountains can be especially hazardous because a pilot may be unable to descend to above freezing temperatures due to terrain elevation. If a pilot approaches a mountain ridge from the windward side, his aircraft may be unable to climb above the mountaintops, or even maintain altitude due to severe ice accumulation. The end result may be a landing crash.

mountain icing hazards airplane

And Finally, The Last Type of Icing: Flying into any visible moisture when the temperature is near, at, or below freezing

Ya, I know, I said the exact same thing above, but I figured it might need repeating.  Stay away from clouds, rain, visible moisture and very humid air when flying in the cold!

How you get rid of it

There are two ways to fight ice.  Anti-ice and De-ice

Examples of De-ice:

  • De-ice boots (inflatable layers (looks like a black rubber lining) on the leading edge of the lifting surface that expands with high-pressure air and “cracks” the ice off the airplane) wing ice boot

Examples of Anti-ice

  • TKS Fluid (constantly pumps alcohol solution through tiny pores on the leading edge of lifting surface, the downside is you eventually run out of fluid) (propellers may also have small nozzles near the propeller hub that can spray fluid onto the propeller blades)
  • Heated Wing / Heated Prop (generally uses high pressure and high temperature bleed air from a turbine engine to flow through a “piccolo” tube and heat the wing to prevent ice from forming) (propeller blades are generally heated with electric heat).

(note: a weeping wing, such as one with TKS fluid may be considered by some to be both de-ice and anti-ice, I think of it more just as anti-ice)

deice wing

Heated Wing Diagram

tks pores

TKS / Weeping Wing

What to do when you have it

Well, here’s the million dollar question…. I have ice on my airplane and no anti-ice or de-ice equipment with the exception of maybe pitot heat and windscreen defrost, what do I do?

Well first of all, you should have used every available resource in your preflight planning and Aeronautical Decision making skills to not fly in the area you are flying in now with ice on your plane!!!!

But since you’re already here, you don’t exactly want to just give up, so let’s look at what we can do to make a bad situation a little better, or at least not worse.

  • Typically, the faster you fly the less ice you will accumulate, plus flying slow increases the angle of attack, allowing more ice to form under the wing disrupting more airflow compared to a low angle of attack just getting ice on the leading edge.
  • If you weren’t picking up ice 5 minutes ago, but now you are, I’d seriously consider high-tailing it back to where you were last where there weren’t icing conditions and then find a place to land ASAP!  (180 degree is almost always your best bet!)
  • Fly in the clear, preferably in warmer air (maybe descend), or fly away from the clouds into the sun where although the temperature may still be well below freezing, the sun can sublimate the ice off of the airframe.
  • While you consider your escape route, remember your airplane is now going to have a higher 1G stall speed (stall speed in level flight) than it did before, and the stall warning device (horn or light) probably won’t warn you of an impending stall since it was calibrated for a different angle of attack (now your wing is going to stall at a new angle of attack determined by just exactly what shape and size those icicles are).  I’d recommend not playing the part of test pilot and finding out what the new stall speed is, instead, fly fast, and fly fast out of the icing conditions.
  • Talk with ATC, tell them what is happening and what you need.  Whether it is reports of clear air from other pilots or have them to tell you where the nearest airport is, use them as your resource (your tax dollars pay them, put them to work for you!)  Don’t hesitate to declare an emergency and ask for all the help you need.
  • REMEMBER: If the Pitot tube gets completely iced over, you will no longer have accurate airspeed information.  Instead, you will have the airspeed indicator frozen on the last measured airspeed, and changes in airspeed will not be reflected on the instrument.  However, with the Pitot tube completely covered and the static port still open, the Airspeed Indicator will act as an altimeter, showing an increase in airspeed with an increase in altitude, and a decrease in airspeed with a decrease in altitude.  This all happens irrelevant of the actual airspeed you are flying when you have both the inlet and drain hole on the Pitot plugged with ice or plugged with anything else for that matter.
  • Plan for a long landing roll out since you’ll be flying a much faster approach speed than normal, and also plan a flap-less landing.  Remember up above when we said “lifting surface” instead of just wing? Well, your tail is a lifting surface that is quite crucial to you flying in a horizontal manner rather than a plummeting manner.  (see diagram below)

tail stall airplane

All airplanes fly with the center of gravity slightly forward of the center of pressure (fancy way of saying forward of where the wing does its lifting from).  Since the center of gravity is forward of the wings, the airplane naturally wants to nose over and point at the ground.  This is a good thing, as it makes it easier for us to recover from a stall (when the wing stops “flying” the airplane points its nose at the ground lowering the angle of attack and making recovery much easier and possible to boot, an aft CG may make it impossible to lower the nose and recover from a stall!).  To keep the airplane flying level the tail also creates lift, but in a downward direction to balance out the “weight” in front of the wings.  If we were to lose the lift from the tail, the airplane would enter a very steep nose down attitude (this bad attitude would be “fixed” upon impact with the ground).

So none of that sounds good, how do we avoid that?  Well, the tail stalling is not really a concern for us on a normal day of flying.  However, if there is ice accumulating on the tail, then that will change the airflow over the tail and make it stall at a higher airspeed.  What will aggravate this, even more, is lowering the flaps.  This is because lowering the flaps deflects air downward and disrupts the smooth airflow over the tail every time you lower them.  Normally, not such a big deal, but if your tail is already on the verge of stalling, lowering the flaps can be the straw that breaks the camel’s back (or the straw to break your back if you happen to be an occupant in the aforementioned airplane).  So what are we to do when in icing conditions?

  • Fly faster than normal
  • Don’t use the flaps
  • Get the heck out of icing conditions FAST!

Simple enough right?

Statistics

The most disappointing fact in the statistics about icing accidents for me is that almost 40% of icing accidents occur on takeoff due to ice or frost already being present on the airplane prior to flight.  This is obviously VERY avoidable.  There is no such thing as an EMERGENCY takeoff! You can always stay on the ground and go back and get the airplane de-iced, or stay on the ground and drive instead of fly that day.

REMEMBER: Just a thin layer of frost (the thickness and texture of coarse sandpaper) on the wings is enough to prevent an airplane from being able to safely takeoff or climb more than just a few feet above the ground (increasing drag by 40%-80%).  Make sure there are NO traces of ice or frost on your airplane BEFORE you fly!

Bonus:

How cold can water actually get before freezing (what is the absolute freezing temperature of water)?