September 2005/
You might think that this season of the year is an unusual time to have a discussion about aircraft icing. Not really. All you have to do is climb a few thousand feet, enter some visible moisture and you can easily have an ice issue on your hands.
Late summer and autumn are the best times to have this discussion because it allows you time to make sure your airplane is in good shape for the upcoming icing season and that your knowledge and training is up to the challenge.
By now pilots should know that it is a bad idea to fly through icing conditions and get that stuff on their airplanes. It seems simple enough: stay out of ice and you stay out of trouble.
Icing forecasts are spotty at best and lots of times you’ll run into icing where you least expect it. Once in it, your three options of staying in the icing, climbing above it, or descending out of it aren’t as simple as they may seem.
Even though you might have good anti-icing and de-icing equipment, sometimes it isn’t enough to keep you safe. Climbing out of ice is usually the best idea because once the outside air temp gets below freezing you are out of danger, but that presupposes that your airplane still has the capability of climbing. One half-inch of airframe ice can reduce lift and increase drag by over 50 percent, so climbing may not be an option if you get frosted.
In order to have ice you need two things and two things only: Cold and Wet. Doesn’t matter if your airplane is providing the cold or fog at your local airport is providing the wet. You gotta have both in some combination to have ice.
“Clear, Rime, and Frost” is Not the Name of a Law Firm
Clear ice is caused by the slow freezing of large supercooled drops of H2O. They spread out when they hit your aircraft and freeze smooth and clear which makes it hard to detect with your naked eye.
In addition to the fact that it is hard to see, it is also very heavy—62 pounds per cubic foot. This ice is usually found in cumulo-bumpy clouds and occurs at temps between 0 and-10 degrees C. Clear ice is very difficult to get off an airframe once it’s on.
When we’re talking about Rime, we are certainly not talking about Leann. We are talking about that opaque, crusty, out-of-shape ice that can stick to your airplane tighter than a tattoo on a biker.
Rime ice instantly forms when small, supercooled drops hit your airplane and instantly freeze. It is brittle stuff, which is good, because it makes it easier to remove. It also forms weird shapes, which is not so good, because it changes the performance of your airfoil and propeller.
Rime happens mostly in stratiform clouds and can form in a wider temperature range. You can get rime ice between 0 and -40 C. Throughout your “circle of life” as a pilot, you can expect to get much more rime than clear.
Frost isn’t just a poet who had to go to the bathroom outside on a snowy evening. It is probably your biggest enemy if you fly General Aviation people movers through the chilly air of this great nation of the free and the brave.
Frost usually forms on parked aircraft and is a royal pain to get off. Make sure you remove it all, because just a quarter inch of the stuff on your wings can totally ruin your day.
The FARs allow you to “polish the frost smooth” before flight, but I’ve checked and Sporty’s doesn’t offer a frost polisher of any kind in their catalog. In my opinion, the very best defense against frost is something we pilots call the “heated hangar.”
More Kinds of Ice
Carburetor icing is something you’ve known about since your student pilot days in that dusky-smelling Cessna 150 when your CFI taught you about the “Gas, Mixture, Carb-Heat” procedure halfway on the downwind.
According to aviation experts, the reason carb ice is such a raspberry seed under everybody’s wisdom teeth is that it occurs in warm air. All you need is a dew point of around 65 degrees F and you’ll be yanking the carb heat handle.
Impact ice is the stuff of which NTSB investigations are made. Ice covering your induction systems, your windshield, static ports and pitot tubes can be more deadly than the stuff blanketing your wings. A Northwest Airlines Boeing 727 crew learned this the hard way in 1974 when ferrying the plane from JFK to Buffalo, N.Y.
During climbout, both pitot tubes were clogged with ice and the airspeed indicators in the jet became, in effect, altimeters. As the aircraft climbed into the thinner air the airspeed indicators, sensing the pressure difference between the clogged pitot tubes and the not-clogged static ports, showed ever increasing airspeed. As they increased pitch to slow down the increasing airspeed, it also increased until they entered a fatal spin.
After the crash it was determined that their pitot heat was never turned on. It was on the taxi checklist, but the switches were never moved. It is easy to fault these guys, but tell me this: you have one pitot heat system on your aircraft… how will you know for sure that it is continuously on, and what will you do if it fails?
The last kind of airframe icing we can talk about today is rarely discussed. It is the stuff that splashes up from the taxiways and runways. That’s right—slush and snow. Get a lot of slush frozen to your flaps and the underside of your horizontal stabilizers and you got trouble. Get enough of it stuck to your landing gear and you won’t be retracting it today. Slush is generally more of a problem than snow is for takeoff and landing because it is much heavier and tends to build up in front of your wheels.
Now that we’ve dispensed with the generalizations and definitions, just how do you deal with this stuff?
Just Don’t Do It
Most of you shouldn’t fly into icing, because it is illegal, immoral and not healthy for you to do so. According to the FAA, unless your airplane is equipped for flight into “known icing,” it is against the law for you to fly in the stuff.
If your aircraft is equipped for flight into known icing, you will know because it will be placarded and the salesperson you bought the bird from will have probably gone on and on about it.
If you are in doubt, you can look up the requirements in FAR part 23, section 34. Then you can peruse Advisory Circular AC 23.1419-2a. They will take pages and pages to tell you this: Your airplane is equipped for flight into “known icing” if it has deicing or anti-icing equipment protecting each propeller, windshield, wing, stabilizing or control surface. You must have ice protection for each airspeed, rate of climb, altimeter and flight attitude instrument system. I think you’ll agree with me that if you have ice protection on all that stuff you have one well-equipped bird.
De-Icing Equipment Won’t Keep You Out of Trouble
Let’s say you have all the equipment that I have on the 767 I fly to pay the rent: heated leading edges on the wings, hot tail, fuel heat, a heated windshield, engine anti-ice and a pitot-static heater system that won’t quit.
Even with all that airborne heat, I avoid icing conditions whenever I can and get out as soon as I can if I get in them. Having ice on your airplane is never a good idea and even the best anti-ice systems can fail or fall behind.
Recent pireps will be passed along by your controller, especially when moderate or heavy icing is reported, and are the best way to find out where the ice is. If you’re in doubt, ask your trusty ATC sidekick if there are any reports of icing in the area.
Your visual orbs are your best line of defense against the frosty stuff. Take a good look out of your windshield. Are you in a cloud? Now look at your outside air temp gauge. If it is at or below 0 degrees C you should be on the lookout for ice.
Every airplane has a best place to look for ice from the cockpit. On the 767, DC-9 and MD-88 it is the windshield wiper bolt. When we see ice forming on it we know it is time to turn on the airfoil anti-ice. For engine anti-ice, we rely on the TAT gauge (total air temp) and a peek outside our window. If we forget, the first sign of engine ice on a turbojet is an increase in EPR (exhaust pressure ratio) because the pitot-probe for that gauge ices over and leads to a really big jump.
Your plane’s icing location will be different. It should be an unheated spot that you can easily see from your seat. By the way, a little ice forming under your windshield wiper blade doesn’t always mean you have airframe ice, but it is a sign that you need to think about it. You usually can’t go wrong by turning on any anti-ice stuff you have ahead of the time that you really need it.
The exception to that rule is when you have pneumatic boots. That is a totally ‘nother animal and I have no expertise to offer you on the subject of boots other than I think you ought to trade up to a jet that has hot wings.
When you find yourself in icing conditions your best bet it to climb out of it if you can. Once you are in cold enough air that the wet stuff hitting your bird is already frozen you’re in good shape.
The problem with climbing, of course is two-fold. First, with an ever-building load of ice you may not be able to climb. Second, unless you are pressurized you are going to need oxygen sooner or later. Descending to find warmer air sometimes works. The problem with that strategy is also a “two-fer”. You may not be able to find warmer air or air warm enough to get rid of existing ice and second, you always have to worry more about the ground the closer you get to it.
Icing is an insidious problem because it is so painless until it is too late to do anything about it. When your airplane is getting frosted you usually aren’t in any turbulence or rain and the ride is deceptively smooth. Unlike thunderstorms which, although they are nasty beasts, are in easily definable spots, icing can be in every cloud along your route and on some really bad days can begin at ground level and go well above the service ceiling of your airplane.
With a little help from your friends in the form of pireps, a look or two at your temp gauge and some common sense you can avoid being the next pile of steaming aircraft wreckage on the six o’clock news.
Kevin Garrison’s aviation career began at age 15 as a lineboy in Lakeland, Fla. He came up through General Aviation and is currently a senior 767 captain. When not frightening passengers, Kevin plays tennis and lives on a horse farm in Kentucky, where he writes unsold humor projects and believes professional wrestling is real and all else is bogus.
