Wx Watch: Icing Rules of Thumb

Tips for surviving the icing season

January 1, 1999

Every pilot knows that airframe, induction system, and carburetor icing is bad news. But unless you plan on remaining ground-bound every time winter rolls around, you've got to have a set of guidelines that will let you form a framework for safe winter flying. Then you have to stick to them. If a forecast's mention of icing conditions makes you uneasy, then staying on the ground may indeed make the most sense. For VFR-only pilots, flight in clouds is prohibited, but that doesn't mean that there's a guarantee of ice-free flying — carburetor icing, for example, can strike no matter what time of year. If you're an experienced instrument pilot with lots of winter actual IFR and have an airplane with ice protection equipment and known icing certification, then your options open up a bit. But — and this can't be overemphasized — there are situations where even the highest-time pilot with the most powerful, capable airplane must exercise extreme caution. Caution to the point of staying on the ground.

So what are some good, reliable rules of thumb for flying, or not flying, when icing conditions threaten? Getting a good preflight weather briefing is a given, of course. Having an instrument rating provides what could be a lifesaving extra measure of skill. But here's some additional advice that has stood the test of time:

Fly in the clear. This is rule number one, and an obvious one. If you avoid clouds by flying on cloud-sparse days, on top, or between cloud layers, then you can't encounter most types of airframe icing. In other words, maintain VFR cloud separation minimums, even if you're flying on an IFR flight plan. Carburetor ice, however, can still pose a problem.

Stay out of the frozen zone. Airframe ice comes in two main varieties: clear and rime. Clear ice is smooth and transparent, and holds tenaciously to leading edges. Rime ice is rough to the touch and milky white in appearance. Rime ice most often forms between minus 20 and minus 10 degrees Celsius (minus 4 and plus 14 degrees Fahrenheit). Clear ice — the worse of the two types — usually occurs when you're flying in clouds and outside air temperatures are between minus 10 and plus 2 degrees Celsius (14 and 35 degrees Fahrenheit). The closer temperatures are to the 0 Celsius mark, the worse any airframe accretions tend to be. These include the double-horn formations that happen in large-droplet cloud conditions. Mixed icing typically occurs near the minus-10-degree-Celsius mark.

As part of your preflight preparations, make sure that you note the temperatures aloft during the weather briefing, then avoid the altitudes in the dangerous part of the frozen zone. Area forecasts, airmets, and sigmets should also give advance warning of any potential icing conditions along your route.

Look for low cloud tops. Cloud top information is mentioned in area forecasts and may also be part of any pireps. What you want is tops low enough that you can safely top them if a climb is necessary. Your ability to top them depends on the weather, as well as your airplane. Cloud tops of 20,000 feet may be toppable if you're flying a turbocharged or turbine-powered airplane, but are certainly out of your league if you're in a normally aspirated piston single.

Also, bear in mind that some of the heaviest icing accumulations can happen right at cloud-top height. For that reason, flying at an altitude that puts you in and out of cloud tops probably is not a good idea.

The good news? In the winter, tops are generally lower than they are in the warmer months of the year. Many times, a good strategy can be to begin your flight by climbing to an on-top altitude, then stay there as long as you can.

Make it warm down low. Above-freezing temperatures at low altitudes are absolutely essential for safe winter flying in instrument meterological conditions. Why? You want to lose ice as you descend, not pack more on. Ideally, above-freezing temperatures should extend all the way up to the minimum en route altitudes (MEAs) along your route. One of the worst icing situations would be one in which you take on a load of ice at altitude, can't maintain altitude, and then have to descend into hostile terrain or shoot an instrument approach to minimums — also in icing conditions. Your ability to climb for a missed approach or any other reason may be nonexistent.

Beware the warm front. Your briefing may mention a warm front along your route of flight. To this, you may think "great, warm temperatures aloft mean that no ice can form." Wrong-o. In a warm front setup, rain falling from the warm front aloft can turn into supercooled droplets as they fall into the cold air slowly retreating ahead of it. This means that you could be flying clear of clouds ahead of a warm front's surface position and still be hit with freezing rain, the absolute worst kind of clear ice.

Assess your airplane. Your ability to handle inadvertent icing encounters depends heavily on your airplane. If it's turbine-powered or turbocharged and has certification for flight into known-icing conditions, then you're fortunate. If it has ice protection equipment that's merely STC'd, then that's certainly better than nothing, but no substitute for a known-ice-certified airplane that's undergone a full battery of icing tests. Airplanes with ice protection gear and plenty of power give their pilots more icing escape options, but a 160-horsepower, fixed-gear single doesn't have a fighting chance if it runs into anything more than the lightest icing. So take a good hard look at your airplane and be honest about its ability to climb or even maintain altitude with a coating of lift-robbing ice.

Have an escape plan. Know ahead of time what you'll do if icing is encountered on any route segment. This could be a climb, a descent, a landing, a 180-degree turn, or any combination of the above. The main thing is that you are mentally prepared for ice and committed to carrying out your plan. Can't form a reasonable plan? Then you probably shouldn't be flying.

Minimize exposure in climb or descent. OK, let's say that you had a good briefing, have been flying on instruments, and suddenly you notice something unforecast: ice is beginning to form on the airplane.

First rule: Don't panic. Maintain control of the airplane, turn on any ice protection equipment, and contact air traffic control for priority handling.

Second rule: Carry out the escape plan.

If you decide to climb or descend, don't dally around in an icing layer. Try to climb at Vy to reach on-top or between-layer conditions as quickly as you safely can. This minimizes any further ice buildups. Some airplanes publish minimum airspeeds for flight in icing conditions, based on avoidance of the kinds of high angles of attack that can put ice under the wing and well aft of the wing leading edges. Ice aft of the leading edges will spoil lift quickly and could easily lead to a stall at airspeeds well above those published.

Check your pilot's operating handbook to see if these minimum airspeeds apply. If they do, then be sure to avoid flying any slower.

Descents should be equally speedy. You want to shed your ice as soon as possible, and that goes for airplanes with or without ice protection equipment.

Use what you've got. All right, so you're flying a Cessna 172 Skyhawk and you suspect that icing could become a factor. Does this mean all is lost? No way. You should carry out your escape plan and turn on all available ice protection equipment. You probably have pitot heat; turn it on, if you haven't already. You have carburetor heat; turn it on, too. Call ATC, let them know of your problem, and find out about the weather for the diversion to an alternate airport. Many are the pilots who simply forgot to turn on their pitot or carburetor heat and paid dearly for these oversights. True, this meager equipment can't pop ice off your wings, but it just may keep you safe in the air long enough to escape icing conditions.

By the way, once you've turned on carburetor heat, leave it on — even though the engine may run rough. It's the melting ice that's causing the engine to falter. The temptation is to turn off the carburetor heat in an attempt to return the engine to its "normal" sound. But doing this will cause more ice to form. Leaving carburetor heat on assures that all the ice will be melted, assuming that you turned it on in time. If there's any subsequent roughness, it can often be eliminated by leaning the mixture slightly.

Known-ice approved? Don't get cocky. Some of the worst icing accidents happened to high-time pilots flying large, powerful, known-ice-approved airplanes. If icing is bad enough, it will fell any airplane, regardless of its equipment list. Escape plans are for turbine and large piston-powered airplanes, too.

Stuck? Try these. Let's say the worst has happened. You're stuck in an icing layer, and it will be awhile until you make your way out. Here are some recommendations:

  • Turn off the autopilot, if you have one. Autopilots can mask abnormal control feel and deny you warnings of control surface icing and/or an impending stall.
  • Don't extend flaps. This can allow ice accretions to form well aft of the leading edges.
  • Avoid abrupt or excessive flight control movements.
  • If your airplane is equipped with ice protection equipment, make sure that it's on and that you're operating it properly. With deice boots, it's vitally important that you allow one-quarter to three-eighths-inch of ice to form on the wings before inflating the boots. (Follow the advice in your POH for exact advice on inflation procedures.) Inflate the boots too often and ice can form a bridge over the boot surfaces, making it impossible to shed.
  • Ask ATC for priority handling to exit icing conditions. I know, that's already been said. But new regulatory language now requires pilots to notify ATC if severe icing is encountered, and pilots are also required to report unforecast hazardous weather per FAR Part 91.183(b). So this advice is more than precautionary in nature. Besides, any ice accumulation is bad and can become severe if you mill around long enough in icing.

Winter flying doesn't need to be scary. In fact, some of the best flying weather occurs in the colder months of the year. Engines perform better, convection is at a minimum, and visibilities run high. With advantages like these, waiting out a major ice-producing weather system remains perhaps the most agreeable option in the ice-avoidance arsenal.

E-mail the author at tom.horne@aopa.org.