Ice Advice

Trace icing is just that: a very thin layer of ice, so thin that it may not be visible. It progresses very slowly, so slowly that you’d have to fly for an hour or so to reach a quarter-inch buildup. Light icing buildups take 15 minutes to an hour to reach a quarter-inch thickness. Moderate icing takes 15 minutes or less to make that quarter-inch accretion. Heavy or severe icing makes a quarter-inch of ice in less than five minutes. Mixed icing is just that—a mix of moderate to severe icing. And the absolutely worst icing is supercooled large droplet (SLD) icing. SLD ice has the largest droplets and hangs around the Great Lakes and Northeast and Northwest quadrants of the United States—where there’s a plentiful source of moisture. It splatters against the airplane, runs back to one-third of the wing chord, then refreezes on wings and other lifting surface.

Water droplet diameters and temperature influence the type of icing you’ll see and the accretion rates. The droplet diameters are defined in terms of microns. To get an idea of just how small micron diameters can be, twirl a sharpened pencil point on a piece of paper. The resulting dot would have a diameter of 500 microns. So, when we talk about droplet sizes, we’re talking small.

Trace and light icing accumulations first look like frost—or thin white lines along the leading edges; they’re associated with 15- to 40-micron droplet sizes. Rime icing has a rough-looking surface that doesn’t run back along the wings; its droplets have 15- to 40-micron diameters. Rime ice is the most common type of ice and is usually found in stratus clouds. Clear icing is just that, a transparent coating of ice created by 20- to 50-micron droplet diameters, of the kind found in cumulus clouds. Runback is common. Mixed icing is a blend of rime and clear ice that has a pebbly surface and forms in cumulus buildups with temperatures in the 5 to 10-degree Celsius range. SLD icing tops the micron chart with 1,000-plus micron droplet diameters—as large as small raindrops. That’s why they splatter noisily on the windshield, readily run back on wing chords, and produce ice buildups in windshield corners. SLD icing forms in temperatures just a few degrees above the zero-Celsius freezing mark, and lives in the same conditions as freezing rain.

So, on icing’s urgent-horror scale we can see trace/light at the low end with its tiny droplets and relatively high temperatures, followed by rime, clear, and mixed icing and their lower temperatures and larger droplets. Topping the chart is SLD and freezing rain, with their near-zero temperatures and monster droplets.

Icing is dynamic. Fly long enough in “light” rime icing and you’ll wind up in the “severe” category. That’s why avoiding or escaping any advertised or real icing condition is the prime directive for winter flying.

So, let’s face a winter fact of life for those of us flying piston-powered airplanes in the northern tier of states: If a chance of icing is in your preflight weather briefing, you’ll probably be stuck on the ground more often than you’d like.

Effect on aircraft, from the 1998 Inflight Icing Plan

^{Several efforts have been made to quantify icing intensity, including this FAA proposal
based on airspeed loss, climb-rate deterioration, power requirements, and control
input and vibration responses.}

There are several sources for getting a reading of the enroute weather aloft, where you’ll be spending most of your time. If you’re old-school and want a telephone weather briefing from a flight service station, go for it. Or you can start with the Aviation Weather Center’s (aviationweather.gov) online icing page, which is on the “Observations” drop-down menu. For selecting an altitude, a slider on the left of the page runs from the surface to FL270. The slider along the bottom of the page gives an 18-hour look at the anticipated future icing conditions, in one-hour increments. You’ll see icing graphics that show where trace, light, moderate, heavy, and supercooled large droplet (SLD) icing are predicted. Icing G-airmets, sigmets, and pireps are also depicted.

The AWC’s winter and terminal weather dashboard pages—on the “Tools” dropdown menu—are also worth a look. So is a satellite’s view of the cloud patterns; use the infrared channel if it’s night. As for weather radar, remember that it’s great for seeing precipitation but it has a hard time seeing clouds, let alone icing clouds.

Apps and tablet-based sources can also give helpful looks at potential icing situations. ForeFlight, for example, can show a cross-section of your planned flight path, as can the Weatherspork app. Select an altitude and you can easily see the predicted icing—and other weather and terrain representations—along your en route segment.

Pireps are another must-see part of any weather briefing. Just note how old they are, and what aircraft type is making the report. A “light” icing report from a Boeing 737 is one thing. On an unprotected piston single it could manifest itself as “heavy” ice accretions. A piston single’s small-radius leading edges are much more efficient at collecting ice than the large-radius leading edges of an airliner.

So, you’ve checked the weather and think there’s good chance of an ice-free flight. Now let’s say you’re in cruise flight and the worst has happened. For whatever reason, you’re flying in a cloud layer with rapidly accumulating ice and no FIKI approval. What to do?

Fly the airplane

Make sure the pitot heat is on.

Call ATC and ask for help. Hopefully they’ll send you to a nearby airport, one with a long, wide runway, and an instrument approach with a decent lighting array. Or the airport you just departed.

Check the OAT gauge and the appearance of any ice accretions. The closer you are to 0 Celsius, the heavier the ice buildups could be. You may have to quickly descend through icing layers as you prepare for the landing. Know your minimum enroute altitude to see your terrain clearance. An off-airport landing may be in the cards if your situation worsens.

Are you losing power? Ice may be blocking the engine air intake. Open the alternate air doors. Cycling the propeller may help shed ice and restore thrust.

If you’re using one, turn off the autopilot and hand-fly. Autopilots can mask feedback from the control surfaces—enough so that it could take you into a stall without warning.

Turn on the defroster to warm the windshield. This will help prevent ice from obscuring your vision ahead—something that could be very helpful during an approach and landing.

Should you climb to on-top conditions? It’s a tough decision. Climbing will increase your angle of attack and cause ice to form on the underside of the wings, further disrupting the flow of air over them. Also, are you positive that on-top conditions will persist until you reach your destination?

For approach and landing, carry extra airspeed, and don’t extend the flaps. That would change the airflow over the elevator. Extending the flaps can lead to a tailplane stall. Because the elevators provide negative lift, the nose will abruptly drop. The control yoke responds to a tailplane stall by slamming forward. Recovering from a tailplane stall is just the opposite of a conventional stall—you have to yank the yoke aft.

Some pilots with experience have flown in icing conditions and lived to tell the tale. This is how bad information gets around. “A small accretion of rime ice along the leading edges actually make the wing perform better,” goes another yarn. “You can’t get ice if you’re flying in clear air,” someone in a bull session may say. Those flying airplanes with deice boots may say to “wait until a quarter or half-inch of ice has built, then blow the boots.”

Icing experts at NASA’s Glenn Research Center have said that small rime accretions can increase lift—and drag—but this effect is short-lived. Within two minutes, lift coefficient can decrease by about 30 percent, stall angle of attack decreases by six degrees or so, and drag can double. So, the advice is to activate the boots the moment you first notice ice. The idea behind waiting for ice to build is based on the notion that too-early or too-frequent boot inflation can cause ice to form a shell-like coating over the boots, leaving them useless as they inflate within a gap between the leading edges and the ice accretion. This phenomenon, known as ice bridging, has been debunked. The ice-bridging myth may have its roots in Ernest K. Gann’s Fate is The Hunter, in which Gann flies a DC–2 over the Appalachians. The airplane is coated in ice, so Gann inflates the boots—and they don’t work. Today, Gann’s problem is explained by the DC–2’s low inflation pressures and pneumatic pump capacities. Modern boot systems work at higher pressure and inflation rates.

No ice in good visibilities? Wrong. A warm frontal surface aloft can produce rain that falls into the colder air below, creating the clear ice of freezing rain. This is the same logic that debunks the advice to climb if you encounter freezing rain. Yes, there is warmer air aloft, but can your airplane climb with a layer of freezing rain? Probably not. Bear in mind that no airplane—not even one with FIKI certification—is certified for flight in freezing rain.

Thomas A. Horne is a former editor at large for AOPA media and the author of Flying America’s Weather.