Cool season challenges: Avoid known icing conditions

During private pilot training, most of us get a detailed lesson on carburetor icing, but not much on airframe icing. The reasoning is we are VFR-only pilots in the beginning and we should not be flying in the clouds. But even in VFR conditions we can deal with frozen precipitation at times, including frost, freezing rain, and snow.

Most single-engine piston aircraft have no icing protection beyond pitot heat. Even those with some ice protection often have placards stating, “Flight into Known Icing Conditions Prohibited.” NTSB report after NTSB report show where icing resulted in loss of control accidents—many fatal. It is a challenge, especially for those early in their flying, to fully grasp known icing conditions. In-flight icing occurs when water droplets from clouds freeze on the surface of an aircraft. “Supercooled” large drops are especially dangerous because they can rapidly create ice, increasing drag and decreasing lift (see “Dangerous Droplets,” December 2022 AOPA Pilot).

Icing can be more of a no-go decision for me than thunderstorms. Oftentimes, we can work to navigate around thunderstorms, but icing layers are much more of a challenge. Frankly, I’m scared of ice in a small aircraft. I never execute a cross-country trip in my Mooney M20J during the cooler months without looking at icing potential along my route several times the day before and right up to the time of my trip. The good news is we have very good resources to help us with our flight planning during the cool season. The more conventional data source is the Aviation Weather Center (aviationweather.gov) and our FAA weather briefings. The Aviation Weather Center provides a new interactive tool that allows pilots to visualize forecast icing levels and pilot reports. Imagery from aviationweather.gov is also available within many electronic flight bags including ForeFlight and Garmin Pilot. The icing analysis and probability chart helps determine the chance for icing with user-selectable altitudes. These are great tools for the near term, but what about a trip several days from now?

Let’s now venture beyond the briefing and look at some nonconventional sources for weather data. One of my favorite sites is pivotalweather.com. The free weather data site has a variety of forecast models to peak into the future weather. One of my favorites for short-term forecasting is the High Resolution Rapid Refresh Model (HRRR). The model goes out 48 hours. You simply click on the pivotalweather.com map how many hours out from model initialization you want to examine.

Once selected, one of the handy features on pivotalweather.com is the ability to click inside the map and pull up a Skew-T chart for the location you wish to query. With this rather complex-looking chart, you might think this is something reserved for weather geeks. It is not, and is probably easier to understand than you think. I will keep it to the point and specific about icing in this article, but you can also forecast cloud layers and severe weather using these tools.

On the Skew-T, the left scale is the altitude in millibars and the scale at the bottom is temperature in Celsius. Look for the zero degree line or freezing line. That’s important. You will also notice a green and red line. The red line is the temperature at each layer of the atmosphere while the green line is the dewpoint temperature. It is where these two lines come together that clouds form. If you see the two lines converge to the left of the zero degree line, this is where you likely have clouds with the temperatures below freezing. Guess what? This is where icing could be troublesome.

In the included example, you can see the circled area where icing would be most likely.

To give you a reference, 850 millibars is around 5,000 feet msl. This icing trouble spot is fairly thick for piston aircraft, extending from roughly 5,000 feet to upwards of 15,000 feet. Where the red and green lines diverge is indicating a drier atmosphere and an absence of clouds. Supercooled large droplets are typically found at altitudes lower than 12,000 feet and can result in severe airframe icing. Unfortunately, this is usually the altitudes where aircraft with limited ice protection fly.

I recommend using this tool and clicking on the map along your route of flight to get a good idea for freezing layers and also where visible moisture could be present. Keep in mind, this is a forecast model and seeing into the future isn’t always accurate. This planning tool is no replacement for looking at pilot reports and a current icing analysis before you start up.

There are other models that are useful in more extended range planning. For example, the North American Mesoscale model extends out to 84 hours and the Global Forecast System model reaches out to 384 hours. The longer range forecast hours tend to be much less accurate because any error is amplified.

As we enter the cool season, be safe. A moving trim wheel, decaying airspeed, and the sudden autopilot disconnection is not how you want to get your first actual icing lesson.

JP Dice is a corporate pilot, flight instructor, and longtime television meteorologist and weather consultant.