Airframe icing is bad stuff. The drag of ice accumulation knocks a bunch of knots off your airspeed. And the rough, uneven surface of most icing alters the performance of the wings and tail, making flying qualities and stall behavior unpredictable.
But for most of us the worst aspect of seeing ice start to form on the leading edges is the uncertainty. There are uncommon situations where you can fly from ice-free air directly into an area that creates almost instant significant ice accumulation. But that is rare. The usual icing encounter starts out with a little frost forming on the leading edges or on the unheated windshield.
That trace of ice won’t really matter to any airplane. That first little bit of ice forms at the stagnation point of the leading edge, the point where the air molecules separate with some going up and over the wing while others slide under, leaving a very small thin line where a few molecules don’t go either way.
But even that first trace of ice raises anxiety in us pilots. Will the ice continue to build? Is what I’m seeing really airframe ice, or is it just snow or other frozen particles stuck at the stagnation point? Should I change altitude? If so, which way?
In all my years of flying piston airplanes, obsessing over icing and trying to decide if the ice is really building, and if so, how fast, has been the most worrisome. Thunderstorms are a different matter. If you get too close you know within seconds you are too close to the storm because the airplane starts climbing with the nose pointed down and you know a giant jolt of turbulence is coming next.
Icing almost always gives you time to think about what to do. Almost always – with the emphasis on almost – the icing will remain minor and not become a problem. Knowing that you usually have time to react can lead you to delay taking action to escape the icing conditions. Reports from other pilots in the area can also comfort you that they have not experienced any icing, or at least nothing bad enough to be of concern.
Delaying the decision, or ignoring pilot reports of icing along your route, is playing with fire, if one can use the metaphor when describing something that only happens when the temperature is below freezing. But we are all guilty of dithering on icing, especially in the springtime.
I was flying back to my home airport in Muskegon, Michigan, from Oshkosh not long ago when the Milwaukee controller told me icing had been reported along my route over eastern Wisconsin and Lake Michigan. And he added, helpfully, that if I wanted to change altitudes to just let him know.
I was not surprised that some pilots had found ice. Spring has been late in arriving in the northern tier of states and the temperature was in the low 40s on the ground at Oshkosh with very light rain when I took off. Often in such situations it is warmer aloft or at least no colder than at the surface, so ice wasn’t a certainty, but a possibility.
Climbing through 5,000 feet the air temperature dropped below freezing but no ice showed up. As I leveled at 7,000 feet, still in solid but reasonably smooth clouds, a trace of ice formed on the leading edges. The weather on the other side of Lake Michigan was good, with broken clouds at 12,000 feet, so I knew somewhere along the route I would fly out of the clouds and the trace of ice.
But the trace didn’t remain a trace. Those lumpy little horns of classic rime ice began to form all over the airplane. I turned on the propeller anti-ice and waited. Finally I had to cycle the boots – several times. I lost 15 knots or more of airspeed from the drag. Other pilots were also reporting icing at my altitude and above all the way to the tops that were around 12,500 feet. Why was I sitting there collecting ice instead of doing something?
The answer is “because it was spring.” Even though it was a cold spring day, I knew there was above freezing air below me and if worse came to worse I would sink into that warmer air. The anxiety of a winter icing encounter was missing because I knew there were several escape paths. Absent the winter anxiety, I sat there for too long before asking for 5,000 feet.
Down at 5,000 the ice stopped forming, but stayed with the airplane on all of those places not protected by the boots. About two-thirds of the way across Lake Michigan I flew into above freezing air and the ice melted off. It’s always fun to watch ice slide off when you know the air ahead is clear and warm.
It would be ridiculous to say icing in the spring is different than icing in the winter, because it’s not – except for the way it looks from the left seat.