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Courtesy:
UCAR
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.
Anti-icing systems are great when they work. Ice can build up fast when
they stop.
The build-up on one propeller de-ice boot took only five minutes in
moderate icing conditions at 6,000 feet over lake Michigan. Courtesy:
Fareed Guyot
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.
Anti-icing systems can help during icing encounters but cannot protect all
surfaces. Courtesy: NASA
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.
This entry was posted in Airmanship, Mac Clellan's Left Seat Blog, Safety.
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