Saturday, December 1, 2007
Avoid flying by rules of thumb
Folklore: A layer of clouds a couple thousand feet thick isn’t a serious icing hazard.
This definitely isn’t the case for a stratocumulus cloud deck. These clouds are often found in the wake of strong cold fronts. Stratocumulus clouds can produce some nasty icing and typically are only a few thousand feet thick. They have characteristics of both stratus clouds and cumulus clouds. They can cover a large geographic area like stratus, but are associated with instability like cumulus. These clouds are “capped” by a strong temperature inversion, which acts as a lid on their growth. Instead of the smooth appearance of the tops of most overcast stratus decks, stratocumulus clouds have a quiltlike appearance.
On January 13, 2006, stratocumulus clouds crippled a Cirrus SR22 southeast of Birmingham, Ala. The pilot departed Birmingham and attempted to climb on top of this “thin” deck of clouds. Just as he broke out on top, he had to activate the Cirrus Airframe Parachute System (CAPS) after losing control of the aircraft because of significant ice accumulation.
According to the NTSB, “The airplane entered the clouds at 5,000 feet on autopilot, climbing at 120 knots. Upon reaching 7,000 feet, the airplane encountered icing conditions. The pilot informed the controller that he’d like to climb to 9,000 feet, which was approved. As the airplane reached the cloud tops in visual flight conditions at 8,000 feet, the airplane began to buffet.”
It’s common to have the highest liquid-water content at the very top of these clouds, where the drops are normally the largest and the temperature is the coldest. In this case, the cloud-top temperature was minus-10 degrees Celsius—perfect for a nasty icing event.
Folklore: Flying into a cloud that’s producing snow isn’t an icing risk.
Snow falling from a cloud base is a good sign that ice crystals exist in the clouds producing them. Thus, it’s easy to conclude that supercooled liquid water doesn’t exist in them, and therefore, there isn’t an icing risk in these clouds.
While there are important exceptions, precipitation falling from a cloud can lessen the icing threat within that cloud. Moreover, if the precipitation is snow, the threat of icing is even less—but not zero. Snowfall can scour out the supercooled liquid water in the cloud, but a snow-producing cloud may still be a mixed-phase cloud. That is, it will contain both ice crystals and supercooled liquid water.
This is especially true when the snow falling is “showery.” Aircraft flying into weather that’s producing snow showers will often report rime ice accretion. Normally, the most significant accretion isn’t from the clouds producing the snow, but from those around the snow-producing clouds. Those normally contain the highest liquid-water content.
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Labels: Accident Statistics, Flight Hazards, Learning Center, Pilot Skills, Safety, Weather, Weather Flying, Weather Skills, Winter Weather