The FAA says that airframe ice can occur in visible moisture between plus-five degrees C and minus-10 degrees C. Ice can even form when the airframe is cooled below freezing and flown through very humid conditions, even in VFR conditions. This type of risk is called sublimation of water vapor.
Basically, there are three kinds of icing: clear, rime and mixed. Clear ice is formed when large, super-cooled droplets hit the airframe, freezing as they spread along the surface, and it’s usually the kind that is most likely encountered in IFR conditions. It’s the most difficult to remove and it may form beyond the area that is covered by the de-icing systems. It’s almost exclusively found in cumulus clouds. Rime ice is formed when smaller, fast-moving, super-cooled droplets hit the airframe and freeze instantly where they hit. Mixed ice is just what it says: A mix of clear ice and rime ice is formed when droplets vary in size or when snow, various-sized droplets and ice pellets make up the mix that is hitting the plane. Mixed ice is the most dangerous kind of airframe icing due to its weight and disruption to the airflow. It also tends to be the fastest accumulating.
|Static Vent Blockage|
|Flight Stage ||Altimeter Reading||VSI Reading||ASI Reading |
|During Climb ||Constant||Zero||Under |
|During Descent ||Constant||Zero||Over |
|During Cruise ||Constant||Zero||Accurate |
|Takeoff ||Constant||Zero||Under |
| || || || |
|Pilot Tube Blockage|| || || |
|Flight Stage||Altimeter Reading||VSI Reading||ASI Reading|
|During Climb||No Effect||No Effect||Zero|
|During Descent||No Effect||No Effect||Zero|
|During Cruise||No Effect||No Effect||Zero|
|Takeoff||No Effect||No Effect||Zero|
There are other types of icing that are of concern. Induction or carburetor ice is formed when moisture is pulled into the carburetor and when fuel is vaporized, causing the temperature to drop (refrigeration cooling and adiabatic cooling). The moisture freezes, causing the induction to choke. Impact ice can also blank off the air filter under some conditions. Instrument ice is when the pitot system or the static system becomes blocked with ice. Some aircraft have pitot and static-system drains that should be checked during preflight. Pitot and static heaters are anti-ice systems and should be on when the wheels are off the ground. “Saving” the anti-ice systems is unwise, especially if you “save” it and discover that it doesn’t work when you really need it. All aircraft over 12,500 pounds are supposed to have its pitot/static heaters on when the wheels are off the ground—you should, too. The tables to the left show what happens when the given side of the pitot/static system becomes blocked by ice.
So, when the first signs of spring come along, and before hopping into your airplane, be mindful of unforeseen icing, rime and snow conditions that may get you in trouble. Doing so won’t only save your life, but it will also allow you to enjoy more of the wonderful flying that spring is known for.
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