AC 00-6A (Aviation Weather) Chapter 11 states:
Do's and Don'ts of Thunderstorm Flying
- To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of -15 C.
I'm trying to understand how flying above an altitude where -15 degrees Celsius occurs would help me avoid icing. I'm also trying to understand how flying below the freezing level would help me avoid icing. What is the explanation?
The icing occurs due to the presence of supercooled liquid droplets. From FAA AC on Icing:
The condition most favorable for very hazardous icing is the presence of many large, supercooled water drops.... Thus, heaviest icing usually will be found at or slightly above the freezing level where temperature is never more than a few degrees below freezing.
For ice to form on the aircraft, the following conditions are required:
The presence of liquid water i.e. rain or cloud droplets.
The temperature at aircraft structure should be at or below freezing point.
Now, as the altitude increases, temperature decreases. Using this, the best way to avoid these conditions would be to either:
Fly above the altitude where the temperature is below -15 degrees, where supercooled water drops are not present (they would've frozen already), or
Fly below the altitude where the temperature is above freezing point, where the water droplets, if present are not supercooled.
Temperature decreases with increasing altitude.
The “freezing level” apparently means level, or altitude, where the temperature equals freezing point. Therefore below it, temperature is higher than freezing point and so no freezing occurs (though keep in mind that temperature above the wing is slightly lower than ambient due to adiabatic cooling, so icing actually starts at temperature slightly above 0°C).
Now to explain the altitude above -15°C we need to mention that icing occurs when the atmosphere contains “supercooled” water droplets that are still liquid at temperatures below freezing that can freeze when they come in contact with the aircraft. But at temperatures below -15°C most droplets have already frozen into tiny ice pellets. Since those won't stick to the aircraft, icing decreases again when it's too cold.
Icing on aircraft relies on liquid water hitting the airplane and freezing onto its surfaces.
The typical outside air temperature range for ice formation is from -20 °C to 2 °C. Water in the atmosphere can become supercooled to below 0 °C, and then freeze into ice upon contact with the aircraft. Below -15 or -20 °C, the water is much more likely to have frozen into ice particles, which do not accumulate as ice would. This is why the colder temperatures will prevent further ice formation.
If the temperature is above 2 °C, it will not be supercooled. While it's possible that the water could freeze to a cold surface, the wing will typically not remain significantly colder than the surrounding air.
I'm also trying to understand how flying below the freezing level would help me avoid icing. What is the explanation?
Even on a warm summer day, temps can be below freezing high up in a towering cumulus cloud. Playing around the edges of a towering cloud, I've had the yaw string of a sailplane get frozen to the windscreen, when it was 85 to 90 degrees or more Fahrenheit on the ground below. Air temps at that level were undoubtedly below freezing. The people on the ground were not experiencing icing, because they were well below the freezing level. They may, however, have been below the sweating level as well.
Perhaps what you are not understanding is that "below" is a reference to altitude, not temperature.
