It’s time to watch the Outside Air Temperature Gauge and determine your altitudes based less on winds and more on how cold it is … or suffer the consequences.
FLYING UNDER THE ICE
Flying small airplanes, we have virtually no protection against ice. The good news is that, as a VFR pilot, if you stay out of the clouds your risk of ice accumulation is greatly reduced. When you climb, watch the OAT gauge and take note of the freezing level. If you *do* start to see a trace of ice on the leading edge of the wing or windscreen, you can descend back down to the altitude that you already know is warmer than the freezing level. This solves the problem — as long as the freezing level is still at a safe altitude.
But what if the freezing level is at the surface?
In the winter many flights will be made entirely in sub-freezing temperatures. Again, VFR pilots can avoid the majority of the ice threat by remaining outside of clouds, but there is one ice problem that can attack you even in clear air: Freezing Rain.
This threat can quickly become hazardous. Freezing rain, as the name implies, attaches and freezes to the airplane on contact. It can quickly add an ice coating to the airplane that will increase the weight of the airplane, freeze control surfaces, and even seal exit doors shut. Freezing rain is unique in formation. Snow forms in air that is already below freezing, but freezing rain first forms as regular rain in air that is above freezing. The rain then falls through air that is below freezing. The raindrops become ‘super-cooled’ and will then freeze instantly on the first surface it touches.
How can warm rain, fall into cold air and produce freezing rain?
When warm air advances on cold air it does so slowly, because the cold air is dense and stubborn to move. The warm air rides up and over the cold air and forms a temperature inversion. Clouds and rain will form in the warm air along the boundary with the cold air. Result: The rain must then fall through the cold air to reach the earth’s surface. Any airplane in that cold air and under the clouds is in jeopardy.
Normally the air gets colder as you go up in altitude, but when a warm front rides over a colder air mass a temperature inversion forms. It is called an inversion because things are inverted or up-side-down from normal. A warm front does not actually have to be warm and, in the winter, they seldom are. To be considered as a warm front, the air behind the front must be just a little warmer than the air it is replacing. Air that is 15 degrees F replacing air that is 10 degrees F would be a warm front. Note: 15 degrees is not ‘warm.’
DEFENSE (Part 1)
First, look at a weather map to see where the warm fronts lie. The line depicts where the warm front touches the surface. The problem will be out ahead of the front and in the cold air below and the effect of a warm front can stretch out for several hundred miles ahead of where the actual warm front line is drawn on the chart.
DEFENSE (Part 2)
Next, compare the different altitude levels on the Winds and Temperatures Aloft chart and locate the temperature inversion. Sub-freezing temperatures low and above-freezing temperatures above are an inversion and a freezing rain threat. When you see a warm front within 300 miles and temperature inversions along your route – look out below!
The best solution to freezing rain is complete avoidance, but if you do get caught in it you must take action to get on the ground fast. If the freezing level is above the surface, you should descend immediately (terrain permitting) and get into above-freezing air. Another solution to a freezing rain encounter is to climb – because the presents of freezing rain means that warmer air is above, but this might not be a practical solution in a light airplane. The warmer air might be much higher than you could climb. In the winter, don’t disregard the warm front!