Finding the Freezing Level…

awclogoFall flying brings us all back to needing to worry about icing on
our flights, but still gives us warm enough days that flights in IFR conditions may sometimes still be able to be completed. We all
know that icing on airframes is a very dangerous thing to have
happen, even in icing equipped aircraft. It is something we all try to avoid while still completing flights that are important to make happen.

Airframe icing is most common when an aircraft is flown in
visibly moisture (think clouds, snow, or rain) at or below freezing temperatures. The worst icing is typically found in temperatures just below the freezing point where the water moisture is, well, wettest. This is an important factor and is an area pilots should try to avoid flying. Do to this, we need to find where the freezing levels are.

One way to do this is to figure out where the freezing levels are for our flights. By determining where the freezing level is, we can do some simple math.

Standard temperature lapse rates will have a degradation of temperature of 2 degrees (Celsius) per 1000 feet of altitude climbed. Sure, there can be inversions, but this is a good general consideration to make when thinking about a flight. If the temperature on the ground is 6 degrees, we can expect the freezing level to be about 3000 AGL (remember to consider AGL, not MSL altitudes). If the METARs are reporting ceilings less than that 3000′ level you know that climbing into the clouds is likely to result in icing. Ideally, when temperatures are colder, you will want to be through clouds and on top of the weather before temperatures fall below freezing levels where icing in the clouds is likely. Doing the math to consider where the freezing level is going to be and how it relates to ceilings and clouds will allow you to plan more effectively for if icing is going to be encountered at your planned flight altitudes.

Using the NOAA Aviation Weather Center Icing Forecasts page online (http://www.aviationweather.gov/adds/icing) can also be a significant help when considering a flight. This page offers resources that will “predict” icing areas along with proving PIREPs, AIRMETs and SIGMETs. One of the great new tools available on this side is the “Flight Path Tool” (http://www.aviationweather.gov/adds/fptapplication) that allows pilots to enter their flight path and have forecasting data evaluate the icing potentials for that flight.

Freezing level charts are also a good basic check for your area of flight. They can be found at http://www.aviationweather.gov/adds/icing/frzgnav. This allows pilots to get a quick look at where the freezing levels are for their flight.

A few resources are available for pilots when trying to consider the probability of icing for an upcoming flight.

AIRMETs and SIGMETs – An AIRMET “Zulu” or SIGMETs for icing are good keys that a pilot should consider the strong possibility that icing may exist if these are issued for the area in which a flight is going to be conducted. AIRMETs are a little less severe, but for most GA aircraft are enough to cause major flight problems if icing is encountered. SIGMETs are typically more severe and if they exist, unless the aircraft is extremely well equipped, most often mean a GA aircraft will be staying on the ground. The one drawback of AIRMETs and SIGMETs is that they are issued for large areas and may be overly broad. They are however a good cue to pilots that they should look more deeply at icing considerations of they cover the area of the intended flight.

PIREPs – These are reports from pilots of actual conditions and many times include valuable information about current conditions, including icing. When considering these it is important to look at their recency and proximity to your route of flight. These can be obtained in a standard weather briefing, but can also be found online at the NOAA Aviation Weather Center at http://www.aviationweather.gov/adds/pireps

Approach/Tower Controllers – Many times are given to controllers in the air by other aircraft that don’t necessarily make it into the PIREP system. Don’t be afraid to ask controllers when you are in the air if they have had any reports. It is even perfectly reasonable to call the tower ahead of time on the ground and if you have their phone number. These controllers will many times have the most current and accurate information available.

A couple of cautions when considering flight in temperatures where icing may be possible are important.

It is easy to have icing conditions force a pilot to fly lower to get out icing conditions. When this happens, terrain becomes more of a factor. Don’t let the potential for icing force you into making a flight at an altitude that is too low for safety.

Getting trapped on top can also be a problem. As a flight progresses, sometimes clouds become more dense. On top in the clear, the flight may be progressing without problems but at some point a descent will need to be made. Thicker clouds mean that the aircraft will need to be in “the soup” longer and have a greater potential to encounter icing conditions. If you are on top of the clouds and it is -2, you should expect to encounter icing in a descent. A better option may be to turn toward an area of warmer air or where clouds are not present.

Even if the temperature is above freezing above the clouds, sometimes, going down can make the temperature go down also. This is definitely the case when there are temperature inversion, but it can also just happen due to loss of the sun’s heating. I have many times experienced temperatures at just above freezing turning into just below freezing as I descended into the tops of clouds and lost the light and heating of the sun. This is something that can be expected. If there is sufficient altitude to continue a descent, this may be manageable, but if those clouds tops are at 3000′ AGL where you will be vectored to fly an approach, it can quickly become a very big problem.

As we all transition into Fall and Winter flying, icing is a bigger factor. One way to keep this in check is to plan ahead, use resources, and figure out where the real freezing level is before we go flying.


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