When Summer Freezes Over

Hawaii evokes images of short sleeves, warm beaches, and endless warm weather. But for astronomers who work on the summit of Mauna Kea, a dormant volcano on the big island of Hawaii, it reminds them of the ice and snow-covered landscape surrounding their observatory (even in summertime).

The peak of that tropical mountain rises to 13,803 feet MSL, which is right around the service ceiling of a Cessna 172. At almost the same latitude as Jamaica, Mauna Kea receives snow even in June.

Ice continues to be a factor in summertime aviation accidents. Understandably, as the surface temperature warms through Spring and Summer, our minds often leave winter weather concerns behind.  

Unfortunately, pilots may also forget that, in the summer, the refreshingly cool air one expects at 10,000 feet MSL could be dangerously cold, wet air. Just because you’re not wearing a parka during preflight doesn’t mean you don’t have to worry about icing.  

On August 16, 2021, a Cessna 208B was damaged substantially in an icing-related accident. According to the pilot, the airplane climbed to the assigned altitude of 10,000 feet MSL with the autopilot engaged and encountered light rime icing conditions.

The pilot stated that the airplane’s deice boots and propeller heat were operating and removing accumulated ice. The pilot climbed to about 10,500 feet MSL to exit the icing conditions but still encountered them. As a result, the pilot began a gradual descent back to 10,000 feet MSL.

During the descent, the autopilot “suddenly” disengaged, and the airplane entered an abrupt, uncommanded right bank followed by a steep nose-down spiraling descent. The pilot stated that she was “fighting” to regain control of the airplane because the aileron controls were “jammed.” The flight ended with no fatalities or injuries.

The NTSB found that the flight had encountered SLD icing conditions, comprised of supercooled water droplets, such as freezing drizzle and/or freezing rain, that measure greater than 50 micrometers in diameter.

There are at least three takeaways from this accident worth noting.

1. This icing happened in August in the Northern Hemisphere. Granted, it happened in Alaska, but such weather at typical GA altitudes can and does occur in the summertime, even in the tropics. Don’t let your guard down.

2. SLD icing is an insidious type that clings instantly to your aircraft. It’s liquid water, below 0° C, but ready to turn to ice the instant it's perturbed by your leading edge. Further, SLD icing tends to accrete beyond your leading edge, possibly affecting control surfaces such as the ailerons, like in the Alaska accident.

3. Not all airplanes have been tested in SLD icing conditions. No matter whether your aircraft has deicing equipment, it has not necessarily been tested in SLD icing conditions. It's always a good idea to ask your aircraft manufacturer if it did this testing.

Always conduct thorough preflight weather planning, taking icing into account even in June.

Know Before You Go

Do you know how to find specific current and forecasted information on SLD icing conditions?

• Go to aviationweather.gov.
• Click on 'Decision Support Imagery' on the right side of the page.
• Select the 'Icing' product.
• Select your Forecast Hour and Forecast type. 
• Select a Level.
• Under 'Field,' select SEVSLD.
• Those red splotches are where SLD icing is lurking.
• Adjust the Forecast Hour and Level to see how the forecasted information changes. 
• For an interactive display of the latest observations and forecasts, take a look at the Graphical Forecasts for Aviation.