Photo © UCAR
Winter usually means great flying weather, but the cold weather also brings its own set of challenges: Snowy runways, cold preflights and dangerous icing conditions. For general aviation pilots, one of the biggest risks of flying in cold weather is the possibility of structural icing.
According to AOPA, aircraft structural icing was the cause of more than 150 accidents over a recent period of 10 years. Icing can be present, even when it's not forecasted. As pilots, it's important to know about weather patterns and what to do should icing begin to develop on your aircraft in flight.
I read an accident case study recently that I think serves as a good reminder to stay alert for icing conditions, even if icing is not in the forecast. You can view the details of this particular accident case study, in which a pilot of a Cirrus SR22 en route from Reno/Tahoe International Airport (RNO) to Oakland, California encountered inadvertent structural icing conditions and crashed into mountainous terrain, here.
By all accounts, the pilot performed his preflight preparation responsibilities normally. According to AOPA's accident analysis, he spent time reviewing the weather, including a full weather briefing, which stated that there were no AIRMETs or SIGMETs or precipitation in the area. The Cirrus SR22 was even quipped with an icing protection system, although it was not approved for flight into known icing. So what went wrong?
The NTSB's probable cause report states that the cause of accident was the pilot's loss of control due to an inadvertent icing encounter. Interestingly, the report also cited an inaccurate weather report from the NWS Aviation Weather Center as a contributing factor.
While there was a low probability for icing conditions according to the NWS, there were a few red flags that may have changed the outcome for this Cirrus pilot if he took notice and reacted. For example, the weather briefer indicated a low freezing level of 6,000 feet and approaching precipitation, although it was dry at the time. Additionally, as the Cirrus departed, an incoming Southwest Airlines 737 reported unforecasted moderate rime icing at 17,000 feet. The weather report also stated cloud tops were between 17,000 and 20,000 feet - well above the service ceiling of the Cirrus. This should have alerted the pilot that climbing above the clouds wasn't an option.
This accident is testimony that as pilots, the more we know about weather and icing conditions, the safer we'll be. While forecasts are helpful, the lack of forecasted icing conditions doesn't always mean that we're in the clear. Below are a few reminders and tips for winter flying.
- Know your aircraft systems: There's a difference between de-icing and anti-ice equipment, and there's a reason that many aircraft with anti-ice systems are still not approved for flight into known icing conditions. Research and learn about your aircraft's specific systems and how it will react in icing conditions.
- Know your weather: There are a few items to pay close attention to in the weather briefing, such as cloud tops, freezing levels, and of course AIRMETs, SIGMETs and PIREPs. But a good review of weather theory is helpful in determining the effects of incoming weather systems and fronts, as well as certain areas like over mountains or near water, where icing is common.
- Know when and where icing occurs: Icing usually forms when the aircraft surface (NOT the outside air temperature) is below freezing AND where there is visible moisture. Icing can also occur inside of a "wet" cloud - a cloud with super-cooled liquid water droplets in it.
- Know what to do when you encounter icing: You have three options if you encounter icing in your aircraft: Climb, descend or turn around. Which one of these you choose will depend on the cloud height, temperatures and your location and terrain. (FYI: Contrary to what many people are told, climbing is not always the best option, as the case study above demonstrates!) If there's even a small chance that you'll encounter icing conditions on your flight, it's best to fully prepare beforehand with multiple exit strategies. Know the cloud tops, the temperatures and the locations of alternate airports. And always communicate to ATC immediately if you find yourself in an icing situation. The pilot in the case above failed to notify ATC when he experienced icing. Instead, he spent 10 minutes trying to trouble shoot on his own. Perhaps the controllers could've helped route him into a warmer, cloudless area had they known more about his situation.
For more tips and tricks about how to prevent icing and what to do if you encounter it, check out AOPA Safety Advisor: Aircraft Icing.