Sunday, January 22, 2012

You May Not Be Able To Prevent Significant Icing From Forming On Your Plane

Even with all the preventative measures in force, you may not be able to prevent significant icing from forming on your plane.

A recent accident in Texas involving a sophisticated turboprop was the result of airframe icing. This plane had pneumatic boots, deicing capability for the props and windshields as well. In spite of that, the heavy mixed icing reported by the pilot brought the plane down. Miraculously, the pilot lived, in spite of having crashed, the plane uncontrollable. Another plane, also recently in Texas, crashed on landing. This was a fancy twin, also with presumed de-icing gear available. The pilot reported having ¼ to ½ inch of ice on the wings (and presumably elsewhere too). He was able to put the plane on the runway, but ran off the end into a fence. I am guessing that he wisely increased his airspeed to account for the negative airfoil effect of the ice, ie stall speed increases significantly due to airfoil changes.

My personnel experience with severe icing came by proxy while resting after a flight at the Burlington, Vt FBO where I worked part time. I was chatting with a Cessna Caravan pilot, who was about to depart on his freight run. The Caravan was a turboprop, large single engine tank of a plane, with "full de-icing". That evening there were two such flights heading out of BTV. Both, I learned later, crashed shortly after take-off with fatal results. Yet, I had flown in to Burlington just a short time before their departures without any icing encountered. It was determined both of these flights had experienced severe icing, the cause of the crashes. Later, Cessna modified the leading edge wing boots so as to extend further back over the cockpit. Unfortunately, it didn't help those two pilots.

So, what’s the problem with having ice on the wings, tail, windshield etc? Mainly two things: the shape of the airfoil is changed and the ice can add significant weight to the plane. “Icing is a cumulative hazard. It reduces aircraft efficiency by increasing weight, reducing lift, decreasing thrust and increasing drag.”* As well, icing can cause instrumentation errors, impair engine performance, cause radio communication problems and more.

What atmospheric conditions are favorable for icing? There must be visible moisture or droplets, and the temperature around the aircraft must be O degrees C or cooler. Under some circumstances however, the OAT may be slightly above freezing and icing still occur. Depending on variations in the above, there are three types of icing that may occur. These are: Clear ice, rime ice and mixed clear and rime. Of the three types, the clear ice may be the hardest to remove by deicing equipment*.

Since I try offer advice what to do if problems are encountered, I will offer this sage bit. Avoidance of ice is the only way to assure a safe outcome. The recent turboprop that crashed after encountering ice (presumed), could have descended to a lower altitude, presumably warmer, and therefore avoided an icing situation. Just because one has all the bells and whistles doesn’t guarantee a good outcome, even if they are all used properly.

One of the ways one can avoid a dangerous weather condition is to listen to the pilot reports on your frequency. They may have been there ahead of you just a few miles in front. Interestingly though, even that is no guarantee of safe passage. There are numerous accounts of several planes flying the same course, separated only by a few miles, each encountering different weather scenarios.

I will offer some advice for those pilots flying a plane with de-icing boots. The latest on how to use the boots is: as soon as any icing is detected on the leading edges of the wings the pneumatic system should be activated. This has replaced old wisdom, which was to wait until a reasonable shell of ice had formed to prevent “bridging” of the ice.

If you want to learn more about the specifics of the type clouds and nitty-gritty of icing consult: Aviation Weather*.

My final advice is to enjoy the ice at the hockey rink, but avoid it all costs when in the air.

*Aviation Weather- For Pilots and Flight Operations Personnel-Revised 1975
DOT, FAA, Flight Standards Service

Another good article:AOPA Safety Advisor, Weather No. 1, Aircraft Icing (Available on the internet)