November 1, 1997
Winter is just around the corner, and once again it's time for pilots to include the risks of icing conditions in their preflight planning and in-flight decision making. Airframe icing and strategies for dealing with it were addressed in last month's column (" Wx Watch: Wrestling the Iceman," October Pilot), so I'd refer you to that article for more information on the subject.
However, airframe icing, per se, isn't the only weather problem that pilots must consider. It's just that airframe icing, and icing accidents, get the most press. Don't get me wrong — airframe icing is a serious problem. But there are other cold-weather meteorological risks out there. Although they are often overlooked, these other risks either cause or contribute to many winter weather-related accidents.
In preparing the AOPA Air Safety Foundation's Safety Review of General Aviation Weather Accidents, I asked the ASF's computer to come up with all the icing accidents between 1982 and mid-1993. The raw statistics and accident summaries came from the National Transportation Safety Board (NTSB), but it was the ASF's Emil Buehler Center for Aviation Safety that crunched the numbers and did the sorting.
It turned out that the NTSB recorded 637 icing accidents in the 10.5-year study period. Of those, 27 percent (172 accidents) involved fatalities.
The ASF's search of icing accidents used ice or icing as keywords. As a result, carburetor icing, induction system icing, and even icy runways turned up in the batch of 637 icing accidents. Some may find fault with this methodology, asserting that airframe icing and all other ice-related phenomena belong in separate categories. Maybe they do, but the decision was made to keep all icing events in the same subset in the interest of consistency and in view of time constraints.
I learned something from the mélange of icing statistics that emerged. Carburetor ice, for example, caused far more accidents than airframe icing encounters. We have no idea of how many carb ice encounters had fatal outcomes, but the accident numbers alone make one sit up and take notice: 326 — or about 51 percent of all "icing" accidents — were attributed to carburetor or induction system icing. Airframe icing was involved in 262 crashes, or 41 percent of icing accidents.
It wasn't so much the carb or induction system ice itself that caused the preponderance of these accidents. Instead, the pilot's failure to use, or improper use of, carburetor heat or alternate induction air was blamed.
Carburetor ice forms when intake air experiences a decrease in temperature as it travels through the carburetor venturi. It can happen in a wide range of temperature conditions — anywhere from 105 to zero degrees Fahrenheit (40 to minus 18 Celsius). No, that 105 degrees was not a typo. This means that if relative humidities are high enough, carburetor ice can form in just about any weather — and you don't have to be in cloud or precipitation for it to build. The envelope for the most severe buildups of carburetor ice is between 65 and 100 percent relative humidity and 25 to 65 degrees Fahrenheit (minus 4 to plus 18 degrees Celsius). That's why carburetor icing happens most often in the colder months of the year.
The initial signs of carburetor ice can include engine roughness, an unexplained drop in revolutions per minute (in airplanes with fixed-pitch propellers) or manifold pressure (in airplanes with constant-speed propellers), and a drop in exhaust gas temperature. Pilots flying airplanes with carburetor temperature gauges can check to see if the instrument's indicator is in the yellow arc, meaning that the carburetor venturi is cold enough for ice to form there.
For advice on proper application of carburetor heat, consult your pilot's operating handbook. Most recommendations advise applying full carburetor heat and leaving the heat on for an extended period of time.
It's essential to apply carburetor heat at the first suspicion of carburetor icing and to avoid any temptation to return the carb heat control to the Off position should any roughness or vibration occur. The roughness is doubtless caused by the water from the melting ice being ingested by the engine. Wait long enough and the roughness should go away. Shut off the carburetor heat prematurely, and the engine will build more ice — and probably quit because of air starvation.
Because carburetor heat is provided by engine exhaust heat ducted to the carburetor, there obviously won't be any carb heat available if the engine quits — a big, big reason to turn on full carb heat and leave it on.
The fact that you fly with a fuel-injected engine doesn't mean you're exempt from induction system icing. True, you have no carburetor and, therefore, no chance of carburetor icing. But ice can still form over the intake air scoop or air filter, depriving the engine of air and thereby causing it to quit.
With icing like this, the first signs are apt to be a loss of power, as evidenced by a drop in manifold pressure or a slight change in engine sound level. This is the signal to activate the alternate induction air door or doors. When these doors open, intake air routes through them, bypassing the ice-blocked normal induction air pathway.
Many alternate induction air systems activate automatically; these designs use spring-loaded doors. Suction in an ice-blocked air intake draws these alternate air doors open. Some older fuel-injected airplanes have alternate air doors that must be manually opened. Knobs or levers — usually located beneath the instrument subpanel — have to be physically moved to the Open position in order for alternate air to reach the engine. With this design, pilots have to take the initiative and take timely action to prevent engine stoppage.
It can be easy to overlook the alternate air in the confusion and stress of an icing encounter. In preparing the ASF's safety review of the Piper Comanche and Twin Comanche, I found numerous instances in which pilots experienced engine stoppages in icing conditions. Most of the time, these pilots forgot to open the alternate air doors. However, there were cases in which alternate air doors in certain Twin Comanches were frozen shut by the very ice they were meant to combat! This unforgivable design flaw was corrected by a service bulletin and an airworthiness directive that required the directing of heated exhaust air to the alternate air door mechanisms.
The moral: Know your alternate air system and use it as part of a checklist item should you inadvertently enter icing conditions.
A pilot of a non-ice-protected airplane learns about icing forecasts and reports during a weather briefing, takes off into or continues flying in the ice, and crashes. At least 68 accidents fit that approximate description in the ASF's 1982 to 1993 study period. The mysteries behind this kind of poor decision making are the subject of ongoing speculation, and are best addressed in a separate forum. These kinds of accidents closely fit the mold of the typical VFR-into-IMC accident (there were 580, 226 of which occurred in the winter months), with the added wonderment of contemplating why anyone would knowingly risk flying into icing conditions. It just goes to show that you can preach icing avoidance until you're blue in the face, but a small group of determined pilots still manage to convince themselves that they're exempt from risk.
These examples prove that it's not just low-time, VFR-only pilots who make serious errors in judgment when it comes to dealing with winter weather. This winter, let's all minimize our risk by really paying attention to warnings of adverse conditions, not just paying them lip service.
E-mail the author at firstname.lastname@example.org.
Safety and Education,
Aircraft Power and Fuel,
VFR into IMC
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