Page 22 - Nov2016TNT Vol 20 No 11

20 • TWIN & TURBINE NOVEMBER 2016From the Flight Deck by Kevin R. DingmanFreezing LevelBoots, Bleeds, Benefits and Beliefslikely won’t explode when you hit the deice switch but it only takes one pneumatic hose or a bleed-air duct coming unfastened for you to lose some, or all, of your system. If the deicing system fails or if it can’t keep up, consider this: In NTSB studies, a pilot diverting due to icing was effective in less than 25% of the cases that reached the required threshold of an accident or reported incident. The predominant sequence of events involves a stall followed by loss of control and impact with the ground. Of the less than 25% that made it to an airport, sufficient performance was lost during the approach so as to force descent below the glide path. Of those that made it to an airport and to a runway, a large number resulted in hard landings. These statistics are why we should avoid or exit all types of icing-- even when our system is working properly.Varieties of IceClear ice is both clear and smooth. Supercooled water droplets or freezing rain strike a surface but don’t freeze instantly. It generally conforms to the shape of the airfoil. Rime ice is rough, opaque and formed by supercooled drops rapidly freezing on impact. Often “horns,” “bowls” or other protrusions are formed and project into the airflow. Rime ice appears white in color. Mixed ice is a combination of clear and rime ice. Frost ice is the result of water freezing while the aircraft is stationary. Frost with a surface texture similar to forty-grit sandpaper is enough to disrupt an airfoil’s boundary layer airflow causing increased drag and a premature aerodynamic stall.SLD ice (Supercooled Large Droplet) is similar to clear ice but because droplet size is large, it extends to unprotected parts of the aircraft and forms larger ice shapes. Runback ice forms when supercooled water moves aft of the surfaces beyond the protected area and then freezes as clear ice. If we encounter any of these typesHe who demands everything that his aircraft can give him is a pilot; he that demands one iota more is a fool.–Len Morgan.You’ve heard it before and have likely said it yourself: being on the ground wishing you were in the air is better than being in the air wishing you were on the ground. Inflight icing can make us wish we were on the ground. Structural icing occurs when supercooled water freezes on impact with any part of the aircraft during flight. Airframe icing can lead to reduced performance, loss of lift, altered controllability and ultimately a stall and loss of control. These are very bad things and wishing won’t help.Pilots that lean more towards VMC flight planning may shake their heads and wonder why anyone would launch into known-ice in the first place. There are two classifications of deicing systems in GA: those for flight in known-icing and “non- hazard” systems that provide time to escape from icing. Known-ice certification is rigorous. In addition to protecting a dozen or so surfaces and components that include the obvious leading edges, windshield, engines and air data probes, the manufacturer must evaluate aircraft tolerance to accumulationon unprotected surfaces such as antennas, landing gear, nose cones, leading edges of flight controls and tip tanks. Most T&T readers operate aircraft in the first classification-- certified for flight into known-ice. The decision to fly into ice lies in our confidence in the system and experience with ice. The worst icing is common in just the top 1,000 feet of cumulus clouds when the temperature is 0°C and lower and ice normally resides in a layer of cloud only two or three thousand feet thick; it can usually be transited quickly. That being said, those that fly in icing know to get out of it as soon as possible, to always use deice and anti-ice equipment and to have a plan if the equipment can’t keep up or if it fails.We Have a ProblemRemember the scene in the movie Apollo 13 when the switch is activated to stir the O2 bottle? The camera follows the electrical current along the wire to the tank. When it reaches the O2 tank, it explodes. When I actuate a control that is seldom used like the alternate air, alternate static or the boots, that scene plays through my mind. You