and are caused by rain, snow or ice crystals falling through a layer of warmer air at lower altitudes. However, the latter’s droplets have a much higher water content. Although aircraft are different, expect icing to occur (in the engine intake area, anyway) whenever the OAT is below 4o C. Otherwise, it can form in clear air when humidity is high. Clear ice is found most often in cumulus clouds and unstable conditions between 0 and –10 degs C, and rime ice in stratiform clouds between –10and –20. Pitot head, static vent and fuel vent heaters should be on whenever you encounter icing, together with anything else you feel is appropriate. Try not to use deicing boots until at least ½ inch of solid (not slushy) ice has formed, otherwise they will merely stretch the ice covering and operate inside the resulting cocoon. Waiting a while at least gives you the ability to crack the ice off. I know that some experts have determined that this is not the case, but, trust me, they’re wrong. If you operate the boots too early, the ice coating on them will merely flake and stay stuck on. Boots on horizontal stabilizers, by the way, will be less effective due to their geometry. You need warmer air to get rid of ice effectively – just flying in clear air can take hours, but I suppose you could at least say you won’t get any more. Climbing out is often not possible, due to lack of performance or ATC considerations, and descending has problems, too – if you’re getting clear ice, it’s a fair bet that the air is warmer above you, since it may be freezing rain, which means an inversion, probably within 1000 feet or so, as you might get before a warm front. In this position, landing on your first attempt becomes more important as you are unlikely to survive a goaround without picking up more of the stuff. You basically have three choices, go up, down or back the way you came. Going up is a good first choice if you know the tops are nearby, if only because you won’t have a chance to do so later, but you do present more of the airframe to icing risk, which is why there is often a minimum speed for climbing in icing conditions. Before going, check the freezing level is well above minimum altitudes, which will help get rid of ice in the descent. Try to make sure the cloud tops are within reach as well, or you have plenty of holes. Turbulence This also exists high up, not so much due to convection, but the passage of fronts or mountain waves. You can’t see the evidence of its existence as there is little moisture to form cloud, hence Clear Air Turbulence. If turbulence is likely, mention it to the cabin crew and advise the passengers to return to, and/or remain in their seats, ensuring their seat belts/harnesses are securely fastened. Catering and other loose equipment should be stowed and secured until the risk has passed. Fly |