Icing Wisdom From TBMOPA Safety Committee
Checking the Weather.
In order to accurately forecast the likelihood of icing along your route of flight you need to know the following; the temperature range and moisture content of the air, the stability of the air – are there fronts or mountain ranges than can lift this air? You must know the cloud tops and bases along the route of flight in case you need to exit icing conditions and of course you must know what he underlying terrain elevations are.
Knowing the reasons for cloud formation along your route; icing can be most intense at the tops of clouds because this is where the air has the most moisture, most cooling and the highest condensation levels. The best signal as to what kind of stability there is in the air mass is given by the kind of
cloud formation, Stratiform or Cumulus Clouds.
Stratiform clouds develop in uniform horizontal layers usually because they are formed by lifting layers of stable air. They generally contain lower amounts of liquid water. They generally cover larger areas, and generally the icing layer in Stratiform clouds is less than 3000 feet and icing is found below 15000 feet. In Stratiform clouds if encountering icing, activate the deicing system and climb or descend 3000 feet.
Cumulus Clouds are formed when there is ample moisture and the air mass is unstable. The vertical growth of these clouds depends on how unstable the air is. They are usually limited in horizontal area. The icing in these clouds can cover many thousands of feet. In cumulus clouds the icing encounters are generally short but severe, and the formation of SLD is more likely. The horizontal extent of these clouds is generally 2-6 miles. Icing is generally found from 27000 feet and below at temperatures from -2c to -20c. Pilots around Cumulus clouds at temperatures between -2 and -20c should remain clear of cloud, navigate laterally around the cloud. Icing in these clouds is normally severe and a substantial risk to flight.
Geographic Effect: Knowing the terrain is important because although icing can occur anywhere, it is greater near large bodies of water. These bodies of water add heat and moisture to the system and induce instability which increases the risk of ice. Topography is also important from the aspect of terrain, this generally limits the pilot decision of climbing or descending out of ice to climbing only. Knowing the direction of the system in mountainous areas is also useful because as air traverses over a range and rises icing encounters are more likely on the windward side. When being routed along a
mountain range always consider re planning the route to be along the leeward side to reduce the possibility of encountering ice.
Frontal Effect: Need to know the kinds of fronts that you will be flying through. What is a front? It is where 2 air masses of different temperatures and or pressures collide. There are 3 kinds of fronts; warm, cold and occluded fronts.
Warm fronts are where warmer air slides over colder air, they are discernible by stratus cloud formations over wide areas. During the winter these can be very dangerous as the warm air rising above the sub-freezing air may result in the formation of freezing rain or drizzle. Signs of approaching the front are where cloud layers build quickly and the clear air between the layers disappears rapidly. So what should you consider when flight planning? If you route will take you along a front, consider crossing the front perpendicular to the direction of movement and flying behind the front. Look at the low altitude and prognosis charts for forecasts of freezing rain.
A cold front is where the colder air under cuts the warmer air. There are 2 types of cold front, a classic cold front and a shallow cold front. A classic cold front is one that is characterized by extensive cumulus cloud development which straddles the front. The classic cold front is also characterized by narrow bands of cumulus clouds, and thunderstorms, with heavy precipitation, turbulence, hail and high levels of super cooled water. If flying in these areas consider flying behind the front, be careful when penetrating these areas because of turbulence and the possibility of substantial icing. Classic cold fronts normally occur in warmer weather.
A shallow cold front is characterized by widespread stratus clouds behind the front and one where the temperature inversion is not very deep. Where possible routing should be attempted ahead of the front, and although significant icing can be encountered, a climb or descent of 3000 feet will generally get you clear.
Occluded fronts occur when warm air is trapped between 2 cold air masses and is forced to a higher altitude. One could experience conditions of both hot and cold fronts. The risk of severe icing is higher in the vicinity of a warm or occluded front.
Before flying in icing conditions know your capabilities and those of the aircraft. Bear in mind that small amounts of ice can result in huge degradations in the performance and handling characteristics of your plane. Climb rate decays, cruise speed decreases, stall speed increases, the plane will have lower service ceilings, and increased fuel consumption resulting in reduced range. Experiments have shown that within 30-60 seconds lift can be reduced by 20-30%, critical angle of attack reduced by 8 degrees which brings about a substantially higher stall speed.
Small amounts of ice change the aerodynamic effects of the wings and control surfaces considerably. Of more significance however, there are no early warnings of the change in handling effects. Small amounts of ice will raise the stall speed of the wing by as much as 15 knots, this may also cause disruptive airflow over the ailerons causing the aircraft to behave in unusual ways, for example ailerons may deflect without pilot input causing uncommanded rolls. Wing stalls normally occur at slower speeds or following premature flap retraction.