In-flight Icing PDF

Module 3 In-flight Icing In-flight icing poses a serious threat to the safety of a flight. Water droplets accumulate on the airframe as ice under specific conditions. The ice deposits alter the wing profile and disrupt the...

Module 3 In-flight Icing In-flight icing poses a serious threat to the safety of a flight. Water droplets accumulate on the airframe as ice under specific conditions. The ice deposits alter the wing profile and disrupt the flow of air. This drastically changes flight parameters such as lift, drag, controllability, etc. As drag increases, lift rapidly decreases - a natural pilot action to compensate for this would be to apply power and increase angle of attack to maintain level, however this leads to even faster ice accumulation as larger airframe surface is exposed. Ice formations on wing and control surfaces can lead to increased stall speed, sudden uncontrolled pitch or roll with difficult recovery and, potentially, eventual loss of control. Moderate or severe icing conditions could overwhelm the anti-ice system commonly fitted on modern aircraft. Safe continuation of the flight could become impossible. The Airframe Ice Accretion Process Ice accretion on an aircraft structure can be distinguished as Rime Icing, Clear/Glaze Icing or a blend of the two referred to as Cloudy or Mixed Icing: Rime Ice Rime ice is formed when small supercooled water droplets freeze rapidly on contact with a sub-zero surface. The resultant ice deposit formed is rough and crystalline and opaque and because of its crystalline structure, it is brittle. It appears white in color when viewed from a distance - for example from the flight deck when on a wing leading edge. Since rime ice forms on leading edges, it can affect the aerodynamic characteristics of both wings and horizontal stabilizers as well as restricting engine air inlets. Clear Ice Clear or Glaze ice is formed by larger supercooled water droplets, of which only a small portion freezes immediately. This results in runback and progressive freezing of the remaining liquid and since the resultant frozen deposit contains relatively few air bubbles as a result, the accreted ice is transparent or translucent. Cloudy or Mixed Ice This blend of the two accreted ice forms in the wide range of conditions between those which lead to mostly Rime or mostly Clear/Glaze Ice and is the most commonly encountered. Its appearance will be determined by the extent to which it has been formed from supercooled water droplets of various sizes. Some other terms which may be encountered in connection with airframe ice accretion include: Supercooled Large Droplets (SLD) "Supercooled large droplets (SLD) are defined as those with a diameter greater than 50 microns” - The World Meteorological Organization. “Supercooled Large Droplet (SLD)....[has] a diameter greater than 50 micrometers (0.05 mm). SLD conditions include freezing drizzle drops and freezing raindrops.2 - FAA AC 91-74A, Pilot’s Guide to Flight in Icing Conditions Droplets of this size are typically found in areas of freezing rain and freezing drizzle. Weather radar is designed to detect large droplets since they are not only an indication of potential in-flight icing but also updrafts and wind shear. Airframe Icing Effects Airframe Icing can lead to reduced performance, loss of lift, altered controllability and ultimately stall and subsequent loss of control of the aircraft. Hazards arising from the presence of ice on an airframe include: Adverse Aerodynamic Effects Ice accretion on critical parts of an airframe unprotected by a normally functioning anti-icing or de-icing system can modify the airflow pattern around airfoil surfaces such as wings and propeller blades leading to loss of lift, increased drag and a shift in the airfoil center of pressure. Blockage of pitot tubes and static vents Partial or complete blockage of the air inlet to any part of a pitot static system can produce errors in the readings of pressure instruments such as Altimeters, Airspeed indicators, and Vertical Speed Indicators. The most likely origin of such occurrences to otherwise serviceable systems has been the non-activation of the built-in electrical heating which these tubes and plates are provided with. Radio communication problems Historically, ice forming on some types of unheated aerials has been the cause of degraded performance of radios but this has not been encountered in the case of modern radio equipment and aerials. Surface Hazard from Ice Shedding Ice shed during in-flight de-icing is not of a size which could create a hazard should it survive in frozen form until reaching the ground below. However, there has been a long history of ice falls from aircraft waste drain masts, a few of which have caused minor property damage and occasionally come close to hitting and injuring people. Source: adapted from https://www.skybrary.aero Este material es de uso exclusivamente didáctico.