First, weight — Page 291, Lesson 276

I want to walk you through the effects of icing on aircraft and then the formation and characteristics of clear ice. Let's start with the general effects. First, weight. In severe icing conditions, ice can form at a rate of one inch in just two minutes. That's a rapid accumulation. Because the ice doesn't build up evenly across the airframe, you get a loss of stability. That uneven distribution can displace the aircraft's centre of gravity, or C of G. Similarly, if ice builds up unevenly on propeller blades, it can cause severe engine vibration. And when that ice breaks off the propellers, the pieces can cause skin damage to the aircraft structure. Next, instrument effects. Ice can block the pressure heads — those are the openings that sense air pressure for your flight instruments. As a result, the readings from your airspeed indicator, or ASI, your vertical speed indicator, or VSI, your altimeter, and your machmeter can all be in error. So you cannot trust your primary flight instruments in icing conditions. Then there are general effects. Windscreens and canopies can become obscured by ice, reducing your visibility. Even a thin film of ice or frost on the surface increases skin friction, which degrades aerodynamic performance. Ice can accumulate in landing gear wells and affect the retraction mechanism. And ice on aerials can cause static interference with your radio communications. Now let's move to clear ice. Clear ice forms when a large supercooled water droplet strikes an aircraft. A supercooled water droplet is liquid water that exists below freezing temperature, typically between 0°C and -20°C. When that droplet hits the aircraft surface, it starts to freeze, and that freezing process releases latent heat — the heat energy given off as the water changes phase from liquid to solid. That latent heat delays the freezing process, so part of the supercooled droplet remains liquid and flows back over the impact surface before it freezes. That flowback is what creates clear ice. Here's a specific number to remember: the amount of a supercooled water droplet that freezes on impact is one-eightieth of the droplet for each degree below freezing. So if the droplet is at -1°C, only 1/80th freezes immediately; the rest flows back. At -10°C, 10/80ths, or one-eighth, freezes on impact, and so on. Clear ice is a transparent form of ice. It appears transparent because there is no air trapped under the flowback icing — the water flows and freezes into a solid, clear sheet. It can be very dangerous. The ice destroys the aerofoil shape of your wings and control surfaces, and because the build-up can be uneven, the weight can cause problems with aircraft control. On propellers, clear ice adheres strongly. When it eventually breaks off, the pieces can be large and cause skin damage, and the uneven weight distribution causes severe vibrations. Clear ice forms in Nimbostratus, or Ns, Cumulus, or Cu, and Cumulonimbus, or Cb clouds, at temperatures from 0°C down to -20°C. This is the most dangerous form of icing because of the speed at which it can build up on the aircraft.