Most of us don’t give much thought to the outside air temperature (OAT) gauge, but it’s there for some very good reasons. It provides the information necessary to make true airspeed, density altitude, and other calculations. It also serves as an early warning of potential icing conditions. Temperature between zero and minus 10 degrees Celsius? Expect clear icing if you happen to enter a cloud or precipitation. Between minus 10 and minus 12 Celsius? That’s where rime icing tends to form.
The OAT gauge is stone-simple, and although it may appear crude, it’s surprisingly accurate. It consists of three main components: a probe, a bimetallic strip, and a dial-and-pointer indicator. The probe picks up the air temperature, then sends it along to a thin strip of metal that’s wound into a coil. The metal is really two different metals, usually steel and copper, fused one on top of the other. Each of these metals responds differently to heating or cooling. To use a term from physics class, they have different coefficients of thermal expansion. As the probe heats the coil, the outer band of copper expands, straightening the coil. With cooling, the inner band of steel contracts, causing the coil to tighten.
Meanwhile, the coil, which is mechanically connected to the OAT’s pointer, moves so as to show the temperature on the dial’s face. Older OATs use the Fahrenheit scale as the primary indication of temperature. Newer ones use the Celsius scale, which is rapidly becoming the coin of the realm in modern meteorology, pilot’s operating handbooks, and aviation in general. Even so, both scales are usually represented, so that all pilots know the score—no matter their preference.
Weather words
Clear ice. A glossy, transparent ice formed by the relatively slow freezing of supercooled water is referred to as clear ice. The terms clear and glaze are used for the same type of ice accretion. This type of ice is denser, harder, and sometimes more transparent than rime ice. Temperatures close to the freezing point, large amounts of liquid water, high aircraft velocities, and large water droplets are conducive to the formation of clear ice.
Rime ice. A rough, milky, opaque ice formed by very rapid freezing of super-cooled droplets as they strike the aircraft is known as rime ice. The rapid freezing results in the formation of air pockets in the ice, giving it an opaque appearance and making it porous and brittle. Low temperatures, lesser amounts of liquid water, low velocities, and small droplets are conducive to the formation of rime ice.
Mixed ice. Mixed ice is a combination of clear and rime ice formed on the same surface. The shape and roughness of the ice is most important aerodynamically.
