WX Watch: The zero point

In many parts of the nation we’re still in icing season, when OAT becomes critical—especially when flying on instruments. That’s when you want to watch for OAT indications in a narrow band right around the 0-degree Celsius mark.

Zero Celsius (32 degrees Fahrenheit) marks the danger zone for the absolute worst icing conditions. Any time temperatures range from minus 5 to plus 5 degrees Celsius in cloud and precipitation, you not only run the risk of encountering clear icing—a slick, transparent layer of ice accretions—but more specifically, clear icing of the sort dubbed supercooled large droplet (SLD) icing. At the relatively warmer temperatures near 0 Celsius, air can hold more moisture than the truly cold temperatures in the minus-20 to minus-40-degree Celsius range, where (rime) icing is unlikely to occur in the moisture-starved air.

More moisture means a higher liquid water content and larger water droplets, so if you’re seeing 0 Celsius on your OAT and you’re in clouds or rain, take a look around for the signs of SLD icing. These include ridges of ice forming at the edges of the windshield, large drops splattering on the windshield and leading edges, and any other unusually large ice accretions on unprotected surfaces such as wing tips, fairings, antennas, strakes, and fences.

What you may not see are signs of the worst manifestations of SLD icing. When supercooled large droplets hit an airplane, they strike the leading edges and, at temperatures near freezing but not yet frozen, have enough mass and fluidity to run back and coat entire airfoils. At some point this runback ice can then freeze into ridges and shapes that can disrupt the flow of air past ailerons and other control surfaces. These ridges and shapes form well aft of the leading edges, so don’t expect deice boots to prevent them. We know this from grim experience. Investigators learned that large droplet icing—then called “supercooled drizzle drops”—was in large part responsible for the fatal October 31, 1994, crash of an American Eagle ATR–72 at Roselawn, Indiana.

Any time temperatures range from minus 5 to plus 5 degrees Celsius in cloud and precipitation, you not only run the risk of encountering clear icing, but clear icing of the sort dubbed supercooled large droplets.Yes, the ATR–72 has flight into known icing conditions (FIKI) certification, but no matter. An asymmetric wing stall situation caused a rolling moment that disengaged the autopilot and sent the airplane spiraling to the surface at 300-plus knots. It didn’t help that the crew extended the first notch of flaps—an unapproved procedure in icing conditions—in an attempt to decrease the airplane’s pitch attitude. The high nose angle should have warned the crew that the airplane was approaching a stall, so lowering the flaps merely served to mask the critical nature of the flight regime. The lesson: Ice protection systems and FIKI certification are no antidotes against clear and SLD icing.

Maybe pilots disregard OAT gauges because they tend to be fairly crude in light airplanes. A probe extends into the relative wind, and a bimetallic spring attached to the temperature pointer expands and contracts to move the temperature pointer. Newer displays use thermocouple sensors to post temperature indications on a primary flight display or multifunction display. Still, there may be erroneous indications. If those errors are on the high side—even by a few degrees—you can’t be sure you’re not flying in or near the 0 Celsius range.

Ram rise can also cause false-high temperature readings, and this is not a phenomenon limited to turbine airplanes flying near Mach 1. Caused by frictional heating of the temperature probe, ram rise can cause airplanes flying as slow as 150 knots to show OAT readings up to three degrees higher than the actual air temperature. Many OAT probes have holed, cylindrical sleeves that try to protect the probe from ram rise effects.

True, turbine-powered airplanes have systems designed to show the true OAT, called the static air temperature (SAT). This is the temperature that would exist should be airplane somehow be stopped in midair. These systems use an algorithm that converts SAT into ram air temperature (RAT) and then converts it into a total air temperature (TAT) reading for use in calculating Mach numbers and other performance data. TAT often is higher than SAT, so many manufacturers recommend turning on pitot and static port heaters based on SAT readings. When SAT plus 10 degrees pops up on the screen, that’s when you turn on the heat.

All of which brings us to the main points. Be aware of the OAT, know that temperatures between plus 5 and minus 5 Celsius are conducive to the worst clear and SLD icing, avoid any icing conditions the best you can, and take immediate measures to escape icing conditions should you encounter them.

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