IFR Fix: Appreciating OATs

November 30, 2011

IFR FixYou’re in instrument conditions at 6,000 feet enjoying a smooth ride. The plan was to fly the route in clear air at 4,000 feet, but the clearance was otherwise.

What made the flight in IMC possible in late fall was a forecast freezing level of 7,000 feet. Nice to log some “actual” at a time of year when the weather isn’t often cooperative.

As your eyes sweep the gauges of the fixed-gear single’s panel, they lock on the airspeed indicator. That indication can’t be right—it’s 10 knots too slow for this cruise power setting. Well, that’s why we keep up the scan (as well as to remain right side up).

A check of manifold pressure and rpm confirms that the power is set correctly. So, what’s going on? It’s not a climb, says the altimeter, corroborated by the VSI. 

A pitot-tube obstruction? Unlikely, but let’s belatedly turn on the pitot heat. Maybe there’s some ice in there.

Seems unlikely—but when you look up at the outside air temperature gauge, in its outcast location, you shiver. The temp sits at 0 degrees C, several degrees lower than forecast for this altitude.

Situationally aware at last, you look around and observe that a thin, draggy line of rime has formed along the horizontal stabilizer’s leading edge. The airspeed slows some more.

Not an atypical beginning to a first encounter with aircraft icing. As you are busy subduing your dismay, ATC calls with instructions: “Climb and maintain eight thousand.”

Treacherous and elusive by nature, airframe ice, the arch-enemy of aerodynamic efficiency and ultimately of safety, demands avoidance. Second best for the unprotected is a prompt escape.

Later, reading up on ice, the pilot would learn that having set a benchmark of 0 degrees C for determining icing risk was incorrect. A few degrees can spell trouble. (What if the rather basic gauge reads a little high?) Then if the minimum en route altitude (MEA) and the cloud bases converge, you can run out of ideas.

Sometimes you see ice first; sometimes you first sense it in a buffet, or read it on an instrument, but that’s cutting things way too close.

Fortunately every aircraft likely has at least one onboard weather system: that outside air temperature gauge. Widen out your cold-weather scan to incorporate the OAT, and plan ahead of the temperature trend for options if your ice-avoidance margin is getting squeezed.