You are flying in instrument meteorological conditions, anxiously monitoring the outside air temperature gauge at your newly assigned altitude, which appears to be very close to the freezing level.
It’s getting dark outside; if only your flight bag were within easier reach, you would fish out your flashlight to inspect the wing’s leading edge again. Trace ice had formed earlier, but it did not increase. Still, that was enough to cause you to ask for a lower altitude, to which the busy center controller replied, "I have your request."
Unable to inspect the leading edge, you can only monitor the outside air temperature and watch for any indication of airspeed decay or altitude loss. Well, the aircraft does seem less responsive in pitch now—but what would that have to do with icing? You discount that as an imagined, stress-induced sensation.
It’s that time of year when freezing levels dip into the lower reaches of the atmosphere and icing becomes a concern for any flight with phases in instrument meteorological conditions or visible precipitation with temperature between plus 2 degrees Celsius and minus 10 degrees Celsius.
It’s fair game on a proficiency check to probe an instrument-rated pilot’s knowledge on that score. But being able to recite reference material by rote does not rate a pilot as risk-reticent.
A two-question quiz may help with that.
First, how much ice on an airfoil’s leading edge does it take to reduce the lifting power of some aircraft by 50 percent, and increase frictional drag by an equal percentage? (A trace? A half-inch? An inch? More?)
The answer, according to Page 11-2 of the Instrument Flying Handbook, is a half-inch. Weigh that against the time available to escape an inadvertent icing encounter so serious that “two to three inches of ice can form on the leading edge of the airfoil in less than five minutes.”
As for that abnormal pitch behavior, is that a warning?
Oscillations, vibrations, or unusual pitch anomalies could indicate an iced-up tailplane. Remember, the tailplane is thinner, and a more efficient ice-accreter than most wings. Typically, it’s also out of the pilot’s sight.
If the tailplane can’t exert the intended tail-down force, the aircraft may react with oscillations, an uncommanded nose-down response, or loss of elevator effectiveness.
"Thus, it is important that the pilot be alert to the possibility of tailplane stall, particularly on approach and landing," the handbook cautions on page 4-15.