October 19, 2012
By Dan Namowitz
Here’s a wing design you may not have seen before. The leading edge protrudes two inches above the top wing surface, and this radical ridge runs rearward roughly a foot. Similar mods grace the tail.
Care to pilot the test flight?
Avoiding that duty will be easier if you focus some attention during winter weather briefings on certain numbers that are trending downward. For refresher purposes, here’s an example. FRZLVL...RANGING FROM 040-135 ACRS AREA 080 ALG 20NE YSC-40ESE HUL 120 ALG 20E HMV-20S SBY-50SE HTO-140ENE ACK.
That’s a typically general prediction of seasonal freezing levels over a sprawling swath of airspace. But don’t dismiss it as area forecast boilerplate. For pilots of many piston singles, those numbers place freezing levels from minimum en route altitudes right up to aircraft service ceilings.
It’s still a degree or two above freezing at your cruise altitude? Nice, but as a student pilot, you learned to build in safety margins for fuel loads, gust factors, and takeoff distances. Why not do the same for outside air temperature?
Here are two good reasons. Your OAT gauge could be slightly off, and—surprise—your aircraft could pick up ice at OATs several degrees above freezing.
“Aerodynamic cooling can lower the surface temperature of an airfoil and cause ice to form on the airframe even though the ambient temperature is slightly above freezing,” notes Chapter 10 of the Instrument Flying Handbook.
Cutting it close on OAT plays a major role in icing accidents, and when you search for accident reports using “freezing level” as key words, what pops up is a list of mostly fatal events. When someone survives the harrowing ride, the account may describe ice “as large as a house brick on the leading edge, extending back on the wing for one foot,” and “approximately 1 or 2 inches thick on the wing.” (This ice had begun forming at one degree Celsius, the report said).
So unforgiving is icing, you can escape an encounter and still make the accident list. That befell a pilot in Mississippi whose necessary addition of carburetor heat cured a rough-running engine but increased fuel burn, leading to fuel exhaustion and a forced landing.
Some pilots shrug and say, “Ice is where you find it.” More precisely, ice finds you, if you give it the chance.
Dan Namowitz is an aviation writer and flight instructor.
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