The FAA’s misinformation about airframe icing is like getting a gift of ice in the wintertime. For years we pilots have been taught that airframe icing can be expected when the outside air temperature is within 10 degrees F. of freezing and we are flying in visible moisture.
The same information is made applicable to jets but in most instances airliners have no anti-ice or deice over their tail surfaces, just heated leading edges of their wings and engine inlets.
Now there is a dearth of information out there of just what a manufacturer has to show the FAA to get Known Icing Certification and for good reason. If you knew, you would ground your airplane in icing conditions.
I was flying my Eclipse Jet out of Pueblo Co. yesterday and into the clouds that I had just vacated on my arrival. Even though the layers on approach were one layer at FL 200 (twenty thousand feet) and another at 12,000 feet there was no ice accumulated.
As I climbed toward the front range of the Rockies on my departure Westbound and looked at those nasty looking clouds over the mountains I remembered the words Orographic Cooling from my distant past and several cases I handled where jets were quickly overcome with ice in the mountains.
Orographic Cooling occurs when the winds are thrust up the windward side of the mountains and as they travel ever faster into the higher elevations the droplets of water that are clouds become supercooled.
If you have the misfortune to fly through some of it, even though there is no warm front overriding a cool surface, and no SLD (supercooled liquid droplets associated with warm rain dropping into cold air below and forming water that forms ice on contact) , you will accumulate ice, usually rime ice, at temperatures and at flight levels you never dreamed of.
So, as a precaution I turned on the engine inlet heat and waited. Well it didn’t take long for the airframe ice to start accumulating. Milky white rime ice on the leading edges of the wings began as the outside air temperature exceeded minus 12 degrees C! The ice continued to accumulate, though the deicing boots shed it quickly and effectively, through FL 240 and OAT of minus 20 degrees C. For those who speak only F, the ice started at 10 degrees F. and ended at about minus ten degrees F., well below any temperature the FAA has told us to expect airframe icing.
- Everything we have been told about airframe icing is useless when flying in, over or near mountains.
- Airframe icing can occur at temperatures well below the “within ten degrees of 32 degrees F”.
- Airframe icing can continue all the way up into the flight levels.
Now many will read this and say that all it means is that flying in the mountains is different than non-mountain flying. That might be true but only two of the jet icing accidents I have handled occurred in the mountains and all of the turboprop icing accidents I have handled were in the flatlands.
Arthur Alan Wolk
January 22, 2020