Icing

Question:  What is your experience with icing of instruments--specifically, pitot probes and TAT sensors?
 
Igor Giterman, New Haven, Connecticut

Answer:  Icing conditions exist with the combination of low visibilities and a temperature range from -10 degrees Celsius to +10 degrees Celsius.  Outside this temperature range or in greater visibility conditions ice doesn't form.  Usually we encounter icing conditions on the ground up to maybe 10,000 feet, although they can go higher.  Much higher, though, and it gets colder than is conducive to ice formation.
 
Every instrument pilot (pilots rated to fly when visual conditions do not exist) encounters icing conditions.  Well-equipped airplanes have both engine heat and wing heat which melts the ice right off.  Wing heat is so powerful pilots can't use it on the ground.  If it's accidentally selected, the airplane will automatically shut it off again until the plane is airborne (on airplanes I've flown).

For those who don't know, by the way, a pitot (pronounced "pee-toe") tube measures air velocity to determine an airplane's speed, while TAT stands for "total air temperature."
 
I only had one experience where the ice accumulation was faster than the airplane could handle, but we climbed out of the cloud layer quickly.  Once we broke into the sunlight, the ice layer peeled right off in a matter of several seconds.

Once, when I was flying Metroliners (18 passenger turboprop) years ago for a commuter airline, I had a perfect dome of ice over my nose (well, not my nose but the airplane's nose).  On a whim, I carefully removed it intact.  It looked like a crystal punch bowl and lasted a couple hours before it melted.

Ice has caused crashes, but usually because someone didn't do their job.  Maybe they didn't clean their airplane of ice before takeoff, or didn't use their anti-icing systems correctly, etc.  Or maybe there was an undetected system failure, although such failures should be annunciated to the pilots on their cockpit displays.
 
Pilots don't fly knowingly into heavy icing conditions and in fact are prohibited from doing so, either by regulations, airline policy, or both.
 
Home