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Avoiding Aircraft Icing

by Darren Smith, CFII/MEI
from IFR Checkride Reviewer, December 2003
CFIDarren Newsletter, January 28, 2011

The conversation went like this:
     Student:  "There's  ice developing on the windshield." 
     Instructor:  "What are you going to do about it?"
     Student:  "I don't know."

    Any ice that develops on an aircraft indicates immediate action is required.  How ice forms on an aircraft and what happens is required knowledge for any IFR pilot.  The potential hazards include:

  • blocking air intakes, static ports, carb air filters
  • jammed flight surfaces 
  • airfoil changes, especially the disruption of the laminar flow during rime ice encounters
  • increased drag resulting in changes in stalling speed
  • increased fuel consumption
  • weight & balance concerns
  • and a reduction in lift.
Significant changes in aircraft performance can occur in less than 10 minutes in severe conditions.  Types of ice include:
  • Clear Ice: a smooth, glassy/clear ice formed by slowly freezing water of large supercooled water droplets.
  • Rime Ice: a frosty/milky color ice formed by quick freezing of small supercooled water droplets.
  • Mixed Ice:  a combination of both types, brittle, hard, rough surface.
Ice conditions are reported as defined in the AIM:
  • Trace:  Ice becomes perceptible.  Rate of accumulation is slightly greater than sublimation.  Deicing/ anti-icing equipment is not utilized unless encountered for an extended period of time (>1hr).
  • Light:  The rate of accumulation may create a problem if flight is prolonged in this environment (>1hr).  Occasional use of deicing/ anti-icing equipment removes or prevents accumulation.  It does not present a problem if the deicing/ anti-icing equipment is used.
  • Moderate:  The rate of accumulation is such that even short encounters become potentially hazardous and use of deicing/ anti icing equipment or flight diversion is necessary.
  • Severe:  The rate of accumulation is such that deicing/ anti icing equipment fails to reduce or control the hazard.  Immediate flight diversion is necessary.
When can you expect icing conditions?  Anytime you fly in visible moisture (clouds & rain) between +2C and -10C. 

Strategies for reducing your risks:

1.  Weather Briefing Tips 

  • Areas of FZRA (freezing rain) and FZDZ (freezing drizzle) are very likely to produce icing conditions. 
  • Convective sigmets imply severe icing potential.
  • A report of 50% relative humidity usually implies icing conditions.
  • Closer to  the center of a low pressure system, moisture moves upwards faster.  This results in icing conditions at higher altitudes.
  • Look for amended TAFs which indicated better information about icing conditions.
  • Obtain pireps for the route of flight but remember that pireps are a snapshot at a given moment.
2.  Never fly with frost on your aircraft.  Using water to remove frost creates clear ice.  Use the sun to remove it and be sure the airframe is dry prior to takeoff. 

3.  Use early warning devices on your aircraft:  corners of windshields, through windshield thermometers, and wing strut steps are all early warning devices for icing conditions. 

4.  Any time you experience visible moisture (fog, clouds, rain), use pitot heat immediately.

5.  Stay clear of clouds if possible.  Clouds are visible moisture and are likely to provide you an icing experience to remember, particularly at the tops.  If you are flying through a cloud which is causing ice buildup, the bottom of the cloud is less likely to give you more ice than the top.  You might be able to descend 1000 feet to clear the cloud or stop the icing.

6.  If you are experiencing ice, leave the area of precipitation, or leave that altitude.  Use the rule of 4000:  it will give you an eight degree shift in temperature which will likely end ice accretion. 

7.  If you are leaving an area of icing by climbing or descending, use gentle control movements.  Climb and descend at moderate angles of attack. 

8.  Induction icing can be just as serious a problem.  Use carb heat when icing conditions exist.  If using carb heat and alternate air, lean the mixture to improve engine performance and increase engine operating temperature. 

9.  If you are experiencing icing conditions, notify and obtain priority handling from ATC. 

10.  Give pireps enroute.  Example:  Cessna 123A over ABC VOR, 2200Z at four thousand a Cessna 172.  We are experiencing trace ice, outside air temperature is 2C.

11.  The standard rate 180 degree turn back out of icing conditions may not produce the result you're looking for so always have a way out.

12.  During landing, use a stabilized approach with minimal use of flaps and avoid large power changes.

Please write to me with your comments and tips for avoiding aircraft icing.


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